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Effects of long‐term straw return on soil organic carbon storage and sequestration rate in North China upland crops: A meta‐analysis

January 2020
Global Change Biology 26(1)

January 2020
26(1)

DOI:10.1111/gcb.15018

Authors:

Berhane Medhn

Tigray Agricultural Research Institute

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Soil organic carbon (SOC) is essential for soil fertility and climate change mitigation, and carbon can be sequestered in soil through proper soil management, including straw return. However, results of studies of long‐term straw return on SOC are contradictory and increasing SOC stocks in upland soils is challenging. This study of North China upland agricultural fields quantified the effects of several fertilizer and straw return treatments on SOC storage changes and crop yields, considering different cropping duration periods, soil types, and cropping systems to establish the relationships of SOC sequestration rates with initial SOC stocks and annual straw C inputs. Our meta‐analysis using long‐term field experiments showed that SOC stock responses to straw return was greater than that of mineral fertilizers alone. Black soils with higher initial SOC stocks also had lower SOC stock increases than did soils with low initial SOC stocks (fluvo‐aquic and loessial soils) following applications of nitrogen‐phosphorous‐potassium (NPK) fertilizer and NPK+S (straw). Soil C stocks under the NPK and NPK+S treatments increased in the greater‐than‐20‐year duration period, while significant SOC stock increases in the NP and NP+S treatment groups was limited to the 11‐20‐year period. Annual crop productivity was higher in double‐cropped wheat and maize under all fertilization treatments, including control (no fertilization), than in the single‐crop systems (wheat or maize). Also, the annual soil sequestration rates and annual straw C inputs of the treatments with straw return (NP+S and NPK+S) were significantly positively related. Moreover, initial SOC stocks and SOC sequestration rates of those treatments were highly negatively correlated. Thus, long‐term straw return integrated with mineral fertilization in upland wheat and maize croplands leads to increased crop yields and SOC stocks. However, those effects of straw return are highly dependent on fertilizer management, cropping system, soil type, duration period, and the initial SOC content.

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2686

Received: 20 September 2019

Revised: 24 December 2019

Accepted: 13 January 2020

DOI: 10.1111/gcb.15018

PRIMARY RESEARCH ARTICLE

Effects of long-term straw return on soil organic carbon

storage and sequestration rate in North China upland crops:

A meta-analysis

Xiaohong Tian

State Key Lab of Crop Stress Bio logy in

Arid Areas/Key La b of Plant Nu trition and

the Agri-envir onment i n Nort hwest China,

Ministry of Agriculture/Coll ege of Natural

Resources and Environment Northwest A&F

University, Yangling, China

Correspondence

Xiaoho ng Tian , State Key L ab of Crop

Stress Biology in Arid A reas/Key L ab of

Plant Nut rition and the A gri-environm ent in

Northwest China, Ministry of Agriculture/

College of N atural Resources and

Environment Northwest A&F University,

Yangling, Shaanxi 712100, China.

Email: txhong@hotmail.com; txhong@nwafu.

edu.cn

Funding information

Nationa l Key R & D Progr am, Grant/Award

Number : 2016YFD0200308;

Key Research and Development Progr am

of Shaanxi, Gra nt/Award Number:

2019ZDLNY01-05- 01; Key Technol ogies

R & D progra m of China, Grant /Award

Number: 2012BAD14B11

Abstract

Soil organic carbon (SOC) is essential for soil fertility and climate change mitigation,

and carbon can be sequestered in soil through proper soil management, including

straw return. However, results of studies of long-term straw return on SOC are con-

tradictory and increasing SOC stocks in upland soils is challenging. This study of

North China upland agricultural fields quantified the effects of several fertilizer and

straw return treatments on SOC storage changes and crop yields, considering dif-

ferent cropping duration periods, soil types, and cropping systems to establish the

relationships of SOC sequestration rates with initial SOC stocks and annual straw C

inputs. Our meta-analysis using long-term field experiments showed that SOC stock

responses to straw return were greater than that of mineral fertilizers alone. Black

soils with higher initial SOC stocks also had lower SOC stock increases than did soils

with lower initial SOC stocks (fluvo-aquic and loessial soils) following applications of

nitrogen-phosphorous-potassium (NPK) fertilizer and NPK+S (straw). Soil C stocks

under the NPK and NPK+S treatments increased in the more-than-20-year duration

period, while significant SOC stock increases in the NP and NP+S treatment groups

were limited to the 11- to 20-year period. Annual crop productivity was higher in

double-cropped wheat and maize under all fertilization treatments, including control

(no fertilization), than in the single-crop systems (wheat or maize). Also, the annual

soil sequestration rates and annual straw C inputs of the treatments with straw re-

turn (NP+S and NPK+S) were significantly positively related. Moreover, initial SOC

stocks and SOC sequestration rates of those treatments were highly negatively cor-

related. Thus, long-term straw return integrated with mineral fertilization in upland

wheat and maize croplands leads to increased crop yields and SOC stocks. However,

those effects of straw return are highly dependent on fertilizer management, crop-

ping system, soil type, duration period, and the initial SOC content.

KEYWORDS

carbon sequestration, crop yield, cropping system, experiment duration, long-term

experiment, meta-analysis, soil organic carbon stock, straw return

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BERHAN E Et Al.

1 | INTRODUCTION

Globally, soil contains the largest carbon (C) pool of any terrestrial

ecos yst em, and so il orga nic carb on (S OC ) is on e of the most impo r t an t

C storage pools. The SOC pool represents a dynamic equilibrium of C

gains and los ses. Even sma ll ch anges in SOC may pot entially add up to

significant changes in large-sc ale C cycling (Fang, Piao, & Zha o, 20 01).

In recent years, a number of studies and countries have paid attention

to C seq ue s tra tio n an d pos si bil it ies for miti gatin g SOC los s by opti mi z-

ing appropriate crop rotation, proper application rates for organic and

inorganic fertilizers, conservation tillage, and integrated soil fertility

management (Srinivasarao et al., 2012; West & Post, 2002).

Increasing the accu mu la tion of organic C in soi ls is a cru cial challenge,

both for soil fertility and for climate change mitigation (Gong, Hu, Wang,

Gong, & Ran, 2008; Lu et al., 2009; Wang, Qiu, et al., 2008). The dynam-

ic s of SOC stock s and the role that the soi l may play in lon g-te rm acc umu -

lation and sequestration of atmospheric CO2 are of incredible concern

because of their great potential for climate change mitigation, sustain-

ability of crop productivity, and soil fertility (Kirchmann, Haberhauer,

Kandeler, Sessitsch, & Gerzabek, 2004; Srinivasarao et al., 2012).

With a yield of 1.04 billion tons, China has a huge amount of crop

straw, almost one-third of the global production in 2015 (Li, Cao,

et al., 2017). But, China's croplands have relatively low SOC content

because of their long cultivation history, low straw return rates (used

instead as fuel and animal feed and burnt in fields), and other losses.

Since abundant nutrients remain in crop residue (e.g., phosphorus

[P], nitrogen [N], potassium [K]), intensified crop residue manage-

ment in farmland helps maintain soil nutrient balance and soil qual-

ity and promotes soil microbial biomass and by-products (Kumar &

Goh, 200 0). Also, China now produces and consumes more chemical

fertilizer than any other country (Sun et al., 2012). Consequently,

the overuse of chemical synthetic fertilizers, coupled with straw re-

moval, have led to soil quality degradation and heavy, adverse agri-

cultural impacts (Liu & Diamond, 2005).

For example, a meta-analysis by Zhao, Sun, et al. (2015) deter-

mined that, compared with straw removal, incorporation of maize,

wheat, and rice residues in Chinese croplands could increase soil C

storage an average of 12%. Likewise, Wang et al. (2018) reported

5.7% higher SOC stock with straw addition compared to no straw

addition after a 33-year-long experiment. Previous research has

also shown that cropping duration has an influence: consistent

straw return boosts SOC accumulation in early years, but that ef-

fect decreases after a decade. Liu, Lu, Cui, Li, and Fang (2014) re-

ported that straw return enhanced SOC sequestration in a trial site

during the first 3 years, but that effect was insignificant in the fol-

lowing 15 years. However, different results have also been reported

(Pittelkow et al., 2015), possibly due to various ecological conditions,

land management approaches, and soil types.

Cropping systems may play a critical role in the change of SOC

stocks by influencing the balance between C inputs through lit-

ter additions and C losses through decomposition (Huang, Sun, &

Zhang, 2012). Double and triple cropping systems are the principal

cropping systems in China, and they have severely degraded China's

agriculture (Dikgwatlhe, Chen, Lai, Zhang, & Chen, 2014). Therefore,

while incorporation of crop straw into soil can be beneficial, it must

be done immediately after harvest because straw's slow biodegra-

dation can lead to unfavorable effect s, such as undegraded straw

interfering with subsequent crop growth, thus disrupting traditional

crop man ag eme nt (Li , Da i, Dai, & Don g, 2018) . Wang et al . (2 01 8), in a

long-term experiment using a double-cropping system in which straw

was returned to the so il along with nit ro gen-p ho sp horous- pot as sium

(NPK+S), fou nd that the SOC stock in th e NP K+S treatment was 5.7%

greater than with mineral NPK alone. However, in a single-maize

cropping system, they found that SOC in the NPK+S treatment was

not significantly different than that in the NPK treatment.

Previous studies have assessed changes in SOC stocks in Chinese

soils and have focused mainly on single fer tilizer management (Ji,

Zhao, Li, Ma, & Wang, 2016); a specific crop (Tian et al., 2015); or a

single-cropping system, particular duration period, and one soil type

(Zha et al., 2015). However, uncer tainties about the long-term ef-

fects of straw return on SOC stocks still exist. Therefore, additional

factors, such as cultivation duration, soil types, cropping systems,

and initial SOC levels, are critically needed to explain why certain soil

C pools react differently to straw return.

Based on the uncertainties and variable reactions of SOC stocks

to straw return under various management practices studied in an

array of single investigations, we conducted a meta-analysis of SOC

stock changes based on data obtained from long-term straw return

and fertilizer management studies conducted in North China up-

land areas that grow wheat and maize crops. A meta-analysis is an

effective statistical method to quantitatively summarize the results

of numerous individual and independent studies and to draw general

conclusions at regio nal and gl obal sc ales to es timate the direction and

magn it ude of a treatm en t ef fect (Guo & Gif ford, 2002). The re fo re, the

main objectives of this study were to (a) quantify the changes in SOC

stocks under various fer tilizer treatments and straw return, in differ-

ent cropping durations, soil types, and cropping systems in 0–20 cm

of th e to p soil; (b) compa re crop yiel ds in long-term fer tilize r an d straw

return treatments; and (c) establish the relationships of SOC seques-

tration rates with the initial SOC stocks and annual straw C inputs.

2 | MATERIALS AND METHODS

2.1 | Data collection

We used keywords related to SOC stock or SOC content in farm-

lands using different fertilizer types and straw return to search peer-

reviewed, English research articles available on Internet databases at

the Northwest A&F University online library (http://www.scien cedir ect.

com, http://link.sprin ger.com, http://apps.webof knowl edge.com). To be

included in the meta-analysis, a study had to meet the following criteria

(A ppe ndix S1): (a) In itial and fin al SOC con te nt or st ock with kn ow n year s

the study st ar ted and ended mus t be provi ded in the public at ion. (b) The

work must have been published in peer-reviewed publications and not

in conference proceedings or books. (c) The study must have been a field

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BERHANE Et Al.

experiment and not a pot experiment or survey study. (d) The soil sam-

pling depth must have been 0–20 cm but, in order to increase the study

count, we included a few papers examining soil depths of 0–30 cm. (e)

The experiment must have included at least one of the following treat-

ment s: Control (no fertilization [CK ]); mineral NP or NPK fer tilizers; min-

eral NP+S or NPK+S; and, to see the effect of fertilizer types, mineral

NPK plus manure (NPK+M) was optional. Only those long-term studies

that were conducted on upland fields in Nor th China and that used con-

ventional tillage practices were included in our analyses.

We compared the SOC response to straw and fer tilizer manage -

ment regimens among dif ferent cropping systems, soil types, and

cropping durations. Cropping systems were categorized as annual

double-cropped winter wheat–summer maize (DC), single-crop sum-

mer maize (SM), and single-crop winter wheat (SW). Experimental

sites were categorized into following three soil types according

to both the general soil classification of China and the Food and

Agriculture Organization of the United Nations classification (in pa-

rentheses): black soils (Luvic Phaeozems), fluvo-aquic soils (Calcaric

Cambisols), and loessial soils (Calcaric Regosols). Cropping duration

was categorized into four intervals, namely 1–5, 6–10, 11–20, and

greater than 20 years, with the minimum and maximum durations

bei ng 1 and 33 yea rs , re spectivel y. Neces sa ry dat a fr om experime n-

ta l si te s, SO C co nt en t or stock, soil bul k de nsity, soil sam pl ing depth ,

cropping duration, cropping system, soil type, and fertilization

regimens were obtained from texts and tables of the publications.

If data were expressed as figures or charts, number values were

extracted using GetData Graph Digitizer Version 2.26 (S. Federov).

Accordingly, we collected 58 published ar ticles with 268 and 139

observations for SOC stock and crop yield, respectively. About 54

of them contained complete SOC data and partial crop yield data,

and the remaining four were ar ticles we included for crop yield data

only. The details of the selected studies and their references are

in Appendices S2, S3, and S4 and site locations are in Figure 1. In

addition, crop yields in the corresponding long-term experimental

sites were collected to qualitatively evaluate the relationships be-

tween SOC stocks at 0–20 cm topsoil depth and crop yields after

straw return and various fertilizer treatments. Using national average

carbon concentrations (National Center for Agricultural Technology

Service [NCATS], 1994), we converted aboveground straw into an

equivalent amount of carbon, assuming 0.4 kg C/kg wheat or maize.

Studies in a meta-analysis are assumed to be independent

(Gurevitch & Hedges, 1999). Therefore, to reduce dependency of

observations, if any study contained duplicate results in different

years for the same field plots, we included only the latest sampling

date in our analysis (except for testing cropping duration effects).

Fertilizer application rates and returned straw quantities were not

considered in our study because of large variations in the amounts

and types of fertilizer and straw used. Additionally, temperature and

precipitation were not considered because of their collinearity with

cropping systems. Therefore, we conducted a meta-analysis using

CK, NP, NPK, NP+S, NPK+S, and NPK+M treatments, but only used

the NPK+M treatment to compare SOC stock changes between

different fertilizer treatments, because of a paucity of data for the

other variables.

2.2 | Data analysis

The following equation was used to convert SOC concentra-

tion in the top 20 cm soil depth to SOC stock (Yang, Mohammat,

Feng, Zhou, & Fang, 20 07):

where SOCS is soil organic C stock (Mg C/ha) and SOCC is soil organic

C concentration (g C/kg), BD is soil bulk density (g/cm3), and D is the

(1)

SOCS=SOCC×BD×

×10−1,

FIGURE 1 Map of China and its

provinces with locations of selected

long-term experimental sites in North

China [Colour figure can be viewed at

wileyonlinelibrary.com]

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BERHAN E Et Al.

measured depth of soil (cm). If a study repor ted soil organic matter, it

was converted to SOC concentration using the coefficient 0.58.

BD is a key parameter used in determining farmland SOC stocks.

Unfortunately, when experimental studies did not report BD values,

we had to use the following equation, developed by Song, Li, Pan,

and Zhang (2005) for upland soils, to estimate BD for those sites:

Because BD ch an ge d due to lo ng-term fer ti lizer use , we also cal cu la ted

initial and final BDs using initial and final SOCC, respectively, for each

treatment in those experimental sites, if it was not provided in the

pertinent publications.

The ca rb on sequ es tration rate (C SR , Mg C ha−1 year−1) was ca lc u-

lated using the following equation:

where SOCf and SOCi are the mean SOC storage during the final year

and in the initial year, respectively, of a specific treatment; and t (year)

is the duration of the experiment.

2.3 | Meta-analysis

Given initial SOC stocks, studies with data including changes to

SOC stocks with different fer tilizer treatments, cropping systems,

soil types, and experimental durations were analyzed using meta-

analysis. The natural log of response ratio (R+) was employed as

effect size (Sanderman & Baldock, 2010). So, for each paired ob-

servation of SOC stocks at the initial and final experimental stages,

we calculated the mean effect size using the following formula

(Hedges, Gurevich, & Curtis, 1999).

where Xf is the mean SOC stock (Mg C/ha) at the final stage, and Xi is

mean SOC stock at the initial stage of each treatment.

To be easily understandable, we repor ted the results as percent age

change, where these changes, influenced by various fertilizer treatments

during long-term investigations, were calculated either by (R+ − 1) × 100 %

or with the following formula. Positive-percentage changes indicate an

increase in SOC stocks, while negative values indicate a decrease.

Means, standard deviations or standard errors, and the num-

ber of replicates are required for meta-analysis (Hedges et al.,

1999; Luo, Hui, & Zhang, 2006), but in most of the studies we

used, the standard deviations or standard errors that would allow

us to calculate sample variances were not provided. Therefore, in

order to include as many studies as possible, we used un-weighted

meta-analysis. For that, the boot strap resampling technique

on MetaWin Version 2.1.4 (Rosenberg Sof tware, Arizona State

University) was used to generate bias-corrected 95% confidence

in te r v als for ea c h mea n ef fe c t siz e ( Ada ms, Gu rev ich , & Rose nbe r g,

1997). Means of the effect sizes of initial SOC stocks compared to

means of their initial levels were considered significant if their 95%

conf id ence in te r vals di d not ove rl ap ze ro (Hua ng et al. , 2012; Zha o,

Sun, et al., 2015).

2.4 | Statistical analysis

We used SPSS 22.0 for Windows (SPSS Inc.) for all statistical analy-

ses and, before analyzing the data set, we performed data qualit y

control and removed outliers. One-way analysis of variance was

used to evaluate the effects that each long-term fertilizer regimen

and cropping system had on crop yields and to compare the effects

of cropping duration, soil type, cropping system, and fertilizers on

SOC stock response ratios between treatment s during long-term

experiments. Linear regressions and logarithmic functions were

used to compare the relationships between straw carbon input and

SOC sequestration rates, and between initial SOC stocks and SOC

sequestration rates.

3 | RESULTS

3.1 | Crop yields in different cropping systems

In the SM cropping system, mean annual maize yield over time and

under different fertilizer regimens were not significantly differ-

ent (p > .05) between the NP, NPK+S, NP+S, and NPK treatments.

Except for NP, all treatments had significantly higher yields (p < .05)

than did CK (Figure 2a). The additional straw return along with min-

eral fer tilizers in the SM cropping system did not affect the result

relative to mineral fertilizer applications alone.

In the SW cropping system, there was no difference between

the average annual wheat yields of the NPK and NPK+S treatment

groups, indicating that straw addition did not affect grain yield in this

cropping system (Figure 2a). We did not include the other fertilizer

treatments used in this cropping system because of too few avail-

able observations or data.

In the DC system, unfertilized (CK) maize had significantly lower

yields than maize in all other fertilizer treatment s. But among fertil-

izer treatments, mean maize yields were not significantly different

between NP+S, NPK , and NPK+S treatment groups. However, while

there was a significant difference between yields in the maize NP

and CK treatment groups, maize yield with the addition of straw in

the NP+S treatment group was significantly greater than that with

NP alone (Figure 2b). In the DC system, wheat yield differences be-

tween the NPK and NPK+S treatment groups were nonsignificant,

but they were significantly higher than yields in the NP+S, NP, and

(2)

=1.377×Exp

(

−0.0048×SOC

C).

(3)

CSR

(

SOCf−SOCi

)

(4)

=ln

(

∕X

i),

(5)

ΔSOC stock =

[

SOCf−SOCi

SOCi

]

100.

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BERHANE Et Al.

CK treatment groups. However, yields in the NPK and NP+S treat-

ment groups were not markedly different (Figure 2b). In summary,

only maize yield under NP fertilization in the DC system increased

significantly with the addition of straw.

In cropping system comparisons (Figure 2c), mean maize yields

wi th NP K app l ica tio n wer e sig nif i can tly hi ghe r in the SM than in the

DC systems (p = .047) and, likewise, between the SM and DC maize

yields with the NP+S treatment ( p = .007). However, we observed

no significant differences in maize yields between the SM and DC

systems in the other treatment groups (CK, p = .396; NP, p = .07;

NP K+S , p = .162). Mean wheat yield under the NPK+S treatment

was significantly lower in the SW system than in the DC system

(p = .034); however, there was no significant difference between

yields in the SW and DC systems for the NPK treatment (p = .34).

Overall, annual crop productivity was significantly greater in

the DC system than in the SM system, regardless of fertilization

treatment except for CK and NP treatments, and also for the NPK

an d NPK+S tr eatme nts in the SW sy ste m (Fi gur e 2d ). In the DC sy s-

tem, the addition of straw clearly increased an nual grain yield over

both NP alone (11.1 vs. 9.2 t/ha) and NPK alone (12.9 vs. 12 t/ha).

3.2 | Response ratios of SOC stocks under different

fertilizer treatments

We estimated the average response ratios of SOC stock to dif ferent

long-term fertilizer treatments (Figure 3). The percentage change of

SOC st oc k fo r th e CK and NP treat me nts was 1% and 4%, resp ect ively,

indicating no significant change in SOC storage between the begin-

ning and the end of long-term experiments (95% bootstrap confi-

dence intervals of means overlap zero in Figure 3). For the NP+S, NPK,

NPK+S, and NPK+M treatments, mean SOC stock changes were 16%,

16%, 31%, and 55%, respectively, thus showing a significant increase

in SOC storage, compared to initial SOC stocks, by the end of the

experiments.

FIGURE 2 Comparisons of means (SE ) of crop yields in dif ferent cropping systems (SM/SW and DC) and different fer tilizer treatments

(CK, control [unfertilized]; NP, mineral nitrogen and phosphorous fertilizer; NPK, NP plus potassium; NP+S, NP plus straw; NPK+S, NPK plus

straw). (a) Dif ferent fertilizer treatments of SM (single-crop maize) and SW (single-crop wheat). (b) Different fer tilizer treatments of double-

crop (DC) winter wheat and summer maize. Bet ween columns, means with different letters (upper case for wheat and lower case for maize)

in (a) and (b) are significantly different (one-way ANOVA, p < .05). (c) Comparisons of yields for each crop in each cropping system and under

various fertilizer treatments. Wheat means with different letters were significantly different and maize means with different letters were

significantly different (one-way ANOVAs, p < .05). (d) Comparisons of annual total yields for each cropping system under each experimental

treatment. Means with dif ferent letters were significantly different (one-way ANOVAs, p < .05). N.A ., no available data

FIGURE 3 Increases in soil organic carbon (SOC) stock (mean,

95% confidence interval) between the beginning (0) and the end

of experiments using different fer tilizer treatments (CK, control

[unfertilized]; NP, mineral nitrogen and phosphorous fer tilizer; NPK,

NP plus potassium; NP+S, NP plus straw; NPK+S, NPK plus straw;

NPK+M, NPK plus manure). (n), number of input data points

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BERHAN E Et Al.

Over all treatments, NPK+M, followed by NPK+S, had signifi-

cantly higher increases in SOC storage than did the other treat-

ments (p < .05). However, there was no significant difference

between the NP+S and NPK treatments (p > .05), and NP and CK

had the lowest and no responses, respectively, compared to the

other treatments. Adding straw affected SOC level changes sig-

nificantly, compared to SOC changes from NP and NPK fer tilizer

treatments alone (p < .05).

3.3 | Response ratios of SOC stocks in different

soil types

Compared to initial levels, SOC stock levels in black soils af ter the

NPK and NPK+S treatments increased significantly (11%, 95% CI

[0.1, 0.23]) and (5%, 95% CI [0.005, 0.1], respectively, Figure 4). For

fluvo-aquic soils, the responses under both NP and NP+S treat-

ments were nonsignificant, with a 3% decrease following the CK

treatment. However, SOC stock changes were significantly higher

in the NPK (18%, 95% CI [0.12, 0.19]) and NPK+S treatments (35%,

95% CI [0.21, 0.35]) compared to initial SOC levels. Also, the mean

SOC stock change following the NPK+S treatment was significantly

higher than all other treatments (p < .05 ). Ne x t , the SO C sto ck le vel s

in loessial soils increased significantly, relative to initial levels, fol-

lowing NPK (21%), NP+S (20%), and NPK+S (34%) treatments, but

not significantly in the CK (9.5%) and NP treatments (3.5%). The

SO C stock i n c r e ase foll owi ng the NP K+S tr eat ment wa s sig nif icant ly

higher than the increases following the NP and CK treatments, but

not significantly different than the changes under the NPK and

NP+S treatments (p > .05). Finally, black soils were not responded

significantly to CK (0.3%) and NP (6%) treatments relative to initial

SOC level. While NP+S treatment for black soils was not reported

in our results due to limited data points.

Following both the NPK and NPK+S treatments, we observed

no significant difference in SOC stock changes between fluvo-aquic

and loessial soils (p > .05), and black soil had a significantly lower re-

sponse ratio than the other soils (p < .05) following those treatments.

Meanwhile, loessial soil following the NP+S treatment responded sig-

nificantly relative to their initial SOC levels (Figure 4). However, there

were no significant differences between fluvo-aquic and loessial soils

under all fertilizer treatments, except that loessial soil had greater

changes in SOC stock following the NP+S treatment than did the flu-

vo-aquic soil. In both black and fluvo-aquic soils, straw added to the

NPK treatments (NPK+S) resulted in greater SOC stock increases than

in th e NPK treatment alone (11% vs . 5% an d 35% vs. 18% for black and

fluvo-a quic soils, respe ctively). Howe ver, we sa w no signifi cant effe cts

from straw addition in loessial soil. Overall, the SOC stock response

ratios in black soil were lower than those of the other soil types.

3.4 | SOC stock response in different

cropping systems

Following both the CK and NP treatments, we observed no signifi-

can t ch a nges in SO C sto cks in bo t h DC an d SM system s (si g nific ance

occurs when confidence intervals do not overlap zero, Figure 5).

FIGURE 4 SOC (soil organic carbon) stock changes (mean,

95% confidence interval) under different fertilizer treatments

(CK, control [unfertilized]; NP, mineral nitrogen and phosphorous

fertilizer; NPK, NP plus potassium; NP+S, NP plus straw; NPK+S,

NPK plus straw) in major soil types. Due to limited data, changes in

the NP+S group in black soil was not used. The ver tical-dashed line

indicates the SOC levels’ starting points before treatments began.

Sample sizes are next to each bar

FIGURE 5 SOC (soil organic carbon) stock changes from

initial levels in each fertilizer treatment in the three cropping

systems. DC, double-cropped summer maize–winter wheat; N.A.,

no available data; SM, single-crop maize; SW, single-crop winter

wheat. See the legend in Figure 2 for the definition of the three

cropping systems and the five fertilizer treatments. The bars

represent 95% confidence intervals. The sample size for each

cropping system is shown next to the bar

2692

BERHANE Et Al.

Following the NPK treatment, the SOC response ratios of two crop-

ping systems, SM and DC, increased significantly (6% and 21%, re-

spectively) compared to their initial levels. But, for this treatment

the increase in the SW system (20%) was nonsignificant relative

to its initial SOC level. Overall, both the SW and DC systems had

significantly greater increases than did the SM cropping system in

NPK (p < .05). Compared to the initial levels in the NP+S treatment,

mean SOC response ratios increased significantly in both the SM

and DC systems (10%, 95% CI [0.05, 0.13] and 18%, 95% CI [0.012,

0.25], respectively) but there was no significant difference be-

tween those cropping systems (p > .05). For the NPK+S treatment,

response ratios increased significantly compared to initial levels in

the SM (14%, 95% CI [0.05, 0.2]) and DC (38%, 95% CI [0.22, 0.37])

systems, but the response in the SW system (15%) was nonsignifi-

cant (Figure 5).

In contrast, response ratios in the DC system were significantly

higher than those in the SM system for both NPK and NPK+S treat-

ments, with a 21% increase in the DC system and a 6% increase in

the SM system following the NPK treatment ( p = .001), and a 38%

increase in the DC system and a 14% increase in the SM system for

the NPK+S treatment (p = .027). However, SOC stock responses to

the NPK treatment in the DC and SW systems were similar (21% and

20%, respectively), while the responses to the NPK+S treatment in

the DC system increased more (38%) than it did in the SW system

(15%), but the difference was not significant between the cropping

systems. Because the SW system had small sample sizes, resulting

in very large confidence intervals, the comparisons with that system

were not as precise as they would have been with a larger sample.

In the DC system, straw added to chemical fertilizer treatments re-

sulted in greater SOC stock response increases than those following

chemical fertilizer treatments alone: NP+S was markedly greater

than NP alone (18% vs. 5%, p > .05) and NPK+S was significantly

greater than NPK alone (38% vs. 21%, p < .01). However, in the SM

system, only the NP+S treatment was significantly higher than NP

(10% vs. about 1%, p < .05), and the SW system did not respond sig-

nificantly to added straw (NPK+S treatment vs. the NPK treatment,

p > .05).

3.5 | Effect of experiment durations on SOC stocks

We grouped each study's experimental periods into four duration

ranges based on the amount of time that each study was con-

ducted and then compared the response ratios of SOC stocks for

each fer tilizer t re at me nt in eac h dur at io n. In t he CK , NP, and NP+S

treatments, most duration groups showed nonsignificant SOC

stock increases (Figure 6). The groups with significant increases

included under 11- to 20-year groups in the NP (22%) and NP+S

(53%) treatments, but in longer periods more-than-20 year groups

lowered to 4% and 13%, respectively, in NP and NP+S. Moreover,

in CK SOC increases were significantly greater than initial levels in

the 1- to 5-year duration group (95% confidence intervals did not

overlap zero).

Under the NPK treatment, response ratios increased signifi-

cantl y co mp ar ed to initial levels (12% [1–5 years], 10% [6–10 yea rs] ,

14% [11–2 0 years] , an d 22% [ m o r e -t han- 20 years] ) , bu t we o b ser v e d

no significant differences between the duration groups. Under

the NPK+S treatment, the response ratios in three groups (6–10,

11–20, and more-than-20 years) increased significantly (22%, 29%,

and 63% , respective ly) compa re d to their initia l levels. The cha nges

between duration groups in the NPK+S and NPK treatments were

significantly higher in the more-than-20-year group than in the 1-

to 5-, 6- to 10-, and 11- to 20 -year groups (p < .05) but not signifi-

cantly different bet ween the 1- to 5-, 6- to 10-, and 11- to 20-year

groups . We obse rved no soil C satu ration tr ends in any of the crop-

ping durations in both the NPK and NPK+S treatments, thus indi-

cating that carbon sequestration may have occurred in the fields

of the studies with the longest duration (more-than-20 years).

However, SOC stock changes increased in the 11- to 20-year dura-

tion group for both the NP and NP+S treatments but decreased in

the more-t han-20-years duration grou p. Additionally, only in the 1-

to 5-year duration group was the SOC stock response in the NP+S

treatment significantly greater than that in the NP treatment alone

(p < .01), while the NPK+S treatment group's SOC stock increase

was significantly greater than that of the NPK treatment group in

the 11- to 20-year duration group (p < .05), but not in the other

duration groups (Figure 6).

3.6 | Relationships between SOC sequestration

rates and annual C inputs from straw

When measuring straw C input s to soils, we examined only the

estimated aboveground C inputs retained by the soil and those

FIGURE 6 Changes in SOC (soil organic carbon) stocks from

their initial levels (0) under different fertilizer treatments (CK,

control [unfertilized]; NP, mineral nitrogen and phosphorous

fertilizer; NPK, NP plus potassium; NP+S, NP plus straw; NPK+S,

NPK plus straw) and across four experimental durations (mean,

95% confidence interval). The sample size for each duration range is

shown next to the bar. N. A., no available data

2693

BERHAN E Et Al.

amounts ranged from 0.05 to 7.36 Mg C ha−1 year−1 and 0.45

to 6.36 Mg C ha−1 year−1 in the NPK+S and NP+S treatments, re-

spectively. In examining the long-term experiments, we observed

significantly positive linear relationships between annual soil C se-

questration rates and the annual straw carbon inputs to the soils

experiencing both straw addition treatments: NP+S (SOC sequestra-

tion rate = 0.13 × annual straw C input + 0.00 4) and NPK+S treat-

ments (SOC sequestration rate = 0.16 × annual straw C input + 0.08;

Figure 7). This implies that the soil in those treatments still had the

capacity to store external carbon. Similar linear relationships were

detected in meta-analyses (Wang, Wang, Xu, Feng, Zhang, & Lu,

2015; Zhao, Sun, et al., 2015) and in observations from Northwest

China (Shuo et al., 2016). Moreover, the slope of the linear equation

was lower for NP+S (0.17) than for NPK+S (0.31), thus demonstrating

that about 17% and 31%, respec tively, of annual straw C input into

the soil was transformed to SOC. We attributed those SOC seques-

tration increases to the C supplied by the returned straw.

4 | DISCUSSION

4.1 | Straw and fertilizer management effects on

SOC stocks

Our meta-analysis specifically examined multiple year observations

of upland grain crops of northern China, focusing on winter wheat

and summer maize. However, differing sample sizes may have af-

fected significance levels, especially when 95% confidence intervals

of small sample sizes are much wider than those derived from large

sample sizes, an effec t experienced in similar studies (Han, Zhang,

Wang, Sun, & Huang, 2016; Tian et al., 2015). With that said, we

found that NP+S, NPK, NPK+S, and NPK+M treatments significantly

increased SOC sequestration, thus indicating that adequate fertiliza-

tion with balanced chemic al and organic fertilizers sequestered SOC

better than did unbalanced fer tilizers (NP) and no fertilization (CK)

during long-term experiments (1–33 years). Potassium is an essen-

tial plant nutrient, second in importance only to N, that increases

crop yield, improves crop qualit y, and is required in large amounts by

plants for proper growth. In our results, crops grown with added K

fertilizer produced more biomass that was returned to the soil than

did crops grown without K fertilizer. Therefore, SOC levels increased

more in those soils that received K fertilizer than in those that did

not. The largest SOC increases, NPK+M (55%) followed by NPK+S

(31% ; Figur e 3), we r e like l y asso cia ted wi t h th e adde d C inp u t of lon g-

term manure and straw additions. Indeed, a global meta-analysis

conducted by Han et al. (2016) revealed that balanced chemical

fertilizers combined with manure or straw resulted in greater SOC

stocks than with chemical fertilizers alone. A similar result was re-

ported by Liang et al. (2019), and Han et al. (2016) claimed that a

substantial C sequestration potential of applying mineral fertilizer

with straw and with manure is very important for either improving

or maintaining current SOC stocks across all agro-ecosystems. Liu

et al. (2010) also repor ted that manure application in combination

with mineral fertilizers more effectively improves soil organic mat-

ter than do mineral fertilizers alone. Synthetic fertilizers integrated

with manure (e.g., NPK+M) further mediated adverse effect s of both

soil acidification and reduction in base saturation caused by chemi-

cal fertilizer inputs alone (Zeng et al., 2017). Thus, supplying enough

nutrients to soil through organic fertilizers is preferred for both soil

fertility improvement and C sequestration enhancement.

Soil nutrients are depleted gradually during continuous cropping

with no fer tilizer, which af fect s SOC dynamics and stability (Banger,

Kukal, Toor, Sudhir, & Hanumanthraju, 2009; Zhang et al., 2010). We

determined that no fer tilizer application (CK) and application of NP

over the long-ter m di d not sig ni fi cant ly cha nge SOC stocks (the ir 95%

confidence intervals overlapped zero), as the stocks stayed at initial

levels (Figure 3). This static result in the CK group was likely due to

relatively high-atmospheric N deposition (Wang, Zhang, et al., 2008).

Later, Zha et al. (2015) showed, in a 19-year experiment without fer-

tilizer, that SOC content decreased by 7%. The difference between

our analysis and that finding may be because only one soil type

(fulvo-aquic) in that small-scale study with only one cropping sys-

tem (wheat–maize cropping system) was examined versus our study,

which averaged results of different soil types (black, fluvo-aquic, and

loessial) and different cropping systems (SM, SW, and DC). Our re sult

is supported by Chen, Zhao, Feng, Li, and Sun (2015) who reported

that long-term unfertilized soils maintain their initial SOC content.

Incorporating organic matter (e.g., crop stubble and root biomass)

FIGURE 7 Relationships between

annual straw carbon (C) inputs and annual

SOC (soil organic carbon) sequestration

rates in the (NP+S, mineral nitrogen and

phosphorous fer tilizer plus straw; NPK+S,

NP plus potassium plus straw) treatments

of different studies using various crop

types [Colour figure can be viewed at

wileyonlinelibrary.com]

2694

BERHANE Et Al.

into agricultural soils comp ens ates for SOC losses throu gh decompo-

sition, especially with double cropping every year. Thus, soil requires

organic C input to increase SOC content.

Our analysis also found that long-term addition of straw, along

with inorganic fertilizers, significantly increased SOC stocks over that

of inorganic fer tilizer applications alone. That is, compared to their ini-

tial SOC stock levels, NPK+S and NP+S treatment groups had greater

increases in SOC stocks (31% and 16%, resp ec tively) tha n did the NPK

and NP treatment groups (16% and a negligible percent, respectively).

Th e same in cre a sin g effe ct wa s obs e r ved in a met a-ana lys is co ndu c t ed

on long-term paddy crops (Tian et al., 2015) in which the NPK+S treat-

ment group sequestered about 1.9 times more SOC than did the NPK

group. Because mineral fertilizers may enrich C inputs only through

increasing root and stubble biomass, they did not directly contribute

organic matter into the soil like straw does. Straw addition provides

large amounts of organic C to cropland soils, thereby promoting soil

microbial biomass and activities, hence increasing soil organic matter

(Lal, Follet t, Stewart, & Kimble, 2007). Therefore, we concluded that

the add it io n of cr op residues resul ts in gr eater C sto ra ge than does the

application of mineral fertilizers alone.

A global scale meta-analysis (Han et al., 2016) and observations

from other studies conducted in upland areas of central China (Zha

et al., 2015) reported SOC increases following straw incorporation.

Th os e stu dies ha d bro ad diff ere nc es in cli mate, soi l t yp e, st udy dura -

tion, and rotation systems, yet the results of SOC increases following

straw addition fell in a narrow range (19.4%–25.2%, Table 1). The

results from our study using data from upland areas of Nor th China

agreed with the above studies since the relative change of SOC

(23.5%) with long-term straw incorporation fell within the range

of those studies (Figure 3, Table 1). In contrast, relative increases

in SOC stocks with straw addition in our study were slightly lower

than those of studies conducted in sub-Saharan croplands (Powlson,

Stirling, Thierfelder, White, & Jat, 2016) and in China (Zhang et al.,

2016) in which the combined range of SOC increases was 26%–38%.

However, our observed increases were greater than those found

by a global scale meta-analysis of upland areas (Liu et al., 2014), a

meta-analysis conducted on a national scale of Chinese croplands

(Zhao, Sun, et al., 2015), and one of European and Nor th American

upland soils (Powlson, Glendining, Coleman, & Whitmore, 2011),

with a combined range of 11%–13.3% (Table 1). The differences

between those studies and ours were probably due to variations in

crop types (from winter wheat in Europe and North America to rice,

wheat, and maize in sub-Saharan Africa to winter wheat and sum-

mer maize in our study), durations (6–56 years in Europe and North

America, 20–30 years in China, and 5.7 years in sub-Saharan Africa

vs. 1–33 years in our study), coverage of the study (from global and

national scale meta-analyses to regional meta-analysis and single-site

studies), and climatic conditions of the various sites (Han et al., 2016;

Liu et al., 2014; Powlson et al., 2011, 2016; Zha et al., 2015; Zhang

et al., 2016; Zhao, Sun, et al., 2015). This array of differing variables

can produce results that disagree with one another, including the

amounts that SOC increases due to straw addition. So, our study's

con cl us io ns sug ge st that we need to con si de r ma ny fac to rs that con-

tribute to variations in outcomes (e.g., crop rotation, study duration,

soil type, initial C concentrations, and climatic conditions) in future

studies. Furthermore, another study (Reicosky et al., 2002) also

found no increases in SOC stock after long-term straw returns, and a

study in North China by Wang et al. (2018) demonstrated that long-

term straw application maintained, and sometimes increased, SOC

compared to the initial SOC level. Perhaps, soil carbon sequestration

is limited by cer tain aspect s of initial soil quality associated with low

inputs of C, initial SOC content, pH, or the total carbon and nitrogen

ratios (Wang, Wang, Xu, Feng, Zhang, Yang, et al., 2015). In addition,

we observed significant linear relationships between straw C inputs

and SOC sequestration rates (Figure 7), indicating that the increase

in SOC stocks in the straw-added treatments was directly correlated

with the amount of straw C input s to the soil.

In co r po r at i on of cr o p re sid ue is not only a maj or so urc e of C inp u t,

but it also helps to control air pollution by returning carbon-based

material into the soil rather than burning it and sending it into the

Study region

Duration

(years)

Land use

type

Relative

change

of SOC

stock (%) Reference

Global 10.2 Cropland 1 9.4 Han et al. (2016)

Short to

long-term

Upland 13.3 Liu et al. (2014)

China Short to

long-term

Cropland 12 Zhao, Sun, et al. (2015)

20–30 Cropland 26–38 Zhang et al. (2016)

Sub-Saharan Africa 5.7 Cropland 34.2 Powlson et al. (2016)

Europe and North

America

6–56 Upland 11 Powlson et al. (2011)

Central China 19 Upland 25.2 Zha et al. (2015)

North China Short to

long-term

(12. 8)

Upland 23.5 This study

TABLE 1 Studies of soil organic carbon

(SOC) changes with straw addition

2695

BERHAN E Et Al.

atmosphere. As chemical fertilizers are the main source of nutrients

used for crops in China, their excessive use with a relatively low ef-

ficienc y contributes to environmental problems (Zhang, Wu, & Dai,

20 0 4). Ma ny fin dings that agree d wi th ou r re su lts als o co ncl ud ed th at

the application of organic materials, combined with chemical fertil-

izer, is the best approach for increasing SOC content (Meng, Ding,

& Cai, 2005; Wang, Li, & Qiu, 2004). Farm management practices

greatly influence biomass productivity by changing SOC content,

thus influencing crop yield while possibly increasing atmospheric CO2

concentra tions, which leads to clim ate change (Wang, Qiu, Tang, Li, &

Li, 2007). Generally, the effect of chemical fertilizers combined with

straw not only decreases the required amounts of chemical fertiliz-

ers, but also improves SOC content and, by extension, contributes to

very efficient and environmentally friendly farm management. Thus,

carbon sequestration could be attained through long-term crop resi-

due application or straw return, especially in our study area in China.

4.2 | Response ratios of SOC stocks in different

soil types

Changes in SOC stock levels probably depend on initial levels in

farmlands (Stewart, Paustian, Conant, Plane, & Six, 2007). In our

meta-analysis, we obser ved a lower SOC stoc k response ratio in the

long-term duration experiments (range 1–33 years) in cropland soils

that had greater initial SOC stocks, regardless of soil types. Black

soils had the highest initial SOC stock in our results (Figure 8); how-

ever, its SOC stock response ratio was the lowest of the three soils

measured in most of the fer tilizer treatments (Figure 4). This is con-

sistent with the result of a meta-analysis by Li, Shi, et al. (2017) of a

long-term experiment looking at fertilizer treatments and different

upland and paddy (rice fields) soil types. It shows that, for both up-

land and paddy fields, SOC stock change depends on th e ini tial SOC

stock . In our stud y, higher response ratios in fluvo-a quic and loe ssial

so ils (Fig ure 4) ma y be at tribu ted to ini t ia l SOC st ock leve ls (2 2 .5 an d

26.2 Mg C/ha, respectively), which were lower than those of black

soils (35.1 Mg C/ha). The lower the initial C content, the far ther a

soil is from C saturation level (Stewart et al., 2007).

In addition, we used a logarithmic function to describe the rela-

tionship bet wee n an nu al SOC se qu est ra ti on rates an d in it ia l C stock s

(Figure 9). SO C seque stration rates in all treatme nt s wer e wea k, with

R2 ran gi ng fro m .11 to .19, but sig nific an tl y negativel y corr el at ed wit h

initial SOC stock amounts, except in the NP+S treatment in which

there was a positive, but weak and nonsignificant , correlation. These

results were mainly due to large differences in C sequestration rates

caused by differences in C inputs, fertilizer rates and types, dura-

tions of fertilizer regime, and cropping systems (Appendix S2). A

meta-analysis by Li, Shi, et al. (2017) revealed a significant negative

correlation bet ween SOC sequestration rates and initial SOC stocks

in upland soils planted in maize and wheat and in paddy soils planted

in rice. However, another meta-analysis in China paddy soils using

single-cropped rice, double-cropped rice, and double-cropped rice–

wheat found no apparent relationship ( Tian et al., 2015). The differ-

ences in similar meta-analyses may be due to climatic conditions and

cropping system variations bet ween upland and paddy soils, espe-

cially since decomposition in anaerobic paddy soils is low, even if ini-

tial SOC contents are high (Li, Shi, et al., 2017). Moreover, initial SOC

levels probably control the sequestration of soil C in farmlands (Sun,

Huang, Zhang, & Yu, 2010). Initial soil C concentrations influence

the stabilization of added C, and perhaps soils having lesser C stocks

have the greatest potential and ability to sequester C because they

are farther from their saturation levels than are soils with greater C

stocks (Stewart, Puastian, Conant, Plane, & Six, 2008). Thus, SOC

sequestration rates will likely depend on initial SOC stocks.

4.3 | Crop rotation systems, SOC stocks, and

crop yields

In this study, greater SOC sequestration occurred in all cropping sys-

tems under the NPK, NP+S, and NPK+S treatment s than in the CK

and NP treatments (Figure 5), mostly because of greater C inputs

resulting from higher crop productivity in the fields treated with

adequate fertilizer and straw return (Mandal et al., 2007). This indi-

cates that soils that received sufficient nutrients from either fer tiliz-

ers or both fertilizers and straw return experienced enhanced SOC

ch an ges , due main ly to gre ate r C in put s . The se in put s aro se fr om crop

residues and root-related C, and from straw addition resulting from

higher crop productivity in the NPK, NP+S, and NPK+S treatments

than in the CK and NP treatments. In comparison, SOC changes in

the DC system were significantly higher than those in the SM sys-

tem, compared to their initial levels, using both NPK and NPK+S fer-

tilization, with increases of 21% in DC versus 6% in the SM systems

FIGURE 8 Initial SOC (soil organic carbon) stocks (Mg C/ha) in

the top 20 cm of soil in the selected studies of major agricultural

soil types in Nor th China. The thick lines within the boxes represent

medians. Vertical bars show minimum and maximum values and

* and ° denote outliers. Numbers in parentheses are numbers

of input data points and the bars with different letters indicate

significant differences (one-way ANOVA, p < .05) [Colour figure can

be viewed at wileyonlinelibrary.com]

2696

BERHANE Et Al.

under the NPK treatment (p = .001), and up 38% in DC versus 14%

in the SM systems under the NPK+S treatment (p = .027). This can

be partially explained by the lower whole-year crop productivity and

less plant-derived C input in a single-cropping system (Huang et al.,

2012). Moreover, under NPK treatments, the SW system response

was significantly higher than that of the SM system, but nonsignifi-

cant under the NPK+S treatment. This indicates that SM cropping re-

sponds less to SOC than do the other cropping systems. Wang et al.

(2018) compared data from different experimental sites in northeast

China and suggested that the continuous, single-cropping maize sys-

tem is less efficient at sequestering C from added C inputs than are

systems that include two crops per year. Single cropping provides

less stored SOC, even in paddy crops, as Huang et al. (2012) found

in a national scale meta-analysis of different cropping systems used

in paddy fields in China. They observed lower SOC sequestration in

the rice single-cropping system than in the double-cropping system

using rice–wheat and rice–rice. Our result was also supported by the

linear relationships between the SOC sequestration rates and the

straw C inputs (Figure 7), and Wang, Wang, Xu, Feng, Zhang, Yang,

et al. (2015) reported a significant correlation between changes in

SOC and the amount of C input. This implies that SOC stock re-

spo nses to crop ping syst ems, in paddie s as we ll as in uplan ds , depend

on the amount of C input supplied to the soil throughout the year. In

addition, Mandal et al. (2007) suggested that SOC stocks could be

enhanced in cropping systems that provided more C than the criti-

cal value of C losses. In contrast, the SM system experienced lower

SOC stock increases than did the SW system, perhaps because the

decomposition rates of wheat and maize residues dif fer because

of their different climatic conditions. The high temperatures of the

maize-growing season foster a faster decomposition rate of organic

materials generated by maize root systems than the decomposition

rate during the cooler wheat-growing season, resulting in a lower re-

tention coefficient for the maize season than for the wheat season

(Wang, Wang , Xu, Feng , Zhang , Yang , et al ., 2015; Zheng et al ., 20 09).

So, while SOC increased in both the DC and SW systems under both

the NPK and NPK+S treatment s, the increase for the SW system was

statistically nonsignificant (p > .05) , po s sib ly be c aus e the SW syste m' s

small sample size resulted in large confidence intervals (Gurevitch &

Hedges, 1999; Han et al., 2016).

Adding straw to the fertilizer regimes of the DC system clearly

resulted in significantly greater SOC stock responses, compared to

initial SOC levels, than those with chemical fertilizers alone. While

single-cropping systems responded slightly to straw return, except

in the SM system in which the SOC increase under NP+S (10%) was

significantly greater than the NP treatment increase (1%). The signif-

icant SOC stock increases in the DC system was most likely due to

the higher annual amounts of straw returned to the soil from double

crops. Therefore, both straw addition and cropping system affected

SOC stock change. Because of low C inputs, single-cropping systems

(SM and SW ) ha d le ss sto re d soil C tha n di d th e DC system (Zh ao , Su n,

et al., 2015 in a previous meta-analysis conducted at a national level).

We also examined grain yields in different fertilizer treatments

and cropping systems. We found that the yields of both the wheat

and maize crops in the DC system fertilized with either NPK,

FIGURE 9 Correlations between the annual soil organic carbon (SOC) sequestration rates and initial SOC stocks in each of the five

fertilizer treatments (CK, control [unfertilized]; NP, mineral nitrogen and phosphorous fertilizer; NPK, NP plus potassium; NP+S, NP plus

straw; NPK+S, NPK plus straw) [Colour figure can be viewed at wileyonlinelibrary.com]

2697

BERHAN E Et Al.

NP+S, or NPK+S were greater than the yields of crops with NP

fertilization and no fertilization (CK; Figure 2). Similarly, examining

a wheat–maize rotation system, Zhao et al. (2014) reported lower

grain yield in NP treatments compared to NP treatments combined

with K fer tilizer or returned straw. That yield difference was asso-

ciated mostly with the essential nutrients in the balanced chemi-

cal fertilizers and the returned straw compared to the insufficient

nutrients in the NP and CK systems. However, we saw no yield

differ ences between treatments in the SM systems, except that the

CK yield was significantly lower, even though SOC stocks varied

with treatments. Partially in line with our results, Chen et al. (2015)

found that all fer tilizer treatments significantly enhanced crop pro-

duc tion compared to unfe rtil ize d crops , but the yield dif feren ce be-

tween fertilized treatments was not significant for both the wheat

and maize crops. They suggested that inorganic fertilizers, straw,

and manures, when N, P, and K are applied at equivalent rates, have

similar effects on crop yields. In our met a-analysis, due to a large

range in the quantities of applied fertilizer or returned straw in the

sel ec te d st ud ies, we did not consider the tot al amounts of nutrien ts

applied for each treatment. Therefore, the total nutrients applied

in these treatments may be similar and may have resulted in similar

grain yields. When comparing cropping systems, maize yield in the

SM system was significantly higher than it was in the DC system

with NPK (p < .05) and NP+S (p < .01) applications. Most likely, pro-

ductivity was lower than in a single-season system because of the

moisture lost by tilling twice per year and the intensive cropping

and high nutrient utilization in the DC system. However, the mean

yi el d of wh eat in the NPK+S tre at m en t was signif icantl y low er in the

SW system (3.92 t/ha) than in the DC system (6.2 t/ha, p < .05). This

difference in wheat yields may have been because the DC system

benefited from two seasons, cold and warm, in which the mobiliza-

tion of available nutrients from fertilizers and soils differed (Wang,

Wang, Xu, Feng, Zhang, & Lu, 2015 in a previous meta-analysis).

Therefore, the higher wheat yield in the DC system might be at-

tributed to two seasons of applied nutrients and tillage frequency.

In the DC system, the combination of straw and chemical

fertilizers resulted higher grain yields than those produced with

chemical fertilizers alone: NP+S yield (11.1 t/ha) was higher than

the NP yie ld (9.2 t /ha) and NPK+S yield (12.9 t/ha) was higher than

the NPK yield (12 t/ha). These overall yield increases under straw

return may have been due to additional nutrient inputs and im-

provements in the soils’ biophysical and physicochemical proper-

ties (Liu et al., 2014). During the past few decades, huge amounts

of chemical fertilizers have been used to maximize crop yields,

thus improving food security throughout the world (Savci, 2012).

Ho w eve r, th ose me t h o ds c an degr ade so i l th roug h com p a c t ion and

acidification, leading to crop yield reduction (Horrigan, Lawrence,

& Wal ke r, 200 2) . Us e of ch emica l fer tilizers ha s als o decre as ed so il

bacteria community richness and diversity and has significantly

changed soil bacteria community structure (Ramirez, Craine, &

Fierer, 2010; Sun et al., 2016). Similar to our result s, Zhao, Jiang,

et al. (2019), in their eight-consecutive-year field experiment using

the wheat–maize cropping system in northwest China, observed

significantly higher crop yields in a combined straw and chemical

fertilizer treatment than with no straw addition. Integrated appli-

cation of chemical fertilizer and crop straw is an important field

management practice that can help maintain both soil quality and

productivity (Bhattacharyya, Kundu, Prakash, & Gupta, 20 08;

Shuo et al., 2018; Zhao, Ning, et al., 2019). It improves the phys-

ical, biological, and chemical properties of the soil (e.g., porosity,

moisture, and constituent substrates), thus aiding the activity and

communit y structure of soil bacteria (Zhao, Wang, & Jia, 2015).

Straw return likely produces increased plant-derived C, sequesters

SOC, and, at the same time, achieves optimum crop productivity

(Shuo et al., 2016). Similar to the results obtained in our study, a

meta-analysis of upland and paddy crops in China compared crop

yields and SOC stock changes with and without straw returns and

found that long-term, continuous straw return leads to overall in-

creases in both yields and SOC (Wang, Wang, Xu, Feng, Zhang, &

Lu, 2015).

Average, year-round, overall crop yields were higher in the DC

system under all treatments than in both the SM and SW crop-

ping systems. So next, we considered the effec t of cropping sys-

tem on both SOC change and annual total crop yield, and found

that the DC system is preferable to both single-cropping systems

per year. The winter wheat–summer maize cropping system is

the most common and important cropping system in the North

and North-Central China Plain. Double-cropped fields in this re-

gion cover 16 million ha and account for about 25% of the na-

tional grain yield (Zhang et al., 2015). Because China's agricultural

production is so large, especially of wheat, it not only feeds its

own population but also exports wheat beyond its borders. Our

study area in North China is very important for that produc tion

and, interestingly, this study has revealed that when straw was

combined with chemical fertilizers, SOC stock increases in the DC

system were greater than those in both single-cropping systems,

an ef fect mainly associated with greater amounts of crop residue

being returned to the soil in double cropping compared to that

returned from just one crop per year. Abundant nutrients (e.g., N,

P, K) remain in crop residues and by intensif ying management of

th ose re sid ues , fa rme r s c an he l p mai ntai n soi l nutr ien t bala nce an d

quality and can promote soil microbial biomass and beneficial mi-

crobial by-product s (Kumar & Goh, 2000). Thus, straw return is

an effective way of promoting agricultural production and, along

wit h annu al dou bl e-croppin g whea t an d maize, is an adv anta geous

and recommended method that can enhance both soil fertility

and annual crop yields. In general, we observed lower responses

of grain yield to different fer tilizer treatments and cropping sys-

tems than to SOC response. North China is a dryland area that

receives low annual rainfall, hence crop productivity and fertilizer

effect on grain yield are highly limited by precipitation and the

low productivity response is linked to dry weather (Fan, Xu, Song,

Zhou, & Ding, 2008). Finally, all fertilizer treatments in our study

included mineral N fertilizer, which decreases decomposition of

organic matter, thus leaving greater amounts of undecomposed

crop residues than in soils without mineral N fertilization. This, in

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BERHANE Et Al.

turn, increases the efficiency of SOC sequestration in the soil (Li,

Jia, et al., 2017).

4.4 | SOC stock responses and

experimental durations

The SOC response ratios of four experimental cropping duration pe-

riods (1–5, 6–10, 11–20, and more-than-20 years) varied among fer-

tilization treatments and were slightly time-dependent. SOC stock

response ratios in the CK treatment did not change significantly in

longer durations but did increase by 6% in the 1- to 5-year period.

Declines in SOC stock over time may be due to the lack of C inputs

and supplementary fertilizers, which would both compensate for C

losses during nutrient consumption by growing crops. For the NP and

NP+S treatment groups, SOC stock responses were higher, relative to

th e ini tia l SOC stoc k lev els , in th e 11- to 20 -ye ar pe rio d (22 % and 53% ,

respectively), but the responses in the more-than-20-year periods

wer e only 4% and 13%, respecti vely (Fig ur e 6). Similar to our NP+S re-

sults, a meta-analysis across major agricultural zones of China (Wang,

Wan g, Xu, Feng , Zh ang , & Lu , 2015 ) ob ser ved th at lon g-te rm st raw re -

turn increased SOC stocks during the 10- to 20-year duration period,

but then the increases declined in longer periods. This result indicates

that SOC enhancement decreases after 20 years of fertilization with-

out K. This persistent lack of K may eventually adversely affect plant

growth, thus reducing C input from stubble, roots, and rhizodepo-

sitions. In contrast, the annual average straw C input following the

NP+S and NPK+S treatments was 3.21 and 2.84 Mg C ha−1 year−1,

respectively, with average annual SOC sequestration rates of 0.4 and

0.7 Mg C ha−1 year−1, respec tively (Figure 7). Th at indicated a higher C

input but less sequestration potential in the NP+S than in the NPK+S

treatment. Moreover, SOC enhancement in longer periods may be

influenced by higher than average, initial SOC stocks in the NP and

NP+S treatments compared to those in NPK and NPK+S treatments

(Figure 10). However, the relationship between the C sequestration

rate and the initial C content in the NP+S treatment (Figure 9) was

weak (R2 = .01) and differed from other treatments, possibly because

the NP+S treatment had a smaller sample size than did the NPK+S

treatment. According to the soil C saturation hypothesis, changes in

SOC stocks may be affected by the initial soil C content levels: the

lower the initial C content, the farther from saturation (Stewart et al.,

2007). Considering that hypothesis and the upland fields included in

our meta-analysis, it seems that SOC sequestration in longer duration

periods was influenced by the initial soil C levels.

In NPK and NPK+S treatments, the SOC stock response ratio in-

creased incrementally in almost all duration periods (Figure 6). For the

NPK treatment group, the highest SOC stock mean response ratio

occurred in the more-than-20-year duration period, but there was a

slight dif ference between duration periods. A study in North China

by Gao, Yang, Ren, and Hailong (2015) determined that application of

balanced inorga nic fer tilizer s, with or wit ho ut adde d manur e or straw,

can significantly increase SOC content over long periods. Similarly,

our NPK+S treatment group had a significantly higher response

ratio (63%; p < .05) in the more-than-20-year duration period than

in periods less than 20 years. A meta-analysis by Tian et al. (2015)

also suggested that application of mineral NPK plus straw increased

the decomposition and mineralization rates of crop residues. This

resulted in greater C storage, compared with the application of NPK

only, as applications continued through time before reaching a new

equilibrium. This increase in SOC in their NPK treatment group was

due mostly to high C input from crop stubble and remaining roots,

whereas in the NPK+S treatment group, the increase was most likely

due to high C input from returned straw, as well as from crop residue

resulting from higher crop productivity (Tian et al., 2015).

In the long-term experiments, we observed no C saturation trend

in all cropping duration periods in the NPK and NPK+S fertilizer treat-

ment groups, indicating that soils in those treatment groups required

more C input from balanced chemic al fertilizer in order to sequester

C continuously over the long-term. In addition, the positive relation-

ship between SOC se que stration rate an d straw C input in the NPK+S

treatment in the included studies indicates that soil has the poten-

tial to sequester C for at least 33 years. Our findings are supported

by a long-term study of the effec ts of chemical fertilizers combined

with straw and organic manure (Fan et al., 2008). Those researchers

found, during their 26-year experiment, that dr yland soils had not

reached their C sequestration upper limit. Moreover, we analyzed

the relationship between clay content and SOC storage for the two

treatments with straw return and found a significant positive correla-

tion (y = 0.733x + 16.29, R2 = .24, p < .01) for the NPK+S treatment,

but the NP+S correlation increased to some extent and then declined

(Appendix S5). However, due to limited studies reporting clay con-

tent, the sample size for the NP+S was likely too small to analyze

FIGURE 10 Initial SOC (soil organic carbon) stocks in the

top 20 cm of soil in selected studies based on different fertilizer

treatments in North China (CK, control [unfer tilized]; NP, mineral

nitrogen and phosphorous fertilizer; NPK, NP plus potassium;

NP+S, NP plus straw; NPK+S, NPK plus straw). The thick lines

within the boxes represent medians, the vertical bars show

minimum and maximum values, and * and ° denote outliers.

Treatments with different letters are significantly different (one-

way ANOVA, p < .05)

2699

BERHAN E Et Al.

effectively. Soils with high clay content may experience greater SOC

st or age tha n soi ls with les s clay conten t (Jaga dam ma & La l, 2010) . Th is

is because organic C molecules adsorb to clay surfaces because of

clay's large surface area and the presence of polyvalent cations that

form organo-mineral complexes that protect SOC from microbial and

enzymatic decay. But, the clay content in our study was low, 10%–

32%, indicating that North China soils may best sequester SOC if kept

under balanced fertilizer management and straw return (NPK+S) pro-

grams. However, the relationships between SOC sequestration rates

an d th e cor re spo ndi ng init ia l SOC sto ck leve ls wer e neg at ive in al mos t

all tre at me nt s, thu s sugges ti ng the possibil it y of SOC satur ation in th e

study area in the future. Thus, our study indicates that balanced fer-

tilizers with straw return could be very important for both crop yields

and SOC stock increases over longer periods in dr yland areas.

Overall, we found straw return to be an effective practice for

sustaining soil fertility and crop productivit y. Our regional and cli-

mate specific study revealed that fertilizer management, cropping

duration, cropping system, initial SOC levels, and soil types all influ-

enced SOC dynamics in relation to straw return. However, we found

very few studies spanning periods greater than 30 years, regardless

of whether straw return was examined and especially in dryland

areas. Future research considering many fac tors in managed exper-

iments are required to fully understand the benefits of straw return

to agricultural fields. Such factors might include different fertilizer

rates, more soil types, climatic conditions, straw types and quanti-

ties, and dif ferent durations. Continued experimental study would

help resolve uncertainties observed in meta-analyses. This knowl-

edge can aid future cropland management and study, especially for

upland crops in North China.

ACKNOWLEDGEMENTS

This work was supported by the National Key R & D Program

(2016YFD020 0308), Key Research and Development Program of

Shaan x i (2 0 19 ZDLN Y01- 0 5- 0 1), and th e Key Tec h n ologi e s R & D pr o -

gram of China during the 12th 5-year plan period (2012BAD14B11).

DATA AVAIL AB I LI T Y STATE MEN T

The data that support the findings of this study are available in the

supplementar y material of this article.

ORCID

Xiaohong Tian https://orcid.org/0000-0003-1383-4218

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With the development of grain production technologies and improvement of rural living standard, the production and utilization of straw have significantly been changed in China. More than 1 billion tones of straw are produced per year, and vast amount of them are discarded without effective utilization, leading various environmental and social impacts. Straw return is an effective approach of the straw utilization that has been greatly recommended by government and scientists in China. This paper discussed the current status of the straw return in China. Specifically, the production and models of straw return were explored and their environmental impacts were extensively evaluated. It was concluded that straw could be positively effective on the improvement of the soil quality and the grain production. However, it appeared that the straw return also had several neglect negative effects, implying that further research and assessment on the returned straw are required before its large-scale promotion in China.

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Core Ideas Straw return could increase SOC stock and improve soil enzyme activities. Both wheat and maize straw returned enlarged SOC stock, soil quality, and crop yield. The single return of crop straw was better for improving the economic sustainability Straw return is a promising method for managing soil organic carbon (SOC) in agricultural soils and thus may promote sustainable land use. However, the optimal straw‐return strategy for economically sustainable grain production in intensive winter wheat ( Triticum aestivum L.)–summer maize ( Zea mays L.) cropping systems remains uncertain. A 4‐yr field experiment was conducted in the central North China Plain to determine the effects of straw return on SOC stock and lability, soil enzyme activity, and grain yield, and to further evaluate the sustainability of crop production in intensive wheat–maize double‐cropping systems. Four treatments were tested: no straw return (NR), return of wheat straw (WR), return of maize straw (MR), and return of both wheat and maize straw (WR+MR). Compared with the initial value, SOC stocks significantly increased by 2.8, 5.4, and 13.1% in WR, MR, and WR+MR, respectively, and significantly decreased by 2.9% in NR. Stocks of KMnO 4 –oxidizable C and microbial biomass C; activities of soil invertase, protease, and dehydrogenase; and the C management index were significantly higher with than without straw return and followed the order NR < WR < MR < WR+MR. Yields of wheat and maize were highest in WR+MR, but economic sustainability was higher in WR and MR relative to NR and WR+MR. Our results indicate that return of either wheat or maize straw is sufficient to maintain SOC levels and may be the optimal strategy for economically sustainable grain production in the central North China Plain.

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Sep 2017
SOIL BIOL BIOCHEM

Nitrogen fertilization may affect the decomposition of soil organic matter (SOM) and plant residues in soil, but this effect is still very uncertain and depends on living plants. We investigated the effects of mineral N (Nmin) availability on SOM and plant residue decomposition in wheat (Triticum aestivum L.) growing soils in a pot experiment. Five treatments were assessed: (1) Control [no maize (Zea Mays L.) residues and no N fertilization]; (2) ¹⁵N-urea addition; (3) ¹⁵N maize leaves; (4) ¹⁵N maize leaves + urea; and (5) ¹⁵N-urea + maize straw. The decomposition of SOM and plant residues was traced by the changes of N and C in the light fraction (density < 1.80 g cm⁻³) during the 127 days. Urea fertilization decreased the decomposition of SOM and maize residues, as indicated by remaining N and C in the light fraction compared to soil without urea. The C decomposition was tightly coupled to that of N in the light fraction SOM. In soils with maize residues, both maize- and SOM-derived light fractions decomposed slowly with N fertilization. Soil microbial biomass N content was increased by maize residues but was unaffected by urea addition. Under low soil Nmin levels, microbes met their N demand by increasing an acquisition from accelerated decomposition of organic sources. To mine N in the Nmin limited soils, soil microbes might have directly taken up more N-containing organics and thus facilitated SOM decomposition. For such an acceleration of SOM decomposition, the presence of N uptake by living plants was especially important, which decreased the Nmin in soil and so, increased N limitation for microorganisms. We concluded that N fertilization decreases SOM decomposition and increases the efficiency of C sequestration in soil through higher portion of un-decomposed crop residues.

Effects of long‐term straw return on soil organic carbon storage and sequestration rate in North China upland crops: A meta‐analysis

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