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Review paper
The effects of simulated nitrogen deposition on plant root traits:
A meta-analysis
Weibin Li
a
,
b
, Changjie Jin
a
, Dexin Guan
a
, Qingkui Wang
a
, Anzhi Wang
a
, Fenghui Yuan
a
,
Jiabing Wu
a
,
*
a
State Key Laboratory of Forest and Soil Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China
b
University of Chinese Academy of Sciences, Beijing 100049, China
article info
Article history:
Received 1 August 2014
Received in revised form
3 January 2015
Accepted 3 January 2015
Available online 16 January 2015
Keywords:
Fungal colonization
Meta-analysis
N deposition
Root biomass
Root morphology
Root turnover
abstract
Global atmospheric nitrogen deposition has increased steadily since the 20th century, and has complex
effects on terrestrial ecosystems. This work synthesized results from 54 papers and conducted a meta-
analysis to evaluate the general response of 15 variables related to plant root traits to simulated nitro-
gen deposition. Simulated nitrogen deposition resulted in significantly decreasing fine root biomass
(<2 mm diameter; 12.8%), while significantly increasing coarse root (2 mm diameter; þ56.5%) and
total root (þ20.2%) biomass, but had no remarkable effect on root morphology. This suggests that
simulated nitrogen deposition could stimulate carbon accumulation in root biomass. The root: shoot
ratio decreased (10.7%) suggests that aboveground biomass was more sensitive to simulated nitrogen
deposition than root biomass. In addition, simulated nitrogen deposition increased the fine root nitrogen
content (þ17.6%), but did not affect carbon content, and thus decreased the fine root C:N ratio (13.5%).
These changes delayed the decomposition of roots, combined with increasing of the fine root turnover
rate (þ21.4%), which suggests that simulated nitrogen deposition could increase carbon and nutrient
retention in the soil. Simulated nitrogen deposition also strongly affected the functional traits of roots,
which increased root respiration (þ20.7%), but decreased fungal colonization (17.0%). The effects of
simulated nitrogen deposition on the plant root systems were dependent on ecosystem and climate zone
types, because soil nutrient conditions and other biotic and abiotic factors vary widely. Long-term
simulated experiments, in which the experimental N-addition levels were less than twofold of the
average of atmospheric nitrogen deposition, would better reflect the response of ecosystems under at-
mospheric nitrogen deposition. These results provide a synthetic understanding of the effects of simu-
lated nitrogen deposition on plant root systems, as well as the mechanisms underlying the effects of
simulated nitrogen deposition on plants and the terrestrial ecosystem carbon cycle.
©2015 Elsevier Ltd. All rights reserved.
1. Introduction
With the increased use of fossil fuels and artificial fertilizers
(Davidson, 2009), atmospheric nitrogen deposition has increased
by three-to five-fold over the past century (IPCC, 2007) and even
further in some regions (Galloway et al., 2004; Dentener et al.,
2006). Global annual nitrogen deposition rates are projected to
increase by a factor of 2.5 by the end of the century (Lamarque et al.,
2005). Atmospheric nitrogen deposition has many negative effects
on terrestrial ecosystems, such as the loss of biodiversity (Vitousek
et al., 1997; Maskell et al., 2010; De Schrijver et al., 2011). Excessive
nitrogen deposition can also affect terrestrial ecosystems through
soil acidification (Hoegberg et al., 2006), loss of base cations, and
nitrate leaching (Hoegberg et al., 2006; Dise et al., 2009). However,
atmospheric nitrogen deposition can stimulate plant growth,
thereby increasing carbon sequestration in plant biomass and soil
carbon pools (Pregitzer et al., 2008; Reay et al., 2008; Thomas et al.,
2010). Plant root systems play critical roles in terrestrial carbon
cycling because they not only take up water and nutrients from soil
for plant production, but also release carbon through root respi-
ration and rhizodeposition. Atmospheric nitrogen deposition pro-
duces strong impacts on plant root systems by influencing
morphology (e.g., root length and diameter), biomass, and func-
tions related to carbon cycling (e.g., root respiration) (Nadelhoffer,
*Corresponding author. Tel.: þ86 24 83970336; fax: þ86 24 83970300.
E-mail address: wujb@iae.ac.cn (J. Wu).
Contents lists available at ScienceDirect
Soil Biology & Biochemistry
journal homepage: www.elsevier.com/locate/soilbio
http://dx.doi.org/10.1016/j.soilbio.2015.01.001
0038-0717/©2015 Elsevier Ltd. All rights reserved.
Soil Biology & Biochemistry 82 (2015) 112e118
2000; Rasse, 2002). Therefore, the effects of atmospheric nitrogen
deposition on root traits can better explain the underlying mech-
anisms of nitrogen deposition on ecosystem carbon cycling.
Atmospheric nitrogen deposition influences plant root systems
directly by injuring tissues and indirectly by changing soil nitrogen
availability (Mcquattie and Schier, 1992; Galloway et al., 2004). Soil
nitrogen availability plays an important role in plant root dynamics
(Vogt et al., 1995). Many studies have suggested that fine root
biomass decreased as the nitrogen availability increased
(Nadelhoffer, 2000; Hendricks et al., 2006). When soil nitrogen
availability increases, the responses of fine root lifespans are
inconsistent, both increases (Vogt et al., 1986) and decreases
(Nadelhoffer et al., 1985; Pregitzer et al., 1995) were observed. An
increase in soil nitrogen availability also alters the root C: N ratio,
thereby influencing carbon accumulation in belowground biomass.
In addition, an increase in soil nitrogen availability can lead to other
nutrient limitations (e.g., phosphorus) (Güsewell, 2004; Vitousek
et al., 2010). Soil acidification caused by atmospheric nitrogen
deposition can change the environment surrounding plant roots
and root fungi (Majdi and Persson, 1993). Therefore, atmospheric
nitrogen deposition integrates various factors, which are difficult to
quantify but can be revealed by exploring the effects of simulated
nitrogen deposition on plant root traits.
Different ecosystem and climate zone types involving different
abiotic and biotic factors have various responses to nitrogen
deposition (Vogt et al., 1995). In addition, the responses of root
systems are markedly different between short- and long-term
simulated nitrogen depositions (Persson and Ahlstrom, 1990;
Hendricks et al., 2006). The impacts of different experimental N-
addition levels on root systems also significantly differ because of
the different degrees of soil nitrogen availability (Nadelhoffer,
2000).
Numerous individual studies have been conducted to investigate
the effects of simulated nitrogen deposition on root systems, but
data synthesis is still unavailable. The present study compiled 15
variables related to plant root traits from 54 experimental studies. A
meta-analysis was conducted to identify the general patterns of the
responses of plant root traits to simulated nitrogen deposition,
investigate the differences among different settings of simulated
nitrogen deposition experiments (e.g., ecosystem types, climate
zone types, treatment durations, and experimental N-addition
levels), and evaluate the response of root traits to increased atmo-
spheric nitrogen deposition under the global change scenario.
2. Materials and methods
2.1. Data collection
In this meta-analysis, data were collected from 54 peer-
reviewed journal articles (Supporting Information, Appendix S1)
published since 1990 using the Web of Science resource. The search
terms were “nitrogen deposition”and “root”or “fertilization”and
“root”. A total of 15 variables (Appendix S2 and S3) related to plant
root traits were compiled from experiments in the control and
simulated nitrogen deposition treatments. The following criteria
were applied to select proper observations: (1) Only field simulated
nitrogen deposition studies were selected and laboratory incuba-
tion studies were not included; (2) The control and treatment plots
were established to have the same abiotic and biotic conditions; (3)
At least one of the selected variables was measured and values
calculated by models were excluded; (4) For multifactorial studies,
only the control and simulated nitrogen deposition treatment data
were selected and the interacting effects were excluded; (5) In N-
fertilization experiments, the fertilizers only contained nitrogen
and no other nutrients (e.g. K, P, Ca, and Mg); (6) The means,
standard errors (SE) or standard deviations (SD) and sample sizes
(n) were reported.
Considering the complexity of the dataset, the selected variables
were categorized into four groups as follows: (1) Root morphology
and biomass (i.e., fine root diameter, fine root length, specific root
length, fine root biomass, fine root density, coarse root biomass,
total root biomass, and the root: shoot ratio); (2) Root carbon and
nitrogen contents (i.e., fine root nitrogen, fine root carbon, and the
fine root C:N ratio); (3) Root turnover (i.e., fine root production, fine
root turnover rate, and fine root respiration); (4) Fungal coloniza-
tion. These four groups' plant root traits can well describe the status
of roots and their ability to exchange carbon, water, and other
nutrients between roots and soil; thus, these traits play critical
roles in determining carbon accumulation in plant and soil carbon
pools (Brunner and Godbold, 2007). Data were compiled directly
from Tables and extracted by Engauge software (4.1) from
Figures in the published articles.
To avoid confounding the responses of variables to experimental
treatments, the variables of each study were categorized according
to the environmental and simulated factors into the following four
groups: climate zone types (tropical, subtropical, temperate, and
boreal climate zones), ecosystem types (forest and grassland),
experimental N-addition levels [N-addition levels ranged from
10 kg N ha
1
yr
1
to 560 kg N ha
1
yr
1
, which were divided into
low (<100 k g N ha
1
yr
1
), medium (100 and
<200 kg N ha
1
yr
1
), and high levels (200 kg N ha
1
yr
1
)], and
treatment durations [experimental N-addition treatment durations
ranged from 0.2 years to 13.6 years, which were divided into short
(<three years), medium (three and <ten years), and long terms
(ten years)]. In addition, experiment location, ambient nitrogen
deposition, mean annual temperature (Ta), and mean annual pre-
cipitation (P) were also obtained (Appendix S2).
2.2. Meta-analysis
We used the natural log-transformed response ratio (RR),
defined as the “effect size,”as an index to weigh the response of
root traits to simulated nitrogen deposition (Hedges et al., 1999).
The RR was calculated as the ratio of its value in the N fertilization
treatment group (X
t
) to that in the control group (X
c
) (Equation
(1)). The logarithm of RR was carried out to improve its statistical
behavior in meta-analyses (Hedges et al., 1999):
ln RR ¼lnX
t
X
c
¼lnX
t
lnX
c
(1)
The variance (v) of logarithmic effect size was calculated by:
v¼S
2
t
n
t
X
2
t
þS
2
c
n
c
X
2
c
(2)
where S
t
and S
c
are the standard deviations (SD ¼SE ffiffiffi
n
p) for the N-
addition treatment and control groups, respectively, and n
t
and n
c
are the sample sizes for the N-addition treatment and control
groups, respectively.
The weighting factorðwÞof each observation was calculated as
the inverse of the variance (Eq. (3)). Because some study cases
contain two or more observations, we adjusted the weights by total
number of observations per study (Bai et al., 2013), and used the
total weighting factorðw0Þ to estimate the mean effect size (RR
þþ
)
(Eqs. (4)e(6)):
w¼1
v
(3)
W. Li et al. / Soil Biology & Biochemistry 82 (2015) 112e118 113
w0¼w=n(4)
ln RR0¼w0$ln RR (5)
RR
þþ
¼P
i
ln RR
0
i
P
i
w
0
i
(6)
where ln RR
0
is weighted effect size, nis the total number of ob-
servations per study, w
i
and ln RR
0
i
are wand ln RR
0
of the ith
observation, respectively.
Afixed effects model was used to determine whether simulated
nitrogen deposition significantly affects each variable using Meta-
Win software (2.1) (Rosenberg et al., 2000). Bootstrapping with
9999 iterations was used to generate the confidence intervals (CIs)
of effect sizes, because confidence limits based on bootstrapping
tests are wider than standard confidence limits when the number
of observations is lower than 20 (Adams et al., 1997; Hedges et al.,
1999). If the 95% CI value of RR
þþ
for a variable does not cover zero,
simulated nitrogen deposition has a significant impact (positive or
negative) on the variable; if not, simulated nitrogen deposition has
no significant impact on the variable. We also used the percentage
transformed from the mean effect size to explain the response of
simulated nitrogen deposition:
e
RR
þþ
1100% (7)
A continuous randomized effects model of meta-analysis was
applied to test the relationships between ln RR
0
and treatment
durations and the experimental N-addition levels. Variables were
selected only when the sample numbers are greater than 20. Total
heterogeneity in ln RR
0
among studies (Q
T
), the difference among
group cumulative effect sizes (Q
M
), and residual error (Q
E
)were
used to report the statistical results (Rosenberg et al., 2000).
3. Results
Considering the entire dataset of root morphology and biomass,
simulated nitrogen deposition significantly increased total root
biomass (þ20.2%), and significantly decreased fine root biomass
(12.8%), but did not significantly change fine root length, and
specific root length (Fig. 1). For root carbon and nitrogen contents,
simulated nitrogen deposition significantly increased fine root ni-
trogen content (þ17.6%), and significantly decreased the fine root
C:N ratio (13.5%), but did not significantly change fine root carbon
content. For root turnover, simulated nitrogen deposition signifi-
cantly increased fine root respiration (þ20.7%), but did not signifi-
cantly change fine root production. Simulated nitrogen deposition
also significantly decreased fungal colonization by 17.0%. The
observed numbers of coarse root biomass, fine root diameter, root:
shoot ratio, fine root density and fine root turnover rate were less
than 20. Based on the current limited number of observations, re-
sults indicate that simulated nitrogen deposition significantly
increased coarse root biomass (þ56.5%) and fine root turnover rate
(þ21.4%), significantly decreased the root: shoot ratio (10.7%), but
did not significantly change fine root diameter, and fine root density.
In forests, simulated nitrogen deposition had significantly
negative effects on fine root biomass (13.5) and fungal coloniza-
tion (19.2%), whereas these effects were not significant in grass-
land ecosystems (Fig. 2). The effects of simulated nitrogen
deposition on fine root respiration were significantly positive both
in forest (þ18.1%) and grassland (þ29.1%) ecosystems. The effects of
simulated nitrogen deposition on the fine root C:N ratio was
significantly negative in forests (13.5%), and the small number of
observations limited the power of meta-analysis in the grassland
ecosystems.
Although the low number of observations from some climate
zone types limited the power of this meta-analysis, the responses of
each variable to simulated nitrogen deposition varied across
climate zones. Overall, for the response of fine root biomass to
simulated nitrogen deposition, tropical, subtropical, and temperate
climates did not undergo a significant change, whereas it signifi-
cantly increased in the boreal climates (þ24.0%) (Fig. 2a). The fine
root C: N ratio was most pronounced in tropical climates (17.3%)
under simulated nitrogen deposition, followed by the temperate
(14.3%) and boreal climates (9.8%) (Fig. 2b), although the change
in the temperate climates was not statistically significant. The in-
crease in fine root respiration under simulated nitrogen deposition
was most significant in the tropical climates (þ35.9%), followed by
temperate climates (þ15.5%) (Fig. 2c). The effects of simulated ni-
trogen deposition on fungal colonization significantly decreased in
temperate climates (19.2%), but was not significant in subtropical
climates (Fig. 2d).
The results of meta-analysis with the continuous randomized
effects model show a significantly negative correlation between
treatment durations and the effect sizes of simulated nitrogen
deposition on fine root respiration (Table 1). Experimental N-
addition levels had a significantly positive correlation with the ef-
fect sizes of simulated nitrogen deposition on fine root biomass,
fine root length, specific root length and fine root respiration
(Table 1).
4. Discussion
4.1. Root morphology and biomass
Fine roots play important roles in ecosystem carbon and nitro-
gen cycling by absorbing water and nutrients and releasing
Fig. 1. Mean effect sizes of simulated nitrogen deposition on four group variables
related to plant root traits. Error bars represent 95% confidence intervals (CIs). The
number in parentheses represents the sample size for each variable. Solid points are
variables with 20 observations, and hollow points are variables with <20 observa-
tions (CRB: coarse root biomass, FRB: fine root biomass, TRB: total root biomass, FRD:
fine root diameter, FRL: fine root length, R:S: root:shoot ratio, SRL: specific root length,
FD: fine root density, FRN: fine root nitrogen, FRC: fine root carbon, C:N: fine root C:N
ratio, FRP: fine root production, FRTR: fine root turnover rate, FRS: fine root respiration,
and FC: fungal colonization).
W. Li et al. / Soil Biology & Biochemistry 82 (2015) 112e118114
exudates (Brunner and Godbold, 2007; Bader et al., 2009). Our
meta-analysis showed that simulated nitrogen deposition did not
significantly change fine root length, diameter, and density but
decreased fine root biomass (12.8%) (Fig. 1), which suggests that
the decrease in the quantity of fine roots possibly resulted in a
decrease in fine root biomass. However, the insignificant response
of fine root length and diameter to simulated nitrogen deposition
suggests that plant roots have not changed the surface areas in
attempt to alter the exchange rates of resources across the
plantesoil interface (Eissenstat, 1992). Simulated nitrogen deposi-
tion significantly increased total root biomass (þ20.2%), reflecting
the balanced results of significantly increased coarse root biomass
(þ56.5%) and significantly decreased fine root biomass (12.8%)
(Fig. 1). Thus, coarse roots were responsible for all of the increase in
total root biomass. Our meta-analysis did not find an impact on the
change in fine root biomass, fine root length and specific root
length that was caused by the duration of simulated nitrogen
deposition treatments, but did reveal a positive effect caused by the
levels of experimental N-addition (Table 1). This suggests that the
effects produced by the accumulation of simulated nitrogen
deposition through duration and quantity were different.
Fine root biomass decreased by 12.8%, and the response of fine
root biomass to simulated nitrogen deposition is dependent on
biotic or abiotic responses. Note that most data for fine root
biomass from our database were extracted from studies performed
in forest ecosystems (Fig. 2a). The effect was significant in forests
but not in grasslands, possibly because of the difference in demands
for nitrogen nutrients in different ecosystems and the limited
number of investigations in the grassland ecosystems. Simulated
nitrogen deposition significantly increased fine root biomass in the
boreal climates, but did not change fine root biomass in tropical,
subtropical and temperate climates (Fig. 2a). This finding may be
attributed to one of the following reasons: the number of studies in
these climate zones was limited; the sensitivity of different vege-
tation types to simulated nitrogen deposition differed; and soil
temperature and moisture may be important factors that control
Fig. 2. Mean effect sizes of simulated nitrogen deposition on fine root biomass (a), fine root C: N ratio (b), fine root respiration (c), and fungal colonization (d). The variables are
categorized into different groups according to ecosystem types, climate zone types, treatment durations, and N-addition levels. Error bars represent 95% confidence intervals (CIs).
The number in parentheses represents the sample size for each variable.
W. Li et al. / Soil Biology & Biochemistry 82 (2015) 112e118 115
fine root biomass. We also found a positive correlation between
experimental N-addition levels and the effects of simulated nitro-
gen deposition on fine root biomass (Table 1), indicating a direct
effect of nitrogen availability in soil on fine root biomass.
It is very important to predict how plants allocate above- and
belowground biomass and alter C distribution in ecosystems during
their response to global change scenarios. Our results indicated that
simulated nitrogen deposition significantly decreased the root:
shoot ratio (Fig. 1), which was consistent with the synthesis result
published by Vogt et al. (1986), who reported that aboveground
biomass is more sensitive to simulated nitrogen deposition than
root biomass. The sensitivity differences between above- and
belowground biomass to simulated nitrogen deposition could be
important indicators of changes in soil and plant carbon storage
resulting from atmospheric nitrogen deposition. In addition, the
lower root: shoot ratio suggested that simulated nitrogen deposi-
tion has greater stimulatory effects on aboveground biomass than
root biomass; this result is consistent with findings of Eissenstat
and Yanai (1997) who reported that more nutrition was typically
devoted to aboveground under conditions with higher soil nutrient
availability.
4.2. Fine root C and N contents
Simulated nitrogen deposition decreased the fine root carbon
content (36.9%), but the effect was insignificant. However, it
significantly increased the fine root nitrogen content by 17.6%
(Fig. 1). These results are consistent with the reduced fine root C: N
ratio that was observed with simulated nitrogen deposition in all
climate types, experimental durations and N-addition levels
(Fig. 2b). Simulated nitrogen deposition increased nitrate avail-
ability to plant roots, thus more nitrogen became available for root
uptake and stock in fine root tissues (Hyvonen et al., 2008; Reay
et al., 2008). Greater nitrogen uptake resulted in higher nitrogen
content and metabolism in fine roots, thereby accelerating the cycle
of carbon and nitrogen in terrestrial ecosystems (Hendricks et al.,
1993; Nadelhoffer, 2000). This result is consistent with the
increased fine root turnover rate with simulated nitrogen
deposition in our meta-analysis. We did not find that the treatment
durations or N-addition levels of simulated nitrogen deposition had
an impact on the change in fine root carbon or nitrogen contents as
a result of simulated nitrogen deposition (Table 1).
Our meta-analysis results showed that simulated nitrogen
deposition significantly decreased the fine root C: N ratio by 13.5%
(Fig. 1), and all these study samples were derived from forests
(Fig. 2b). Given that simulated nitrogen deposition did not signifi-
cantly change the carbon content of fine roots, the decreases in the
fine root C: N ratio were dominated by increases in the fine root
nitrogen content. Fan and Guo (2010) proposed that carbon quality
and nitrogen inhibition hypotheses can better explain the variance
of root decomposition processes, and the mycorrhizal hypothesis
was expected to apply only to ectomycorrhizal species. Guo et al.
(2004) suggested that carbon limitation (the carbon quality hy-
pothesis) should be more severe under a lower C: N ratio. Previous
studies have reported that higher root nitrogen inhibited the decay
of fine roots (the nitrogen inhibition hypothesis) (Bouma et al.,
1994; Fan and Guo, 2010). Therefore, in our meta-analysis, the
lower C:N ratio and higher nitrogen content of fine roots under
simulated nitrogen deposition delayed the decomposition of roots
and elevated the carbon retention in soil. The effects of simulated
nitrogen deposition on the fine root C:N ratio varied in different
climates (Fig. 2b), which may be attributed to the differences in the
physiological characteristics of plants and living conditions in
different climates (Harrington et al., 2001; Kaspari et al., 2008).
4.3. Fine root turnover
Fine root turnover is considered as a critical pathway for carbon
inputs into the soil and it is also a central process of nutrient cycling
in ecosystems. The turnover of fine roots is sensitive to global
change scenarios (Eissenstat et al., 2000; Gill and Jackson, 2000). In
our meta-analysis, simulated nitrogen deposition did not signifi-
cantly change fine root production, but it significantly increased the
fine root turnover rate and fine root respiration by 21.4% and 20.7%,
respectively (Fig. 1). Fine root production at the beginning of the
growing season completely differed from that at the late growing
season (Pregitzer et al., 2000; King et al., 2002); thus, the differ-
ences in sampling date complicated the present analysis.
Smithwick et al. (2013) suggested when plants reach the saturation
of nitrogen under simulated nitrogen deposition, the marginal gain
caused by taking up additional nitrogen could be of less value than
the marginal cost of carbohydrates expended on root maintenance,
leading to shorter root lifespan. This could explain our results that
the simulated nitrogen deposition increased the fine root turnover
rate and decreased fine root biomass. Many previous studies pro-
posed that root respiration increases proportionately with root
nitrogen content (Burton et al., 2002; Zhang et al., 2014). Our re-
sults also demonstrated that simulated nitrogen deposition
increased both fine root nitrogen content and fine root respiration.
The mechanism causing this probably is that typically 90% of the
nitrogen in plant cells is bound in proteins, which require energy
(carbohydrates) for replacement and repair (Bouma et al., 1994).
However, the relationships between fine root respiration and fine
root nitrogen content do not hold under chronic simulated nitrogen
deposition, the respiration per unit nitrogen could became lower
(Burton et al., 2012).
Our results indicated that the effect of simulated nitrogen
deposition on fine root respiration was significantly positive both in
forest and grassland ecosystems (Fig. 2c). Root respiration is a
substantial part of autotrophic respiration (root, rhizosphere, and
mycorrhizal respiration). Many studies actually refer to root
respiration as the sum of root and rhizosphere respiration because
of the limitation of measuring methods. Janssens et al. (2010)
Table 1
Relationships between the effect size of simulated nitrogen deposition on plant root
traits, treatment duration, and N-addition level.
Q
T
Q
M
Q
E
Slope P-value
Treatment duration
Fine root biomass 63.185 0.584 62.601 0.0071 0.445
Total root biomass 45.669 3.487 42.182 0.0220 0.062
Fine root length 23.728 1.119 22.609 0.1234 0.290
Specific root length 14.592 0.475 14.117 0.0428 0.491
Fine root nitrogen 112.961 0.029 112.932 0.0010 0.865
Fine root carbon 9.499 0.623 8.876 0.0261 0.430
Fine root C: N ratio 24.000 2.110 21.890 0.0265 0.146
Fine root production 9.075 0.419 8.657 0.0226 0.518
Fine root respiration 66.439 26.350 40.089 0.0378 0.000
Fungal colonization 20.691 2.561 18.130 0.0853 0.110
N-addition level
Fine root biomass 63.185 11.995 51.190 0.0012 0.001
Total root biomass 41.193 0.764 40.429 0.0003 0.382
Fine root length 29.065 14.101 14.964 0.0029 0.000
Specific root length 14.592 8.997 5.595 0.0014 0.003
Fine root nitrogen 110.835 0.393 110.442 0.0002 0.531
Fine root carbon 28.732 3.493 25.239 0.0008 0.062
Fine root C: N ratio 20.955 0.326 20.629 0.0003 0.568
Fine root production 9.075 0.528 8.548 0.0013 0.468
Fine root respiration 66.439 32.791 33.648 0.0011 0.000
Fungal colonization 20.470 1.505 18.965 0.0005 0.220
Q
T
,Q
M
, and Q
E
represent total heterogeneity in effect sizes among studies, the dif-
ference among group cumulative effect sizes, and residual error, respectively. The
relationship is significant if P<0.05.
W. Li et al. / Soil Biology & Biochemistry 82 (2015) 112e118116
compiled 17 studies and conducted a meta-analysis in the forest
ecosystems, and suggested that simulated nitrogen deposition
significantly decreased root respiration. However, our analysis with
experimental data in forest ecosystems presented a completely
different conclusion (Fig. 2c). This difference may be caused by two
factors. First, the different criteria applied to sample selection for
meta-analysis. The majority of data of the former analysis were
calculated through equations, whereas only measured data were
compiled in our analysis. Second, our work synthesized much
greater number of experimental results.
In our meta-analysis, simulated nitrogen deposition decreased
fine root biomass by 12.8%, but increased fine root respiration by
20.7%. At the ecosystem level, an increase in the supply of carbo-
hydrates to roots from aboveground photosynthesis makes up this
difference (Hyvonen et al., 2007; Li et al., 2010). We also found a
positive correlation between the experimental N-addition levels
and the effects of simulated nitrogen deposition on fine root
respiration, and a negative correlation between treatment dura-
tions and the effects of simulated nitrogen deposition on fine root
respiration (Table 1). These findings suggest that the effects differed
between short-term plus high nitrogen treatment and long-term
plus low nitrogen treatment. Therefore, considering the distur-
bance of the environment and economic factors, simulated exper-
iments of N-addition that the amount of N-addition were less than
twofold of the average of atmospheric nitrogen deposition and
long-term treatment can be reasonably used in combination to
identify the possible impacts of global atmospheric nitrogen
deposition scenarios.
4.4. Fungal colonization
Fungal colonization is a critical indicator of root turnover
(Eissenstat et al., 2000), and plays a key role in carbon and nitrogen
cycles in ecosystems (Simard et al., 2003; Adam Langley et al.,
2006). In our meta-analysis, simulated nitrogen deposition signif-
icantly decreased fungal colonization by 17.0% (Fig. 1). This result
was consistent with a previous meta-analysis of Treseder (2004),
who found that fungal colonization in roots decreases by an average
of 15% under nitrogen fertilization. Simulated nitrogen deposition
increased nitrate mobility in the soil and hence diffusion and mass
flow may supply adequate nitrogen in nitrate-rich systems
(Treseder, 2004). Under these circumstances, plant investment in
mycorrhizal fungi may be minimal and result in a reduction of
ectomycorrhizal mycelia growth and production (Nilsson and
Wallander, 2003; Sims et al., 2007). Additional mechanisms could
be that simulated nitrogen deposition accelerated soil acidification
(Wallenda and Kottke, 1998) and this toxic soil shifted the
composition of the ectomycorrhizal community (Lilleskov et al.,
2002; Avis et al., 2003).
The effects of simulated nitrogen deposition on fungal coloni-
zation were significantly negative in the forest ecosystems or
temperate climates. However, this was not the case for the grass-
land ecosystems or subtropical climates (Fig. 2d). Our results also
demonstrated that fungal colonization decreased with an increase
in experimental N-addition (Fig. 2d), consistent with previous
findings of Treseder (2004), who found that mycorrhizal fungi are
more abundant where plants are more limited by soil nutrients.
Therefore, those differences between different ecosystems or
climate types were dominated by the differences of soil nutrient
status.
5. Conclusions
Atmospheric nitrogen deposition generates complex responses
of ecosystem structure and function, but this study suggests
coherence in many plant root traits responses under simulated
nitrogen deposition. The results revealed that simulated nitrogen
deposition altered root biomass, root chemical composition, root
turnover, and fungal colonization, but did not alter root
morphology. Increased root biomass and decreased the root: shoot
ratio suggest that more photosynthate sequestered in plant than
soil carbon pool with elevated atmospheric nitrogen deposition.
Fine root turnover rate and fine root respiration were substantially
increased under simulated nitrogen deposition. These responses
could potentially accelerate the nutrients cycling. Moreover, an
enhanced root turnover with lower root decomposability could
potentially facilitate retention of carbon and nitrogen in soil. In
different ecosystem and climate types, the differences in soil
nutrient conditions and other biotic and abiotic factors varied the
effects of simulated nitrogen deposition on plant root traits. Our
meta-analysis further revealed that the chronic simulated experi-
ments, in which the amount of N-addition were less than twofold of
the average annual atmospheric nitrogen deposition rate are
enough to reveal the practical effects of atmospheric nitrogen
deposition on terrestrial ecosystems. This meta-analysis provides a
comprehensive understanding of the effects of simulated nitrogen
deposition on plant root systems, and will deepen our under-
standing of the mechanisms underlying the effects of simulated
nitrogen deposition on plants and the terrestrial ecosystem carbon
cycle.
Acknowledgments
This work was supported by the Strategic Priority Research
Program of the Chinese Academy of Sciences (Grant No.
XDB15010301), the National Natural Science Foundation of China
(Grant No. 41375119), and the Youth Fund for Creative Research
Groups, Institute of Applied Ecology, CAS (Grant No. LFSE2013-11).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.soilbio.2015.01.001.
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