A Fundamental Role of Slope Aspect and Elevation in Controlling Diversity Patterns of Soil Bacterial Communities: Insights from an Arid-Montane Ecosystem in China

Abstract

In montane ecosystems, slope aspect and elevation are the main topographic parameters that produce environmental heterogeneity related to microclimate, pedogenic processes, and vegetation patterns. However, their effects on belowground microbes are not well understood. In particular, there are few studies on how bacteria community responds to slope aspect. Here, we selected a shaded north-facing slope and a sunny south-facing slope, and investigated the influences of slope aspect and elevation on bacterial communities along transects at 2400 to 3800 m in the Qilian Mountains, a typical arid-montane ecosystem of northwestern China. The results showed that bacterial alpha and beta diversity differed significantly with slope aspect and elevation. North-facing slope had higher bacterial richness and abundance than south-facing slope, and the bacterial community composition differed significantly between slope aspects (stress = 0.062, R2 = 0.849, p < 0.001) as revealed by non-metric multidimensional scaling analysis. Bacterial richness and diversity increased significantly with elevation and then decreased on both north-facing and south-facing slopes, with the highest values at 3500 m, and the community composition differed dramatically along elevation, as shown with quadratic relationships (R2south-facing = 0.78; R2north-facing = 0.66) between beta diversity indices and elevation. Redundancy analysis further revealed that the variations in soil pH, soil organic carbon, and soil carbon/nitrogen ratios induced by slope aspect and elevation contributed significantly to the diversity patterns of soil bacterial communities. These findings indicated a fundamental role of slope aspect and elevation in controlling diversity patterns of bacterial communities in arid-montane ecosystems, providing new insights into microbial relationships with topography.

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Vol.:(0123456789)
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Journal of Soil Science and Plant Nutrition
https://doi.org/10.1007/s42729-022-01002-8
ORIGINAL PAPER
A Fundamental Role ofSlope Aspect andElevation inControlling
Diversity Patterns ofSoil Bacterial Communities: Insights
fromanArid‑Montane Ecosystem inChina
Long‑FeiChen1· Jun‑QiaKong1· Zhi‑BinHe1· Wen‑ZhiZhao1· Ming‑DanSong2· Yue‑MeiLi2· YuanGao1·
Shu‑PingYang1
Received: 5 April 2022 / Accepted: 12 September 2022
© The Author(s) under exclusive licence to Sociedad Chilena de la Ciencia del Suelo 2022
Abstract
In montane ecosystems, slope aspect and elevation are the main topographic parameters that produce environmental het-
erogeneity related to microclimate, pedogenic processes, and vegetation patterns. However, their effects on belowground
microbes are not well understood. In particular, there are few studies on how bacteria community responds to slope aspect.
Here, we selected a shaded north-facing slope and a sunny south-facing slope, and investigated the influences of slope aspect
and elevation on bacterial communities along transects at 2400 to 3800m in the Qilian Mountains, a typical arid-montane
ecosystem of northwestern China. The results showed that bacterial alpha and beta diversity differed significantly with slope
aspect and elevation. North-facing slope had higher bacterial richness and abundance than south-facing slope, and the bacte-
rial community composition differed significantly between slope aspects (stress = 0.062, R2 = 0.849, p < 0.001) as revealed by
non-metric multidimensional scaling analysis. Bacterial richness and diversity increased significantly with elevation and then
decreased on both north-facing and south-facing slopes, with the highest values at 3500m, and the community composition
differed dramatically along elevation, as shown with quadratic relationships (R2south-facing = 0.78; R2north-facing = 0.66) between
beta diversity indices and elevation. Redundancy analysis further revealed that the variations in soil pH, soil organic carbon, and
soil carbon/nitrogen ratios induced by slope aspect and elevation contributed significantly to the diversity patterns of soil bacte-
rial communities. These findings indicated a fundamental role of slope aspect and elevation in controlling diversity patterns
of bacterial communities in arid-montane ecosystems, providing new insights into microbial relationships with topography.
Keywords Bacteria· Community diversity and composition· Biogeographic patterns· Topography· Slope aspect· Arid-
montane ecosystems
1 Introduction
Despite accounting for only 12% of the terrestrial surface,
mountain ecosystems provide a major habitat and refuge
for biodiversity (Körner 2007; Moret etal. 2019; Hagedorn
etal. 2019), and biogeographic patterns of plants, animal,
and macrofauna and their interactions with environmental
factors have been widely studied (Coblentz and Riitters
2010; Moret etal. 2019; Tan etal. 2021). By comparison,
the diversity patterns of belowground microbes, in terms
of their complex diversities and compositions, are not well
understood (Delgado-Baquerizo etal. 2018; Tajik etal.
2020; Ivashchenko etal. 2021).
Growing evidence demonstrates that soil, plant, and cli-
matic characteristics are often among the most important
environmental predictors of microbial diversity and compo-
sition in soils (Nielsen etal. 2010; Siles and Margesin 2016;
Nottingham etal. 2018; Shen etal. 2019). In montane eco-
systems, slope aspect and elevation are the main topographic
parameters that produce environmental heterogeneity related
to microclimate, pedogenic processes, and plant traits; thus,
Long-Fei Chen and Jun-Qia Kong contributed equally to this work.
* Zhi-Bin He
hzbmail@lzb.ac.cn
1 Linze Inland River Basin Research Station, Chinese
Ecosystem Research Network, Northwest Institute ofEco-
Environment andResources, Chinese Academy ofSciences,
Lanzhou730000, China
2 Academy ofAgriculture andForestry Sciences, Qinghai
University, Xining810000, China

Journal of Soil Science and Plant Nutrition
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they are likely to contribute to soil microbial variability
(Coblentz and Riitters 2010; Seibert etal. 2007; Stage and
Salas 2007; Mendez-Toribio etal. 2016; Zeng etal. 2019;
Ivashchenko etal. 2021). For example, slope aspect alters
net solar radiation received, and creates different microen-
vironment and opportunities for soil formation and devel-
opment, and vegetation establishment (Sidari etal. 2008;
Bennie etal. 2008; Liu etal. 2013). Climatic gradients along
elevation can modify hydrothermal processes, further affect-
ing plant traits and soil-forming processes (Wang etal. 2003;
Deng etal. 2019; Chen etal. 2022). Accordingly, a greater
understanding is needed of topographic controls on soil
microbial communities, especially in montane ecosystems.
Soil bacteria, ranking among the most abundant and
diverse group of soil microorganisms, play an important role
in maintaining multiple functions of terrestrial ecosystems,
including nutrient and carbon cycling, plant production,
and greenhouse gas emissions (Tiedje etal. 1999; Bardgett
and van der Putten 2014; Delgado-Baquerizo etal. 2018).
The immense diversity of soil bacterial communities has
stymied efforts to characterize their biogeographic patterns
(Delgado-Baquerizo etal. 2018). Recently, an increasing
number of researchers began to explore the role of topo-
graphic factors in controlling bacterial diversity patterns.
However, different and sometimes contradictory results have
been produced. For example, bacterial community diver-
sity along increasing elevations include decreasing (Li etal.
2016), increasing (Margesin etal. 2009), unimodal (Praeg
etal. 2020), and hollow (a dip in diversity at mid-altitude)
patterns (Singh etal. 2014; Liu etal. 2016). Fewer work
focused on the influence of slope aspect on bacterial commu-
nities compared with elevation, and the limited results were
inconclusive. For example, some studies reported greater
richness of arbuscular mycorrhizal fungi (AMF) and bacteria
on sunny south-facing slope than on shaded north-facing
slope (Chu etal. 2016; Liu etal. 2017; Wei etal. 2021).
However, Ai etal. (2018) and Xue etal. (2018) reported
that fungi and bacteria were more abundant on north-facing
slope, while Schlatter etal. (2018) and Tajik etal. (2020)
reported relatively minor differences in bacterial communi-
ties over varied slope aspects. Those inconclusive results
indicated that the biogeographic patterns of bacterial com-
munities were more complex than expected; thus, new work
on bacterial communities facilitates a better understanding
of the microbial relationships with topography.
The Qilian Mountains, constituting a major biodiversity
hotspot in the arid northwestern China, are marked by com-
plicated topography with abrupt elevations, creating high
heterogeneity in climate, soil, and vegetation. Moreover,
the mountains have been identified as a National Nature
Reserve since 1988, greatly limiting human interference
and enhancing the capacity for investigating the effects of
topography on microbial communities. Thus, we selected a
shaded north-facing slope and a sunny south-facing slope,
and investigated the influence of slope aspect and elevation
on bacterial communities along transects at 2400 to 3800m
in the Qilian Mountains. In particular, we addressed two
main questions: (1) whether slope aspect and elevation had
significant effects on bacterial communities and (2) which
environmental variables associated with slope aspect and
elevation contributed to the biogeographic patterns of bacte-
rial communities.
2 Methods
2.1 Study Area
The study sites were situated in the Dayekou watershed
(100°03′E–100°23′E, 38°23′–38°48′N, 2250–3980m above
sea level) in central Qilian Mountains, northwestern China.
Native vegetation in the catchment was shaped by slope
aspect and elevation, and forms two distinctly vegetation
zones (Chen etal. 2016): grasslands on south-facing slopes
and grassland-forest-shrubland on north-facing slopes (Picea
crassifolia forests are distributed at elevations between 2500
and 3300m, and shrublands are found at elevations from
3250 to 3650m). Soil type is dominated by Haplic podsol
according to the FAO classification system.
2.2 Experimental Design andSampling
In August 2013, five sample sites were set up at about 2400,
2800, 3200, 3500, and 3800m on selected north-facing
and south-facing slopes. Site characteristics were given in
TableS1. Three replicate sampling plots (30m × 30m) were
randomly established in each site, and the distance between
each plot was at least 50m. Mean annual precipitation
(MAP) and mean annual temperature (MAT) for each plot
were monitored by standing tipping-bucket pluviographs and
thermo-hygrometers, respectively.
In early August 2018, one composite sample compris-
ing twelve soil cores at depths of 0–20cm was collected
for each plot, giving a total of 30 soil samples. Subse-
quently, visible roots and litter debris were removed from
each soil sample, which was then sieved through a 2-mm
soil sieve. Then, samples were divided into two portions:
one portion for physicochemical analysis was air-dried and
the other portion for molecular analysis was immediately
stored at − 80°C. At the same time, five undisturbed soil
cores were obtained from each plot for determining soil
bulk density (BD). The details for vegetation survey were
present in Chen etal. (2016).
For each plot, twelve litter nets (1.0 × 1.0 m2) were ran-
domly installed 50cm above the ground to collect the above-
ground litter of trees and shrubs, ten quadrats (1.0 × 1.0 m2)

Journal of Soil Science and Plant Nutrition
1 3
were randomly selected to collect the aboveground litter of
herbs and understory vegetation, and twelve soil cores (9cm
in diameter and 20cm in depth) were collected, and fine
roots (< 2mm in diameter) were gently separated from the
soil manually. Collected litter and fine roots were ground
finely to determine carbon and nitrogen.
2.3 Analysis ofSoil Physicochemical Properties
andPlant Characteristics
Carbon (C) and nitrogen (N) concentrations in collected lit-
ter and fine roots were detected using a CHNS/O Elemental
Analyzer (PerkinElmer, USA). Detailed analysis methods
and procedures of soil pH, soil organic carbon (SOC), total
nitrogen (TN), ammonium nitrogen (NH4+-N), nitrate nitro-
gen (NO3−-N), total phosphorus (TP), and available phos-
phorus (AP) were present in He etal. (2018).
2.4 DNA Extraction andSequence Analysis
Soil total DNA was isolated with the MoBio Power Soil
DNA Isolation kit. The primer set 338F/806R was adopted
to quantify the V3–V4 region of the 16S Rrna (Zhu etal.
2018). PCR reactions were conducted for amplification, and
the amplification conditions and programs were present in
Zhao etal. (2019). The PCR amplicons were purified using
a gel extraction kit, quantified using Qubit fluorometer, and
then paired-end sequenced using an Illumina Miseq PE 250
platform (San Diego, CA, USA).
The raw reads were clustered into the same OTUs (opera-
tional taxonomic units) at 97% nucleotide similarity with the
UPARSE software (http:// drive5. com/ usear ch/). 16S rRNA
sequences were assigned to a taxonomic unit based on the
bacterial SILVA reference database (Release138 http:// www .
arb- silva. de) using the RDP classifier v.11.5 (http:// rdp. cme.
msu. edu/). We identified a total of 13,682,118 high-quality
16S sequences, ranging from 30,012 to 74,319 sequences
per sample, and these sequences were classified into 8313
OTUs at 97% similarity level.
2.5 Statistical Analysis
Taxonomic diversity indices were estimated with
MOTHUR v.1.34.4 (http:// www. mothur. org/). Two-way
analysis of variance (ANOVA) was used to detect the dif-
ferences in soil physicochemical properties, plant charac-
teristics, and bacterial alpha diversity between slope aspect
and elevation, and multiple comparisons were performed
by the Duncan’s new multiple range tests. The differences
in bacterial beta diversity at OTU level were explored by
non-metric multidimensional scaling (NMDS) analysis
based on Bray–Curtis distances, and the significance of the
observed differences was estimated by Adonis using 999
permutations. The compositional variance within groups,
measured as distances to centroids, was evaluated using the
betadisper function. The relationships between distances to
centroids and elevation were evaluated by ordinary least
squares (OLS) regression. Redundancy analysis (RDA) was
performed to estimate the correlations among environmen-
tal variables and bacterial community. The environmental
variables with variance inflation factor (VIF) > 10 were
considered to have strong collinearity with other environ-
mental variables, and removed from the RDA analysis. The
VIF values of MAT, MAP, BD, TN, NO3−-N, and AP > 10
were removed. In addition, Pearson correlation (PC) anal-
ysis was adopted to estimate the relationships between
distances to centroids and environmental variables, and
between bacteria abundance and environmental variables.
The ANOVA, NMDS, Adonis test, OLS, RDA, and PC
analyses were performed using “multcomp,” “vegan,”
“vegan,” “basicTrendline,” “vegan,” and “psych” packages
in R v.3.2.3, respectively (Boix-Amorós etal. 2016; Ziegler
etal. 2017; Zhao etal. 2019).
3 Results
3.1 Soil Physicochemical Properties andPlant
Characteristics
Generally, soil pH, BD, SOC, soil C/N ratios, NO3−-N, and
AP varied significantly with slope aspect and elevation, TN
and NH4+-N varied significantly with elevation, while TP
showed no significant difference with slope aspect or eleva-
tion (Table1). Notably, SOC, soil C/N ratios, NO3−-N, and
AP on north-facing slope were significantly higher than
those on south-facing slope, while soil pH on north-facing
slope was significantly lower than that on south-facing slope
(Table1). Furthermore, SOC, soil C/N ratios, NO3−-N, and
AP on both south-facing and north-facing slopes increased
initially and then decreased with elevation, while soil pH and
BD on both south-facing and north-facing slopes decreased
initially and then increased with elevation (Table1).
Nutrient levels of aboveground litter and fine roots also
changed with slope aspect and elevation. Specifically, the N
concentrations and C/N ratios of aboveground litter and the
C concentrations of fine roots varied significantly with slope
aspect and elevation; the C concentrations of aboveground
litter varied significantly with slope aspect; the C/N ratios of
fine roots varied significantly with elevation (Table2). Nota-
bly, the C concentrations and C/N ratios of aboveground
litter and the C concentrations of fine roots on north-facing
slope were higher than those on south-facing slope, and the
C/N ratios of aboveground litter on both south-facing and
north-facing slopes increased initially and then decreased
with elevation (Table2).

Journal of Soil Science and Plant Nutrition
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Table 1 Soil physicochemical properties along an altitudinal gradient on south-facing and north-facing slopes
Values of p are the significance by the two-way ANOVA. S38, S35, S32, S28, and S24 represent sites at 3800, 3500, 3200, 2800, and 2400m on south-facing slope; N38, N35, N32, N28, and
N24 represent sites at 3800, 3500, 3200, 2800, and 2400m on north-facing slope. BD, SOC, TN, C/N, TP, NH4+-N, NO3−-N, and AP indicate soil bulk density, soil organic carbon, total nitro-
gen, soil carbon/nitrogen ratios, total phosphorus, ammonia nitrogen, nitrate nitrogen, and available phosphorus. The data were expressed as mean (SE). Lowercase letters following the mean
values indicated significantly different between elevations within slope aspect, and uppercase letters following the mean values indicated significantly different between aspects at the same
elevation
*** p < 0.001; **p < 0.01
Soil pH BD (g cm−3) SOC (g kg−1) TN (g kg−1) C/N TP (g kg−1) NH4+-N (mg kg−1)NO3—N (mg kg−1) AP (mg kg−1)
South-facing S38 7.76 (0.11) dA 0.84 (0.03) cA 80.30 (3.24) bB 7.82 (0.29) bB 10.27 (0.39) bB 1.51 (0.26) bA 3.54 (0.68) aA 95.99 (3.80) bB 13.68 (0.56) bB
slope S35 7.63 (0.10) dA 0.76 (0.03) cA 92.56 (2.77) aB 8.04 (0.14) bB 11.51 (0.15) aB 1.33 (0.06) bB 4.18 (0.77) aA 113.88 (8.14) aB 15.77 (0.47) aB
S32 8.03 (0.03) cA 0.75 (0.07) cA 90.19 (3.23) aB 9.11 (0.25) aA 9.90 (0.15) bcB 2.90 (0.75) aA 2.40 (0.67) aA 115.10 (5.94) aB 15.65 (0.55) aB
S28 8.57 (0.09) bA 0.95 (0.06) bA 41.63 (1.79) cB 4.27 (0.13) cB 9.75 (0.14) cB 1.75 (0.30) bA 3.91 (1.26) aA 52.55 (7.27) cB 8.43 (1.25) cB
S24 8.99 (0.11) aA 1.08 (0.05) aA 21.98 (1.70) dB 2.28 (0.20) dB 9.63 (0.09) cB 1.83 (0.45) bA 4.32 (0.55) aA 26.27 (5.46) dB 6.11 (0.54) dB
North-facing N38 7.46 (0.07) cB 0.76 (0.03) bcB 91.22 (0.46) cA 7.96 (0.01) bB 11.46 (0.07) cA 1.98 (0.46) aA 4.32 (1.02) abA 106.31 (5.13) bcA 15.21 (0.62) bcA
slope N35 7.19 (0.16) cB 0.71 (0.03) cA 123.36 (9.55) aA 10.20 (0.20) aA 12.09 (0.43) cA 2.10 (0.14) aA 3.02 (1.64) abA 161.71 (7.31) aA 23.64 (1.24) aA
N32 7.75 (0.04) bB 0.73 (0.02) bcA 102.49 (6.89) bA 5.02 (0.85) cB 20.45 (2.25) aA 1.37 (0.09) aB 2.29 (0.13) bA 125.21 (0.91) bA 17.40 (1.04) bA
N28 7.97 (0.07) bB 0.81 (0.02) bcB 84.28 (1.61) cA 5.26 (0.16) cA 16.22(0.62) bA 1.84 (0.48) aA 5.09 (1.10) aA 96.11 (6.39) cA 14.02 (0.68) cA
N24 8.64 (0.14) aB 1.01 (0.07) aA 34.31 (1.97) dA 2.97 (0.08) dA 11.57(0.95) cA 1.59 (0.17) aA 3.26 (0.27) abB 47.16 (10.43) dA 7.51 (0.81) dA
Slope aspect *** *** *** 0.28 *** 0.53 0.82 *** ***
Altitude *** *** *** *** *** 0.28 ** *** ***
Slope aspect × altitude 0.08 0.24 *** *** *** *** 0.13 *** ***

Journal of Soil Science and Plant Nutrition
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3.2 Patterns ofSoil Bacterial α‑Diversity
Generally, the number of OTUs and Chao index varied sig-
nificantly with slope aspect and elevation, and the Shannon
index varied significantly with elevation (Table3). Specifi-
cally, the number of OTUs at 2400, 2800, and 3200m on
north-facing slope was significantly higher than that on
south-facing slope, and the Chao index on north-facing
Table 2 Carbon and nitrogen concentrations and their ratios for aboveground litter and fine roots along an altitudinal gradient on south-facing
and north-facing slopes
Values of p are the significance by the two-way ANOVA. S38, S35, S32, S28, and S24 represent sites at 3800, 3500, 3200, 2800, and 2400m
on south-facing slope; N38, N35, N32, N28, and N24 represent sites at 3800, 3500, 3200, 2800, and 2400m on north-facing slope. Clitter, Nlitter,
and C/Nlitter indicate the carbon and nitrogen concentrations and their ratios for aboveground litter; Croots, Nroots, and C/Nroots indicate the carbon
and nitrogen and their ratios for fine roots. The data were expressed as mean (SE). Lowercase letters following the mean values indicated signifi-
cantly different between elevations within slope aspect, and uppercase letters following the mean values indicated significantly different between
aspects at the same elevation
*** p < 0.001; **p < 0.01; *p < 0.05
Aboveground litter Fine roots
Clitter (mg g−1) Nlitter (mg g−1) C/Nlitter (mg g−1) Croots (mg g−1) Nroots (mg g−1) C/Nroots (mg g−1)
South-facing S38 365.08 (55.77) aA 14.28 (2.71) aA 26.58 (0.98) bB 387.20 (67.19) aA 7.69 (0.90) aA 50.11 (3.02) aA
slope S35 376.88 (20.58) aB 13.12 (2.94) aA 28.78 (0.37) aA 490.38 (29.40) aB 8.75 (0.80) aA 56.41 (7.03) aA
S32 369.10 (45.52) aB 14.91 (1.68) aA 24.73 (0.36) bcB 407.21 (67.23) aA 8.67 (0.86) aA 45.59 (7.91) aA
S28 359.19 (24.66) aB 14.75 (1.22) aA 24.37 (0.35) bcB 460.35 (57.55) aA 9.91 (1.21) aA 46.76 (7.14) aA
S24 363.78 (24.55) aA 15.11 (0.96) aA 24.08 (0.21) cB 473.47 (51.18) aB 8.97 (0.77) aA 53.02 (7.09) aA
North-facing N38 426.03 (32.44) aA 14.79 (1.45) aA 28.66 (0.17) cA 478.41 (44.69) abA 8.91 (0.70) aA 53.83 (5.09) bcA
slope N35 424.45 (39.92) aA 14.09 (0.36) aA 32.03 (2.07) cA 582.69 (34.23) aA 9.31 (0.32) aA 62.71 (5.71) aA
N32 502.64 (93.06) aA 9.77 (0.76) cB 51.13 (5.63) aA 457.89 (54.88) bA 8.81 (1.06) aA 51.97 (1.19) bcA
N28 413.28 (14.25) aA 10.17 (0.94) bcB 40.54 (1.55) bA 480.03 (38.40) abA 9.61 (0.27) aA 49.93 (2.83) cA
N24 383.18 (38.82) aA 13.32 (1.94) bA 29.83 (2.37) cA 584.09 (37.21) aA 9.40 (0.40) aA 62.15 (3.56) aA
Slope aspect ** ** *** *** 0.24 0.08
Altitude 0.374 * *** ** 0.06 *
Slope aspect × altitude 0.476 * *** 0.53 0.48 0.89
Table 3 Bacterial richness and
diversity estimators along an
altitudinal gradient on south-
facing and north-facing slopes
Values of p are the significance by the two-way ANOVA. S38, S35, S32, S28, and S24 represent sites at
3800, 3500, 3200, 2800, and 2400m on south-facing slope; N38, N35, N32, N28, and N24 represent sites
at 3800, 3500, 3200, 2800, and 2400 m on north-facing slope. The data were expressed as mean (SE).
Lowercase letters following the mean values indicated significantly different between elevations within
slope aspect, and uppercase letters following the mean values indicated significantly different between
aspects at the same elevation
*** p < 0.001; *p < 0.05
Observed OTUs Chao1 estimator Shannon index
South-facing S38 2701 (167.29) abA 3344.73 (73.37) aB 6.70 (0.03) aA
slope S35 2834 (184.86) aA 3727.99 (296.68) aA 6.71 (0.02) aA
S32 2410 (94.62) bB 3292.71 (88.17) aB 6.56 (0.03) bA
S28 2049 (105.67) cB 2770.79 (219.97) bB 6.38 (0.05) cA
S24 2038 (133.97) cB 2525.04 (86.17) bB 6.27 (0.08) cA
North-facing N38 2769 (201.58) abA 3695.73 (193.64) abA 6.65 (0.04) aA
slope N35 2980 (98.09) aA 4060.01 (140.37) aA 6.68 (0.04) aA
N32 2755 (121.97) abA 3839.04 (110.79) abA 6.51 (0.08) abA
N28 2715 (74.36) abA 3798.12 (133.88) abA 6.43 (0.05) abA
N24 2471 (101.71) bA 3597.15 (104.72) bA 6.37 (0.09) bA
Slope aspect *** *** 0.91
Altitude *** *** ***
Slope aspect × altitude * * 0.34

Journal of Soil Science and Plant Nutrition
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slope was significantly higher than that on south-facing
slope, except at elevation of 3500m. However, no signifi-
cant differences were detected in Shannon index between
north-facing and south-facing slopes (Table3). The number
of OTUs, Chao index, and Shannon index on both south-
facing and north-facing slopes increased significantly with
elevation up till 3500m and then decreased, exhibiting a
unimodal pattern (Table3).
3.3 Patterns ofSoil Bacterial β‑Diversity
NMDS analysis revealed significant differences in bacterial
beta diversity with slope aspect (stress = 0.062, R2 = 0.849,
p < 0.001) (Fig. 1a). Furthermore, NMDS analysis also
revealed significant differences in bacterial beta diversity
with elevation on both south-facing slope (stress = 0.069,
R2 = 0.748, p < 0.001) and north-facing slope (stress = 0.059,
R2 = 0.706, p < 0.001) (Fig.1b, c). On south-facing slope,
bacterial communities at elevations of 3800 and 3500m
grouped together, and were obviously separated from those
at 3200, 2800, and 2400m, which were also separated from
each other (Fig.1b). On north-facing slope, bacterial com-
munities at elevation of 3200 and 2800m grouped together,
and were obviously separated from those at elevations of
3800, 3500, and 2400m, which were also separated from
each other (Fig.1c). A regression analysis showed that dis-
tances to centroids were significantly correlated with eleva-
tion on both south-facing and north-facing slopes, and the
relationships were described by quadratic models (Fig.2a,
b; TableS2).
3.4 Patterns ofSoil Bacterial Community
Compositions
The top 10 dominant phyla were Actinobacteria
(17.58–32.63%), Proteobacteria (15.35–36.56%), Acido-
bacteria (8.99–23.09%), Chloroflexi (8.07–16.47%), Bacte-
roidetes (2.76–4.42%), Gemmatimonadetes (1.96–5.62%),
Fig. 1 Non-metric multidimensional scaling (NMDS) ordination
of bacterial communities based on Bray–Curtis similarities for the
overall (a), south-facing slope (b), and north-facing slope (c) and
the significance of the observed differences was estimated by Adonis
using 999 permutations. S38, S35, S32, S28, and S24 represent sites
at 3800, 3500, 3200, 2800, and 2400m on south-facing slope; N38,
N35, N32, N28, and N24 represent sites at 3800, 3500, 3200, 2800,
and 2400m on north-facing slope
Fig. 2 Relationships of eleva-
tion with distances to centroids
on south-facing slope (a) and
north-facing slope (b). The
quadratic models were selected
by comparing adjusted R2 and
AIC

Journal of Soil Science and Plant Nutrition
1 3
Firmicutes (0.18–7.84%), Nitrospirae (0.39–4.25%), Verru-
comicrobia (0.59–2.35%), and Cyanobacteria (0.04–7.31%)
(Fig.3a). Notably, Actinobacteria, Chloroflexi, Gemmati-
monadetes, and Cyanobacteria were more abundant on
south-facing slope than those on north-facing slope; while
Proteobacteria and Nitrospirae were less abundant on south-
facing slope than those on north-facing slope (Fig.3b).
The top 10 dominant classes were Actinobacteria
(17.58–36.16%), Acidobacteria (8.99–23.09%), Alp-
haproteobacteria (11.54–18.39%), Betaproteobacteria
(1.27–11.36%), Gemmatimonadetes (1.98–5.62%), Thermo-
microbia (0.25–8.69%), Deltaproteobacteria (1.80–3.93%),
Nitrospirae (0.39–4.25%), Bacilli (0.07–7.54%), and Cyano-
bacteria (0.04–7.31%) (Fig.4a). Notably, Actinobacteria,
Thermomicrobia, Gemmatimonadetes, and Cyanobacteria
were more abundant on south-facing slope than those on
north-facing slope, while Alphaproteobacteria, Betaproteo-
bacteria, and Nitrospirae were less abundant on south-facing
slope than those on north-facing slope (Fig.4b).
3.5 Relationships ofSoil Bacterial Community
withEnvironmental Variables
RDA indicated that slope aspect (r2 = 0.929, p < 0.001),
soil C/N ratios (r2 = 0.805, p < 0.001), SOC (r2 = 0.270,
p < 0.05), and soil pH (r2 = 0.234, p < 0.05) were the major
environmental variables significantly affecting bacterial
community composition (Fig.5a; Table4). For south-facing
slope, elevation (r2 = 0.840, p < 0.001), soil pH (r2 = 0.886,
p < 0.001), SOC (r2 = 0.772, p < 0.001), and soil C/N ratios
(r2 = 0.677, p < 0.01) were the major environmental vari-
ables significantly affecting bacterial community compo-
sition (Fig.5b; Table4). For north-facing slope, elevation
(r2 = 0.890), soil C/N ratios (r2 = 0.779), soil pH (r2 = 0.673),
and SOC (r2 = 0.549) were the major environmental vari-
ables significantly affecting bacterial community compo-
sition (Fig.5c; Table4). This finding was confirmed with
regression analyses, which demonstrated that aspect, soil
C/N ratios, SOC, and soil pH were significantly correlated
with distances to centroids. For both south-facing and north-
facing slopes, elevation, soil pH, SOC, and soil C/N ratios
were significantly correlated with distances to centroids
(TableS3).
4 Discussion
To our knowledge, the present study is as one of the few
studies in arid-montane ecosystems to investigate the
influence of slope aspect on soil bacterial communities.
Our findings revealed that bacterial communities dif-
fered significantly between north-facing and south-facing
Fig. 3 Distributions of the top 10 dominant phyla at the different
sampling sites (a) and the difference in the relative abundance of the
dominant class between on south-facing and north-facing slopes (b).
***p < 0.001; **p < 0.01; *p < 0.05. S38, S35, S32, S28, and S24 rep-
resent sites at 3800, 3500, 3200, 2800, and 2400m on south-facing
slope; N38, N35, N32, N28, and N24 represent sites at 3800, 3500,
3200, 2800, and 2400m on north-facing slope

Journal of Soil Science and Plant Nutrition
1 3
slopes, and that bacterial richness and abundance were
higher on the north-facing slope than on the south-facing
slope. Furthermore, we also detected remarkably eleva-
tional diversity patterns of soil bacterial communities
on both north-facing and south-facing slopes. In that,
bacterial richness and diversity increased significantly
with elevation up to 3500m, and then decreased, and
community composition differed dramatically along ele-
vation as shown with the significant quadratic relation-
ships between beta diversity indices and elevation. These
findings indicated a fundamental role of slope aspect and
elevation in controlling diversity patterns of soil bacte-
rial communities in arid-montane ecosystems. RDA fur-
ther revealed that slope aspect has the greatest effect on
Fig. 4 Distributions of the top 10 dominant class at the different
sampling sites (a) and the difference in the relative abundance of the
dominant phyla between south-facing and north-facing slopes (b).
***p < 0.001; **p < 0.01; *p < 0.05. S38, S35, S32, S28, and S24 rep-
resent sites at 3800, 3500, 3200, 2800, and 2400m on south-facing
slope; N38, N35, N32, N28, and N24 represent sites at 3800, 3500,
3200, 2800, and 2400m on north-facing slope
Fig. 5 Redundancy analysis identifying the relationships between
bacterial community structures and environmental variables for the
overall (a), south-facing slope (b), and north-facing slope (c). S38,
S35, S32, S28, and S24 represent sites at 3800, 3500, 3200, 2800,
and 2400m on south-facing slope; N38, N35, N32, N28 and N24
represent sites at 3800, 3500, 3200, 2800, and 2400 m on north-
facing slope. AS, EL, GR, SOC, C/N, TP, NH4+-N, Clitter, and Nroots
indicate aspect, elevation, gradient, soil organic carbon, soil carbon/
nitrogen ratios, total phosphorus, ammonia nitrogen, carbon con-
centrations for aboveground litter, and nitrogen concentrations for
fine roots, respectively

Journal of Soil Science and Plant Nutrition
1 3
bacterial community composition for the whole catchment.
At this spatial scale, elevation has no significant effect on
bacterial community composition. However, within both
south-facing and north-facing slopes, elevation was the
most important environmental variable affecting bacterial
community composition. These results indicated that the
effect of slope aspect and elevation on bacterial commu-
nity composition depends on spatial scale in arid-montane
ecosystems.
The importance of elevation in controlling soil microbial
diversity and community composition has been shown in
other montane ecosystems; nevertheless, different and even
contradictory elevational distribution patterns have been
documented (Liu etal. 2016; Shen etal. 2019). A recent
synthesis of more than 20 studies revealed that elevational
trends of microbial diversity were related to the tree line
(Shen etal. 2019). Studies began above the tree line and
extended upwards that tended to show declining diversity
trends with elevation (Li etal. 2016); whereas others that
extended across the tree line showed other diversity trends
with elevation, including increasing (Margesin etal. 2009),
unimodal (Peng etal. 2018; Praeg etal. 2020), and hollow
patterns (Singh etal. 2014; Liu etal. 2016). A unimodal
pattern was documented in this study for bacterial diversity
from 2400 to 3800m on both north-facing and south-facing
slopes (tree line was at about 3300m a.s.l). Our findings
supported those of Peng etal. (2018) and Praeg etal. (2020)
from Taibai Mountain in China and the Central European
Alps, respectively.
Fewer studies focused on microbial relationships with
slope aspect than with elevation. Slope aspect is the main
topographic parameter generating environmental heteroge-
neity by altering the effects of solar radiation and hydrother-
mal processes (Sidari etal. 2008; Bennie etal. 2008; Liu
etal. 2013; Chen etal. 2016). It has been well documented
that slope aspects play a primary role in shaping soil bio-
geochemical processes and vegetation patterns (Coblentz
and Riitters 2010; Xue etal. 2018). Recently, the influence
of slope aspect on AMF communities also attracted atten-
tion because of the observed close association between
plant diversity and AMF communities (Hiiesalu etal. 2014;
Prober etal. 2015; Liu etal. 2017). Previous studies have
revealed that aspect-induced changes in plant communities
had strong direct effects on AMF community diversity (Chu
etal. 2016; Ai etal. 2018; Wei etal. 2021). However, little is
known about the response of bacterial communities to slope
aspect. Interestingly, in this study, significant differences
were observed in soil bacterial diversity and composition
with slope aspect in the arid-montane ecosystem, support-
ing earlier evidence of the importance of slope aspect in
regulating the diversity pattern of bacterial communities in
arid-montane ecosystems.
The significant influence of slope aspect on bacterial com-
munity composition was demonstrated by the significant
difference in dominant bacterial abundance between north-
facing and south-facing slopes. Notably, Actinobacteria,
Gemmatimonadetes, Cyanobacteria, and Thermomicrobia
within Chloroflexi were more abundant on south-facing slope
than those on north-facing slope, while Alphaproteobacteria
and Betaproteobacteria within Proteobacteria and Nitros-
pirae were less abundant on south-facing slope than those on
north-facing slope. Actinobacteria and Thermomicrobia are
oligotrophic groups, and prefer nutrient deficient conditions
(Eichorst etal. 2007; Sorokin etal. 2012; Lazcano etal. 2013;
Song etal. 2018), while Alphaproteobacteria, Betaproteo-
bacteria, and Nitrospirae have copiotrophic life history strat-
egies, and are more abundant in nutrient-enriched environ-
ment (Fierer etal. 2007; Chu etal. 2010; Goldfarb etal. 2011;
Table 4 Correlation between soil properties and bacterial communities (OTU abundance) as evaluated by redundancy analysis
VIF, SOC, C/N, TP, NH4+-N, Clitter, and Nroots indicate variance inflation factor, soil organic carbon, soil carbon/nitrogen ratios, total phospho-
rus, ammonia nitrogen, carbon concentrations for aboveground litter, and nitrogen concentrations for fine roots, respectively
*** p < 0.001, **p < 0.01, *p < 0.05
Overall South-facing slope North-facing slope
VIF Axis 1 Axis 2 r2pAxis 1 Axis 2 r2pAxis 1 Axis 2 r2p
Aspect 3.055 0.946 − 0.325 0.929 *** / / / / / / / /
Altitude 4.857 − 0.686 − 0.728 0.053 0.480 − 0.898 0.439 0.840 *** − 0.200 − 0.980 0.890 ***
Gradient 1.523 − 0.299 − 0.954 0.008 0.910 − 0.587 − 0.810 0.130 0.447 − 0.953 − 0.304 0.025 0.835
Soil pH 8.492 0.987 0.163 0.234 * 0.978 − 0.210 0.886 *** 0.335 0.942 0.673 **
SOC 5.509 − 0.945 − 0.327 0.270 * − 0.984 0.176 0.772 *** − 0.547 − 0.837 0.549 *
C/N 2.835 − 0.934 − 0.359 0.805 *** − 0.601 0.799 0.677 ** − 0.942 0.336 0.779 ***
TP 1.639 0.637 0.771 0.028 0.691 0.852 0.523 0.037 0.768 0.321 − 0.947 0.219 0.210
NH4+-N 1.253 0.333 − 0.943 0.020 0.784 0.260 − 0.966 0.204 0.249 − 0.922 − 0.388 0.028 0.857
Clitter 1.766 − 0.998 0.062 0.174 0.121 − 0.727 0.686 0.002 0.993 − 0.737 − 0.675 0.139 0.433
Nroots 1.961 0.335 − 0.942 0.114 0.196 0.400 − 0.917 0.068 0.667 − 0.591 0.807 0.319 0.106

Journal of Soil Science and Plant Nutrition
1 3
Daims etal. 2015; Wang and Hua 2022). Thus, the higher
abundance of Alphaproteobacteria, Betaproteobacteria, and
Nitrospirae and lower abundance of Actinobacteria and Ther-
momicrobia on north-facing slope than on south-facing slope
can be attributed to higher availability of substrate and nutrient
supply on north-facing slope. Our interpretations were further
confirmed with a Pearson correlation analysis, in which Act-
inobacteria and Thermomicrobia were significantly negatively
correlated with SOC and available nutrient contents, while
Alphaproteobacteria, Betaproteobacteria, and Nitrospirae
were significantly positively correlated with SOC and avail-
able nutrients. In addition, Cyanobacteria are phototrophs and
have been demonstrated be more abundant in south-facing
slope (Kuritz 1998); Gemmatimonadetes prefer arid condi-
tions (Chanal etal. 2006; DeBruyn etal. 2011), explaining
their higher abundance on drier south-facing slope than on
moister north-facing slope.
Furthermore, our results also revealed that the variations
in soil pH, SOC, and soil C/N ratios caused by slope aspect
and elevation contributed significantly to the diversity pat-
terns of soil bacterial communities in this arid-montane
ecosystem; this finding was confirmed with correlation
analyses, which demonstrated that soil pH, SOC, and soil
C/N ratios were significantly correlated with alpha and beta
diversity indices of bacterial community. Soil pH-driven
elevational patterns of microbial diversity and composition
have been described across a variety of spatial scales (Fierer
and Jackson 2006; Delgado-Baquerizo etal. 2018; Malard
etal. 2019). Our observations were in line with studies men-
tioned above, and emphasized the importance of soil pH in
mediating the influence of slope aspect on bacterial diversity
and community composition in arid-montane ecosystem.
SOC, as the fundamental substrate and energy source for
soil microbes, and soil C/N ratios, indicating substrate qual-
ity (Nilsson etal. 2012; Deng etal. 2018; Zhao etal. 2021),
are supposed to influence elevational diversity patterns of
microbial communities by affecting their metabolism (Smith
etal. 2002; Xiang etal. 2014; Peng etal. 2018). Our obser-
vations supported the role of slope aspect in controlling bac-
terial diversity and composition by altering SOC and soil
C/N ratios.
5 Conclusions
Our work is as one of the few studies in arid-montane to
explore the influence of slope aspect on bacterial communi-
ties in arid-montane ecosystems. The results revealed that
bacterial alpha and beta diversity significantly differed with
slope aspect and elevation, indicating a fundamental role
of slope aspect and elevation in regulating diversity and
composition of bacterial communities in the arid-montane
ecosystems. The strong effect of slope aspect on bacterial
communities was demonstrated by the shifts in dominant
bacterial abundance between north-facing and south-facing
slopes. Our results further emphasized the importance of soil
pH, soil organic carbon, and soil carbon/nitrogen ratios in
mediating the influence of slope aspect on bacterial diversity
and community composition in arid-montane ecosystem.
Overall, our findings provide new insights into microbial
relationships with topography and have important implica-
tions for biodiversity conservation in arid-montane ecosys-
tems in China.
Supplementary Information The online version contains supplemen-
tary material available at https:// doi. org/ 10. 1007/ s42729- 022- 01002-8.
Funding The study was funded by the Key Program of the Chinese
Academy of Sciences (QYZDJ-SSW-DQC040), the Second Tibetan
Plateau Scientific Expedition and Research (STEP) program (Grant No.
2019QZKK0303), and the Strategic Priority Research Program of the
Chinese Academy of Sciences (CAS) (XDA23060301).
Declarations
Conflict of Interest The authors declare no competing interests.
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