Divergent responses of soil bacterial communities in erosion-deposition plots on the Loess Plateau
Introduction
Soil erosion, a serious eco-environmental problem exacerbated by anthropogenic perturbations, is the most widespread form of land degradation (Lal, 2003, Pimentel et al., 1995). Globally, ~75 Gt of soil is redistributed by erosion per year, of which 70–90% is deposited in various low-lying areas (Stallard, 1998). In general, soil erosion breaks down aggregates and selectively transports fine or light particles and labile organic fractions (Lal, 2003, Muller-Nedebock et al., 2016), which deteriorates soil structure and results in soil organic carbon loss (Berhe and Kleber, 2013). In contrast, depending on temporal surface runoff and sediment, deposition increases nutrients and wetness, reduces aeration and potentially alters pH and redox conditions, which contribute to the storage and stability of carbon and nitrogen in the soil (Berhe et al., 2018, Berhe and Torn, 2016). These variations in local hydrologic, pedologic and microclimatic conditions can profoundly impact productivity and stability of soil ecosystems (Gregorich et al., 1998, Kirkels et al., 2014, Yue et al., 2016).
Soil microbes (especially bacteria) are ubiquitous and abundant in soil ecosystems and act as biogeochemical engineers of the earth's biosphere (Bardgett and van der Putten, 2014). Soil microbial communities perform crucial roles in ecosystem functions, including organic matter transformation and decomposition, as well as nutrient and carbon cycling (Allison and Goulden, 2017, Takriti et al., 2018). As reported, soil microbes can contribute considerably to soil organic matter, and microbial anabolism can exert critical control over soil organic carbon stabilization in the global carbon cycle (Liang et al., 2017). Soil bacterial communities are influenced by their soil habitats, and changes in the soil environment can drive dramatic responses in bacterial communities (Griffiths and Philippot, 2013). Since erosion causes soil loss while deposition increases soil resource availability, and both processes probably lead to shifts in bacterial functional groups, taxonomic diversity, and community structure (Banerjee et al., 2016). Additionally, soil erosion and deposition can induce soil spatial heterogeneity that may alter bacterial interactions and co-occurrence patterns. Previous studies performed by Mabuhay et al., 2004, Helgason et al., 2014 have separately investigated the responses of bacterial communities to soil erosion or deposition; and Xiao et al. (2017) demonstrated that erosion homogenizes bacterial communities along eroding slopes. However, the effects of soil erosion and deposition on bacterial communities have not been comprehensively investigated. Moreover, the response characteristics of bacterial community structure and co-occurrence patterns during soil erosion and deposition processes have been largely ignored.
The Loess Plateau of China, covering 640,000 km2 and featuring numerous loess hills and gullies, is one of the most eroded regions in the world. The complex terrain, intense storms, and anthropogenic disturbances including deforestation, overgrazing and intensive agricultural practices have accelerated the erosion of this inherently highly erodible soil (Shi and Shao, 2000), producing an erosion modulus of 100–20,000 t km−2 year−1 (Fu, 2010). In this region, slopes ranging from 8° to 35° are found to account for 45.5% of the total land area and responsible for 82% of the total soil loss. Slopes < 5° cover 43.2% of the total land area and have a lower erosion rate of <100 t km−2 y−1 (Fu, 2010, Fu et al., 2011). Recently, the effects of soil erosion and deposition on soil physiochemical properties have been reported (Rousk et al., 2010). Given the variation in soil physiochemical properties with soil erosion and deposition, we hypothesized that soil bacterial communities may display prominent responses to soil erosion and deposition on the Loess Plateau.
In the present study, a field experiment simulating soil erosion and deposition was conducted on the Loess Plateau from 2015 to 2017. Using high-throughput sequencing, we aimed to (1) assess the effects of soil erosion and deposition on bacterial alpha diversity, community structure and co-occurrence patterns, and (2) determine the influences of soil physicochemical properties and slope gradients on the divergence of bacterial communities during soil erosion and deposition processes.
Section snippets
Site description and experimental design
The study site was located at Changwu State Key Agro-Ecological Experimental Station (35°13′ N, 107°40′ E, 1220 m a.s.l.) in Changwu County, Shaanxi Province, China, which is a typical eroded gully of the south Loess Plateau (Fig. 1a). This site is characterized by heavy rainstorms during hot summer and cold dry weather in winter. The mean annual precipitation is 560 mm, 60% of which occurs from July to September, and the mean annual air temperature is 9.4 °C (according to meteorological data
Variation in runoff, sediment, erosion rate and soil physicochemical properties
Runoff, sediment yield, and erosion rate for the three slope gradients during 39 measurable erosion events are listed in Table 1. Runoff increased with slope gradients; slopes of 10° and 20° generated 30–115% and 48–207% more runoff than the 5° reference slopes, respectively. Similarly, the annual sediment yield of 10° and 20° slopes was 243–550% and 221–780% more than that of reference slopes, respectively. Erosion rate increased with increasing slope gradients; the most severe erosion
Both erosion and deposition induce changes in soil physiochemical properties
Compared with the reference slopes, the greater soil nutrients (C, N, P) in the depositional zones and the according decrease of organic matter and dissolved organic carbon on the eroding slopes clearly demonstrated the erosion- and deposition-induced spatial variation of soil physiochemical properties over 3 years. This variation mainly resulted from the substantial loss of runoff and sediment from the eroding slopes to the depositional zones (Table 1). Light organic matter and more labile
Conclusions
In the 3-year field simulation experiment, soil erosion and deposition had contrasting effects on bacterial communities. Soil erosion altered bacterial community structure rather than alpha diversity, whereas deposition increased bacterial alpha diversity but did not alter community structure. Increasing slope gradients enlarged the differences in both bacterial alpha diversity and community structure between eroding slopes and depositional zones. Erosion- and deposition-induced soil moisture,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by the National Natural Science Foundation of China (No. 41371279, No. 41701318), and the Fundamental Research Funds for the Central Universities, Northwest A&F University (2452017191).
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