Abstract
Soil microorganisms are the main driving force in soil ecological and biogeochemical processes, such as carbon mineralization, humus formation, and nutrient cycles. Soil erosion may dramatically alter microbial properties. However, it is still unclear how soil microbial characteristics affected by soil erosion. In hilly areas, soil erosion is highly attributed to topographic characteristics such as slope position and slope gradient. Herein, we compared the soil microbial characteristics between different slope gradients and positions in the farmland in the typical black soil area of Northeast China. The relative abundances of major microbial phyla are illustrated. Proteobacteria, Acidobacteria, Gemmatimonadetes, Planctomycetes, and Actinobacteria accounted for 83% of the total bacterial sequences, and Ascomycota, Basidiomycota, and chytridiomycosis accounted for 78% of total fungal sequences in the studied area. Microbial alpha-diversity (Bacteria and Fungi) was similar among the slope gradients or slope positions. The bacterial and fungal community composition was only affected by slope gradient, which was significantly lower in the highest slope gradient level than in others. This study suggests that slope gradient but not slope gradient affecting microbial community composition in the cropland of black soil region.
Keywords: Black soil; soil microbial community; bacterial; fungal; slope.
Introduction
Land degradation refers to deterioration of soil properties related to crop production, infrastructure maintenance, and natural resource quality (Lal, 2001). More than 20% of cultivated areas in the world experience degradation, and its severity extends to many parts (Bai et al., 2008). The NRC (1994) highlighted that land degradation is complex and implicates transformations in the soil and vegetation’s physical, chemical, and biological properties. Soil erosion, one of the considerable critical environmental problems, is the primary process leading to soil degradation worldwide (Wilkinson and Mcelory, 2007), particularly in hilly areas, highly attributed to topographic effects such as slope position, and slope gradient (Khormali et al., 2009). Topographic properties determine the receipt and redistribution of light, heat, water, and sediments over the land surface, affecting the variations in soil properties and leading to reduced productivity (FAO, 2015).
Soil microorganisms are the main driving force in soil ecological and biogeochemical processes, such as carbon mineralization, humus formation, and nutrient cycles. Soil erosion may dramatically alter microbial properties and enzyme activities. Research has indicated that microbial counts, biomass, diversity, and enzyme activity decline drastically during soil erosion (Salam et al., 2001; Moreno-De Las Heras, 2009; Mabuhay et al., 2004; An et al., 2008). Erosion directs to the loss of fertile topsoil and nutrients, which sustain microbial habitats and the ecosystem services function, resulting in reduced productivity (Van Oost and Bakker, 2012). The composition, structure, and functionality of soil microorganisms are sensitive to environmental change and disturbance (Chang et al., 2007). Soil carbon and nutrient availability to microbes would be modified through rain splash and surface wash, which is more pronounced in sloped land. The available substrates would move with fine particles along the slope.
Wang et al. (2017) found that the bottom slope position is rich in soil moisture, clay, and organic carbon content compared with the top slope position, leading to a larger microbial community and greater microbial biomass and enzyme activity in the bottom slope position. The varying soil water and substrate conditions can further lead to distinctive vegetation coverage, above- and below-ground plant biomass on different topographic positions (Wang et al., 2015), which would generate a variety of substrates for different microbial communities through fresh C supply (Li et al., 2017). Soil loss would increase with the increase of slope gradient because of the more extensive surface runoff and lower infiltration rate under a higher slope gradient (Zhang and Hosoyamada, 1996). Du et al. (2020) observed that relative abundances of bacterial phyla at eroding slopes (10° and 20°) and depositional zones differed from reference slopes (5°). The changes in soil microbiota due to erosion would profoundly impact the environment, such as nutrient cycling (Li et al., 2019). Due to the significance of soil microbiota for ecosystem service function, they should be considered when exploring the mechanisms behind the response of soil to erosion (Qiu et al., 2021).
Hence, few studies have thoroughly examined the soil microbial characteristics, extracellular enzyme, and soil physicochemical process, which represent the main understanding gap given that soils are a complex system. The main objectives of our study are to find out the effect of slope gradient and slope position on soil physicochemical properties, microbial community composition and diversity, and enzyme activity. Our hypothesis is higher slope gradient and position would have lower microbial community composition and diversity, and enzyme activity due to the deterioration of soil properties than lower slope gradient and position.