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.