Depth-dependent effects of tree species identity on soil microbial community characteristics and multifunctionality

Soil microbes play key roles that support forest ecosystem functioning, while their community characteristics are strongly determined by tree species identity. However, the majority studies primarily focus on soil microorganisms in the topsoil, resulting in limited understanding of the linkages between tree species identity and the microbial communities that inhabit deep soils. Here we investigated the diversity, structure, function, and co-occurrence networks of soil bacterial and fungal communities, as well as related soil physicochemical properties, to a depth of two meters in dryland forests dominated by either Pinus tabuliformis, a native coniferous species, Robinia pseudoacacia, an exotic broadleaf and nitrogen-fixing species, or both. Tree species identity had stronger effects on soil multifunctionality and microbial community structure in the deep layers (80-200 cm) than in the top layers (0-60 cm). In addition, fungal communities were more responsive to tree species identity, whereas bacteria were more sensitive to soil depth. Tree species identity strongly influenced microbial network stability and complexity, with higher quantities in R. pseudoacacia than the other plantations, by affecting microbial composition and their associations. The increased in microbial network complexity and the relative abundance of keystone taxa enhance the soil multifunctionality of microbial productivity, sugar and chitin degradation, and nutrient availability and cycling. Meanwhile, the relative abundance of keystone taxa was more representative of soil multifunctionality than microbial diversity. Our study highlights that tree species identity significantly influences soil microbial community characteristics and multifunctionality, especially in deep soils, which will help us understand soil nutrients processed in plantation forest ecosystem and provide a reference for tree species selection in ecological restoration.

Vertical distribution and influencing factors of soil organic carbon in the Loess Plateau, China

Accurate analysis and evaluation of the spatial distribution and the primary factors that affect regional soil organic carbon (SOC) together make an important step in assessing carbon sequestration potential. However, little information is available on distribution of regional SOC in deep soil layers. To analyze the spatial distribution of and factors influencing SOC in a 500 cm soil profile, 1440 soil samples were collected from 90 sites on the Loess Plateau in China. The primary factors dominating the spatial distribution of SOC were quantified using principal component analysis with multiple linear regression (PCA-MLR) analysis. Results showed that the mean SOC of the 500 cm soil profile ranged from 1.20 to 3.37 g kg^-1, decreasing with increasing soil depth. The SOC in the deep soil profile decreased across the types of land use in the following order: forestland > cropland > grassland. Based on the factors analyzed in this study, land use accounted for 22% of the variation in SOC and was the dominant factor controlling the spatial distribution of organic carbon in shallow soils (0-100 cm); while soil factors (including soil clay, soil water content, and soil bulk density) were dominant in deep soil layers (200-500 cm), averagely accounting for 44.3%. The SOC stock in the 0-20 cm soil layer was 1.34 kg m^-2, accounting for only a small proportion (8%) of the total carbon in the entire 500 cm soil profile. SOC stock in the 200-500 cm layer was 7.62 kg m^-2, accounting for 44% of the total carbon in the 0-500 cm soil profile. This study demonstrates that a large amount of organic carbon is stored in deep soil, indicating that a better understanding of the reserves and cycles of deep soil carbon is a critical factor in the effective management of terrestrial ecosystems.