Conversion of grassland to cropland altered soil nitrogen-related microbial communities at large scales

Land-use changes may dramatically disturb underground microbial biodiversity, especially in the fragile ecological areas. However, the impact of conversion of grassland to cropland on soil nitrogen (N)-related microbial communities is not fully understood in the farming-pastoral ecotone of northern China. Therefore, 24 paired grassland and cropland soil samples were collected in this region to investigate the community structure and assembly processes of soil N-related microorganisms via amplicon sequencing of nifH, archaeal and bacterial amoA, and nxrB genes. The results showed higher ammonia-oxidizing bacteria (AOB) alpha diversity but a lower nitrite oxidizing bacteria (NOB) diversity in cropland soil compared to grassland soil. Non-metric multidimensional scaling ordinations revealed that diazotroph, AOB and NOB communities differed considerably between grassland and cropland soil. Soil microbial co-occurrence networks showed that conversion of grassland to cropland significantly lowered the average degree, average clustering coefficient, total nodes and links, resulting in less complex microbial networks in cropland soil. Land-use change altered AOB community assembly processes, resulting from a stochasticity-dominated process in grassland soil to a determinism-dominated process in cropland soil. In contrast, deterministic processes were dominant in diazotroph community assembly, whereas stochastic processes were dominant in constructing ammonia-oxidizing archaea and NOB communities in both grassland and cropland soil. These results provide novel evidence that the conversion of grassland to cropland altered the diversity and assembly processes of soil microbial communities involved in soil N-cycling processes, which has important implications for the potential changes in soil functions under land-use changes. (c) 2021 Published by Elsevier B.V.

Automated summary

This study investigated the impact of grassland conversion to cropland on soil nitrogen-related microbial communities in the farming-pastoral ecotone of northern China through amplicon sequencing. The results showed changes in microbial diversity and assembly processes, with higher AOB alpha diversity but lower NOB diversity in cropland soil. The microbial co-occurrence networks became less complex in cropland soil, indicating a shift from stochastic to deterministic processes in AOB community assembly.