Microbial Community and Their Potential Functions after Natural Vegetation Restoration in Gullies of Farmland in Mollisols of Northeast China

Although huge numbers of gullies have been widely formed and have severely decreased the quality of farmlands in mollisols, it is still unclear how the microbial community distributes after natural vegetation restoration (NVR), which highly relates to the ecological functions in the farmland. In this study, both the microbial community and their potential ecological functions after NVR were reviewed, together with the environmental factors relating to microbial evolution which were detected in two gullies of mollisols situated on farmland in Northeast China. The main results showed that NVR improved the microbial diversity and complexity of the co-occurrence network in gullies, and promoted bacterial community composition to be similar between the gully and deposition area. Moreover, the soil organic matter (SOM) regulated the microbial diversity by balancing soil available phosphorus (AP), soil moisture (SM), and pH, thus stimulating the key bacterial biomarkers of gullies (Rhizobiales, Microtrichales, TRA3-20) and regulating the bacterial composition, as well as indirectly enriching the function of bacteria to perform denitrification, C fixation, and phosphorus transport in gullies. In addition, abundant Dicotyledons in gullies mainly regulate the fungal community composition, and increased fungal richness in 0–20 cm soil depth, but decreased bacteria richness in 0–20 cm soil depth. Our findings revealed the repair mechanism of NVR on soil bacterial and fungal communities, especially on bacterial functionality, which should be given further attention in nutrient cycling across eroding mollisols in gullies.