Dynamics of soil microbial recovery from cropland to orchard along a 20-year chronosequence in a degraded karst ecosystem.

The 'Grain for Green' project (GGP) is the largest ecological rehabilitation project in China. A large body of croplands has been abandoned or converted to shrubs or grasslands since 1999. Soil microbes are recognized as sensitive responders of environmental changes, therefore, they are considered as a key component of ecological rehabilitation. However, very limited field experiments have been conducted to investigate the responses of soil microorganisms to restoration projects, especially in karst regions of China. In order to evaluate the response of soil microbial community to ecological restoration, we determined soil microbial community composition by means of qPCR, PLFAs, and high-throughput amplicon sequencing following conversion of cropland to Chinese prickly ash (Zanthoxylum bungeanum Maxim) orchard (CP) along a 20-year chronosequence in a degraded karst ecosystem. Our results showed that soil nutrient contents significantly increased following cropland to CP conversion. qPCR results showed that the highest bacterial abundance was found in the 20-year CP, but bacterial abundance decreased during the first 5-year land-use conversion. Conversion of cropland to CP strongly impacted soil microbial community composition, despite the cropland sites having a long cultivation history (>50 years). However, soil bacterial diversity remained unchanged within a 20-year land-use conversion. Actinobacteria, Proteobacteria, and Acidobacteria were the main bacterial phyla in all land-use sites. In particular, various members of Actinobacteria (e.g., Solirubrobacteraceae) tended to increase their relative abundances in responding to land-use conversion, which may imply that the shifts of soil microbial communities associated with recovering of ecological conditions. Overall, given the rapid yet differential response to ecological restoration, investigation of the belowground microbial community can provide an effective way of assessing ecological recovery of restoration projects in the karst region.