Cropland conversion changes the status of microbial resource limitation in degraded karst soil

Cropland conversion has been suggested as a major strategy to improve the fertility of degraded croplands, but whether cropland conversion changes the status of microbial resource limitation remains poorly understood. Here, we studied three common cropland conversion strategies in a karst area of southwest China (i.e., conversion with i) Toona sinensis (TS), ii) Guimu-1 hybrid elephant grass (GG), and iii) a combination of Zenia insignis and Guimu-1 hybrid elephant grass (ZG) using cropland (CR) with maize-soybean rotation as reference. Enzymatic stoichiometry was calculated and used as an indicator of microbial resource limitation. Soil enzyme activity was altered after cropland conversions, which was strongly related to changes in soil properties. Enzymatic stoichiometry further showed that microbial carbon (C) limitation was mitigated under all three conversion strategies, but nitrogen (N) and phosphorus (P) limitations were aggravated under TS and GG. The mitigated limitation of microbial C may be due to the elevated soil C content, and the aggravated microbial N and P limitation may have been due to increased soil C/N and C/P, respectively. This study suggests that cropland conversion can change microbial resource limitation, which should be considered in the assessment and management of rebuilding ecosystems after cropland conversions.