Differences in rhizosphere enzyme activities and microbial metabolic limitations among overstory trees, understory shrubs and herbs are regulated by growing season

Abstract Identifying microbial metabolism at the root-soil interface of coexisting species is crucial for plantation ecosystems. Soil extracellular enzyme activities (EEAs) serve as reliable indicators of microbial nutrient limitations. However, how plant life forms (overstory trees, understory shrubs, and herbs) affect EEAs and microbial metabolic limitations, and how these effects vary with the growing season, remain unclear. We measured seven hydrolytic enzyme activities related to carbon (C), nitrogen (N), and phosphorus (P) acquisition in bulk and rhizosphere soils of different plant life forms in monoculture plantations of Cunninghamia lanceolata, Pinus massoniana, and Pinus elliottii in subtropical China. We found that rhizosphere enzyme activities were generally higher than those in bulk soils, with this stimulation intensifying as the growing season progressed. Notably, overstory and understory rhizosphere enzyme activities showed contrasting seasonal responses. During the plant vigorous growing season, rhizosphere C- and P-acquiring enzyme activities of trees were higher than those of shrubs, but this trend reversed during plant slow growing season. These limitations decreased from the slow to vigorous season across all plant life forms, except for microbial C limitation in tree rhizosphere. Microbial metabolic limitations were mainly regulated by seasonal variations in the ratios of available nutrients and microbial biomass. These findings reveal life-form-dependent patterns of rhizosphere microbial metabolism, and highlight the regulatory role of the growing season in shaping these patterns in subtropical plantation ecosystems.