Shrub encroachment regulates soil carbon via soil enzyme activities and iron in a temperate grassland

This study aimed to discover the mechanisms of ecological effects of shrub encroachment by investigating the changes in soil microbial carbon (C), nitrogen (N) and phosphorus (P) acquiring enzyme activities down to a depth of 1 m and disentangling the effects of various driving forces to elucidate the potential mechanism of carbon sequestration mediated by soil metals. Significant effects of shrub encroachment and/or its interactive effects with soil depth were detected on soil metals. Iron (Fe) was identified as the most pivotal metal simultaneously influencing soil C-, N- and P-acquiring enzyme activities. The structural equation model showed that Fe and microbial biomass carbon governed the effects of shrub encroachment on enzyme activities and soil organic carbon (SOC). Remarkably, Fe acted as a prominent controlling factor of the variation in SOC. The model illustrated the Fe-mediated mechanism showing that shrub encroachment stimulated soil hydrolytic enzyme (e.g. carbon-acquiring enzyme) activities and facilitated the sequestration of soil carbon (the increase of 11-42% following shrub encroachment). Longer shrub encroachment could mitigate N limitation and simultaneously increase P limitation of microorganisms in deep soils. The study provides a novel mechanism model highlighting the iron-enzyme-carbon links under globally expanding colonization of grasslands by shrubs.