Towards a Mechanistic Understanding of Legume Functioning in Natural Restoration of Degraded Ecosystem: Legume‐Specific Impacts on Nitrogen Transformation Processes

Legumes have important functions in degraded ecosystems as they can mediate atmospheric nitrogen (N) inputs and increase soil N availability. However, it remains unclear whether legumes affect N availability only through biological N fixation or stimulating microbial N transformations. In this study, nine native legumes and four non-legumes were collected following a 9-year natural vegetation restoration experiment in a karst rocky desertification area. Leaf N/phosphorus (P) ratios and various soil N pool compositions were analyzed and gross N transformation rates were determined by ¹⁵N tracing techniques. Legumes exhibited higher leaf δ¹⁵N values and increased contents of total N, microbial biomass N and inorganic N compared to non‒legumes. Legume leaf N content and N/P ratio (26.7 g kg^‒1 and 20.7) significantly exceeded those of non‒legumes (14.2 g kg^‒1 and 14.5). Our results indicate that legumes increased soil N availability and decreased plant N limitation after 9 years of natural vegetation succession, with effects varying between species and related to soil N transformation processes. Species with low plant N limitation exhibited high rates of organic N mineralization (M_Norg) and ammonium oxidation to nitrate (O_NH4), both of which increase inorganic N supply (especially nitrate). This effect was more pronounced in rhizosphere than bulk soil. M_Norg and O_NH4 rates were positively correlated (p < 0.01) with soil organic carbon, total N, water holding capacity, calcium content and microbial biomass as well as with leaf N:P ratios, indicating legumes improve soil quality and inorganic N supply, thereby alleviating plant N limitation. Our results highlight the importance of legumes in soil N cycling and availability, which is often a limiting factor for natural restoration of degraded ecosystems.