Nutrient-acquisitive legume species stimulate soil free-living N fixation and organic N mineralization to alleviate N limitation in the degraded ecosystem

Legumes play a crucial role in vegetation restoration by mediating atmospheric nitrogen (N) inputs and enhancing soil N availability, especially in degraded ecosystems. A deeper understanding of the mechanisms through which legumes accelerate vegetation restoration will help provide a reference for the government to formulate ecological restoration strategies and enhance ecological service functions. In degraded ecosystems, low soil N availability intensifies plant-microbe competition for N, thereby impairing vegetation restoration processes, even under long-term monoculture afforestation. Although legumes are known to influence soil N availability through biological N fixation and the stimulation of microbial N transformations, the integrated mechanisms underlying these effects remain poorly understood. Here, we investigated eight native legume species differing in nutrient utilization strategies following nine years of natural restoration in a degraded karst ecosystem of southwest China characterized by severe rocky desertification. Four non-legume species served as controls. We found that four legume species, including Lespedeza juncea, Indigofera mengtzeana, Sophora davidii, and Indigofera pseudotinctoria, exhibited nutrient-acquisitive traits, whereas the remaining four legume species of Dalbergia hupeana, Bauhinia brachycarpa, Bauhinia comosa, and Solanum viciifolia showed nutrient-conservative strategies. Integrated analyses of plant leaf N/phosphorus ratios and vector-threshold angles revealed that both plant and soil microbial growth associated with nutrient-acquisitive legume species were no longer N-limited, whereas severe N limitation persisted under nutrient-conservative legume species. These contrasting patterns were primarily explained by the changes in the rates of soil free-living N fixation and inherent N transformation processes that control inorganic N production. Specifically, soils associated with nutrient-acquisitive legume species exhibited significantly higher rates of free-living N fixation, gross N mineralization, and gross ammonium immobilization, corresponding with reduced plant and microbial N limitation. Structural equation modeling further indicated that nutrient-acquisitive legume species enhanced inorganic N supply capacity by increasing soil energy and substrate availability, microbial biomass, and the abundance and activity of free-living diazotrophs, thereby effectively alleviating ecosystem N limitation. Beyond elucidating species-specific pathways and mechanisms through which legumes alleviate N limitation, our results provide critical guidance for species selection and management in the ecological restoration of degraded karst ecosystems.

Keywords: Degraded ecosystem; Free-living N fixation; Legume; Plant and microbial N limitation; Soil inorganic N supply