Translocation and Allocation of Smooth Vetch‐Derived Nitrogen Along the Plant–Soil Continuum in Citrus Orchards

ABSTRACT The dynamics and distribution of legume‐derived nitrogen (N) throughout the phenological cycle in legume‐based agroforestry systems remain poorly understood, limiting our understanding of N allocation and use efficiency. Here, an in situ 15 N labeling approach was used to trace the dynamics of smooth vetch‐derived N transfer to citrus under fertilized (F) and unfertilized (NF) conditions during the co‐growth stage. At citrus fruit maturity, corresponding to the smooth vetch decomposition stage, 15 N enrichment was quantified in particulate organic matter (POM), mineral‐associated organic matter (MAOM), and microbial necromass fractions to assess the allocation and stabilization of smooth vetch‐derived N in soil organic pools. N‐cycling functional gene analysis was further used to identify potential microbial pathways mediating the transformation and redistribution of smooth vetch‐derived N, thereby linking soil N retention processes with citrus N uptake. The results showed that, during the co‐growth stage of citrus and smooth vetch, contribution indices remained greater than 1 under both treatments, indicating that citrus N uptake was mainly supported by soil‐derived N, with limited direct contribution from smooth vetch‐derived N. During the vetch decomposition stage, 15 N recovery was lower in citrus leaves (F: 1.09%; NF: 1.70%) than in fruits (F: 2.97%; NF: 4.20%), and fruit recovery under no fertilization was approximately 1.5 times that under fertilization, suggesting greater reliance on smooth vetch‐derived N when external N input was limited. However, a larger proportion of smooth vetch‐derived 15 N was retained in the 0–10 cm soil layer (F: 9.17%; NF: 9.09%) than recovered by citrus. Fertilization promoted the accumulation of smooth vetch‐derived N in POM (3.26%) and MAOM (5.07%), which were 2.2 and 2.4 times higher than under no fertilization, respectively, while reducing its retention in microbial biomass N by 33.6% (F: 1.54%; NF: 2.32%). Path analysis further suggested that POM‐associated smooth vetch‐derived N contributed to citrus N uptake, potentially through fungal‐mediated processes. Overall, this study reveals a phenology‐driven shift in the citrus‐smooth vetch system from limited direct N transfer during co‐growth to residue‐mediated fruit N supply and soil N stabilization during decomposition. These results support a management strategy combining smooth vetch cover with appropriate fertilization, which leverages phenological complementarity and staggered nutrient return to meet citrus growth demands while improving soil fertility and ecosystem N retention.