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

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 15N 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, 15N 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, 15N 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 15N 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.