Plot level nitrogen stock is enhanced by a young agroforestry system in a mediterranean environment Mahmud, M. Bertrand, I. Ngao, J. Martin-Blangy, S. Bazot, S. Delpierre, N. Leadley, P. Vincent, G. Marsden, C. /Dugué, Rémi Morfin, A. Forest, M. Marmagne, A. Barthes, L. Black locust Safety net N-15 %Ndfa Biological fixation Microbial biomass A 2-years assessment of nitrogen (N) stocks, tree N safety nets (from a 15N labeling experiment), and biological N fixation was conducted in a Mediterranean agroforestry system in southern France. The study aimed to quantify N retention in agroforestry. The study area is characterized by a skeletic rhodic luvisol soil, a mean annual temperature of 15.5 degrees C, and an average annual precipitation of 556 mm. N and 15N were quantified across all system components, which are N-fixing black locust trees, crops, weed plants, understory vegetation strips (UVS), rhizospheric soil, and soil microbial biomass. A split-plot experimental design was used, with agroforestry (AF) and monoculture (MC) plots. 15N labeling was applied to track N fluxes and determine tree nutrient absorption over time. We hypothesized that tree root growth and nutrient leaching would enhance the safety-net effect, improving N retention. Results showed that crops in AF had lower N stocks (13-30% less than MC), but tree and UVS contributions compensated for the deficit, leading to a 62% increase in total plot-level N stock in 2022. The Relative Nitrogen Content was 0.97 in 2021 and 1.63 in 2022. While no 15N was detected in trees in the first year (70 days after labeling), 2% of applied 15N was recovered in trees 14 months post-labeling. % of N derived from atmospheric N2 (%Ndfa) ranged from 52 to 68%, with trees fixing 14-18 kg N ha-1. We confirmed that agroforestry enhances N retention, but further research is needed to quantify leaching and gaseous losses of nitrogen. 2025 text https://www.documentation.ird.fr/hor/fdi:010094918 fdi:010094918 Mahmud M., Bertrand I., Ngao J., Martin-Blangy S., Bazot S., Delpierre N., Leadley P., Vincent G., Marsden C., Dugué Rémi, Morfin A., Forest M., Marmagne A., Barthes L.. Plot level nitrogen stock is enhanced by a young agroforestry system in a mediterranean environment. 2025, 99 (7), 182 [16 p.] EN FRANCE ZONE MEDITERRANEENNE