Legacy effects of 41 years of tillage and fertilization on biological nitrogen fixation and nutritional status of hairy vetch in a continuous cotton system

Abstract Legume cover crops (CCs) rely on a dynamic contribution of soil‐derived nitrogen (N) and biological nitrogen fixation (BNF) to maintain optimum N status, yet how long‐term agronomic management shapes this relative contribution remains poorly understood. This study leveraged a 41‐year continuous cotton ( Gossypium hirsutum L .) system to evaluate how legacy effects of tillage and N fertilization influence BNF, legume N nutritional status, and N sources in hairy vetch ( Vicia villosa ). Using the 15 N natural abundance method and multivariate analyses, we found that tillage was the dominant factor affecting N dynamics: no‐till (NT) plots exhibited higher N status, soil organic matter, soil‐derived N, and total N uptake than conventional tillage. Fertilization effects were less pronounced but modulated BNF efficiency. However, NT alone was insufficient to overcome N limitation under unfertilized conditions, as indicated by similarly low nitrogen nutrition index values and overlapping distributions in both CT‐0N and NT‐0N treatments, highlighting the need for integrated management. These findings underscore how legacy effects of long‐term practices shape N acquisition by legumes and support combining NT with crop rotation‐specific N fertilization to optimize N inputs and sustain CC performance in low‐input systems. Core Ideas Hairy vetch grown under conventional tillage was more limited by N than no‐till, with differences widening at higher N fertilization rates. Atmospheric N contribution was higher under conventional tillage than no‐till, with greater differences at higher fertilization rates. No‐till was associated with higher soil organic matter and soil N uptake, supporting improved legume N status. No‐till alone was insufficient to prevent N limitation in hairy vetch under unfertilized conditions.