Plant species diversity and abundance of functional groups influence net nitrogen mineralization along productivity gradients in montane <i>Calluna</i> ‐dominated heathlands

Societal Impact Statement Heathlands are globally important ecosystems that provide key functions and services but are increasingly threatened by land use changes. We investigated how plant diversity and productivity influence nitrogen cycling under different management regimes. Our findings reveal that extensive grazing modulates the relationship between plant diversity and soil nitrogen dynamics, with contrasting effects in grazed versus abandoned sites. Increased plant species diversity and abundance of annual and perennial herbs in grazed sites were associated with enhanced N mineralization rates. These insights highlight the need for adaptive management strategies to address the challenges posed by land use changes and biodiversity loss in heathland ecosystems. Summary Abiotic controls on soil N‐transformations in heathlands are well understood; however, the effects of plant species diversity–productivity on N cycling remain unclear, particularly concerning grazing management. We evaluated the feedbacks among plant species diversity, functional group productivity and soil properties, as well as their relationship with net N mineralization rates, in six heathlands dominated by Calluna vulgaris with differing management history (grazed at low intensity and abandoned). Our findings revealed a pronounced increase in soil organic matter content with plant diversity in abandoned, species‐poor heathlands compared to the stronger feedback with soil total N in grazed, species‐rich sites, suggesting trade‐offs in the carbon‐to‐nitrogen ratio. Mean net ammonification, nitrification, and mineralization rates showed no significant differences between species‐rich and species‐poor heathlands. However, increased plant species diversity and abundance of productive functional groups in grazed sites were associated with enhanced N mineralization rates. In contrast, high species diversity in abandoned heathlands led to increased woody species productivity with traits related to nutrient conservation, which was linked to slower N mineralization rates. These findings enhance our understanding of the effects of the diversity‐productivity relationship on nitrogen cycling in heathlands, providing insights for anticipating ecosystem responses to changing management regimes.