Management rather than climate and biodiversity shapes long-term δ15N dynamics in grassland soils

The bulk soil δ 15 N signature in grassland soils serves as a fingerprint of nitrogen inputs, turnover and losses, reflecting the influence of land management, climate, plant biodiversity and abiotic soil properties. Although natural abundance δ 15 N has been proposed as an indicator for these factors, previous studies yielded mixed results regarding the importance of its drivers. In this study, we quantified changes in soil δ 15 N across 72 German grassland sites after 12 years (2011 vs. 2023) and analysed relationships with detailed records of land use intensity, plant biodiversity, climate, and soil properties. Land use intensity was quantified on a continuous scale based on mowing, grazing and fertilisation. In the top 10 cm of soil, δ 15 N changes ranged from −0.86 to + 2.49‰, equating to an average increase of + 0.35‰ per decade. Over two-thirds of our study sites showed a long-term increase in δ 15 N associated with high land use intensity. Our findings indicate that fertiliser application and grazing are the dominant drivers of δ 15 N increases, while a biodiversity-mediated decrease via reduced N losses was not observed. The slow decadal-scale shifts suggest that single soil δ 15 N measurements without known initial values are only informative for long-term management processes and not for assessing recent management impacts.