Long-term organic fertilisation shifts N2O production towards denitrification in an alpine grassland soil

Organic fertilisers including farmyard and liquid manure supply organic matter and nutrients to grassland soils, with potential benefits for organic carbon (C) storage, soil fertility and ultimately, productivity. However, these benefits may be partially offset by changes in nitrogen (N) turnover and associated emissions of nitrous oxide (N₂O), a potent greenhouse gas, contributing to climate change. Here, we investigated legacy effects of long-term fertilisation on N transformations and N₂O production pathways from an alpine grassland soil from Styria, Austria, subjected to organic (ORG), mineral (NPK), or no fertilization (NIL) since 1971. Combining the ¹⁵N pool dilution and the ¹⁵N gas flux method enabled to quantify gross rates of mineralisation, nitrification, and dissimilatory nitrate reduction to ammonium (DNRA), together with N₂O production pathways and the reduction to environmentally benign dinitrogen (N₂) in a soil microcosm experiment. Long-term organic fertilisation increased soil organic C and CO₂ emissions compared to NPK and NIL, consistent with increased rates of mineralisation, nitrification, and increased N retention via DNRA. Under the conditions of the experiment, long-term fertilisation showed no effect on magnitude of N₂O and N₂ emissions. Denitrification was the main pathway of N₂O production across treatments, with its contribution increasing from 65% under NIL and NPK, to >85% under ORG. The main product of denitrification was N₂, accounting for 95% of N₂O+N₂ under NIL and NPK. Organic fertilisation however shifted the N₂O:N₂ ratio towards N₂O, accounting for more than 15% of N₂O+N₂ emitted. These results show a clear legacy effect of long-term organic fertilisation on N₂O production, which may be explained by higher C availability, fuelling microbial activity and O₂ consumption, shifting N₂O production towards denitrification. Even though not reflected in overall amounts of N₂O and N₂ emitted, the shift in the N₂O:N₂ ratio towards N₂O under organic fertilisation denotes an increased risk for N₂O emissions, likely amplified by increased N supply via nitrification. Our findings demonstrate a clear increase of N substrate supply via mineralisation and nitrification turnover under long-term organic fertilisation and highlight the need to consider potential environmental offsets for alpine grassland management in the form of N₂O emissions, driven by denitrification.