Long-term effects of nutrient addition on yield, diversity, and nutritional quality in a Mediterranean grassland under variable rainfall

Global eutrophication and increased precipitation variability affect the dynamics of Mediterranean grasslands, with significant implications for their stability, conservation, and functions, such as providing livestock feed and preserving biodiversity. This study investigates the long-term effects of multiple nutrient additions and the inter-annual precipitation variability on Mediterranean grassland productivity, diversity and nutritional quality. A long-term experiment was conducted using a randomised block design, where N, P, and K were applied in a full factorial combination. The study included two normal precipitation years (2016 and 2018, similar to the long-term average) and two dry years (2017 and 2019). We analysed the effects of nutrient addition and annual precipitation on yield, functional group composition, and plant diversity, while nutritional quality was analysed in 2019. The results reveal nutrient co-limitation effects on grassland productivity. The PK, NP and NPK treatments yielded 736.8 ± 38.1 g DM m⁻², 2.6-fold higher than the remaining treatments (284.6 ± 15.7 g DM m⁻²). Grassland yield declined by 28 % in dry years, irrespective of nutrient addition. Annual precipitation significantly influenced plant species richness, which decreased from 18.4 species m⁻² in normal precipitation years to 12.6 species m⁻² in dry years. Nutrient addition, particularly NPK, provided grasses a competitive advantage, leading to lower plant species richness. The interaction between nutrient addition and annual precipitation influenced legume abundance, with a positive response observed under P and PK treatments, depending on water availability. The proportion of forbs, and the Shannon-Wiener index showed particularly low values in dry years under NPK treatment. Nutrient addition had no significant effect on grassland nutritional quality. These findings underline the long-term effects of multiple nutrient additions and reduced precipitation on Mediterranean grassland diversity and productivity, providing valuable insights for management and conservation policies to sustain these ecosystems under changing environmental conditions.