Nutrient addition in a Mediterranean grassland decreases species diversity, increases productivity but does not affect stability

Increased nutrient inputs and climate change are affecting ecosystems worldwide. However, there is a dearth of knowledge on how the interacting effects of multiple nutrient inputs and climatic variability may affect ecosystem functioning including grassland species and functional diversity, productivity or resilience to disturbances. This is particularly important in the Mediterranean Basin, a hotspot of climate change, where the frequency of autumn and spring droughts is projected to increase.

 

We conducted a 6-year nutrient addition experiment in an annual grassland site, in Portugal, that is part of a globally distributed experiment called the Nutrient Network (http://www.nutnet.org/). We added high rates of nitrogen, phosphorus and potassium to 5 × 5 m plots, following a full factorial combination in a complete randomized three block design. We established three treatments of one, two and three added nutrients and maintained control plots without addition of nutrients. We examined how a decrease in nutrient limitation and inter-annual climatic variability affected grassland productivity and diversity. We determined the community functional structure (e.g., Community Weighted Mean) and functional diversity (e.g., Function Dispersion) of key morphological and physiological leaf traits associated with the leaf economics spectrum, resource acquisition and water use strategies.

 

Our 6-year study period was characterized by contrasting climatological years, including two dry years (2017 and 2019). We found that grassland productivity was co-limited by multiple nutrients and that species richness decreased with nutrient enrichment. Dry years reduced productivity and species richness and were a critical factor reducing functional diversity of most of the studied traits. Species with competitive characteristics dominated nutrient enriched communities and were related to ecosystem stability by increasing mean biomass production relative to the standard deviation of biomass over time. Contrary to expectations species richness was not related to stability.

 

This study shows that mechanisms underlying ecological functioning of Mediterranean grasslands depend on interactions of multiple nutrient addition and precipitation variability. Understanding these mechanisms is crucial to anticipate potential effects of global changes on Mediterranean grasslands.