Global impacts of fertilization and herbivore removal on soil net nitrogen mineralization are modulated by local climate and soil properties

Soil nitrogen (N) availability is critical for grassland functioning. However, human activities have increased the supply of biologically limiting nutrients, and changed the density and identity of mammalian herbivores. These anthropogenic changes may alter net soil N mineralization (soil net N-min), that is, the net balance between N mineralization and immobilization, which could severely impact grassland structure and functioning. Yet, to date, little is known about how fertilization and herbivore removal individually, or jointly, affect soil net N(min)across a wide range of grasslands that vary in soil and climatic properties. Here we collected data from 22 grasslands on five continents, all part of a globally replicated experiment, to assess how fertilization and herbivore removal affected potential (laboratory-based) and realized (field-based) soil net N-min. Herbivore removal in the absence of fertilization did not alter potential and realized soil net N-min. However, fertilization alone and in combination with herbivore removal consistently increased potential soil net N(min.)Realized soil net N-min, in contrast, significantly decreased in fertilized plots where herbivores were removed. Treatment effects on potential and realized soil net N(min)were contingent on site-specific soil and climatic properties. Fertilization effects on potential soil net N(min)were larger at sites with higher mean annual precipitation (MAP) and temperature of the wettest quarter (T.q.wet). Reciprocally, realized soil net N(min)declined most strongly with fertilization and herbivore removal at sites with lower MAP and higher T.q.wet. In summary, our findings show that anthropogenic nutrient enrichment, herbivore exclusion and alterations in future climatic conditions can negatively impact soil net N(min)across global grasslands under realistic field conditions. This is an important context-dependent knowledge for grassland management worldwide.