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

Abstract

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.

Keywords: NutNet; anthropogenic change; global grasslands; grazers; nitrogen; phosphorus; potassium; potential and realized soil net nitrogen mineralization; precipitation; temperature.