Unexpected Diversity in Ecosystem Nutrient Responses to Experimental Drought in Temperate Grasslands

The responses of ecosystem nitrogen (N) and phosphorus (P) to drought are an important component of global change studies. However, previous studies were more often based on site-specific experiments, introducing a significant uncertainty to synthesis and site comparisons. We investigated the responses of vegetation and soil nutrients to drought using a network experiment of temperate grasslands in Northern China. Drought treatment (66% reduction in growing season precipitation) was imposed by erecting rainout shelters, respectively, at the driest, intermediate, and wettest sites. We found that vegetation nutrient concentrations increased but soil nutrient concentrations decreased along the aridity gradient. Differential responses were observed under experimental drought among the three grassland sites. Specifically, the experimental drought did not change vegetation and soil nutrient status at the driest site, while strongly reduced vegetation but increased soil nutrient concentrations at the site with intermediate precipitation. On the contrary, experimental drought increased vegetation N concentrations but did not change vegetation P and soil nutrient concentrations at the wettest site. In general, the differential effects of drought on ecosystem nutrients were observed between manipulative and observational experiments as well as between sites. Our research findings suggest that conducting large-scale, consistent, and controlled network experiments is essential to accurately evaluate the effects of global climate change on terrestrial ecosystem bio-geochemistry.