Effects of simulated drought and nitrogen fertilizer on plant productivity and nitrous oxide (N2O) emissions of two pastures

Aims As a consequence of global climate change, increases in the frequencies and severities of drought are anticipated for many parts of the world. Soil moisture and nitrogen (N) are among the major factors limiting grassland productivity. In pastures, N fertilizer returns by grazing animals are spatially and temporally heterogenous, and we therefore hypothesized that responses of plants and soil processes to drought may differ at the patch level. Methods Using rain-exclusion roofs, we simulated severe summer drought in a three-year field experiment replicated at two grassland sites contrasting in climate and management intensity. The study included a factorial N application treatment encompassing the application of cattle urine and mineral nitrogen. Responses of plants, soil microbes, and soil organic matter were assessed (carbon and nitrogen pools). N2O emissions were measured on 72 dates, and soil N2O concentration profiles on 44 dates. Results Plant productivity responded negatively to drought and positively to N application. Interestingly, no or only small drought-effect were found on plant productivity when cumulated over the entire experimental duration, despite large effects during and shortly after the period when rain-exclusion roofs were installed. We further did not find evidence for compensatory growth after drought, and drought-effects did not differ between fertilizer hot spots and unaffected areas. In the short-term, soil microbial biomass responded positively to drought, but no long-term effects were detected. Nitrous oxide (N2O) emissions originated primarily from fertilizer hot spots, and these emissions were massively reduced under drought, with effects lasting throughout most of the growing season. On a growing season basis, N2O emissions were estimated to be 1 to 2 orders of magnitude lower under drought. Conclusions Overall, our data suggest that even severe summer drought may have relatively little effect on plant productivity in the type of grassland and climate investigated, at least when considered on an annual basis. In contrast, drought may result in a large and sustained reduction of N2O emissions.