Nitrogen deposition increases susceptibility to drought - experimental evidence with the perennial grass Molinia caerulea (L.) Moench

Aims We investigated the response of the perennial grass Molinia caerulea (L.) Moench to combined effects of fertilization (N, P) and drought events. We hypothesized that N fertilization increases, and drought decreases productivity, but that N addition strengthens negative effects caused by drought. Methods Within a full-factorial 2-year greenhouse experiment we measured biomass productivity and allocation, tissue nutrient concentrations and nitrogen allocation patterns using N-15 as a tracer. Results N fertilization caused a strong increase in productivity, but effects of drought were almost insignificant. However, we found strongly interrelated, non-additive effects of fertilization and drought, expressed by a strong increase of necrotic tissue. Dead aboveground biomass showed the highest values for N and N-15. Conclusions Accelerated productivity of aboveground tissue under N fertilization resulted in increased evaporative demands and thus higher drought susceptibility. In addition N-15 allocation patterns showed that fertilization-drought treatments disenabled plants' control of their N allocation. Molinia was unable to withdraw leaf N during the dieback of aboveground tissue. Due to the lack of an adaptive strategy to the combined effects of fertilization and drought, increasing summer drought may weaken the competitive performance of species with traits comparable to those of Molinia in N-fertilized environments.