Altered precipitation regime and associated changes in drought intensity and frequency may have profound impacts on ecosystem structure and functions, especially in semiarid regions. Drought sensitivity of ecosystem functions, such as productivity, are likely dependent on soil nutrient availability, which will be influenced by nitrogen deposition. Thus understanding patterns and mechanisms of responses of ecosystem stability to varied drought frequency and intensity is critical for sustainable ecosystem management and ecosystem restoration. We manipulated different drought periods (15 days, 30 days, 45 days and 60 days) in both unfertilized and fertilized treatment (10 g N m-2 yr-1) to test the effects of N addition on grassland drought sensitivity of productivity in a meadow steppe in northeast China. The dynamic responses of drought sensitivity of productivity were quantified by measurements of biomass, ecosystem carbon exchange and leaf gas exchange.
Results/Conclusions
With the intensification of drought intensity, above-ground phytomass and ecosystem carbon exchange decreased in patterns of sigmoidal curve. N addition induced changes in drought sensitivity positively depends on drought period. The N addition plots suffered severe drought stress were unable to fully recovered within the experimental year. One year after drought events, we observed no legacy effects of drought in both the fertilized or unfertilized plots, which suggests a quick recovery capability of the studied grassland to frequent and extensive drought. Although N addition reduced root:shoot (R/S) ratio, the N-induced differences of R/S ratio were continue to decrease with the prolongation of drought period. It is conflict with the trend of N-induced alteration of drought sensitivity of productivity. Furthermore, we found that drought sensitivity was positively correlated with the aboveground phytomass in non-drought treatments. Our findings, together with the results of previous studies, suggest that drought sensitivity depends on the variation of reference standard (potential maximum productivity) caused by N addition. A plurality of mechanisms determining drought sensitivity of grassland ecosystems need to be further tested by more multi-site manipulative experiments.