Effects Of N Addition On Drought Sensitivity Of Aboveground Productivity In Songnen Grassland

Nearly all global change drivers,including chronic pressure like global warming and nutrients enrichment and discrete disturbances like extreme events,have been altered dramatically in the past century as a consequence of rapid human population growth and expanding human activities.Grasslands cover more than a quarter of Earth's total terrestrial area,contributing 30 percent of global primary productivity.Changes in resource availability will influence ecosystem stability either directly by changing plants physiological and morphological traits or indirectly via changing interspecific association and community composition.Chronic recourses alteration could thereby trigger unpredictable interactions with other global change drivers.Understanding the response of grassland productivity to stochastic drought events,and assessing the effects of continuous N addition on grassland ecosystem resistance and resilience are not only critical for economic development of human society,but also related to the feedback to climate change and the trend and extent of future environmental change.Here,we manipulated precipitation redistribution in growing-season by installing rainout shelters and irrigating to assess the response of grassland to intra-annual precipitation variability.We introduced different drought episodes in long-term nutrient addition experiments to test the interaction of N addition and drought on grassland productivity.In addition,a ¹³CO₂ pulse labelling experiment was conducted to trace C processes,which was used to reveal the underlying mechanisms of the negative effect of N addition on plant drought tolerance and ecosystem stability.The main results of this study are as following:(1)We introduced growing-season drought treatments of different lengths(15,30,45 and 60 day drought)by delaying growing-season precipitation in a long-term nitrogen addition experiment in Songnen meadow steppe in Northeast China.Response variables of productivity included aboveground biomass(AGB),ecosystem net carbon exchange(NEE),and leaf net carbon assimilation rate(A).Drought significantly decreased AGB after 45 day drought treatment,and N addition significantly increase drought sensitivity of productivity.In the unfertilized plots,drought decreased AGB by13.7%after a 45 day drought and 31.7%after a 60 day drought,in the fertilized plots drought decreased AGB by 19.5%after a 45 day drought and 47.6%after a 60 day drought.Progressive increases in the drought response of NEE were also observed.Although N addition treatment significantly increased AGB,no significant effects on evapotranspiration were observed during the drought treatments.The effects of N addition on the drought response of productivity increased as drought duration increased,and these responses were a function of changes in productive potential and biomass allocation.However,no significant effects of drought occurred in the fertilized or unfertilized plots in the growing season a year after the experiment.N addition did limit the recovery of AGB from severe drought during the remainder of the current growing season,resulting from N addition and drought interacted to alter(reduce)the density of the dominant species,and there were greater proportions of dead leaves in the drought-N treatment than drought-only treatment after 60 days of drought.(2)We manipulated precipitation redistribution,imposing extreme droughts(45days)of different season,in a long-term N addition experiment to test whether the effects of N addition on drought sensitivity varied along growing season.The spring drought reduced AGB by 28%in the unfertilized plots and by 33%in the fertilized plots,and the effects persisted during the subsequent post-drought period within the same growing season;however,the summer drought had no significant influence on AGB.Our results indicate that the drought sensitivity of productivity likely depends on the phenological and morphological traits of the single highly-dominant species(L.chinensis)in this meadow steppe.For the later period of the spring drought event(in June),L.chinensis was in the jointing stage to the flowering stage,over which it exhibits a quick growth rate and thereby more water demands.By contrast,the rapid growth and high AGB production of L.chinensis during spring could have contributed to increased drought resistance in the summer,and more carbohydrates thereby could be allocated to roots to cope with summer drought stress.Our result implies that richness is a less contributor to the stability of ecosystems due to highly-dominance(>90%)of L.chinensis.The rapid recruitment from the underground bud bank of the dominant species ensures the long-term stability of ecosystem function.(3)To address the interaction of N addition and drought on plants C allocation strategy,we conducted ¹³CO₂ pulse labelling experiment to trace C transport through the plant-soil system.We found that N addition significantly decreased belowground C allocation under either ambient or drought conditions.In combination,however,drought and N addition resulted in offsetting effects on C allocation.At the peak point,drought significantly increased C allocation to roots by 13%,N addition significant decreased C allocation to roots by 10%,together,the N plus drought treatment just increased C allocation to roots by 1.3%,relative to the control treatment.The drought response of C allocation depends on the retrenchment in respiration loss rather than reducing C allocation to shoots.Greater AGB under N addition resulted in higher C loss via aboveground plant respiration,such that belowground C investment could not alleviate drought stress.Compared to the concept of active phenotype adjustment in traditional optimal partitioning theory,our results emphasize that passive regulation by pre-drought allometry,which changed under the long-term resource addition,probably played an unexpected role in the ecosystem stability under changing environmental conditions.Overall,even in the absence of changes in total precipitation amount,the effects of rainfall variability on ecosystem function may become more significant as within-year drought episodes varied in duration or timing,particularly when interacting with N enrichment.Although N addition will increase productivity,it could also increase drought sensitivity and probably limit recovery from drought event.Our results implied that the destabilizing effects of anthropogenic N enrichment on ecosystem appear to be a consequence of the simultaneous increases in productive potential and declines in belowground biomass allocation.However,stability mechanisms may be context dependent.Thus,relative to richness,the effect of N addition on the drought tolerance of the dominant species could be the most significant contributor to the stability of low-diversity ecosystems.This study not only has important implications for understanding the grassland response to future environmental change,but also provides a guiding theoretical basis for the sustainable management of grassland ecosystems in the future.