| With the progress of science and technology,the influence of human activities on the environment is becoming more and more serious.Anthropogenic activity intensification induced global warming has significantly altered global rainfall pattern.Climate models predict that the amount of annual rainfall will increase in mid-latitude regions of northern hemisphere.Moreover,fossil fuel buring and the utilization of nitrogen fertilizers will cause significant increase in nitrogen deposition in the midlatitude of northern hemisphere.Increased nitrogen deposition and precipitation are likely to change vegetation composition and ecosystem structure,which will subsequently alter response of ecosystem functions to extreme climate events,such as drought,which are predicted to happen more frequently in the future.Unfortunately,studies on the combined impacts of long-term global change effects and short-term extreme climate change on ecosystem processes are very scarce.Therefore,we designed the presented study to test ecosystem carbon exchange drought sensitivity in a Leymus chinensis dominated meadow steppe that have been treated with 3 years nitrogen and water addition.We measured vegetation composition,biomass,soil water and nutrient content,leaf gas exchange and ecosystem carbon exchange during both drought phase and recovery phase.The objective is to understand how ecosystem carbon exchange response to drought when the vegetation composition has been changed by long-term nitrogen and water addition.The primary findings are:i.Compared to the control plots(CK),nitrogen addtion(N)and nitrogen and water addition(CK)caused a significant increase in height,coverage and density in L.chinensis.However,water addition had no significant impacts on these parameters.Plant diversity in the N,WN and W plots was significantly lower than in the CK plots.ii.Aboveground biomass in the N and WN plots was apparently greater than in the CK plots;however water addition didn’t induce a significant enhancement in aboveground biomass.Moreover,leaf area index and vegetation water content in the N and WN plots were also significantly enhanced relative to the CK plots.Both nitrogen and water addition caused reduction in root/shoot ratio,however the mechanisms are different.Nitrogen caused increase in both aboveground and belowground biomass,but increase in aboveground biomass was much greater in belowground biomass,therefore decreased root/shoot ratio.Water addition had no apparent influence on aboveground biomass,but significantly decreased root biomass.iii.Leaf nitrogen content in L.chinensis was significantly enhanced in both N and WN plots,which subsequently caused increase in leaf photosynthetic rate and leaf instantaneous water use efficiency.The drought sensitivity of leaf net assimilation rate was greater in the N and WN plots as compared to the CK plots.iv.Relative to the CK plots,N and WN caused increase in both gross ecosystem production(GEP)and ecosystem respiration(ER),but the magnitude of increase in GEP was much greater than in ER,which caused an increase in net ecosystem CO2 exchange(NEE).During the drought treatment,NEE in all treatments decrease with the reduction in soil water content.However,NEE in both N and WN plots had a much greater reduction in both rate and magnitude,which suggest NEE in nitrogen addition plots is more sensitive to drought than in the CK plots.For both drought and rewetting periods,ER maintained relatively constant,which indicates that L.chinensis community control water lost through closing stomata when facing drought threat,but maintaining basic respiration.This is important for L.chinensis community to quickly recover once drought stress was relieved.v.The observed greater drought sensitivity in nitrogen and water addition plots may attributed to long-term resource addition induced reduction in plant diversity,which is often associated with loss in compensatory effects among different plant functional types.Nitrogen addition caused increase in aboveground biomass and leaf area index which will enhance water loss per unit land area.At last,nitrogen and water addition induced redution in root/shoot ratio will constrain L.chinensis community to obtain water from deeper soil layers,which will obviously intensify drought sensitivity.In summary,nitrogen and water addition altered plant diveristy,biomass and biomass allocation in L.chinensis community.These changes in vegetation exaggerated drought sensitivity of ecosystem carbon exchange and finally influenced ecosystem production and functions.We explored drought response and adaptation mechanisms in meadow steppe from both leaf and community aspects.The findings are important and useful for future grassland management under the interactive effects of multiple global chan... |
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