Responses Of Soil Respiration To Simulated Drought An Nitrogen Addition In A Subtropical Evergreen Broad-leaved Forest

Evergreen broad-leaved forest is not only a zonal vegetation in the subtropical region of China,but also was one of the most important biota of terrestrial ecosystems,which accounting for nearly 27%of China's total land area.Global change significantly increases the frequency and intensity of drought and nitrogen?N?deposition in the subtropical region of China,which has a profound impacts on the carbon?C?cycle of China and even the global.As the second biggest C flux between terrestrial ecosystems and atmosphere,soil respiration?Rs?plays a key step in regulating feedback of the C cycle to climate change.However,we have important knowledge gaps in understanding response patterns of Rs to drought and N deposition as well as the potential mechanisms in subtropical evergreen broad-leaved forests.In this study,we have conducted the field throughfall exclusion experiment?control and70%reduction of throughfall?and took advantage of N and phosphorus?P?addition?including control,low N,high N,P,P+low N,and P+high N treatments?platform in Zhejiang Tiantong Forest Ecosystem National Observation and Research Station,to explore how long term drought and N addition affect Rs.We measured Rs dynamic and changes in soil microclimate,soil nutrients,microbial community and root system to probe the potential mechanism.Furthermore,we applied a data-model fusion method to combine the experimental observation data with terrestrial ecosystem carbon-nitrogen coupling model?TECO-CN?to simulate the comprehensive effect of drought and N addition on Rs in subtropical forest ecosystems,and predict the future change trends and explore the driving mechanism.The main results were shown as below:?1?Effects of simulated drought on soil respiration:A 4-year field experiment demonstrated that the effect of simulated drought significantly varied with time.Specially,simulated drought had no significant effect on Rs in the first year,but significantly decreased it in the second,third,and four years by 11.8%,18.3%,20.8%,respectively.Simulated drought significantly decreased Rs in the growing season,but has no effect in the non-grwoing season.Meanwhile,simulated drought significantly increased fine root biomass and caused the dominant community to shift from bacteria to fungi compared with those in the control.Microclimate,fine root and soil microbes could jointly explain the variance of Rs in the control and drought treatments by 77%and 65%,respectively.The shifted microbial community and increased fine root biomass were the main drivers in mediating the responses of Rs to simulated drought.?2?Effects of nitrogen?N?and phosphorus?P?addition on soil respiration:Short term?less than 1 year?N addition significantly increased Rs,while long term?7-8year?N addition significantly decreased it.The negative effect of long term N addition on Rs could be enhanced with the addition amount with decreases of 4.5%and 13.6%by low N?LN?and high N?HN?,respectively.On the contrary,P addition could stimulate Rs?+20.2%?than that in the control.The stimulated effects of P addition on Rs could by largely decreased by N addition.Specifically,P+LN and P+HN treatments increased Rs by 15.84%and 7.69%,respectively.Meanwhile,N addition significantly increased ammonium and nitrate nitrogen?i.e.,NH₄⁺-N&NO₃^--N?,but had no effects on soil soluble organic nitrogen?DON?as well as available phosphorus?AP?.Rs exhibited a significantly negative correlation with NH₄⁺-N,but positive correlation with soil AP.These results showed that the long term N addition significantly decreased Rs,mainy stemming from the increased soil inorganic N,while P addition relieved the inhibitory effect of N addition on Rs by changing the content of available phosphorus in soil.?3?Comprehensive effect of drought and N deposition on Rs:A data-model fusion method was used to simulate the comprehensive effect of drought and N deposition on Rs of Tiantong forest ecosystem.We predicted the comprehensive effects of drought and N deposition on Rs based on the constrained model parameters?i.e.,the well constrained parameters could account for 67%of all parameters?as well as the validated model results via measured Rs?i.e.,the fitting degree between the observed value and the simulated value of drought and N deposition were 69%and 68%respectively?.Our results showed that the effects of drought,N deposition and drought+N deposition on Rs significantly varied with time.Specifically,drought,N deposition,and drought+N deposition signifiantly decreased Rs from 2011 to 2040year by 8.40%,11.16%and 7.41%,resepectively,and by 5.58%,6.18%and 4.25%resepectively from 2041 to 2100.The comprehensive effect of drought and N deposition on Rs in short term simulation phase was regulated by fine root C:N and microbial turnover,while regulated by the turnover of woody C and slow SOM at long term predicting period.These results highlighted that drought and N deposition significantly decreased Rs of subtropical evergreen broad-leaved forest,and the potential mechanisms was differ from short and long term.Taken together,the long term drought and N deposition significantly decreased Rs.The effect of drought on Rs was regulated by the changed fine root biomass and the shifted soil microbial community.N addition significantly decreased Rs via increasing soil inorganic N.The simulated comprehensive effect of drought and N deposition on Rs was significantlty lower than those by drought or N deposition singlely.These results could largely deepen our understanding of the response patterns of Rs and the associated mechanisms,and provide a strong theoretical and scientific basis for strengthening C sink management of terrestrial ecosystems in the future.