Effects And Mechanisms Of Warming,Nitrogen And Phosphorus Additions And Their Interactions On CO₂ And CH₄ Fluxes In Hani Peatland

Peatlands,which only account for 3%of the global land area,but store approximately one-third of the word’s soil carbon,play a pivotal role in global carbon cycle of terrestrial ecosystems and mitigating climate warming.The Changbai Mountains region is one of the important distribution areas of peatland in China,and also the main research area of peatland ecology research in China.However,experimental studies on greenhouse gas flux monitoring and ecosystem carbon sink function have been lacking in the area all the time.On the one hand,global changes including current and future climate warming,nitrogen（N）deposition and phosphorus（P）input from agricultural production will greatly change peatland environment;on the other hand,global changes may change the vegetation composition of peatlands,thereby effecting the CO₂ and CH₄ flux and threatening the stability of peatland carbon sink function.In this study,an experimental platform with simulated climate change for 12years in Hani peatland of the Changbai Mountains was used.In the platform,3 factors:warming,nitrogen addition and phosphorus input were designed,18 treatments were set up and a total of 72 plots was deployed.During two years of experiment,I measured CO₂ and CH₄ fluxes,estimated net CO₂ exchange（NEE）,ecosystem respiration（ER）and gross primary productivity（GPP）,and investigated vegetation cover,enzyme activity,physical and chemical properties of peat,to study the influence mechanism of warming,N addition and P input and their interaction on CO₂and CH₄fluxes from the peatland.In addition,an indoor incubation experiment was conducted to study the influencing of litter change caused by vegetation alteration under the backgound of global changes on temperature sensitivity of peat（with different qualities namely in different depths）decomposition.The main results and conclusions of the study are as follows:（1）The open top chambers（OTCs）used in this study achieved an increase of about 0.5°C in the average daily air temperature,and the 0.5°C warming had no significant effect on CO₂ and CH₄ fluxes.Vegetation investigation,however,showed that the warming had an inhibitory effect on the growth of Sphagnum mosses,but it had a promoting effect on the growth of shrubs.（2）Long-term high level of N addition(10 g m^-2 y^-1)significantly reduced NEE and transformed the peatland from carbon sink into carbon source,mainly driven by a decrease in aboveground plant productivity.There was a significant interaction between warming and N addition on NEE,and warming could alleviate the negative effect of N addition on peatland carbon sink function.In addition,long-term N addition significantly promoted CH₄ emission.Long-term N addition caused the cooling effect of peatlands to disappear,the global warming potential（GWP）of natural peatlands was-1361 g CO₂ m^-2 yr^-1,and the GWP of peatlands treated with low(5 g m^-2 yr^-1)and high nitrogen additions was 311 g CO₂ m^-2 yr^-1 and 2169 g CO₂m^-2 yr^-1respectively.（3）Long-term P addition significantly decreased NEE,and high P addition(1 g m^-2 yr^-1)led to a disappearance of carbon sink function,mainly due to increased ER.There was a significant interaction between warming and P addition on NEE,and warming alleviated the negative effect of P addition on peatland carbon sink function.ER increased with P addition,and it was about 92%higher at high P addition compared with the control.Long-term P additions lead to a weakening or even disappearance of the cooling effect of peatlands on climate warming,mainly due to increased CO₂ release.（4）There were significant interactions between N and P addition on NEE,ER and GPP,and significant interactions between warming and N,P addition on NEE.Long-term co-addition of N and P reduced NEE significantly and the peatlands changed from carbon sink to carbon source.Warming further amplified the negative effects of N and P co-addition on NEE.N addition relieved the positive effect of high P addition on ER.P addition relieved the negative effects of N addition on GPP.Another,P addition alleviated the positive effect of N addition on CH₄ emission.There was a significant positive correlation between CH₄ flux and GPP.Long-term co-addition of N and P resulted in the cooling effect of peatlands disappeared,and the warming intensified the warming potential（GWP）of peatlands.（5）The CO₂ production rate of shallow peat（8-13 cm）was significantly higher than that of deep peat（35-40 cm）,but the temperature sensitivity of deep peat decomposition was higher.The addition of Sphagnum moss litter increased the CO₂production rate of shallow peat,but had little effect on the CO₂ production rate of deep peat,and even reduced the temperature sensitivity of deep peat decomposition.Betula leaf litter increased both the shallow and deep peat CO₂ production rates and the temperature sensitivity of peat decomposition.Furthermore,Peat with lower moisture content had higher temperature sensitivity of peat decomposition.Our research showed that long-term N and P additions both have negative effects on the carbon sink function of the peatland but with obsiously different mechanisms.N addition mainly reduced the productivity of above-ground vegetation,while P addition mainly accelerated the decomposition of peat.Warming,N addition and P addition all inhibited the growth of Sphagnum mosses,but promoted the growth of vascular plants’to varing degrees,which leads to alteration in peatland litter.However,vascular plant litter promoted shallow and deep peat decomposition and increased the temperature sensitivity of peat decomposition.Thus,continuous warming,N deposition and P input will not only result in the loss of new carbon（carbon in litter and recently formed peat）,but also accelerate the mineralization of old carbon（carbon in deep peat）since accelerated surface peat decomposition will lead to high-quality litter to contact and prime exposed deep peat.These processes will strongly threaten the stability of the peatland carbon sink function,and is expected to further aggravate global warming.