The Source And Sink Function Of Nitrous Oxide In Hani Peatland Under The Background Of Global Change

Peatlands cover only 3%of the global land area,but store approximately 30%of carbon（C）and 16%of nitrogen（N）in land soil,playing a crucial role in the global C and N budget.Nitrous oxide（N₂O）is known as an important greenhouse gas whose warming potential is 273 times greater than carbon dioxide（CO₂）,and it is the main greenhouse gas that destroys the ozone layer.Peatlands dominated by Sphagnum usually have low concentrations of available nutrients such as N and P due to the slow decomposition and limitations of environmental conditions.Therefore,although peatlands are a huge N pool,peatlands are not a significant source of N₂O.Current warming and the increase of atmospheric N,P deposition caused by global change will lead to nutrient increased in the peatland,break the original environment and nutrient limitation,and threaten the N₂O source/sink function of the peatland.For a long time,the research on the response of greenhouse gas emissions from peatland to global change has been mainly concentrated in the high latitude region,and the research on the impact of long-term warming,N,P addition and their interaction on N₂O emissions from mountain peatland in the middle temperate zone is still rare.At the same time,due to the special environmental conditions of peatland,N₂O production and emission characteristics of peatland are different from other ecosystems,and the research on N₂O is relatively less than that on CO₂ and CH₄,the mechanism of N₂O emission is still unclear.Based on this,we selected the long-term（12 years）simulated global change experimental sample plots of Hani peatland in Changbai mountain,and tried to explore the impact of warming,N,P addition and their interaction on the N₂O source/sink function of mountain peatland under global change,as well as the response of biological and abiotic factors in peatland to global change by in situ and laboratory experiments.Our research can not only understand the pattern of peatland greenhouse gas emissions to global change,but also may enrich our understanding of peatland climate change ecology and peatland N cycle,which is of great significance for estimating the total greenhouse gas emissions of peatlands in Northeast temperate regions of China under the global change,and even provide data support for its theoretical development.The main results and conclusions of this study are as follows:（1）Hani peatland is not a significant source of N₂O due to the special environmental conditions of long-term low temperature,flooding and decomposition resistance with a flux of-38±62 g m^-2.The temperature increase of 0.6℃in the growth season will stimulate the activity of soil enzymes,accelerate the decomposition of peat,promote the loss of soluble carbon components（DOC）,improve the nutrient conditions of peat soil,change the composition of vegetation in peatland,promote the growth of vascular plants,inhibit the growth of Sphagnum,and finally lead to a significant N₂O emissions,with a flux with 54±43 g m^-2.（2）Long-term low level P addition(5 kg P ha^-1 yr^-1)accelerates the decomposition of peat by stimulating the activity of hydrolase,improves the availability of soil nutrients,increases the vascular plants cover,changes the composition and quality of litter input in peatland,promotes the production and emission of N₂O,and causes the peatland to approach a source of N₂O with a flux of 38±24 g m^-2.The high level of P addition(10 kg P ha^-1 yr^-1)also promotes the decomposition of peat by stimulating the activity of extracellular enzymes,changes the soil stoichiometry,and alleviate the nutrient restriction of denitrification.Compared with the low level of P addition,the high level of P addition brings more suitable conditions,which causes N₂O to be reduced to N₂,and finally leads to the peatland becomes a sink of N₂O,with a flux of-39±49 g m^-2.The interaction between P addition and warming combines their positive effect on N₂O production and emission,especially under warming conditions,high level of P addition strongly promotes N₂O emission from peatland,changes the source/sink function of N₂O in peatland,and resulting in the peatland becomes a significant source of N₂O,with a flux of 101±30 g m^-2.（3）Long-term low level N addition(50 kg N ha^-1 yr^-1)and its interaction with warming significantly increase the N₂O emissions by changing the vegetation type of peatland,stimulating the activity of extracellular enzymes,and accelerating the decomposition of peat,resulting in the peatland becoming a source of N₂O,with a flux of 67±28 g m^-2and 131±23 g m^-2,respectively.Although high level N addition(100kg N ha^-1 yr^-1)and its interaction with warming have the same positive effect on N₂O flux in peatland as low level N addition,due to the limitation of water conditions and vegetation composition,N₂O flux in growing season shows a strong uptake effect,making the peatland a significant N₂O sink,with a flux of-164±48 g m^-2and-124±72 g m^-2,respectively.In addition,the seasonal variation of N₂O flux in peatland is obvious,and the seasonal variation of water table is the key environmental factor leading to these changes.（4）The emission and uptake of N₂O in peatland are regulated by different biological and abiotic factors,and the joint addition of N and P at different doses has different effects on the production,emission and uptake of N₂O.Under the co-addition of N and P,N addition mainly affects the soil enzyme activity,accelerates the decomposition rate of peat,changes the soil stoichiometry and the concentration of available nutrient substrate,and then regulates the net N₂O flux in peatland,while P addition affects the net N₂O flux by changing the composition and quality of plant composition and litter input in peatland.The co-addition of low level N and different levels of P addition has no significant effect on the source sink/function of N₂O in peatland,and the co-addition of low level N and high level P addition even makes the peatland tend to a N₂O sink with a flux of-45±47 g m^-2 in the growing season.The combined addition of high level N and different levels of P addition significantly stimulated N₂O emissions from peatland,making peatland a significant source of N₂O,with a flux of 145±44 g m^-2 and 115±6 g m^-2,respectively.（5）Nutrient condition is still the most important control factor affecting N₂O production and emission in the oligotrophic peatland.The N₂O flux without N addition is almost zero,while the treatment with N addition has significant N₂O emission in incubation experiment.The second important factor are water conditions,and N₂O emissions first increase and then decrease with the increase of soil water-holding capacity.With or without plant aerenchyma has the smallest impact,and the treatment with aerenchyma has stronger but not significant N₂O emissions than the treatment without plant aerenchyma under the same conditions.We compared the field and experiment experiment found that the accumulation and instantaneous effect of high level N addition on N₂O flux under flooded condition were different,the instantaneous effect caused by N addition would have pulse N₂O emission.There was no significant difference in DOC concentration among the treatments in short-term experiment,which may indicate that the control factors affecting peat decomposition are different in different periods of global change.This study explored the impact of global change on the environment and the source\sink function of N₂O in temperate mountain peatland by long-term simulated warming,different levels of N,P addition and their interaction.We also explored the contribution of nutrients,water and simulated vegetation conditions to the N₂O flux of peatland through short-term incubation experiments,as well as the difference between the long-term and instantaneous impact of global change on the N₂O source and sink function of peatland.Our research shows that warming will strongly affect the N₂O flux of the peatland,resulting in the peatland becomes a significant source of N₂O.Different levels of N and P addition have different effects on N₂O source and sink functions,and their effects may be indirectly realized by affecting hydrology or vegetation.Increasing temperature will amplify the positive effect of N and P addition on N₂O flux,leading peatland become a strong source of N₂O.Nutrient is the most important factor to affect N₂O flux in Hani peatland,and N₂O instantaneous pulse emission caused by N deposition will threaten the N₂O sink function of peatland.We have reason to believe that with the climate warming and the intensification of human activities,the increase of nutrient availability will seriously affect the circulation of C,N,P,vegetation composition and microbial extracellular enzyme activity in peatland,which will stimulate the N₂O emission potential of peatland,make it become a strong source of N₂O,and further accelerate the speed of global warming.