Effects Of Simulate Different Extreme Rainfall Frequencies On Soil Nitrogen Components And N₂O Flux In Gahai Wetland

The alpine wetland is an important part of the Qinghai-Tibet Plateau,which has extremely important ecological significance in biodiversity conservation,water conservation,especially in soil nitrogen cycle and N₂O emission.In the context of global warming,it has been proved that climate warming will trigger and intensify the occurrence of extreme precipitation events,and the precipitation pattern over land has also changed significantly.In recent years,global climate change has led to an increasing trend of extreme rainfall events and rainfall over the Qinghai-Tibet Plateau.The input of extreme rainfall will have a profound impact on soil nitrogen components and N₂O emission in wetland.Based on this,this study selected the alpine wetland in Gahai-Zecha national natural wetland as the research object.According to the historical and future precipitation trends,in the growing season（May to October）,through the collection of natural rainfall,on the basis of receiving natural rainfall in all treatments,four different gradient extreme rainfall frequencies with a single irrigation of 25 mm were added to simulate the input of extreme precipitation in this region:（CK:only natural rainfall is accepted,the total amount of ambient rainfall from May to October is 600 mm;DF1:watered once a week;DF2:watered once every two weeks;DF3:watered once every three weeks;DF4:watered once every four weeks）.By changing the rainfall frequency and rainfall amount in Gahai wetland during the plant growing season（May to October）in 2019,the experiment explored the soil nitrogen components（total nitrogen,TN;Nitrate nitrogen,NO₃^--N;Ammonium nitrogen,NH₄⁺-N;Microbial biomass nitrogen,MBN;Soluble organic nitrogen,DON）,soil physical properties（soil water content,SWC;Soil temperature,ST;Bulk density,BD）,aboveground biomass（AGB）,N₂O flux and GWP _N2O under different extreme rainfall treatments.The main conclusions are as follows:1.The increase in the frequency of extreme rainfall significantly increased the aboveground biomass of Gahai wetland.The aboveground biomass of DF1,DF2,DF3and DF4 treatments increased by 61.2%,44.5%,34.9%and 11.8%compared with CK treatments,respectively.Aboveground biomass increased from 85.82 g·m^-2 to 245.79g·m^-2.The results indicated that increasing the frequency of extreme rainfall could effectively increase the vegetation community coverage during the growing season.Soil density decreased with the increase of frequency of extreme rainfall,and the soil density of DF1,DF2,DF3 and DF4 treatments decreased by 28.2%,18.3%,8.4%and2.7%compared with CK treatment,respectively.The increase of extreme rainfall frequency significantly increased the soil water content.Compared with CK treatment,the soil water content of DF1,DF2,DF3 and DF4 increased by 39.6%,12.2%,13.3%and 4.7%.The results indicated that soil bulk density decreased due to the increase of total vegetation roots and soil water absorption expansion,and thus improved soil water holding capacity.2.The increasing frequency of extreme rainfall had significant effects on soil nitrogen composition in Gahai wetland（P<0.05）.The increase in the frequency of extreme rainfall increased the contents of NH₄⁺-N and NO₃^--N in the soil of 0～40 cm layer of Garhai wetland,and decreased the contents of TN,DON and MBN.NO₃^--N,NH₄⁺-N,DON and MNB showed obvious seasonal dynamic changes in the whole growing season,in which NO₃^--N,NH₄⁺-N and MBN showed a trend of first increase and then decrease,and DON showed a trend of"rise-fall-rise".With the increase of rainfall frequency,the peak values of NO₃^--N,NH₄⁺-N,and MBN in soil increased significantly.Among different frequencies of extreme rainfall,with the increase of rainfall frequency,the contents of NH₄⁺-N,NO₃^--N,DON and MBN in soil from 0 to40 cm layer showed a trend of first increase and then decrease,while the contents of TN showed a trend of first decrease and then increase.The TN proportion of soil NH₄⁺-N under each extreme rainfall frequency treatment was higher than that of NO₃^--N,and the TN proportion of DON in nitrogen component was the highest,and the contents of NH₄⁺-N,NO₃^--N,DON,MBN and TN in soil under different rainfall frequency treatment decreased with the increase of soil depth.3.The increasing frequency of extreme rainfall had a significant effect on the N₂O emission flux in Gahai wetland（P<0.01）.Compared to CK,DF1 high-frequency extreme rainfall treatment significantly reduced the N₂O flux and its cumulative emissions,significantly reduced by 42.3%and 59.7%compared with CK,and GWP_N2O significantly decreased by 59.8%.With the increase of the frequency of extreme rainfall,the N₂O flux,cumulative emission and GWP_N2O increased first and then decreased.In the whole growing season,the N₂O fluxes in Gahai wetlands showed obvious seasonal changes,sometimes from"source"to"sink",but in general it was the"source"of N₂O.4.Among different environmental factors,N₂O flux showed a significant positive correlation with SWC,NO₃^--N and NH₄⁺-N（P<0.01）,and a significant positive correlation with AGB（P<0.05）.The quadratic regression equation could well describe the relationship between N₂O flux,SWC and AGB.SWC,NO₃^--N and NH₄⁺-N are the key driving factors of N₂O emission in Gahai wetland.In conclusion,alpine wetland is very sensitive to extreme rainfall events.The increase of extreme rainfall frequencies effectively increased the aboveground biomass,at the same time,it can promote the accumulation of soil inorganic nitrogen,which is conducive to the growth of vegetation.Under the condition of increasing extreme rainfall events,the content of NH₄⁺-N in soil was higher than NO₃^--N,and soil nitrogen showed obvious surface aggregation phenomenon.The high frequency extreme rainfall events（DF1）significantly inhibited the emission of N₂O and GWP_N2O,indicating that the high frequency extreme rainfall events would not increase the emission of N₂O in the alpine wetland of Qinghai-Tibet Plateau and cause environmental pressure,and the increase of precipitation events in the future may weaken the contribution of N₂O emission in the alpine wetland to global warming.