Effects Of Livestock Excreta Returning On Nitrogen Dynamics And N₂O Emission In Seasonal Freezing And Thawing Alpine Steppe

Livestock excreta is an important source of nitrogen in natural grassland and plays an important ecological role in maintaining soil fertility and ecosystem productivity.For seasonal freezing-thawing grassland,the return of livestock excreta will further change the soil nitrogen availability and freezing-thawing pattern of the grassland during freezing-thawing period,which may generate more complex effects on soil nitrogen dynamics and nitrous oxide（N₂O）emission.However,the coupling effect and mechanism of livestock excreta return and freeze-thaw in grassland soil active layer on soil nitrogen dynamics and N₂O emission remain unclear,which lead to the limited understanding of the relationship between livestock excreta and nitrogen biogeochemical cycle in grassland during freezing-thawing period.Therefore,relying on Shenzha Alpine Steppe and Wetland Ecosystem Observation and Experimental Station of Chinese Academy of Sciences,this study planned to take seasonal freeze-thaw alpine steppe and livestock excreta as the research object,and the treatments include control（CK,no dung urine is applied）,yak dung（YD）,Tibetan sheep dung（TSD）,yak urine（YU）and Tibetan sheep urine（TSU）were arranged to conduct the field expernent.Static box gas chromatography was used to collect greenhouse gas samples from different soil treatments at different freezing-thawing periods（Freeze period,stable freeze period,thaw period,and complete thaw period）,and to determine soil physicochemical properties and nitrogen dynamics under the treatment of cattle and sheep dung urine.The N₂O emission flux and variation rule were analyzed,and the nutrient changes in the degradation process of cattle and sheep dung urine during different sampling times were explored,as well as the soil nitrogen dynamics and N₂O emission characteristics under the dung and urine return in alpine steppe.The main research results are as follows:（1）The concontrations of total nitrogen（TN）,organic carbon（OC）and ammonium nitrogen（NH₄⁺-N）decreased rapidly before the freezing-thawing period,and tended to be flat in the freeze-thaw period.After one year of freeze-thaw degradation,the dry matter loss rate of yak dung was 2.5 times that of Tibetan sheep dung.The contents of total nitrogen,NH₄⁺-N and pH of degraded yak dung were always higher than those of Tibetan sheep dung in different sampling times.The contents of two kinds of dung nitrate nitrogen（NO₃^--N）increased gradually with the extension of degradation time,and the continuous occurrence of dung organic nitrogen mineralization and nitrification may be the main cause of NO₃^--N accumulation.According to the differences of different freezing-thawing periods,the peak NO₃^--N content of yak and Tibetan sheep dung appeared in the end of complete melting period and the end of ablation period,respectively.（2）The difference of dung and urine nitrogen content and the coupling effect with soil freeze-thaw were the key factors leading to the dynamic differential response of soil nitrogen in the active layer of alpine steppe.The contents of NH₄⁺-N and NO₃^--N in soil were significantly affected by freezing-thawing period,dung and urine treatment and their interaction（P<0.01）.Yak dung recharge soil water slowly and for a long time,resulting in soil water content under YD treatment was significantly higher than that under other treatments（P<0.05）.There was a negative correlation between soil depth and soil mineralized nitrogen content,and a significant positive correlation between soil NO₃^--N content and soil water content（P<0.01）.Compared with Tibetan sheep dung urine,yak dung urine had a more prominent effect on soil available nitrogen content.The content of NH₄⁺-N in 0～5 cm soil under YD and YU treatments was significantly higher than that under the other three treatments in the end of freezing and ablation periods（P<0.05）.Moreover,yak urine increased soil NO₃^--N content significantly,mainly due to its higher nitrogen input,rapid hydrolysis of urea to produce mineralized nitrogen and nitrification,which resulted in continuous increase of NO₃^--N content.The daily freeze-thaw cycle of soil and the increase of soil water content caused by rising temperature during the ablation period resulted in a gradual increase of NO₃^--N content in soil 5～10 cm under each treatment.In general,the increase of soil mineralized nitrogen content during the ablation period is helpful to provide necessary nutrients for the growth of alpine steppe vegetation,and the differential response of soil NH₄⁺-N and NO₃^--N content may change the nitrogen uptake and utilization strategies of plants in alpine steppe,thus affecting vegetation greening and its phenological characteristics in different degrees during the growing season.（3）The return of dung and urine from cattle and sheep changed the soil nitrogen availability and microbial activity in freezing-thawing period to varying degrees,leading to the different emission characteristics of N₂O in different freeze-thaw stages of alpine steppe.Freezing-thawing period,dung urine treatment and their interaction had significant effects on N₂O emission in alpine steppe（P<0.001）.The alpine steppe in northern Tibet was a N₂O emission source throughout the year,and the N₂O emission flux had a significant positive correlation with soil temperature（r=0.548;P<0.01）.Compared with dung,urine application rapidly increased soil mineralized nitrogen supply and changed its REDOX environment in a short period of time.Enhanced denitrification may be the main reason for the higher N₂O emission flux in soil after urine return.The annual cumulative N₂O emissions of YD and YU treatments（760.27and 788.69 g N/ha）were significantly higher than those of CK（587.99 g N/ha）,TSD（628.10 g N/ha）and TSU（596.93 g N/ha）treatments（P<0.05）.Freezing-thawing period（freezing period+thawing period）makes a great contribution to the annual N₂O emission of alpine steppe.Except for YD treatment,N₂O emission from other treatments in freezing-thawing period accounts for more than 35%of the annual emission.In addition,the proportion of N₂O emission of each treated soil in melting period is between 52%and 67%in freeze-thaw period.The strong daily freeze-thaw cycle during the ablation period made the N₂O emissions of all treated soils account for more than 40%of the total emissions in non-growing season.In general,the N₂O emission factor and dry weight emission index of yak dung were 0.047%and 0.009%,respectively.The N₂O emission factor and dry weight emission index of Tibetan sheep dung were 6.3 and 1.8 times of that of yak dung,and the N₂O emission factor of yak urine and Tibetan sheep urine were 0.42%and 0.25%,respectively.The results of this study can provide theoretical reference for optimizing the management mode of livestock excreta in different freezing-thawing periods,formulating scientific and reasonable grassland soil nutrient regulation strategies,and promoting the sustainable development of grassland ecosystem and greenhouse gas emission reduction.