Effectsof Simulated Climate Change On Soil Nitrous Oxide Flux In Typical Forest Ecosystems In Qinling Mountains

Nitrous oxide?N₂O?is the third most important greenhouse gas and plays an important role in global warming.The forest is the main component of the terrestrial ecosystem,the soil N₂O flux from which may control the atmospheric N₂O concentration to a great extent.Meanwhile,the increase of air temperature,the change of precipitation pattern,and their interaction may in turn affect the soil in a global climate change scenario in the future,thus regulating the dynamics of soil N₂O emission.Therefore,simulation research on the response of soil N₂O flux to climate change?warming,drought and their interactions?in typical forest is beneficial not only to deeply understand nitrogen cycle of forest soil under global climate change condition,but also to provide inventory data of the global greenhouse gas emissions.As yet,the underlying mechanisms regarding the effects of simulated climate change on forest soil N₂O flux are not clear.Qinling Mountains locate in the south-north division of China,and play crucial roles in nursing biodiversity,holding freshwater resource,conserving soil and water,and sequestering atmospheric carbon dioxide..However,the mountain region is sensitive to climate change,implying that increasing climate change characterized by precipitation pattern change and global warming might give to some extent effects on soil N₂O flux from the forest.Up to date,soil N₂O efflux in the forest ecosystems of Qinling Mountains and its response to climate change remain unknown.A field study was therefore conducted in three representative forest types,Quercus acutidentata,Pinus tabuliformis,and Pinus armandii in Huoditang area of Qinling Mountains to investigate the spatiotemporal variations of soil N₂O flux and its response to simulated warming,drought?intercepting 30%precipitation,IP1;intercepting 60%of precipitation,IP2?,and their interactions.The objectives of this study are to?1?provide the basic data for greenhouse gas emissions inventory at different scales,and to?2?understand the forest soil nitrogen cycles under climate change scenarios.The main results are as follows:?1?During the two years of monitoring,all three forest types are"sources"of N₂O emissions.The annual average emission of soil N₂O in the Q.acutidentata forest was the highest,reaching 1.65±0.45 kg N₂O ha^-1 year^-1,followed by P.tabuliformis forest with the annual average emission of N₂O 1.52±0.47 kg N₂O ha^-1 year^-1.The annual average emission of N₂O in the P.armandii forest was the lowest,with the emission rate of 1.24±0.41 kg N₂O ha^-1 year^-1.Obvious seasonal variation of soil N₂O emission was observed in the three forest types,and summer and autumn are the major emission periods,which accounted for 60%and 80%of the annual emissions,respectively.The soil N₂O emission in the three forest types was significantly correlated with soil temperature and moisture,but with slightly correlation coefficiency with soil moisture.Strong correlation between the N₂O emission and soil ammonium and nitrate nitrogen was also found in the Q.acutidentata and P.tabuliformis forests.?2?Experimental warming increased soil N₂O emission for all three forest types have been increased by 28.3%,13.9%and 10.4%respectivity for P.tabuliformis,Q.acutidentata and P.armandii forests.Under the simulated warming condition,the highest annual average N₂O emission was observed in the P.tabuliformis forest,reaching 1.95±0.57 kg N₂O ha^-1 year^-1,followed by Q.acutidentata forest of 1.88±0.44 kg N₂O ha^-1year^-1.P.armandii forest soil had the lowest rate of N₂O emission with 1.37±0.24 kg N₂O ha^-1 year^-1.In addition,the soil N₂O emission increased significantly in summer,and decreased significantly in autumn and winter in the P.tabuliformis forest.Simulated warming increased the soil N₂O emission in the whole year,and increased the most in spring in Pinus tabuliformis forest.However,the soil N₂O emission increased the most in winter in the Pinus armandii forest.After the warming treatment,the N₂O emissions in three forest types were correlated with soil temperature and soil moisture.There was significant correlation between N₂O emission and available nitrogen in the Quercus acutidentata forest.?3?The effects of IP1 on soil N₂O emission varied greatly with forest types,seasons and years.In compared with the control,the soil N2O emission decreased by 11%and2%for P.tabuliformis and P.armandii forests but the soil N₂O emission increased by 4%for Q.acutidentata.Under the IP1,the annual average N₂O emission was the highest in the Quercus acutidentata forest,reaching 1.71±0.45 kg N₂O ha^-1 year^-1,followed by Pinus tabuliformis forest of 1.35±0.65 kg N₂O ha^-1 year^-1.Pinus armandii forest was the lowest with the N₂O emission rate of 1.22±0.26 kg N₂O ha^-1 year^-1.After decrease of 30%precipitation,the N₂O emission increased in June and August,but decreased in most of July and a few times in June and August in Q.acutidentata forest.The N₂O emission increased in April and September,but decreased in June,July and August in P.tabuliformis forest.The N₂O emission increased in March and May,but decreased in June and July in Pinus armandii forest.After 30%precipitation decrease,there was significant relationship between the N₂O emissions and soil temperature and soil moisture in the three forest types,and the correlation with soil temperature was stronger.The correlation between N₂O emissions and available nitrogen was significant in the Q.acutidentata forest,and the correlation between N₂O emissions and ammonium and nitrate nitrogen was significant in the P.armandii forest.?4?The soil N₂O emission decreased for all three forest types by IP2 comperaed with control,with by 28.9%,30.9%and 17.7%respectivty for P.tabuliformis,Q.acutidentata and P.armandii forests.Under the condition of 60%precipitation decrease,the annual average N₂O emission was the highest in the Q.acutidentata forest,reaching to 1.14±0.46 kg N₂O ha^-1 year^-1,followed by P.tabuliformis forest of 1.08±0.35 kg N₂O ha^-1year^-1.Pinus armandii forest was the lowest with the N₂O emission rate of 1.02±0.25 kg N₂O ha^-1 year^-1.After decrease of 60%precipitation,the N₂O emission increased in a few time of August,but decreased in May,June and most of September and October in Quercus acutidentata forest.The N₂O emission decreased in the whole year,and decreased most in July and August in P.tabuliformis forest.The N₂O emission increased in May,July and a few time in September,but decreased in March and July in P.armandii forest.After 60%precipitation decrease,there was significant relationship between the N₂O emissions and soil temperature in the three forest types.The correlation between N₂O emissions and available nitrogen was significant in the Q.acutidentata forest,and the correlation between N₂O emissions and nitrate nitrogen was significant in the Pinus armandii forest.?5?Under the combined influence of temperature increase and 30%precipitation decrease,the soil N₂O emission varied with foreste types and season.The soil N₂O emission decreased by 11.3%and 5%for P.armandii and P.tabuliformis forests,increased by 4%for Q.acutidentata forest.With the experimental condition,the annual average N₂O emission was the highest in the Q.acutidentata forest,reaching to 1.70±0.32 kg N₂O ha^-1 year^-1,followed by Pinus tabuliformis forest of 1.44±0.51 kg N₂O ha^-1year^-1.P.armandii forest was the lowest with the N₂O emission rate of 1.10±0.28 kg N₂O ha^-1 year^-1.After temperature increase and 30%precipitation decrease,the N₂O emission increased in spring and autumn,but decreased in summer and winter in Q.acutidentata forest.The N₂O emission decreased in the whole year,except in autumn in P.tabuliformis forest.The N₂O emission decreased in the whole year in P.armandii forest.After combination of temperature increase and 30%precipitation decrease,there was significant relationship between the N₂O emissions and soil temperature in the three forest types.The correlation between N₂O emissions and nitrate nitrogen was significant in the Q.acutidentata forest.?6?The combined effected of soil warming and rainfall reduction?60%?significantly decreased N₂O emission by 40%,23%and 21.8%for P.tabuliformis,Q.acutidentata and P.armandii forests.Under the combined influence of temperature increase and 60%precipitation decrease,the annual average N₂O emission was the highest in the P.tabuliformis forest,reaching to 1.27±0.42 kg N₂O ha^-1 year^-1,followed by Quercus acutidentata forest of 0.91±0.37 kg N₂O ha^-1 year^-1.P.armandii forest was the lowest with the N₂O emission rate of 0.97±0.24 kg N₂O ha^-1 year^-1.After temperature increase and 60%precipitation decrease,the N₂O emission decreased in spring,summer and autumn,but increased in winter in Q.acutidentata forest.The N₂O emission increased in spring,but decreased in summer and winter in P.tabuliformis forest.The N₂O emission decreased in the whole year,except in spring in Pinus armandii forest.After combination of temperature increase and 60%precipitation decrease,there was significant relationship between the N₂O emissions and soil temperature in the three forest types.The correlation between N₂O emissions and ammonium nitrogen was significant in the Q.acutidentata forest.?7?No matter temperature increase alone,30%or 60%precipitation decrease,or combination of temperature increase and precipitation decrease,there were significant positive relationship between N₂O emissions and soil temperature and moisture in the three different forest types,but the positive relationship between N₂O emissions and soil ammonium and nitrate nitrogen was depended on the forest types.In a conclusion,the soils in the three typical forest types in this study were sources of N₂O emission,which were controlled by forest types,soil ammonium and nitrate nitrogen,and the seasonal changes of soil temperature and moisture.The simulated temperature increase significantly increased the soil N₂O emission,but the response to simulated temperature increase was different in different seasons.The simulated drought?30%and60%of precipitation decrease?significantly reduced the soil N₂O emission from three forest types,except for Quercus acutidentata,in which 30%of precipitation decrease increased soil N₂O emission.The degree of the decrease in soil N₂O emission was related to the forest type and the season.The interaction between simulated temperature increase and simulated drought all reduced the soil N₂O emission from three forest types to some extent.These results show that climate warming will increase,while the drought will reduce the soil N₂O emission from forest in Qinling Mountains in climate change scenario in the future.