Effects Of Nitrogenand Phosphorus Addition On Soil Respiration Of Alpine Grassland In Southeast Tibet

The impact of global change on the alpine ecosystem of the Qinghai-Tibet Plateau has become a hot spot for researchers.With the development of social economy,atmospheric nitrogen deposition and phosphorus deposition will continue to affect alpine ecosystems,making the already fragile ecosystems face greater challenges.Therefore,we studied the response mechanism of soil respiration in alpine ecosystems under different nitrogen and phosphorus addition treatments is of great significance for the management and protection of Qinghai-Tibet alpine ecosystems.In this study,the alpine grassland of the Nyingchi Sejila Mountains in Tibet was set up,including control（no nutrient addition）,N addition(NH₄NO₃,15 kg·hm^-2·a^-1),P addition(Ca（H₂PO₄）₂,75kg·hm^-2·a^-1),and both N and P addition(NH₄NO₃15 kg·hm^-2·a^-1N,Ca（H₂PO₄）₂,75 kg·hm^-2·a^-1).To study the effects of nitrogen and phosphorus deposition on soil respiration.Combined with the changes of soil temperature,soil moisture,soil physical and chemical properties,microbial mass and enzyme activity,the response mechanism of soil respiration to nitrogen and phosphorus deposition in alpine grassland ecosystems was revealed,which provided reference and theoretical basis for the study of carbon cycle and global change in alpine grassland ecosystems.The main conclusions are as follows:（1）Effects of nitrogen and phosphorus addition on soil respirationThe diurnal variation of soil respiration rate in the middle of the growing season（August）reached the highest level,which showed that nitrogen addition had the greatest effect on soil respiration.Nitrogen addition significantly increased the average soil respiration rate in June～October by 69.94%,92.20%,157.42%,60.59%and 60.59%（P<0.01）,phosphorus addition only significantly promoted soil respiration rate in August and September,increasing by 144.75%and 40.91%（P<0.01）,and nitrogen and phosphorus mixed addition did not have a significant effect on soil respiration rate（P>0.01）.Soil respiration did not change significantly under each fertilization treatment in May.In May,the soil respiratory temperature sensitivity coefficient increased significantly under different fertilization treatments,but in September,it only had a promoting effect under nitrogen addition,and the mixed addition of phosphorus and nitrogen and phosphorus showed inhibition,and the soil respiratory temperature sensitivity coefficient in June,July,August and October was significantly inhibited under different fertilization treatments.（2）Effects of nitrogen and phosphorus addition on soil respiration.The addition of nitrogen and phosphorus had a significant effect on soil organic carbon,total nitrogen,ammonium nitrogen,and available phosphorus.soil nitrate nitrogen only had a significant promoting effect under nitrogen addition（P<0.05）,and soil total phosphorus content only had a significant effect under mixed addition of nitrogen and phosphorus（P<0.05）.The p H showed acidification across all treatments.Compared with the effects of nitrogen and phosphorus addition on soil nutrients,we found that the effect of nitrogen addition on soil chemistry was greater than that of phosphorus addition（P<0.05）.Besides,nitrogen and phosphorus mixed addition performed best changes of organic carbon,and total phosphorus（P<0.05）.Soil microbial biomass carbon and nitrogen increased significantly under treatments（P<0.05）.Under each treatment,the effect of nitrogen addition on microbial biomass was greater than that of phosphorus addition.Nitrogen and phosphorus mixing had the least effect on the nitrogen content of microbial mass.The activities of soil sucrase,glucosidase and urease increased significantly under fertilization（P<0.05）.Sucrase and glucosidase had similar changes under fertilization treatment,and both had minimal effect under phosphorus addition treatment.Urease was the most active under phosphorus addition,followed by nitrogen and phosphorus addition,and nitrogen addition.（3）The relationship between soil respiration and influencing factors under nitrogen and phosphorus addition.There was a significant difference between soil temperature and soil respiration rate（P<0.01）,Soil temperature changes could be explained by 82.0%,77.2%,74.5%and 68.4%in each experimental group,respectively.Throughout the growing season,the daily variation of soil respiration showed a unimodal curve,which was consistent with the trend of soil temperature change.In the monthly changes of soil respiration,soil respiration gradually increased with time from May～August,soil respiration rate gradually decreased in August～October.The overall variation of soil respiration was in te order of August>June>September>July>October>May.The relationship between soil moisture and soil respiration was not significant（P>0.01）.soil total nitrogen,nitrate nitrogen,ammonium nitrogen,available phosphorus and microbial nitrogen showed a significant positive correlation（P<0.05）with soil respiration,with correlation coefficies of 0.97,0.67,0.76,0.76 and 0.86,respectively.and soil carbon-nitrogen-associated enzyme activities had no significant correlation on soil respiration.In conclusion,N and P addition can promote soil respiration rate under the alpine grassland of Sejila Mountain in southeast Tibet.Soil total N,nitrate N,ammonium N,available P and microbial biomass N were the key factors affecting soil respiration.The key factors affecting soil respiration may promote the carbon emission in the grassland ecosystem of Tibet,which could provid a reference for maintaining the ecological security of Tibet in the future and studying the healthy and green development of the grassland ecosystem soil of the Qinghai-Tibet Plateau.