Evolution Characteristics Of Soil Organic Nitrogen Mineralization During Degradation Of Zoige Alpine Wetland

Soil organic nitrogen mineralization and nitrogen availability restrict the primary productivity of wetland ecosystems,and also profoundly affect regional climate.Zoige marsh is the largest alpine wetland in the world.However,the alpine wetland has undergone reverse succession due to the effects of climate warming and human factor in recent decades.Therefore,exploring the evolution characteristics of soil organic nitrogen mineralization during wetland degradation will be conducive to fully understand the mechanism of alpine wetland degradation,and provide theoretical basis for nitrogen regulation in the process of wetland ecological restoration.In this paper,there were selected four wetland soils in Zoige Nature Wetland Reserve,which included Pristine marsh（RPM）,Light degradation marsh（LDM）,Moderate degradation marsh（MDM）,Heavy degradation marsh（HDM）.The characteristics of organic nitrogen mineralization in different degraded wetland soils were analyzed by indoor simulated culture method,mixed first-order and zero-order models.The exponential model was used to analyze the response of soil nitrogen mineralization rate to temperature.Response of soil nitrogen mineralization rate to hydrothermal interaction was analyzed by two-factor model.The coupling relationship between mineralized nitrogen and organic nitrogen components,nitrogen mineralizing enzyme,by RDA redundancy analysis and regression analysis,illustrated the effects of soil organic nitrogen components and nitrogen mineralizing enzyme on mineralized nitrogen in alpine wetland degradation process.The main results are as follows:（1）The accumulation of nitrate and net nitrogen in four types of wetland soils increased with the increase of temperature,were insensitive to water change between40-70%saturated water content（SSM）.The accumulation of nitrate and net nitrogen in different degraded wetland soils were followed by MDM>HDM>RPM,LDM.Mixed first-order and zero-order models N_t=N₀(1-exp^-kt)+C_rt could well describe the dynamics of wetland soil nitrogen mineralization.（2）Nitrification rate and net nitrogen mineralization rate of four kinds of wetland soils increased with the increase of incubation temperature,but changed little between 40%and 70%saturated moisture content（SSM）.Compared with incubation temperature at 5℃,wetland soil nitrification rates incubated at 10,15,20 and 25℃increased by 10.1-98.4%,1341-267.2%,253.1-466.5%and 478.4-648.6%,respectively;net nitrogen mineralization rates incubated at 10,15,20 and 25℃increased by 45.5-68.8%,283.4-358.3%,393.2-569.4%and 531.0-683.2%times,respectively.The exponential model could well simulate the change of mineralization rate with temperature（R²>0.874,P<0.05）.The hydrothermal two-factor model F（T,M）=A*e(^B*T)*（C*M²+D）could well reflect the response of mineralization rate to hydrothermal change（R²>0.869,P<0.05）.（3）With the increase of incubation time,the contents of ammonia nitrogen and amino acid nitrogen decreased in four types of wetland soils;the contents of amino sugar nitrogen also decreased in LDM and HDM soils,but decreased first and then increased in RPM and MDM soils;the contents of unknown nitrogen first increased and then decreased in RPM,LDM and MDM soils,but gradually increased in HDM soils during mineralization culture.After 56 days of incubation,the contents of acidolysis nitrogen,non-acidolysis nitrogen and acidolysis nitrogen components decreased by 7.32-24.4%,1.4-56.4%and 8.2-33.3%respectively compared with those before incubation.Wetland degradation reduced the contents of organic nitrogen components except amino sugar nitrogen.Compared with RPM,the acidolysis nitrogen content in LDM,MDM and HDM decreased significantly by 53.3-58.3%,41.8-55.4%,72.3-78.8%respectively;non-acidolysis nitrogen content in LDM,MDM and HDM decreased significantly by 33.2-49.4%,30.6-57.9%,81.9-89.6%respectively;acidolysis ammonia nitrogen content in LDM,MDM and HDM decreased significantly by 48.0-59.9%,35.6-50.8%,72.4-76.9%,respectively;amino acid nitrogen content in LDM,MDM and HDM decreased significantly by55.7-63.5%,43.1-57.1%,76.5-82%,respectively;the contents of unknown nitrogen decreased significantly by 51.6-63.2%,38.7-65.4%and 71.8-85.8%,respectively.（4）With the increase of culture time,urease activity in 4 types of wetland soils,protease activity in RPM soil,β-1,4-acetyl-glucosaminidase activity in RPM,LDM and MDM soil tended to increase first and then decrease;protease activity in LDM,MDM and HDM,L-glutaminase in RPM and LDM,lβ-1,4-acetyl-glucosaminidase activity in HDM tended to decrease.Wetland degradation reduced the activity of nitrogen mineralase except urease.Compared with RPM,protease activity in LDM,MDM and HDM decreased by 25.0-69.2%,7.7-69.2%,78.6-87.4%respectively,L-glutaminase activity decreased by 37.9-55.5%,27.0-52.0%,34.2-66.7%respectively,andβ-1,4-acetyl-glucosaminidase activity decreased by 40.5-87.5%,13.2-79.3%,77.1-93.7%,respectively.But only the urease activity in MDM was significantly different from that of RPM,which was 46.1-369.0%higher than that of RPM.（5）Redundancy analysis showed that there was significant positive correlation between mineralized nitrogen and urease in RPM,and significant negative correlation between mineralized nitrogen and acid hydrolyzed ammonia nitrogen（P<0.01）.LDM soil mineralized nitrogen was negatively correlated with protease activity,L-glutaminase activity and acidolysis ammonia nitrogen（P<0.01）.MDM soil mineralized nitrogen was negatively correlated with protease activity and amino acid nitrogen（P<0.01）.Mineralized nitrogen in HDM was positively correlated with acidolysis unknown nitrogen,negatively correlated withβ-1,4-acetyl-glucosaminidase and non-acidolysis nitrogen（P<0.01）.The results showed that wetland degradation changed the contribution of nitrogen components and nitrogen cycling enzyme activities to soil mineralized nitrogen.