Effects Of Nitrogen Addition On Soil Carbon,Nitrogen And Enzyme Activity In Great Hing’an Mountain

The peatland in the permafrost region of the Great Hing’an Mountains is an important carbon and nitrogen reservoir.Because low temperature limits the decomposition of soil organic matter,nitrogen is mainly fixed by microorganisms in organic form or stored in permafrost,so less inorganic nitrogen is available.Under the influence of climate change and human activities,endogenous nitrogen released from organic matter decomposition and exogenous nitrogen input increase soil nitrogen availability,which may accelerate the carbon cycle of peatland in permafrost regions and weaken the ability of peatland as a carbon pool.Soil enzymes are important players in the carbon and nitrogen cycle,and are sensitive to environmental changes and can quickly form feedback effects.Therefore,this study selected the peatland in the permafrost area of the Great Hing’an Mountains,and added three different concentrations of nitrogen in the field(N1:6 g N m^-2 year^-1,N2:12 g N m^-2 year^-1,N3:24 g N m^-2 year^-1)and indoor nitrogen addition(50 mg N kg^-1 dry soil)and warming culture experiments simulate the changes in the nitrogen nutrient environment of peatland in permafrost regions.Analysis of differences in the content of carbon and nitrogen fractions and soil enzyme activities related to carbon and nitrogen conversion with their correlations,which under nitrogen addition for nine consecutive growing seasons on surface（0 cm-20 cm）and subsurface（20 cm-40 cm）soils and under short-term N addition and warming（5°C）environments.To reveal the effects of environmental changes in nitrogen nutrients on the carbon and nitrogen pools and the activities of enzymes related to carbon and nitrogen conversion in peatland soils in the permafrost region of the Great Hing’an Mountains.The results of the study are as follows:（1）Long-term nitrogen addition had no significant effect on total soil carbon content.N2 and N3 treatments resulted in decrease of 41.64%and 20.19%in surface soil microbial biomass carbon content,respectively,and N2 treatment led to 33.90%decrease in subsurface soil dissolved organic carbon content.Nitrogen addition at different concentrations increased the content of dissolved organic nitrogen in the subsurface soil and the total nitrogen content in the surface soil,and decreased the total nitrogen content in the subsurface soil and the microbial biomass nitrogen content in the surface and subsurface soils.The total phosphorus content of soil showed a decreasing trend with the increase of nitrogen addition concentration.Long-term nitrogen addition increased the contents of easily extractable glomalin-related soil protein and total glomalin-related soil protein in both soil layers,and the contents of both soil layers were higher.In the two soil layers,the total phenols content of the soil increased gradually with the increase of nitrogen concentration,and the N3 treatment increased by 59.59%and 83.04%compared with the control group.（2）Under the influence of long-term nitrogen addition,the activities of soil invertase and urease in both soil layers increased first and then decreased with the increase of nitrogen concentration.N1 and N2 treatments increased the invertase activity in the surface soil,among which the N2 treatment was the highest with an increase of 45.98%,and the N2 treatment led to an increase of 133.33%and 69.23%in the surface and subsurface soil urease activities,respectively.Affected by nitrogen addition,the activity ofβ-glucosidase in peatland soil in permafrost regions was positively correlated with the microbial biomass carbon and dissolved organic carbon in the soil,and the activity of acid phosphatase was positively correlated with the content of total carbon and microbial biomass carbon.Invertase andβ-glucosidase activities were negatively correlated with glomalin-related soil protein and phenols contents.（3）Nitrogen addition and warming increased soil CO₂ and N₂O release rates in permafrost peatlands.The CO₂ release rate of the peatland soil treated with nitrogen addition at 15℃was higher than that of the control,and the dynamic change was similar.The cumulative release of CO₂ increased by 1.78 times under the interaction of nitrogen addition and warming.At both temperature levels,the N₂O release rate from peatland soils treated with nitrogen supplementation was higher and earlier than that in the control group.The cumulative release of N₂O increased by 108 times under the interaction of nitrogen addition and warming.Warming led to 18.30%increase in soil dissolved organic carbon.Under the combined effect of nitrogen addition and warming,the content of soil dissolved organic nitrogen decreased by 31.21%,and the contents of ammonium nitrogen and nitrate nitrogen increased by 74.40%and 41.96%,respectively.Nitrogen addition treatments had no significant effect on the temperature sensitivity Q₁₀ of soil CO₂ and N₂O release.The average release of soil CO₂ and N₂O was positively correlated with the contents of ammonium nitrogen and nitrate nitrogen.（4）Nitrogen addition and temperature increase inhibited the activities ofβ-1,4-glucosidase,cellobiohydrolase and acid phosphatase in peatland soil,while soilβ-1,4-N-acetylglucosaminidase activity was significantly increased by the combined effect of nitrogen addition and temperature increase.Nitrogen addition decreased the temperature sensitivity Q₁₀ of soil cellobiohydrolase and increased the temperature sensitivity Q₁₀ of soilβ-1,4-N-acetylglucosaminidase.Activity of soilβ-1,4-N-acetylglucosaminidase was positively correlated with the average CO₂ release rate,ammonium nitrogen and nitrate nitrogen content,and the activities of cellobiohydrolase and acid phosphatase were negatively correlated with the average CO₂ and N₂O release rates,ammonium nitrogen and nitrate nitrogen content.Therefore,under the influence of long-term nitrogen addition,the soil active carbon pool and microbial biomass nitrogen pool of the peatland in permafrost areas significantly decreased,and the glomalin-related soil protein and phenols increased significantly;under the interaction of short-term nitrogen addition and warming,the soil CO₂ and N₂O release rate and active nitrogen pool increased,soil active carbon pool decreased.It shows that the change of soil nitrogen nutrition environment will lead to aggravation of soil carbon and nitrogen transformation process,and a large amount of carbon and nitrogen will be released.High-concentration nitrogen addition can increase the content of soil phenols substances involved in enzyme locking,thereby inhibiting the activities of soil invertase andβ-glucosidase.The response of soil enzyme activity to nitrogen addition in peatland in permafrost region is related to nitrogen addition concentration,duration and types of enzymes.Moreover,soil enzyme activities are closely related to carbon and nitrogen components,which can be used as an important indicator to reflect the transformation mechanism of soil carbon and nitrogen in peatland in permafrost regions under the influence of changes in nitrogen nutrient environment.