Nitrogen Transformation Characteristics And Influencing Factors Of Forest Soil In Qinghai-Tibet Plateau

Nitrogen(N)is one of the restrictive element of ecosystem productivity.Soil nitrogen cycle,which including biological nitrogen fixation,mineralization,nitrification,denitrification and inorganic nitrogen assimilation and other important processes,is an important part of the ecosystem nitrogen cycle.Among them,free-living nitrogen fixation is one of the important sources of N in natural ecosystems.In addition,N mineralization and nitrification are the two key processes for the supply and maintenance of soil available N.They are also commonly used for evaluating soil N supply capacity and losing risk.It is of great significance for maintaining the productivity of ecosystem and protecting the ecological environment.However,the spatial distribution characteristics of soil N cycling on the Qinghai-Tibet Plateau are still not well studied.Based on this,the forest soils of different altitude gradients in the southeastern of the Qinghai-Tibet Plateau and the Qilian Mountains were sampled to reveal the spatial distribution characteristics and key driving factors of the soil nitrogen cycle in typical forests on the Qinghai-Tibet Plateau.The nitrogen fixation rate,net mineralization and net nitrification rate were measured by indoors incubation experiments,and its relationship with factors such as soil physical-chemical properties,climate and vegetation were studied to provide a theoretical basis for assessing and predicting the soil nitrogen supply potential of the Qinghai-Tibet Plateau and the risk of soil nitrogen loss under the background of climate change.The main research conclusions are as follows:(1)The soil nitrogen fixation rate of the three snow mountains(Meili,Baima and Haba Snow Mountains)in Southeastern Tibet increased with the increase of altitude(1707 m?4312 m),from 0.05 nmol·g^-1·d^-1 at low altitude to 0.38 nmol·g^-1·d^-1 at high altitude.Among them,soil organic matter content and water content are the main factor affect the rate of nitrogen fixation.Soil physical and chemical factors(W_H2O,p H,SOM,TN,C/N)can explain 80%of the variation in nitrogen fixation rate(P<0.01).(2)Meili Snow Mountain and Baima Snow Mountain are dominated by NO₃^--N content in low and medium altitude range(1707 m?3198 m),and the ratio of NH₄⁺-N to NO₃^--N is 0.11.The ratio of NH₄⁺-N to NO₃^--N is dominant in the high-altitude range(3215 m?4312 m),and the ratio of NH₄⁺-N to NO₃^--N is 60.Accordingly,the soil nitrification rate in the high-altitude range of Meili Snow Mountain and Baima Snow Mountain was significantly lower,which made the inorganic nitrogen content in the soil dominated by NH₄⁺-N content.This is likely related to the change of vegetation type,and its formation mechanism needs to be further analyzed.(3)The inorganic nitrogen content of the soil in the Qilian Mountain is 49.47±4.28mg·kg^-1,which is 54.9%lower than that of the southeastern part of Tibet.The net mineralization rate,net nitrification rate and nitrogen fixation rate of the soil are also significantly lower than that of the southeastern part of Tibet by 68.0%,68.5%and 50.0%,this difference is mainly related to the climate difference between the two places.In general,the net mineralization rate and nitrogen fixation rate are significantly positive correlated with annual average temperature,annual average precipitation,and humidity index(P<0.001),and are significantly positively correlated with soil organic matter content.There is a significant negative correlation between the net mineralization rate and soil p H.The net nitrification rate mainly depends on the net mineralization rate of the soil,indicating that environmental factors may affect the soil's physical and chemical properties(such as p H and organic matter)and then affect the nitrogen conversion rate in the soil,which ultimately determined the content and form of inorganic nitrogen in the soil.These results showed that the nitrogen fixation rate and net mineralization rate of forest soil in southeastern Tibet are significantly higher than that in Qilian Mountain,which indicated that the forest soil in southeastern Tibet had higher nitrogen supply potential.At the same time,the net nitrification rate in southeast Tibet is obviously higher,due to the southeast Tibet is relatively warm and humid,while the Qilian Mountain is relatively cold and dry.Therefore,the risk of nitrogen loss is higher in southeastern Tibet,especially in the high-altitude areas of Meili Snow Mountain and Baima Snow Mountain,where large accumulation of NH₄⁺-N content may lead to a large loss of nitrogen and an increase in N₂O emissions under the scenario of future temperature rise.