Spatial Distribution Characteristics And Influencing Factors Of Soil Nitrogen In The Middle And Upper Reaches Of The Wujiang River

Soil nitrogen in the watershed is not only an important soil component,but also an important factor affecting the nitrogen load of the watershed aquatic system.Therefore,the spatial pattern of soil nitrogen in the watershed and its influencing factors have been widely concerned,and it is one of the hot issues in current environmental geology and ecology.With the development of social economy,the active nitrogen input by human activities in the soil of the basin is increasing.With the erosion of rainwater and rivers and the loss or leaching of various biogeochemical processes into the aquatic system and the atmosphere of the basin,it has become an important factor affecting the regional aquatic environment and atmospheric environment.Clearing the spatial distribution pattern of soil nitrogen and its influencing factors is the basis of controlling and preventing nitrogen non-point source pollution and reducing water nitrogen load level,which has important environmental significance and global change significance.At present,there is still a lack of relevant research on the spatial distribution and influencing factors of watershed nitrogen.The existing research focuses on forest and grassland ecosystems,and the research on soil nitrogen in the watershed is mostly concentrated in the small watershed.However,the research on soil nitrogen in the karst watershed is still scarce,and the basic data are few.The Wujiang River Basin is located in the typical karst area of China,and it is also the transition zone from the second largest cascade in the western plateau mountainous area to the first largest cascade in the eastern hilly plain.The soil in the basin is mainly calcareous soil,which has many characteristics such as thin soil layer,loose structure,obvious influence by weathering and denudation and poor continuity of soil.At the same time,due to the influence of continuous cascade development,the slope of both sides of the basin is large,resulting in serious soil erosion and rocky desertification.Based on this,in order to clarify the spatial distribution and influencing factors of soil nitrogen in karst watershed,this paper analyzed the contents of different nitrogen forms(including nitrate nitrogen,ammonium nitrogen,easily mineralized organic nitrogen,and soluble total nitrogen)in soil by collecting soil samples in karst watershed.Combined with soil environmental parameters and topographic and geomorphic data,the spatial distribution law of soil nitrogen forms in karst watershed was mastered,and the test method of calcareous soil nitrogen in karst was improved through conditional experiments.The influence of different land use modes on the spatial distribution of soil nitrogen in karst watershed was clarified,and the spatial law and principle of soil nitrogen changing with altitude were discussed.It revealed the causes of soil nitrogen loss and leaching in the valley-type river-reservoir system watershed in southwestern karst area.The main results are as follows:(1)Effects of different drying methods on soil nitrogen speciation in karst watershedDifferent pretreatment methods have different effects on the analysis of nitrogen content in different types of soil samples,while currently there are few studies revealing their effects on the determination of nitrogen content influenced by different mechanisms in lime soil in the karst basin.Hence,we collected surface soils(0-20 cm)from four different soil textures in the middle and upper reaches of the Wujiang River in the karst basin,compared the effects of different drying methods in the laboratory and field extraction on the results of nitrogen content determination.The results demonstrated that different morphologic N contents varied following the order of drying soil>air-dried soil>fresh soil in the drying process of calcareous soil,indicating that the N contents after drying and air-drying treatments were higher than the extraction.The contents of NH₄⁺-N and organic nitrogen were positively correlated with temperature under different drying treatments that might be dominated by various nitrogen transformation mechanisms in changeable temperature.It demonstrated that the higher temperature conditions would promote the processes of mineralization and the high nitrogen contents might originate from the decomposition of microorganisms under high-temperature conditions(105°C).The contents of NO₃^--N had little significant differences with different drying treatments while significantly higher than that of fresh soils.Additionally,the highest NO₃^--N contents in oven-dried soils at 40°C were related to the concomitance of nitrification and mineralization led to the accumulation of NO₃^--N.The differences of microbial species and activities of various soils in drying processes would be dominant the increasing contents of nitrogen species of different types of soil textures.The increase of nitrogen forms in different soil textures during drying is also different,which may be due to the difference of microbial species and activity in different soils.The improvement of the pretreatment method clarified that different forms of nitrogen in calcareous soil in karst basin had different characteristics from those in other regions under different pretreatment methods.According to the results of this study,in order to more accurately determine the content of soil nitrogen form,this paper used the method of field extraction to pretreat the soil nitrogen content,which ensured the accuracy of the test results.(2)Vertical distribution characteristics of surface soil nitrogen in different land use typesThere were significant differences in nitrogen content among surface soils of different land use types in the middle and upper reaches of Wujiang River Basin(P<0.05).NO₃^--N content from upstream to downstream showed farmland>woodland>wasteland at different points,while NH₄⁺-N content showed woodland>farmland>wasteland.The content of NO₃^--N in soil of different land use types decreased gradually from upstream to downstream.The content of NO₃^--N in farmland and woodland in river section was significantly higher than that in reservoir area,which may be related to lower slope and less nitrogen leaching in river section.In addition to the terrain slope in the upper and lower reaches,it may also be affected by the soil dissolved oxygen conditions under the control of altitude elevation.From upstream to downstream,the altitude gradually decreases,and the soil dissolved oxygen at low altitude is high,which is not conducive to the accumulation of soil nitrogen.The content of NH₄⁺-N in farmland and wasteland decreased continuously along the river basin,and there was no obvious change in the content of NH₄⁺-N in forest land along the river basin.It is possible that the dense degree of trees in forest land and humus thickness have greater influence on NH₄⁺-N content than terrain slope.The TSN in the soil of different land use types decreased from upstream to downstream.The TSN in the soil of different land use types was mainly NO₃^--N,followed by NH₄⁺-N.The TSN in the soil of different land use types also showed the order of farmland>woodland>wasteland in the four sampling sites,which was similar to the variation of NO₃^--N content in the soil,indicating that agricultural fertilization was the main source of nitrogen in farmland and woodland.(3)Distribution characteristics of surface soil nitrogen forms in different land use typesThe NO₃^--N,NH₄⁺-N and TSN surface soil of different land use types at the same altitude of Hongjiadu Reservoir were significantly different(P<0.05),and the distribution law was obvious.The surface soil NO₃^--N and TSN of the three land use types showed farmland>woodland>wasteland.The application of nitrogen fertilizer and organic fertilizer in farmland led to the accumulation of soil nitrogen in the surface soil,and the nitrogen content was higher than that of other land use types.Because of crop planting and perennial tillage,the soil was loose,mostly loam,and the permeability was good,which was beneficial to the mineralization of nitrogen,and the nitrogen content was higher than that of other land use types.The surface soil NO₃^--N content in different land use types accounted for the main proportion of TSN,so the soil TSN was greatly affected by the change of soil NO₃^--N content.The content of NH₄⁺-N in surface soil of three land use types showed forest land>farmland>wasteland.NO₃^--N,NH₄⁺-N and TSN in topsoil of different land use types decreased with the decrease of altitude,and the vertical law was obvious.The content of NO₃^--N,NH₄⁺-N and TSN in different land use types decreased significantly from the highest altitude to the lowest altitude.The change of altitude may cause the change of soil hydrothermal conditions and climate conditions.With the decrease of altitude,the soil water content is also decreasing.The water content is in a certain range,and the decrease of water content is not conducive to the mineralization of soil,resulting in the difference of nitrogen content at different altitudes.And with the change of altitude,at the top of the mountain,due to the slow slope,soil surface nitrogen is less affected by leaching,so the higher nitrogen accumulation,the lower altitude,slope increases,soil leaching is easier.(4)Vertical distribution characteristics of soil nitrogen in different land use profilesThe contents of NO₃^--N and NH₄⁺-N in soil profiles of different land use types at the same altitude in Hongjiadu Reservoir decreased continuously along the soil layer.The contents of NO₃^--N,and NH₄⁺-N in farmland were significantly different in different soil layers(P<0.05).There was no significant difference between 20-40 cm and 40-60 cm(P>0.05),indicating that the contents of NO₃^--N and NH₄⁺-N in the surface soil of farmland were leached downward and accumulated in the soil profile,with the leaching depth reaching 60 cm.The contents of NO₃^--N and NH₄⁺-N in the surface soil of wasteland and woodland were also leached downward,with the leaching depth reaching 40 cm and no leaching phenomenon was found at 40-60 cm.The NO₃^--N content in the same soil layer with different land use types at the same altitude was farmland>woodland>wasteland,and the NH₄⁺-N content in the same soil layer was not obvious.The main reason may be that the farmland surface soil due to the input of exogenous nitrogen,soil surface nitrogen content accumulated more,and farmland soil is mostly loam,perennial farming,soil particles loose,easy to leaching down.The contents of NO₃^--N and NH₄⁺-N in soil profiles of the three land use types at different altitudes decreased with the decrease of altitude,and the vertical distribution was obvious.From the top to the bottom of the mountain,the slope gradually increased,and the leaching effect in the soil was strengthened.In addition to farmland,the contents of NO₃^--N and NH₄⁺-N in the soil of wasteland and woodland from 20-40 cm to 40-60 cm decreased with the decrease of altitude,and the variation range was small.There was no significant difference in the contents of NO₃^--N and NH₄⁺-N may be that the basic properties of the soil in the 40-60 cm soil layer were relatively stable,and they were less affected by altitude.The study on the distribution of nitrogen forms in soil profiles of different land use types can reduce the leaching risk of soil nitrogen,as well as the loss of nitrogen in soil,and provide basic data and theoretical basis for the contribution of soil nitrogen to water basin.(5)Influencing factors of nitrogen morphology distribution characteristics of different land use typesNO₃^--N content in soil was significantly correlated with soil p H(P<0.05).From the soil NO₃^--N content of different p H in the study area,it showed a slightly alkaline soil>slightly acidic soil>acidic soil.Most of the nitrogen in the soil existed in the form of organic nitrogen,and the microorganisms involved in organic nitrogen decomposition were suitable to survive in neutral or near neutral environments.In this environment,the activity of microorganisms was high,which was helpful to the mineralization and decomposition of organic matter and improve the release rate of nitrogen.The contents of NO₃^--N,NH₄⁺-N and TSN in soil were significantly correlated with soil water content at 0.05 level.With the decrease of altitude,the soil water content was also decreasing.Within a certain range of soil water content,the amount of nitrogen mineralization in soil increased with the increase of soil water content.Soil water can change the air flow,microbial activity and community structure in soil,thereby increasing the content of nitrogen in soil.The contents of NO₃^--N and NH₄⁺-N in different soils were significantly affected by soil texture,and the order was loam>clay loam>sandy loam>clay>sand,different soil textures can change the soil aeration performance and hydrothermal conditions,thereby affecting the activity of microorganisms on organic nitrogen substrate,resulting in significant differences in soil nitrogen content distribution in different textures.For the Wujiang River Basin,with the establishment of continuous cascade reservoirs,and the topographic conditions of valley type caused a large slope of soil slope,the slope of soil in the middle and upper reaches of the Wujiang River gradually increased from the river section of the basin to the reservoir area close to the basin.For the Hongjiadu Reservoir,the slope change from the top to the bottom of the reservoir area was also gradually increasing.The contents of NO₃^--N and NH₄⁺-N in different land use types in the basin decreased continuously along the river(from upstream to downstream).The contents of NO₃^--N and NH₄⁺-N in different land use types also decreased with the increase of slope from high altitude to low altitude in Hongjiadu Reservoir area.The increase of slope increased the leaching and leaching effect of soil nitrogen,and the slope was the key factor affecting the distribution of nitrogen content in different land use types.