| The soil soluble nitrogen is the most active component in the soil nitrogen.It is the important source of the soil effective nutrient,but may pollute the surface water and shallow groundwater through the surface runoff and deep percolation respectively.The content and form of soil soluble nitrogen are jointly affected by the fertilization,irrigation,planting mode,soil texture,terrain,underground water level and other conditions,especially the soil at the plough layer.Jianghan Plain is the important commodity grain production base in China.It has the various planting modes,broken piece of paddy field and larger difference in the farmland management way.The research of spatial-temporal differentiation of soluble nitrogen at the small watershed scale is of great significance for guiding the precise nitrogen application in this region.In addition,there are large farmland fertilizing amounts;,rich rainfall and shallow underground water level in this region,and the agricultural production has caused the problem that the nitrogen concentration of a large area of surface water and shallow underground water exceeds standard.It is urgent to carry out the related research of the spatial-temporal differentiation characteristics of farmland soil soluble nitrogen,thus providing the theoretical support for the reasonable nitrogen application and farmland nitrogen loss control.This paper takes the typical agricultural small watershed(43 hm2)of Jianghan Plain as the research object,carries out the even layout of 120 sampling points through the gridded stationing method,collects 0-2 m deep soil profile sample and 0-20 cm plough layer soil sample of different seasons respectively,reveals the change law of the content of farmland soil nitrate nitrogen,ammonium nitrogen and total soluble nitrogen in terms of the time(different seasons)and space(soil profile)through the method of combining classical statistics with geostatistics,analyzes the influence of farmland types,crop strains,terrain,underground water level and other factors upon the spatial-temporal differentiation of nitrogen,and evaluates the soil soluble nitrogen level of various farmland types and its potential risk degree of polluting water body,which can provide the scientific basis for the reasonable agricultural fertilization and irrigation.This research mainly obtains the following results:(1)Spatial differential characteristics of soluble nitrogen in the soil profileThe range of the average value of nitrate nitrogen content in 0-2 m deep soil profile,ammonium nitrogen content and total soluble nitrogen content is 1.20-5.55 mg·kg-1,2.60-12.24 mg·kg-1 and 6.02-25.53 mg·kg-1 respectively.In terms of the variation characteristics,except that the variation coefficient of ammonium nitrogen content at 80-160 cm deep soil layer is larger than 100%,belonging to the strong variation,the variation coefficient of nitrate nitrogen,ammonium nitrogen and total soluble nitrogen at all the rest layers is 10%-100%,belonging to the medium variation.The spatial structure of soil nitrogen is good,and all variables have the medium to stronger spatial autocorrelation on the whole.Affected by the fertilization mode(surface fertilization),the soluble nitrogen content of soil profile presents the trend of high at the surface layer and low at the deep layer on the whole,and the spatial distribution characteristics of the underlayer nitrogen tend to be stable.The content of nitrate nitrogen presents the trend of decreasing progressively from southwest to northeast;The content of ammonium nitrogen and total soluble nitrogen presents the distribution characteristics of high in the east and low in the north,west and south,and especially the content of underlying soil in the eastern region increases significantly,which may cause the pollution of underground water in this region.(2)Spatial-temporal differentiation characteristics of soil soluble nitrogen at the plough layerIn terms of the spatial variability characteristics,the variation coefficient of soil soluble nitrogen content at the plough layer in the research area is 50-100%,and the seasonal difference is small,belonging to the medium variation.But affected by the fertilization,the seasonal difference of soluble nitrogen content in the soil is significant and presents the temporal distribution of spring<autumn<summer on the whole.The ranking of spatial structure from good to ordinary is spring>summer>autumn.The nitrogen content presents the characteristic of high at the surface layer and low at the low layer in different seasons.The spatial distribution characteristics of soil soluble nitrogen in summer and autumn are similar,the content of nitrate nitrogen has the spatial distribution characteristics of high In the south and north,low in the middle and west,and the content of ammonium nitrogen and total soluble nitrogen presents the trend of decreasing progressively from northeast to southwest;Since the nitrogen content in the soil is lower in spring and is greatly affected by the random factor,there are no obvious spatial distribution characteristics,but the high value area in spring is consistent with that in summer and autumn.(3)Factors influencing content of soil soluble nitrogenThe difference of farmland soil in the small area is smaller,and the natural factors of influencing the content of soil soluble nitrogen are mainly terrain and underground water level,and the human factors are mainly the different cultivation ways,fertilization management measures and farmland management measures caused by the different farmland types and crop strains.In the vertical profile of the soil,due to the low terrain and higher underground water level of rice field,the content of ammonium nitrogen and total soluble nitrogen of the soil presents the trend of significant increase from middle layer to underlayer and is close to the content of surface soil at the underlayer.Compared with other rice fields,the rotation paddy-upland crop field presents the weak increasing trend,and the dry land presents the trend of nitrogen content decreasing progressively from top to bottom.The existing form of nitrogen is different in the different farmland types.The nitrogen in the dryland soil is mainly nitrate nitrogen,while the rice field has the higher content of ammonium nitrogen.The content of nitrogen in the soil is significantly affected by the fertilization in terms of temporal variation characteristics.In spring,except that the content of nitrate nitrogen of quarterly rice field and rotation paddy-upland crop field is higher,the content of nitrogen in the soil of various land types is the lowest in all reasons;The season with the highest nitrogen content of quarterly rice field,rotation shrimp-rice crop field and rotation paddy-upland crop field is summer;The content of nitrogen of double-cropping rice field in autumn and the content of nitrate nitrogen of dryland are higher.The existing form of soluble nitrogen in the soil profile is significantly affected by the terrain and underground water,and the spatial and temporal variability of nitrogen is greatly affected by the fertilization measures and cultivation methods.The leaching amount of nitrogen of dryland in summer and autumn is larger,which easily pollutes the surface runoff;The content of nitrogen in the deep soil of rice field is higher,which easily pollutes the shallow surface water.The trgeted adjustment shall be made in terms of fertilization and farmland management,the fertilizing amount of farmland shall be decreased in summer and autumn,the water environment monitoring in the area with higher nitrogen content shall be strengthened for preventing the nitrogen from entering the nearby water body or underground water through the leaching and side leakage,thus safeguarding the agricultural sustainable development and residents' water safety. |
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