N And P Spatial Distributions And Losses In A Small Watershed Of The Gully Region,the Loess Plateau

The Loess Plateau belongs to arid continental monsoon climatic zone.The landforms are fragmented,and soils are loose and porous with serious soil and water losses.Soil and water losses on the Loess Plateau also take away nutrients in the soil surface and resulted soil impoverishment.Therefore,it is particularly important to study spatial distribution and loss of soil nutrient content in small watersheds in the Loess Plateau,which can provide scientific basis for rational agricultural production activities and soil and water conservation practices.In this study,the spatial distribution and influencing factors of soil nitrogen and phosphorus were analyzed in the small watershed of Wangdonggou in the Loess Plateau.The runoff,sediment and nutrient loss were monitored at the slope scales.The following conclusions could be drawn:(1)The mean values of total nitrogen,nitrate nitrogen and ammonium nitrogen in the soil surface of the Wangdonggou watershed(0-20 cm)were 0.99,9.80 and 2.61 mg/kg,respectively.The distribution of total nitrogen in different land use types was: wood land > orchard > wild grassland > farmland > abandoned orchard;the distribution of nitrate nitrogen was: orchard > farmland > forest land > abandoned orchard > wild grassland;the distribution of ammonium nitrogen was: woodland > wild grassland> orchard > farmland > abandoned orchard.In different geomorphic units,the distribution of soil total nitrogen was: loess tableland> gully> slope;the distribution of nitrate nitrogen was: loess tableland > slope >gully;the distribution of ammonium nitrogen was: gully> loess tableland> slope.Land use types and geomorphic units had significant impacts on the spatial distribution of soil total nitrogen and nitrate nitrogen and presented a significant interaction between the two factors.However,the geomorphic unit and land use patterns had no significant effect on the spatial distribution of ammonium nitrogen.(2)The total phosphorus content of the soil surface layer(0-20 cm)under different geomorphic units in the Wangdonggou small watershed of the Loess Plateau was tableland > slope >gully,and the total phosphorus content of different land use types was orchard>abandoned orchard>farmland>woodland>natural grassland.The impact of land use types and geomorphic units on the spatial variability of total phosphorus was extremely significant and the significance level of land use patterns was higher than that of geomorphic units.This study found that the land use types in Wangdonggou small watershed had been adjusted during the past 14 years,and the content of total phosphorus had been greatly improved.The average content varied from 664 to 850mg/kg.The land use types significantly changed the spatial distribution pattern of total phosphorus.(3)The nitrogen and phosphorus contents in the soil are affected by many factors,such as organic matter content,soil pH,and soil particle composition.The soil organic matter content in Wangdonggou had significant impact on total phosphorus,available phosphorus,total nitrogen and ammonium nitrogen contents,and had a significant positive correlation with total nitrogen content.The pH values were different in different land use types and their order were: abandoned orchard>wild grassland> woodland>farmland>orchard.The soil pH had a significantly positive correlation with and the total phosphorus,available phosphorus,total nitrogen and nitrate nitrogen content in Wangdonggou.The average proportions of sand,silt and clay in Wangdonggou soils were 25.41%,64.75% and 9.85%,respectively.There was a significantly correlation between sand and total phosphorus,available phosphorus and total nitrogen content,respectively.(4)The effects of rainfall intensity,slope and land use types on runoff,sediment and nutrient loss were studied at the slope scale.The results showed that the rainfall intensity was linearly proportional to the runoff and sediment loss,not to the nutrient content in the sediment.Runoff and sediment and the nutrient content increased with increasing slope..The amount of nutrient loss in the sediment presented a threshold slope between 25° and 30°.The nutrient enrichment rate in the sediment was the same as the loss of nutrient in the runoff and sediment and presented a threshold slope between 25° and 30°.Under the two land use types of bare land and grassland,natural grass coverage had a good effect to reduce runoff,sediment and nutrient loss.Different land use types also impact the nutrient enrichment rate of sediment.The total nitrogen and total phosphorus enrichment rates in grassland are obviously higher than those in bare land.