Spatial Variation And Leaching Risk Of Soil Nitrogen And Phosphorus In A Small Hilly Watershed Of The Subtropical China

Excessive fertilizer application had not only significantly reduces the nutrient utilization, greatly increases the risk of soil nutrients to water migration. Soil nitrogen(N) and phosphorus(P)accumulation and its corresponding leaching risk associated with excessive fertilizer application are concerned as one of the major sources of non-point agricultural pollutants.Spatial distribution of soil N and Paccumulation, and N and P leaching risk from the topsoil(0–20 cm) downward were assessed in a representative small hilly watershed for the subtropical region of China. The watershed is located in Jinjing town, Changsha County, Hunan Province and covers an area of about 52 km2. Data for soil N and P were obtained from the analyses of soil samples as 10 sites per km. This paper attempted to establish a quantitatively evaluation criteria of soil nitrogen and phosphorus leaching risk, and estimated the small watershed soil nitrogen and phosphorus leaching risk level and the difference between the different land use patterns.The main results are presented in the following:(1) In Tuojia small watershed, both soil NH4+-N content with an average of 26.82 mg/kg and NO3--N content with an average of 9.94 mg/kgachieved to higher intensity variation.The paddy soil NH4+-N thatwas 2.1 times of that(2.36 mg/kg) for woodland soils was the highest average, and soil undervegetable NO3--N thatwas 7.4 times of that(2.36 mg/kg) for woodland soils was the highest average.The mean soil total soil nitrogen(TSN) content was 1.49 g/kg, up to higher intensity variation.The means of TSN content accumulated in soils under paddy rice, vegetable, forest, tea cultivation and orchardwere1.77, 1.46, 1.29, 1.00 and 0.99 mg/kg. Soil Olsen-P content with an average of 13.31mg/kg and soil total soil nitrogen(TSP) content with an average of 0.51 mg/kg both achieved to higher intensity variation.Olsen-P of vegetable soil was the highest average with 27.5 times of that(2.36 mg/kg) for woodland soils.The means of TSPcontent(g/kg) accumulated in soils under vegetable,paddy rice, orchard, tea cultivation and forest were 0.91, 0.60,0.50, and 0.46.This mean of vegetable was 2.76 times of that(0.33g/kg) for woodland soils.(2) In the study area, small extent of spatial variations and high degree of spatial autocorrelation were found in the NO3--N contents of topsoil(0-20cm).The content of NH4+-N had middlerange of spatial variations and moderate degree of spatial autocorrelation. TSN was wide extents of spatial variations and moderate degree of spatial autocorrelation. The middlerange of spatial variations and high degree of spatial autocorrelation were found in the topsoil Olsen-Pand TSP had middle extents of spatial variations and moderate degree of spatial autocorrelation.(3) Not only in the central and southeast corner of region with relatively low-lying, high levels area of soil NH4+-N content but also appeared in relatively high-lying areas such as the northern part of the top left and right of the middle.TSN high levels district mainly distributed in relatively high terrain of the northern region of the upstream section of the Tuojia watershed.Soil NO3--N, available P(Olsen-P) and TSP have the same spatial distribution. Their low levels regions seen in the woodland district, and the high content areas mainly in the paddy fields, tea plantations and other agricultural land and residential land surrounding.(4)Based on the quantitatively evaluation criteria of soil nitrogen and phosphorus leaching risk,the land proportion with NO3--N leaching risk above the medium degree accounted for 23.7% of the total for the upland area, and in the paddy area was 8.9%. But the land proportion with P leaching risk above the medium degree accounted for 36.4% of the total for the upland area, whereas in the paddy area, only 0.2%.Throughout the study occurred to a low level of phosphorus leaching risk.(5) Five kinds of land use types of soil had certain risk of nitrate leaching with gradually increased from woodland, rice fields, orchards, tea plantations to vegetable.From vegetable, tea plantations, orchards, rice fields to woodland, their phosphorus leaching risk decreased.In addition to the vegetable field, the other four kinds of soil phosphorus leaching risk was low.Thereby, to mitigate agricultural non-point source N and P pollution in subtropical hilly watershed and reduce N and P leaching risk, it is important to reducefertilizer application, particularly for vegetable cultivation.