Soil Nitrogen And Phosphorus Forms And Risk Assessment In An Agricultural Catchment Of Hefei Suburban

In order to investigate the contents of different nitrogen and phosphorus forms and organic matter in the Ershipu River,132 surfer soil samples were collected from the catchment area. Based on the test, we analyzed the contents of bio-available nitrogens and phosphorus in the soil; used the Kriging technology of ArcGIS to analyze the spatial distribution of different nitrogen and phosphorus forms and organic matter; and SPSS software was applied for the statistical analysis of nitrogens and phosphorus; then the risk assessment of nitrogen and phosphorus loss was conducted by nitrogen and phosphoru index and phosphorus leaching risk to estimate the total nitrogen and phosphorus index and determining the soil phosphorus leaching threshold. Giving the research results as follows:(1) The average concentrations of TN, NH4-N and and NO3-N were 1442.353, 156.865 and 17.456 mg·kg-1,TP, Ex-P, Fe/Al-P, Ca-P, IP and Olsen-P were 566.296, 8.220,114.892,110.961,318.126 and 12.53mg-kg-1. The average of soil organic matter content and pH were 5.13% and 6.99. The nitrogen and phosphorus contents were relatively high in the forest and farmland.(2) The Kriging interpolation results showed that:TN content distribution had significantly spatial variability in the study area, but NH4-N and NO3-N was relatively even. Sampling sites located at the two tributaries converge generally contained high contents of TN.The higher content of points were located at the two tributaries. All forms of phosphorus were evenly distributed in the area and relative high concentrations of all forms of P were observed in the upstream of the A tributary, the middle of the B tributary and the local area of the downstream of the area where two tributaries converge. It was mainly vegetable field.(3) The multivariate statistical analysis results showed that:various forms of nitrogen and phosphorus were divided into four types by correlation. The first were Ex-P, Olsen-P, NO3-N, Fe/Al-P, NH4-N and Ca-P. The second were OP and IP. The third and forth were respectively TP and TN. The correlation of TP and other various forms of phosphorus had obvious correlation, and TN with other various forms of nitrogen also had certain correlation. However TN and pH had extremely negative correlation. The principle component analysis showed that the first were TP, IP and Fe/Al-P with contribution rate of 48.783%, the second were TN, NH4-N and OP with contribution rate of 13.570%, the third was NO3-N with contribution rate of 10.633%, and the last was NH4-N with contribution rate of 10.287%.(4) The content of Bioavailable nitrogen was about 174.321mg-kg’1, accountting for 12.602% of TN,the content of Bioavailable phosphorus was 123.113mg-kg-1,accounted for 21.297% of TP. Bioavailable nitrogen were mainly distributed in the upstream of A tributary, the middle of the B tributary and the local area of the downstream of the area where two tributaries converge. However, Bioavailable phosphorus were mainly distributed in the upstream of A tributary and the left of two tributaries converge.(5) Soil nitrogen and phosphorus index in the study area were 5.175 and 4.483 respectively. The high risk of nitrogen and phosphorus were manily distributed in the upstream of A tributary and the minddle of B near the river. The enrichment coefficient of TP, Ex-P, Fe/Al-P, Ca, P, IP, OP and Olsen-P were 3.42,2.76,2.43,15.01,2.94, 4.16 and 15.01, and the descendent order of the enrichment coefficient of the soil phosphorus form was:Ca-P> TP> IP> Ex-P> Fe/Al-P> Olsen-P. The critical value of Olsen-P leaching is 18.388mg·kg-1, and the leaching samples higher than threshold account for 16.6% of total samples. The high-risk areas mainly appear in the upstream of the A tributary, the middle of the B tributary and the local area of the downstream of the area where two tributaries converge.