Simulating And Modeling Nitrogen And Phosphorus Loss In Surface Runoff From Sloping Vegetable Fields Of Red Soils

Hillside region of red soil is one of the most potential agricultural producing areas. It is at subtropic zone with abundant light and heat, which is advantage to the growth and rotation of plants. Due to the shortage of lands and pursuing high economic benefit, overuse of nitrogen and phosphorus fertilizers and frequent irrigations are normal, especially in vegetable fields. Furthermore, the rainy climate enhances the lost nitrogen and phosphorus bond to runoff and sediment from vegetable fields, which leads to water body eutrophication eventually.Vegetable field is also one important land use in the suburbs. More fertilizers were applied to vegetable fields than crop lands and irrigated lands, so it makes a significant sense to study nitrogen and phosphorus loss in surface runoff from sloping vegetable fields with different conditions:the case of Chinese cabbage, in the suburb of cities. It also supplied the suggestions of better fertilizer managements on vegetables fields to reduce nitrogen and phosphorus loss.Based on the chain mechanism among the soil and water erosion—nitrogen and phosphorus loss—water body eutrophication, we studied the following objectives by simulated rainfalls indoor:characteristics of the runoff and sediment loss from Chinese cabbage fields; characteristics of the nitrogen and phosphorus loss in surface runoff; the relationships among runoff, sediment, nitrogen and phosphorus loss through runoff in different growth stages of Chinese cabbage; and transport models of nitrogen and phosphorus loss in runoff in one set of rainfall. The conclusions were as follows:1. Rainfall intensity is the key factor to the distribution of rain water, and its effects could be exaggerated by the increasing coverage ratio. Different contributing factors had multiple and exacerbated impact on runoff and infiltration. Runoff volume related to runoff occurrence time negatively. The key factor to runoff occurrence time is coverage ratio and the key factor to runoff continuation is rainfall intensity. The difference among the runoff volumes in different growth stages decreased as the increasing rainfall intensity or slope.2. Nitrogen loss accorded to two different runoff formations well. Saturation-excess runoff led to more dissolved nitrogen loss and infiltration-excess runoff led to increased percentage of residual nitrogen in total nitrogen loss. Residual nitrogen took the major part of total nitrogen, and the nitrate nitrogen took the major part of dissolved nitrogen in both kinds of runoff. Nitrogen lost mainly in rosette stage and anterior folding stage.3. Fertilizer had more affects of on nitrogen loss in runoff and higher percentage of dissolved nitrogen in total nitrogen on the flatter field. Slope affected more on the contents than concentration of nitrate nitrogen. The five growth stages had the content of lost ammonia nitrogen in the following order:posterior folding stage> middle folding stage> anterior folding stage> rosette stage> seedling stage. The absorption to nitrogen of Chinese cabbage didn't affect the ammonia nitrogen loss significantly. The percentage of ammonia nitrogen in total nitrogen in runoff increased as Chinese cabbage growth and the percentage of nitrate nitrogen in total nitrogen in runoff decreased as Chinese cabbage growth.4. Coverage ratio affected phosphorus loss most, the next factor was slope, and the last was rainfall intensity. Different runoff formation also affected the phosphorus forms in runoff. Infiltration-excess runoff was advantage to particulate phosphorus loss and occurred easily in seedling stage and rosette stage; saturation-excess runoff was advantage to dissolved phosphorus loss and occurred easily in folding stages.5. Contents of particulate phosphorus in surface runoff related closely to sediment content. Increased slope had a weaker impact on dissolved phosphorus loss than on particulate phosphorus loss in the whole growth period of Chinese cabbage. Orders of contributing factors of the impacts on percentage of particulate phosphorus in total phosphorus and particulate phosphorus loss modulus were rainfall intensity> slope> coverage ratio> gravimetric soil moisture content.6. Model of nitrogen and phosphorus loss in runoff from Chinese cabbage fields was built based on rainfall intensity, slope, coverage ratio and gravimetric soil moisture content. It has been tested and showed 75%of the errors between measured values and simulated values were less than 5%, and the relations between them were linear.The innovations of the paper are as follows: 1. The dynamical nitrogen and phosphorus loss were studied during the period from runoff occurrence to runoff end. The results showed concentrations of nitrogen and phosphorus in runoff kept stable throughout the rainfall, but the content increased over rainfall time.2. As the growth of Chinese cabbage, the contents and forms of nitrogen and phosphorus in runoff from fields under rainfalls were different in different growth stages. They can be concluded as follows:nitrogen loss occurred easily in rosette stage and anterior folding stage; particulate phosphorus loss occurred easily in seedling stage and rosette stage, dissolved phosphorus loss occurred easily in folding stages.3. Regression model of nitrogen and phosphorus loss through runoff from Chinese cabbage fields throughout rainfall were built on the basis of rainfall intensity, slope, coverage ratio and gravimetric soil moisture content, respectively. Both of them were tested and showed that 75%of the errors between measured values and simulated values were less than 5%, and the relations between them were linear.