| Hilly red soil region of Southern China is one of the most serious areas of water and soil losses.It is located in the subtropical zone with unique natural environment, such as abundant rainfall, high intensity rainstorms, high topographic relief, weak soil anti-erodibility, frequent water and soil loss, etc. As a consequence, the loss of soil nutrients, the decrease of soil productivity and fertility, water body eutrophication, aggrading rivers and lakes, and floods disaster occurred frequently, which are threatening the living environment of human beings.Red loam from hilly region of zhejiang province were researched, and we used simulated rainfalls indoor as research method, to analyze and investigate characteristics of the runoff and sediment loss and laws of nutrition (nitrogen, phosphorus and total organic carbon) loss through runoff and sediment from sloping vegetable fields of red loam. Therefore, we conducted 46 effective simulated rainfall experiments under 5 slope lengths (lm,2m,3m,4m,5m),6 levels of vegetation coverage ratios (0, 15%,30%,45%,60%,90%) and 3 fertilizer treatments (check (CK), compound N-P-K fertilizer (CF) and organic fertilizer (OF)) conditions.Each simulated rainfall intensity of about 120mm h-1 was employed for about 30 min. First, the dominant factor of water and soil conservation for slope was determined by analyzing the characteristics of runoff and sediment loss;Second, coarsening of slope soil under heavy rainfall was studied through comparative analyzed the changes of soil aggregates stability, particle size distribution of sediments and original soil during the rainfall; Meanwhile, in order to predict whether possibility of secondary pollution in the water and sediment, we analyzed characteristics of nitrogen, phosphorus and total organic carbon loss in runoff and sediment respectively, and also comparatively analyzed nutrient loss modulus;Finally, the dominant factor of N:P and C:N and the main key index for causing non-point pollution in water environment were determined by using principal component analysis and regression analysis, the main conclusions are as follows:1. Analysis of runoff and sediment yield for slope showed that the initial time of producing runoff advanced with the increase of slope length; and delayed with the increase of vegetation coverage ratio,and vegetation coverage ratio is key factor to affect initial runoff time, which was determined by regression analysis. The intervention of vegetation factor can weaken the effects of slope length on the initial runoff time; Vegetation coverage ratio was also a key factor to affect runoff and sediment yield from sloping filed compared with slope length, especially, when vegetation coverage ratio was 60% which could be regarded as effective coverage ratio. Vegetation cover ratio also has important impacts on runoff modulus and erosion modulus than slope length.2.Analysis of coarsening of slope soil showed that eroded sediments were mainly clay (<2μm) and silt particles (2-50μm). The percentage of clay (0-2μm) and fine silt particles (2-20μm) increased from 43.92% in the original soil to 53.24-58.97% in the sediment. While sand particle content increased significantly in slope soil after rain, and the emergence of a certain degree of coarsening for slope soil, sand particles increment gradually decreased with the increase of slope length or vegetation coverage ratio. Vegetation coverage ratio is the key factor affect particle size distribution of sediments than slope length. We got the conclusion by analyzing soil aggregate stability that PAD0.25 (The degree of aggregate dispersibility) and PAD2 increased, but WAS0.25 (>0.25mm soil water stable aggregate content) reduced after rainfall; The effect of vegetation coverage ratio on soil aggregate stability was stronger than slope length; Meanwhile, positive correlation between Mean weight diameter (MWD) of soil particles and MWDsoii aggregate was found, also showed the more stable of soil macro-aggregate was, the more difficult to form coarsening.3.Analysis of nitrogen loss from slope showed that nitrogen loss was mainly associated with runoff, nitrate nitrogen (N03--N) and ammonia nitrogen (NH4+-N) were main parts in total nitrogen (TN) loss by runoff, the ratio of NO3--N and NH4+-N in TN was 59%-83%. NO3--N was main form of TN, the effect of slope length on NO3--N and NH4+-N loss in runoff were stronger than vegetation coverage ratio. Nitrogen in sediment is primarily enriched in< 20μm sediment particles. There are Logarithmic relationship between the TN enrichment ratio (ERTN) in the eroded sediments and soil erosion modulus (Ms). Nitrogen loss in sediment is mainly affected by vegetation coverage ratio.The various forms of nitrogen loss in runoff and sediment followed the order after the application of fertilizer:CF>OF>CK, organic fertilizer (OF) on reducing nitrogen loss is better than compound N-P-K fertilizer (CF)4.Analysis of phosphorus loss from slope showed that phosphorus loss was mainly associated with sediment.Clay particles were primary carriers of phosphorus loss in sediment. Total phosphorus loss in sediment (TP) is mainly affected by vegetation coverage ratio. There are Logarithmic relationship between the total phosphorus (TP) enrichment ratio (ERTP) in the eroded sediments and soil erosion modulus (Ms) Particulate phosphorus (PP) was main form of TP in runoff. The ratio of PP in TP was 61.22%-72.44%. The effect of slope length on phosphorus loss in runoff were stronger than vegetation coverage ratio. The various forms of phosphorus loss in runoff and sediment followed the order after the application of fertilizer:CF>OF>CK.5.Analysis of total organic carbon (TOC) loss in runoff and sediments showed that TOC loss was mainly associated with sediment. ERTOC was positively correlated with ERclay (<2μm) (R=0.6819) and ERfine siit (2-20μm) (R=0.6312). And all the size particle categories of the original soil and eroded sediments, more than 95% of TOC was concentrated in small-sized particles (<50μm). The change trend of TOC loss was similar with erosion sediment loss, but ERTOC was depended on sediment yield. Vegetation coverage ratio is key factor to affect TOC loss.6.Regression model of nitrogen, phosphorus and TOC loss through runoff and sediment were built under different slope length, vegetation coverage ratio, and fertilizer treatments, it was validated, and the test result showed 80% of the errors between measured values and simulated values of TN, TP and TOC were less than 20%, thus the model is suitable to predict TN, TP and TOC loss amount. Slope length has the most prominent effect on N:P and C:N, which were determined by statistical analysis, N:P decreased as the slope length increased; C:N increased with the increase of slope length. We draw the conclusion that the nutrition loss followed the order in the runoff:TOC>TN>TP, phosphorus was the key factor for causing non-point in water environment.Thus controlling phosphorus loss from sloping vegetable fields of red loam is significant for preventing water eutrophication.The innovations of the study are as follows:1.Combined with comprehensive effects of slope length and vegetation coverage ratio on runoff and sediment yield.We researched the coarsening process of slope soil by analyzing particle size distribution of sediments during the rainfall. The regression analysis of effects slope length or vegetation coverage ratio on runoff modulus or erosion modulus,and the results showed that vegetation coverage ratio has a key impact on runoff and sediment yield than slope length.2.Combined with factors of slope length, vegetation coverage ratio and fertilizer treatment, the loss characteristics of N, P and TOC were analyzed from two aspects of runoff and sediment, which revealed the laws of TN, TP and TOC loss. TN loss mainly associated with runoff, TP and TOC loss mainly associated with sediment. Based on calculation results of enrichment ratio (ER) of nutrient in sediments, and further discuss the relationship of nutrient (TN, TP and TOC) loss and particle size distribution of sediments, we obtained the conclusion that clay particles and silt particles were main part of nutrient loss in sediment.3.Based on the hypothesis which nutrient in runoff is incompatible with nutrient in sediments, combined with factors including slope length, vegetation coverage ratio and fertilizer treatment, we built Regression model of TN, TP and TOC loss through runoff. The factors influencing N:P and C:N in runoff were studied by statistical analysis, and results showed that slope length has the most prominent effect on N:P and C:N, N:P decreased as the slope length increased; C:N increased as the slope length increased. |
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