| Eutrophication is one of the most severe problems of the water environment in the world. There’s a significant relationship between eutrophication and the losses of nitrogen and phosphorus. As a major soil type in Three Gorges Reservoir Area, purple soil has been know as shallowly developed and easy to erode. Nitrogen and phosphorus losses are very serious in slop cropland of purple soil and thus increase the risk of eutrophication in the Three Gorges Reservoir Area. To study the losses of nitrogen and phosphorus under different fertilizer application types and tillage methods, the study in the losses of nitrogen and phosphorus was conducted by rainfall simulation through comparison between longitudinal ridge with chemical fertilizer(T1), longitudinal ridge with combined application of manure and chemical fertilizer(T2) and cross ridge with chemical fertilizer(T3) on 15°slop cropland. The mainresults were:(1)For T2 and T3,the runoff began to yield early and the threshold rainfall were larger than T1 under the same rainfall intensity, which indicated that the appliacation of manure and chemical fertilizer and cross ridge had delayed effects. The threshold rainfall to runoff reduced with the increase of rainfall intensity. Camparing longitudinal ridge with chemical fertilizer, application combined manure with chemical fertilizer and cross ridge could reduce the runoff coefficient and increase infiltration rate. Comparing T2 with T1, the runoff coefficient reduced by 34.62%,21.18% and 42.77%, and the stable infiltriation rate increased by 30.91%, 34.91% and 79.66% at the rainfall intensity of 60, 90, 120 mm?h-1 respectively. As of T3, compared with T1, the runoff coefficient reduced by 60.17%,42.46% and 30.06%, and the stable infiltriation rate increased by 62.13%, 38.16% and 47.63% at the rainfall intensity of 60, 90, 120 mm?h-1 respectively. The effect on reducing runfall increased first and the decresed with increasing rainfall intensity. And the effect of cross ridge increasedwith increasing intensity.. The average runoff rate increased with the increase of intensity in T1, T2 and T3. The average runoff rate and rainfall intensity showed a linear relationship(y = ax + b, a> 0, b < 0). The runoff rate increased gradually until stabilization or slight fluctuations in the process of runoff. Among T1, T2 and T3, T1 had the largest runoff rate at the same rainfall intensity. Runoff rate and time of runoff showed a logarithmic function(y = aln(x) + b, a> 0).(2) The sediment concentration of T1 and T2 over timesuring to a peak and suddenly decreased,, and then stabilized. Compared with T1, T2 and T3 could reduce the average sediment concentrations and sediment yield output rates at the same rain intensity. Combined application of manure and chemical fertilizer enhanced the effects of sediment reduction, and cross ridge’s effects of sediment reduction increaseed first and then decreased with the increase of the rainfall intensity. The sediment output yield rates of T1, T2 and T3 were strongly increased with the increase of rain intensity. The sediment output rates and rainfall intensity showed a linear relationship(y = ax + b, a> 0, b <0). The period sediment yield and period runoff showed a power function relationship(y= axb, a> 0, b> 0). The accumulated sediment yield and accumulation runoff showed a linear relationship(y = ax + b, a> 0).(3)The concentrations of TN, TDN, NO3--N, TP, TDP had the similar change process. The initial runoff nutrient increased rapidly to a peak, and went stabilized gradually with the subsequent runoff lasted. NH4+-N concentrations decreases rapidly at the beginning, and followed by slowly stabilized. The concentrations of nutrient and the time of runoff progress showed a power function relationship. The concentrations of nutrient in runoff increased, and the peak value appeared in advance with the increase of rainfall intensity.Compared with chemical fertilizer(T1), combined application of manure and chemical fertilizer(T2) could reduce the concentration of various forms of nitrogen in runoff at 60 mm?h-1 rainfall intensity. T2 could reduce the concentrations of TN, TDN, NO3--N, NH4+-N, TP and TDP by 33.63%, 33.78%, 21.45%, 53.40%, 23.37% and 37.51%,respectively at 60 mm?h-1 rainfall intensity. The concentrations of NO3--N, NH4+-N and TDP were reduced by 18.04%, 45.77% and 37.56%,respectively, and the concentration of TN and TP increased, and TDN concentration had little difference in T2 at 90 mm?h-1. The concentrations of TN and TP increased, and the concentrations of TDN, NO3--N and NH4+-N had little difference in T2 at 120 mm?h-1 rainfall intensity. Compared with longitudinal ridge(T1), the concentrations of TN, TDN, NO3--N, TP and TDP reduced by 30.67%, 22.59%, 11.28%, 45.65% and 25.34% respectively, and NH4+-N concentration was increased; in cross ridge(T3) at the 60 mm?h-1 rainfall intensity. The concentrations of TN, TDN, NH4+-N, TP and TDP reduced by 18.06%, 16.91%, 78.04%, 34.53% and 12.51%, and NO3--N concentration had little difference in T3 at 90 mm?h-1. The different between concentrations of various forms of nitrogen were insignificant, and the concentrations of TP reduced by 20.24% at 120 mm?h-1.(4) T1 had the largest output rates of TN, TDN, NO3--N and NH4+-N at the same rainfall intensity. The loss of output rate of TN were 78.45%, 49.86% and 47.09%, respectively, in T3 than T1 at 60 mm?h-1, 90 mm?h-1 and 120 mm?h-1 rainfall intensity. T3 had larger effect on reducing the output rates of nitrogen than T2. The effect on reducing output rates was broken in T3 in heavy rain. T2 had the weakest effect at 90 mm?h-1 rainfall intensity. The losses of the output rates of TDN, NO3--N and NH4+-N had a similar rule with TN in T2 and T3 than T1.With the increase of rainfall intensity, the output rates of nitrogen increased in T1, T2 and T3. The output rates of nitrogen and rainfall intensity showed a linear relationship(y = ax + b). T1 had the largest output rates of TP and TDP at the same rainfall intensity.T3 could reduce the output rates of TP by 82.26%, 63.93% and 50.96% at 60 mm?h-1, 90 mm?h-1 and 120 mm?h-1 rainfall intensity, respectively. T3 had the larger effect on reducing the output rates of phosphorus than T2. The effect on reducing output rates was broken in T3 in heavy rain. T2 had the weakest effect at 90 mm?h-1 rainfall intensity. The losses of the output rates of TDP had a similar rule with TP in T2 and T3 than T1.With the increase of rainfall intensity, the output rates of phosphorus increased in T1, T2 and T3. The output rates of phosphorus and rainfall intensity showed a linear relationship(y = ax + b).Granular nitrogen and guanular phosphorus were the main forms of nutrient in runoff on slope cropland in purple soil area. The heavier the rainfall was, the higher proportion of granular nitrogen and phosphorus enjoyed. Granular nitrogen produced 59.27%~72.11% of TN, and granular phosphorus produced 95.49%~97.25% of TP at 120 mm?h-1.(5) The NO3--N / TDP ratios were higher than the eutrophication risk upper limit(15.0), and NH4+-N / TDP ratios were lower than the lower limit(6.4), in every treatment at every rainfall intensity. Generally speaking, N / TDP ratio increased, and NH4+-N/ TDP ratios reduced with the increase of time in T1, T2 and T3.The eutrophication risk of T2 and T3 reduced as time increased. The eutrophication risk of T1 increased at 60 mm?h-1 and 90 mm?h-1 rainfall intensity, and reduced but remained at a high level at 120 mm?h-1. Eutrophication risk was highest at the initial stage of rainfall, especially at 90 mm?h-1 and 120 mm?h-1 rainfall intensity. The eutrophication risk of longitudinal ridge with chemical fertilizer was highest at the initial stage of rainfall. Cross ridge could reduce the risk while combined application of manure and chemical fertilizerfailed in reducing the risk. |
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