Water Erosion Of Purple Soil And Its Response To Fertilization And Pastures Cover

The detachment of soil particles from aggregates primarily by raindrops and flowing water, and their transport by runoff water are involved in soil erosion by water. Soil erosion associated with loss of soil particles, plant nutrients, organic matter and pesticides, is detrimental to crop production(causing up to 17% reduction in crop productivity) and becomes an issue of significantly severe societal and environmental concern. This thesis is divided in three main studies:(1) the assessment of purple soil erosion when varying slope angles and rainfall intensities,(2) the soil erosion responses to long term(5 years) chemical and manure fertilization(3) the function of Pastures growth stages in erosion control.Each year, about 75 billion tons of soil is eroded from the world’s terrestrial ecosystems, most from agricultural land. The loss is about 6.6 billion tons of soil per year in India, 5.5 billion tons annually in China, while in the USA, soil loss is more than 4 billion tons per year. These estimations are subject to discussions in some regions as they mainly based on models projections. The accuracy of erosion based models [The Universal Soil Loss Equation(USLE) and its revised version(RUSLE) as examples] to predict purple soil erosion, and the rainfall function on runoff and soil loss from different slope angles still remain uncertain. Those models have been set under areas with gentle slope gradients between 3% and 18%, a normal probability distribution of annual rainfall, and cropping management systems similar to the US. They have been employed in many countries for the assessment of soil loss from agriculture because of their simplicity and low requirements for input parameters. When applied to areas with the different environmental conditions and farming techniques, such as purple soil areas, variables in the USLE/RUSLE models need to be modified to accommodate local characteristics.In this study we carried out laboratory experiments to estimate runoff and sediment yield from different slope angles(5°, 10°, 15°, 20° and 25°) using Norton rainfall simulator and soil boxes. The soil was subjected to different rainfall intensities:0.6, 1.1, 1.5 and 1.8 mmmin-1. Results showed the Pearson correlation(p<0.01) with a negative significant correlation between the time to runoff initiation and the slope angles, as well as between the time to runoff initiation and the rainfall intensities. The time to runoff initiation decrease when slope increase, also the time to runoff initiation decrease when rainfall intensity increase. The correlation is pronounced with the rainfall intensity(82% to 94%) so that rainfall intensity appeared to be the main factor that controls the start of runoff flow. Results on runoff showed a decreased rates in 200 > 150 > 100 > 250 > 50 order, with values ranging from 0.26 mmmin-1 for low rainfall intensity(0.6 mmmin-1) on slope 50 to 1.06 mmmin-1 for simulation experiments with higher rainfall intensity(1.8 mmmin-1) on slope 200. Pearson coefficient showed a strong correlation(p<0.01) between rainfall intensity and runoff rate. Runoff increased with the rainfall intensities which appeared the main factor affecting runoff rate. High intensity produces more runoff on soil surface. The runoff generally occurred quickly on steeper slopes than lower slopes: lower slope shows larger soil surface ponding and water occupied a larger layer, an advantage for more infiltration resulting in less flows. Slope 250 showed less runoff, seemed to have not received the full rainfall due to box inclination and closer position to simulator nozzle. The soil loss data results in a linearly increased with runoff rate and slope angles(from 56.10 gm-2h-1 under rainfall 0.6 mmmin-1 on slope 50 to 1255.66 gm-2h-1 under rainfall 1.8 mmmin-1 on slope 250) in same trend of runoff 200 > 150 > 100 > 250 > 50. However, the highest rainfall intensity 1.8 mmmin-1 sediment loss increase with slope in the natural order of 250 > 200 > 150 > 100 > 50. High erosion occurred timing under highest rainfall and steepest slope. From R2 values, runoff strongly explained for 53 to 81% the variance in sediment detachment. Wherever, slope angle contribute to soil detachment under high rainfall intensity. Study showed a purple soil with highest erosion rate in china of about 1074.4 g.m-2, compared to the loess plateau soil in northwestern with a rate of 531.6 g.m-2, and the rocky soil in northern china soil with lowest erosion rate of about 308.7 g.m-2. This is explained by the purple soil texture which has a comparatively low percentage of grain size with coarse-textured(sands and loams) then easy to be detach than loess or rocky soils. As infiltration rates depend upon the characteristics of the soil, this impact the final results on erosion rates. Assessment of rainfall function on purple soil may help to build conservative measures and strategies in controlling soil loss under different rainfall intensities in Sichuan basin.Studies on purple soil response to fertilization was set to characterize water, soil, nitrogen and phosphorus losses caused by rainfall under different fertilizer application levels in order to provide a theoretical evidence for the agricultural production and coordinate land management which could improve ecological environment. The experiment took place under rotation cropping, winter wheat-summer maize, on a 150 slope purple soil in Chongqing. Four treatments, control(CK) without fertilizer, combined manure with chemical fertilizer(T1), chemical fertilization(T2) and chemical fertilizer with increasing fertilization(T3) were designed on experimental runoff plots for a long term observation aiming to study their effects on soil erosion and nutrients losses. The results showed shows runoff firstly controlled by climate conditions as higher isolated rainfall events generated runoff accounted for more than 1/3 of total runoff or several lower rainfall events. Higher is the rainfall more the kinetic energy, confirmed by significant relationships between rainfall and runoff of the four treatments. The fertilization reduced farm surface runoff and nutrient losses as the CK treatment set with no fertilizer had highest runoff, 1.55, 1.42, and 1.34 times higher than T1, T2, and T3 treatments respectively. T1, T2 and T3 compared to CK, reduced runoff volume by 35.7, 29.6 and 16.8%, respectively. More rainfall water intercept by T1 where combined manure was applied. Over years, the soil conditions in T1 has been favorable for vegetation growth, good coverage, litter production and OM mineralization and the T1 presented a comparative advantage in reducing rain splash, increasing infiltration, and reducing runoff volume. Results showed sediments loss from 4.06 to 13.20 t km-2 varied in CK>T3>T1>T2 order. CK had higher yield 1.67, 1.98, and 1.26 times higher than T1, T2, and T3. The T1, T2, and T3 reduced sediments loss by 40.5, 20.9, and 49.6 %, respectively the loss compared to CK. Regression analysis results indicated that there were significant relationships between soil loss and runoff volume in all treatments. CK with no fertilizer had not good vegetation. T2 had less soil loss. It has been set with optimum fertilizer which met the ecological fertilizations requirements and resulting in good equilibrium in covering and litter production avoiding soil damage and weakness. T3 had increase loss. T3 set with increase fertilizer, chemical can pass into runoff. Also overtime T3 weakness soil, affect soil structure decrease nutrients uptake, influenced the crop development and the soil resistance to the detachment forces. The T1 and the T2 retained much more the nutrients on the plots resulting in associated actions of vegetative growth, fertilizers inputs, fertilizers uptakes, conditions of soil structure and soil chemical reactions with inputs minerals(from fertilizers). More, the amount runoff and detached play a role as soil Nitrogen and Phosphorus in runoff are linked to sediment which carried nutrients in runoff. The T3 had less sediment but high nutrients cause of its high budget of chemical fertilizers which delivered more nutrients in runoff instead of its good continuous vegetation cover. The combined manure with chemical fertilizer(T1) treatment highly reduced total nitrogen and total phosphorus losses by 41.2 and 33.33%, respectively as compared with CK the favorable for controlling runoff sediments losses and preventing soil erosion. Agricultural management practices have a major influence on soil nutrients loss.Vegetation is known as one of the most effective measures for erosion control and for regeneration of the degraded former soil. Ryegrass(Lolium perenne L.) and alfalfa(Medicago sativa L.) are widely-planted pastures. However, their roles in regulating runoff and sediments have not been fully understood. In this thesis we investigated the effects of vegetation at various growth stages on reducing runoff and sediments in soil boxes under simulated rainfall conditions. Growth stages were set as: early stage(late May), middle stage(late June), and late stage(late July). Four indices were used to assess the capacity of reducing runoff and sediments in terms of runoff reduction benefit(RRB), sediment reduction benefit(SRB), ratio between runoff and sediment reduction(RRSR), and soil average/stable infiltration rate. The results showed runoff initiation from early to late stage are in bare soil < Alfalfa < Ryegrass order. At early stage low RRB, 28.1% for alfalfa to 39.2% for ryegrass. At middle stage higher RRB, 64.2%alfalfa, 67.3% ryegrass. RRB relatively gentles at late stage with much more reduction in alfalfa. At the early and middle stages, RRB, SRB, RRSR, and soil average/stable infiltration rate were the largest for ryegrass followed by alfalfa. At the late stage, however, alfalfa performed better than ryegrass. With the growth of vegetation, the capacity of ryegrass and alfalfa in reducing runoff and sediments increased. The RRB and SRB for the late-stage ryegrass were 69.5% and 98.6%, respectively, while they were 61.6% and 95.4% for the late-stage alfalfa, respectively. In particular, the SRB for the two vegetation types at all stages were over 90%, and were apparently higher than the RRB. Difference in RRB and SRB be due to crop characteristics, ground vegetation cover and root system varying with vegetation growing. They cause differences in runoff and sediment yields. Vegetation cover for ryegrass increased gradually over the entire experimental period, greater than alfalfa at different growth stages. Ryegrass has high leaf area index, intercept a large amount of rainfall and delay soil infiltration. Vegetation by its cover plays a role of barrier reduces the energy of raindrops with their canopy. In addition, the RRSR increased greatly from the early to middle growth stage for all vegetative treatments, indicating that the reduction in runoff was more effective than the reduction in sediment when vegetation grew mature. Since the majority of heavy rainstorms fall in the late growth stage, the functioning of artificial grass in reducing slope runoff and sediments should be stressed and investigated in further details.