Effects Of Ecological Grass Furrows In Farmland On Controlling Soil Losses And Non-point Source Pollution In Purple Soil Area

Purple soil is the main planting soil of the Three Gorges Reservoir Area.Due to its special soil properties and geographical characteristics,the soil erosion here is serious,which makes the sloping land in the Southwest become an important nest of agricultural non-point source pollutants in the Yangtze River Valley.The agricultural runoff generated during rainfall is bound to cause non-point source pollution in agricultural areas.During the process of material loss,Non-point source pollution load will get into the runoff,meanwhile,in the continuous process of runoff transport,there will have a dynamic exchange process between the pollution load and the underlying material.Therefore,in the process of non-point source pollution prevention,it is important to consider the control matter between different environmental units and receptors.In this paper,in contrast with traditional soil furrow without any grass(CK),we focus on the land scale and simulate the ecological grass furrows in farmland(T)for auxiliary drainage in the land parcel.The simulated scouring test was used to explore the process of soil and nutrient loss in the agricultural furrow during the over current of the auxiliary drainage ditches,and the effect of ecological grass furrows in farmland on the reduction of non-point source pollution load in sewage runoff was analyzed,too.The results showed that the runoff scouring could inevitably lead to the loss of sediment and soil nutrients,but the grass in the drainage channel would make the change of the underlying surface.On the one hand,grass will have a more obvious protective effect on the furrow soil,on the other hand,agricultural ecological grass furrow will intercept and adsorb contaminants in runoff and it turns out that agricultural ecological grass furrow in reducing runoff sediment content,COD and nitrogen and phosphorus load also has a certain effect.In this study,the soil loss and runoff process of furrows underdifferent slope ratio and different scour flow were analyzed by interactive test.The main results are as follows:(1)After the scouring test,compared with CK treatment,the content of organic matter(OM)in the soil decreased from 0.54% to 42.18% and the agricultural ecological grass furrow would have a stronger protective effect at larger scouring flow(4.5L/min).When the flow rate was 3L/min,the change rate of total nitrogen,nitrate nitrogen and ammonium nitrogen in the furrow soil was more than zero.Compared with CK treatment,the nitrogen in the soil could be significantly protected,and the nitrogen content increased by 32.87% and 20.94% at different slops,respectively.However,when the runoff flow rate was 4.5L/min,the content of ammonium nitrogen in the soil was decreased by 68.33% and 38.87%,indicating that the ability to improve the control of ammonia nitrogen in the soil during the scouring process may be weaker.The control effect of total phosphorus on the surface soil in agricultural ecological grass furrows were similar to that of soil ammonia and it was obviously controlled by the flushing flow.The control ability of agricultural ecological grass furrow was higher when the flow rate was 3L/min than the flow rate was 4.5 L/min.(2)Scouring would result in changes in the proportion of soil particles and soil texture type.Compared with CK treatment,agricultural ecological grass furrow would reduce the loss of clay particles due to vegetation cover protection.The use of ecological grass furrows in farmland as an auxiliary drainage channel in agricultural land parcel could be possible to maintain the content of clay particles in the furrow soil and control the coarsening effect of the soil under the scouring flow.When the slope ratio was 5 °,the soil dispersal coefficients of ecological grass furrows in farmland were low,and T-3 and T-4.5 were 64.21% and 63.28%,respectively,which were lower than CK treatment.The root systems could be used to maintain the soil as well as vegetation cover for the control of soil erosion,which can eventually be reflected in the improvement of soil micro-aggregates.When the slope ratio rose to 10 °,the control effect of slope on soil erosion would be increased,but the smaller size of soil particles are still in a fragile and easily lost state.Therefore,the dispersal coefficients of soil micro-aggregates after scouring have a downward trend,but their degree of decline are limited.(3)Whether or not to fix grass on an auxiliary drainage channel in agricultural land parcel would have a great influence on the effect of mechanical interception.In this study,the initial flow rates of the runoff were reduced by 46% ~ 82%,and the steadyflow rates of the furrows were reduced by 45% ~ 71%.At the same time,sediment yield would be higher when the designed scouring flow rate was 4.5L/min than 3L/min.In each slope,average sediment concentration of the runoff under the different treatment represented as T-3 <T-4.5 <CK-3 <CK-4.5.In the control of CK treatment,under the different scouring flow,the average sediment concentration of T decreased and ecological grass furrows in farmland would reduce 50% to 70% of the amount of sediment loss,and reduce 12.88% to 38.76% of the sewage runoff sediment.(4)The change of nutrient in runoff would also indicate the result of the material subsidence adsorption and surface soil and nutrient loss in the ecological grass furrows in farmland.When the slope ratio is reduced to 5 °,the agro-ecological furrow has a certain effect on the COD load in the runoff(reduction rate was 8.86% to 14.00%),but when the gradient is 10 °,the ecological grass furrows in farmland would increase the COD pollution load in the runoff.Analyze the removal efficiency of total nitrogen in sewage runoff under different slope and different designed scouring flow,it was found that T would have a reduction effect on total nitrogen load in runoff(reduction rate was 2.46% to 33.39%)in most cases.For the phosphorus,the agro-ecological furrows have an absolute control effect on the loss of particulate phosphorus caused by erosion,but similar to nitrogen loss,the slow flow rate reduced by Cynodondactylon(Linn.)Pers.would also increase the loss of mineral phosphorus.