Removal Effect Of Grass Planting In Ditches On Sediment,nitrogen And Phosphorus From Runoff In Intensive Agricultural Area

The coupling effect of excessive fertilization with rainfall runoff causes a large amount of nitrogen and phosphorus loss with the sediment and runoff from farmland and transport to rivers and lakes through ditches,resulting in water quality deterioration.This effect is very severe in the intensive agricultural sloping lands with shallow soil layer and poor water storage of tropics and subtropics.How to control the transport of the non-point source pollutants derived from farmland erosion to rivers and lakes has attracted more and more attention.Therefore,this study constructed a plant interception system in a 500meters long agricultural ditch in the downstream of Nala small watershed,located in the water source area of Kelan Reservoir,Guangxi.The dominant plants,Cynodon dactylon（L.）Pers and Pennisetum purpureum,and the introduced plant Vetiveria zizanioides（L.）Nash,were planted spacing out in the ditch.Water samples were collected from inlet and outlet of ditch,respectively,after rainfall events,and then the removal benefit of grass planting in ditches on reducing slope sediment and nitrogen and phosphorus pollution into the river were evaluated and grass with strong pollutant interception and adsorption capacity was screened.At the same time,the main controlled factors affecting the interception and absorption of agricultural non-point source pollution in grass planting ditches were analyzed by measuring biomass of the grasses,nitrogen and phosphorus contents of plant and ditch sediment at different growth stages and investigating vegetation coverage and root density,providing a theoretical basis for the control of water erosion-induced agricultural non-point source pollutants in the watershed.The main conclusions are as follows:（1）In this study,Sediment,particle nitrogen（PN）and particle phosphorus（PP）concentrations were significantly positively correlated with precipitation（P<0.01）,while Total dissolved nitrogen（TDN）,NO₃--N,NH₄+-N,Total dissolved phosphorus（TDP）concentrations were not and showed a decreasing trend with time.The grass-planting ditches showed better interception effect of sediment and non-point source pollutants,and the sediment and nutrient concentrations in the upstream inlet were significantly higher than those in the downstream outlet（P<0.01）.There was no significant difference in sediment removal rate among the three vegetation ditches,which all reached more than 50%.The PN,TDN,NO₃^--N,NH₄⁺-N,PP and TDP removal rates of Vetiveria zizanioides were significantly higher than those of the other two species（P<0.01）,reaching37.01%,13.91%,15.63%,41.40%,38.86%and 23.32%,respectively.（2）Three grass plants selected by this research were able to adapt to the environment of study area.The biomass and root density of Vetiveria zizanioides was significantly greater than the other two（P<0.01）,and it has the largest root-top ratio.This indicates that Vetiveria zizanioides not only has a thicker aboveground part,but also has a more developed root system than the other two plants.The vegetation coverage was one of the main factors affecting the purification effect of grass planting ditches.The vegetation coverage of the three plants was Vetiveria zizanioides>Pennisetum purpureum>Cynodon dactylon.It was consistent with the conclusion that Vetiveria zizanioides had better removal ability of nitrogen and phosphorus than the other two species.（3）The TN content of vetiver grass was in the descending order of Vetiveria zizanioides>Pennisetum purpureum>Cynodon dactylon,and the nitrogen absorption effect of Vetiveria zizanioides and Pennisetum purpureum was significantly higher than that of Cynodon dactylon（P<0.01）.The TP content of plants from large to small was Cynodon dactylon>Vetiveria zizanioides>Pennisetum purpureum,and the phosphorus absorption effect of Vetiveria zizanioides and Cynodon dactylon was significantly higher than that of elephant grass（P<0.01）.The nutrient storage and nutrient removal amount of Vetiveria zizanioides ditch were significantly higher than those in the other two（P<0.01）.However,there was no significant difference in TN and TP contents in ditch sediment of the ditches of three plant species.These results indicated that Vetiveria zizanioides had better nitrogen and phosphorus fixation ability than the other two plants,and the nitrogen and phosphorus fixation ability of ditches sediment had no significant effect on the nutrient removal ability of grass-planting ditches.The results show that in the intensive multi-runoff agricultural watershed of tropical and subtropical regions,grass planting in ditches linking upstream slopes with downstream rivers or lakes can effectively intercept and remove nitrogen and phosphorus runoff,and Vetiveria zizanioides was preferentially selected as ditch plant.This measure is a simple,feasible and effective approach for preventing and controlling agricultural non-point source pollution in small watershed,and is worthy of popularization and application.