Removal Effect And Management Measures Of Nitrogen And Phosphorus In Agricultural Drainage Ditches

Agricultural nonpoint source pollution caused by massive use ofchemical fertilizers and pesticides in the process of agriculturalproduction has attracted much interest and concern presently. The linearwetland characteristic of agricultural drainage ditches makes it importantin mitigation and interception of agricultural nonpoint source pollutants.Agricultural drainage ditches function as not only the moisture adjuster offarmland ecosystems, but also hydrological regime changer in the area.Meanwhile, drainage ditches intercept and purify pollutants in ditch watervia sediment and soil adsorption, plant uptake, microbial activities etc.The Sanjiang Plain was chosen as the study area. The plot experimentcombined with the field monitoring experiment were employed, theregular monitoring methods was used in this study. The migration andtransformation mechanism of nitrogen and phosphorus in water,sediments and plants within natural drainage ditches in Sanjiang Plainwas studies; the mitigation capacities of planted drainage ditches andinfluencing factors were revealed; the influence of water environment byagricultural drainage was forecasted; ecological drainage ditches whichwere fit for farmland managements of Sangjiang Plain were designed.The main conclusions were shown as follows:（1） Intercepting and accumulating drainage water in drainage ditchesis in favour of increasing the utilization rate of groundwater andgroundwater recharge, and reducing the extraction of groundwater in theSanjiang Plain. （2） Farmland fertilization, artificial drainage, and storm runoff canlead to mutation of water quality in drainage ditches. At present, waterquality of drainage ditches in paddy fields of Sanjiang Plain belongs tocategory V of surface water during the long-term period:ammonia-nitrogen （NH₄-N）, nitrate-nitrogen （NO₃-N） andphosphate-phosphorus （PO₄-P） concentrations are0.21-6.91mg/L,1-4mg/L, and0.04-0.45mg/L, respectively. The results show thatnutrients which drainage water carries in the process of agriculturalproduction, especially in crop growth periods, have the potential pollutionrisk for downstream receiving waters. Hence, it is necessary regulate andcontrol of farmland drainage in Sanjiang Plain.（3） Drainage ditches for the mitigation of non-point-source nitrogenand phosphorus pollutants is mainly by ditch sediment/soil adsorption,plant uptake, and microbial metabolism. Under the existing agriculturalditch management condition, drainage ditches have ability to interceptand purify nitrogen and phosphorus in water.（4） Low flow rate can extend the hydraulic retention time, enhancethe adsorbing NH₄-N and PO₄-P capacity of ditch sediments and soil,increase the survival rate of plant seedlings, and weaken the ditchsidewall scour as well as decreasing water and soil loss. It was useful todry ditch sediments for the short-term drying （8days） and rewetting forthe N removal, which indirectly shows that drying and wetting changescuasing by rainfall runoff is helpful to remove N from ditch water inSanjing Plain. Water level also affects the nutrient removal of drainageditches. High water level decrease the biomass of ditch plants and thenutrient concentrations in pore water, accelerate the landslide of ditchsidewall soil, and lead to the stratification of water quality, however,water level has little influence on variation of TN, TP and Corg contents of ditch sediments. Meanwhile, initial influent concentrations may affectthe N and P removal. Under low initial influent concentrations, theremoval efficiencies of NH₄-N, NO₃-N and PO₄-P were enhanced, andwere about70%, however, their removal rates were increased under highinitial influent concentrations; and N and P concentrations of overlyingwater affect the concentrations of pore water. Besides, the nitrogen andphosphorus removal capacities of drainage ditches were different atdifferent growth stages of plants, and nitrogen and phosphorusconcentrations in pore water were similar at different growth stages ofplants.（5） For avoiding massive landslides of ditch sidewall soil andmaking full use of the adsorption capacity of ditch soil, it is suggestedthat the water level of drainage ditches is no higher than2/3of ditchdepth, and inlet flow velocity of drainage ditches is adjusted by sluicevalves without effect on normal farmland drainage. Without affectingnormal agricultural drainage, it is in favour of pollutant removal to adjustthe flow velocity and water level by sluice valves.（6） Laying matrix dams and planting Phragmites communis canefficiently enhance the removal capacities of NH₄-N, NO₃-N and PO₄-Pin ditch water. Compared with the natural ditch, the removal efficienciesof planted ditches were increased about7-10%. Matrix dams in theplanted ditch can extend the hydraulic retention time, and the matrix ofmatrix dams can adsorb nitrogen （N） and phosphorus （P） from water. Theadsorbing amounts of N and P by the matrix were30.26g and17.81grespectively in the planted ditch. The absorbing amounts of N by plants inditch sidewalls and bottom were139.3kg/hm²and123.3kg/hm²,respectively, and the absorbing amounts of P were29.4kg/hm²and25.7kg/hm², respectively. Because nutrients of Phragmites communis and Echinochloa crusgalli can migrate from aboveground part to roots at theend of plant growth, ditch plants should be harvested at the first andmiddle decade of September for preventing N and P of the plantsreleasing into ditch systems due to plant decomposition in order toprevent plant nutrients releasing to ditch systems.（7） There were obvious stratifications of total organic carbon （Corg）and total nitrogen （TN） contents of ditch sediments, and Corg and TNcontents in the0-10cm layer were significantly higher than that in the10-20cm layer. On the whole, Corg and TN contents did not decrease orincrease during the whole experiment, but total phosphorus （TP） contentsdecrease. The results show that running water, stagnant water, anddrying-wetting changes maybe not affect Corg and TN contents of ditchsediments, but the release of TP contents. Hence, the release ofendogenous phosphorus will be decreased by sediment dredging, and wesuggest that dredging frequency of ditches is once every3-5years.（8） Under the current cultivation mode and farmland management,the extension of ecological ditches can subtracte the peak of pollutantconcentrations, increase the mitigation capacity of nonpoint sourcepollutants from paddy fields, and reduce the output load of non-pointsource pollutants; as an ecological channel, the ecological ditch isbeneficial to the restoration and conservation of regional biodiversity.