Nitrogen Transport Under Sewage Irrigation At Different Groundwater Depth

Experimentaly nitrogen transport and transformation under different groundwater depths isinvestigated in this thesis, the effect of nitrogen transport under sewage irrigation on soil andgroundwater is discussed, and guidance for safe use of sewage are provided. The field experimentalplots are located in Hongmen experimental station of Farmland Irrigation Research Institute, ChineseAcademy of Agricultural Sciences. We take groundwater depth and amount of irrigtation (two irrigationamount and three groundwater depths) into account in the experimental treatments. Observation itemsinclude the profiles of water content, nitrate and ammonium content in soil, nitrate and ammoniumconcentration of soilwater, nitrate concentration of groundwater, pH of soilwater and groundwater.The result of the study indicates: 1) After sewage irrigation, soil nitrate content at different layersincreases noticeably.; the average nitrate content between 0 to 100 centimeter depth in all the treatmentsincreases 4.44, 5.05, 6.03 mg.kg^-1 in sequence when the irrigation amount is B₁; the average nitratecontent between 0 to 100 centimeter depth in all the treatments increases 4.49, 5.49, 9.76 mg.kg^-1 insequence when the irrigation amount is B₁. From above data, we know that the depth of nitrate transpotis affected by groundwater depth and irrigation amount.2)After sewage irrigation, the ammonium of irrigation water is absorbed to soil,and the ammoniumcontent of the soil increases rapidly. Two days after irrigation, the ammonium content of soil decraesesbecause of nitrification. Five days after irrigation, the ammonium content of soil retunrs back to thevalue before irrigation. Ten days after irrigation, the ammonium content of soil increases slightlybecause of the increase of soil water content and nitrification, and the increase of mineralization oforganic matters. The comparison between the ammonium content under different treatments and thesame irrigation amount is not significant.3) Under the different groundwater depth, the nitrate concentration of groundwater increases34.67ï¼…, 24.94ï¼…, 20.88ï¼…when the irrigation is B₁ in sequence, nitrate concentrations of groundwaterincrease 58.42ï¼…, 38.98ï¼…, 27.21ï¼…when the irrigation is B₂ in sequence. The results indicate that nitrateconcentration of groundwate increases as groundwater depth decreases, and the risk of pollution ofgroundwater by nitrate increases under shallow groundwater depth.4) The nitrate content of soil after summer harvest is lower than that in planting, and the averagenitrate content between 0 to 100 centimeter in all the treatments falls 56.91ï¼…, 47.01ï¼…, 60.21ï¼…,43.43ï¼…, 55.38ï¼…, 48.77ï¼…in sequence. The reasons causing the decrease of soil nitrate content includesroot uptake and leaching. Under the low irrigation, the root uptake is the major reason that leads to thedecrease of soil nitrate content. However, under the high irrigation, the nitrate leaching is the majorreason that gives rise to the decrease of soil nitrate content. The change of soil nitrate content under thesame irrigation and different groundwater depths indicates that the optimum nitrate use efficiency mayappear in the low irrigation stage, and the minimum nitrate leaching may appear in the high irrigationstage. The optimal groundwater depth in terms of efficient use of nitrogen appers to be in the range of3m. 5) The soil ammonium content under summer corn after the harvest decreases. The averageammonium content between 0 to 100 centimeter in all the treatments falls 12.52ï¼…, 32.65ï¼…, 35.98ï¼…,41.94ï¼…, 37.47ï¼…, 49.76ï¼…in sequence.6) Using PESTAN model of WHI UnSat Suite Plus, we simulate nitrate transport in soil atdifferent groundwater depths, and compare the simulations against the experimental datas. The resultsindicate that simulations decribes the transport of nitrate and ammonium reasonably well after sewageirrigation. The simulation overestimates the peak concentration. As time increases, the agreementbetween the simulations and the experiments becomes close. The PESTAN model can catch the nitratetransporting in soil water quit well, and predict nitrate concentration of nitrate in effluent satisfactorily.