Effects Of Long-Term Fertilizer Treatments On Nitrogen And Phosphorus In The Direct-Seeded Rice Field Ecosystem

Non-point pollution is a severe problem which leads to the degradation of water quality all over the world.China is the world's largest manufacturer and uses about a quarter of total world's fertilizers.Nitrogen and phosphorus from the agricultural watersheds become the main sources giving rise to non-point pollution.Sustainable development of Chinese economy was restricted by a lot of environmental problems caused by nitrogen and phosphorus runoff from rice fields.Since 1960s,the fate of applied nitrogen and phosphorus in the paddy field and the effect to the environment has become one of the most important research topics for scientists all over the world.Based on a long-term fertilizer experiment,this dissertation aimes to understand the dynamic changes of nitrogen and phosphorus concentrations in the paddy field under different fertilizer treatments,accumulation of nitrogen and phosphorus at different soil profile depths,and the effects on rice production and nitrogen recovery rate.The long-term fertilizer experiment includes five treatments,control(CK), conventional fertilization(CG),reduced fertilization(JL),organic fertilization(YJ), and coated fertilizer(BM),which is located in Pinghu city,Zhejiang province.Main results and conclusions of the study are briefly summarized as follows:Based on periodic observations on nitrogen and phosphorus in the floodwater,it was found that the period within one week after N fertilizer application was the key time to control massive N loss from flooded paddy fields.Dissolved nitrogen(DN) accounted for the major part of nitrogen in the floodwater.The concentration of DN after sodium bicarbonate was applied was the highest as compared with that after urea,manure or coated fertilizer was applied.The risk of nitrogen loss could be reduced mainly by two methods,applying organic fertilizer or coated fertilizer,and cutting down the amount of fertilizer.As organic phosphorus is difficult to be fixed by soil,the concentration of total phosphorus(TP)and dissolved reactive phosphorus (DRP)were in a high level in floodwater under the organic fertilizer treatment during observation.There was a significant correlation between DRP and TP,so DRP could be taken as an monitoring index of TP.Nitrogen and phosphorus concentrations at different soil profile depths of the treatment plots under long-term fertilizer treatments were measured to observe leaching and accumulation of nitrogen and phosphorus.The results indicated that the concentrations of organic matter and total nitrogen in the plough horizon soil from high to low followed the order of BM,YJ,CK,JL,and CG,and BM,JL,CK, YJ,and CG.respectively.However,there were no significant differences among the treatments.Significant correlation between the concentration of organic matter and total nitrogen was found in the plough horizon soil.Except for YJ,the concentration of total nitrogen declined gradually from 0 to 100cm for all other treatments.The concentration of total nitrogen in the 40 to 60cm soil profile layer for YJ was higher than that in the plough horizon.In the 0 to 20cm and 80 to 100cm soil profile layers, there were no significant differences among all the treatments for the concentration of total nitrogen.However,in the 40 to 60cm soil profile layer,there were significant differences among the treatments for the concentration of total nitrogen.The ammonium nitrogen concentrations for different treatments showed a trend of decline with the soil profile depths,but there were no consistent changes for nitrate.The total phosphorus concentration for different treatments displayed similar trend from 0 to 40cm soil profile depths,and the concentrations in the plough horizon varied from 0.70 to 0.78 gram per kilogram soil.However,there were no significant differences for all the treatments.From 2001 to 2006,yield declined gradually for all the treatments.Significant differences were found among the treatments of CK,CG,JL,and YJ for the years from 2002 to 2005,but there were no significant differences among the treatments of CG,JL,and YJ.The yield of BM was also significantly higher than that of CK for the first three years,and there were no significant differences among the treatments of BM,CG,JL,and YJ.However,the yield of BM was significantly lower than those of CG,JL,and YJ for the years from 2004 to 2006,but there was no significant difference between BM and CK.These results indicated that there were no consistently significant effect of fertilization on rice yield.It was therefore suggested that the amount of nitrogen application(CG)could be reduced or replaced by organic fertilizer,while the quantity of coated fertilizer application should be determined by the situation of local soil basic properties.The nitrogen balance and recovery rate were analyzed based on the data obtained from the soil and rice plant analysis.Rresults indicated that the treatments for the indexes of apparent utilization rate(RE)and agronomic utilization rate(AE)from high to low followed the order of JL,BM,YJ,and CG,and JL,YJ,CG.,and BM respectively.The highest rates for RE and AE were 29.71%and 8.55kg / kg N respectively,and the lowest rates were 20.82%and 6.91kg / kg N respectively.The nitrogen recovery rates for all the treatments were less than 65%of the national average rate of China.The result of nitrogen balance analysis indicated that part of the BM nitrogen deposited in the plough horizon and thereby reduced nitrogen loss.Nitrogen of sodium bicarbonate and urea was easily lost from the paddy fields through ammonium volatilization,leaching and runoff,which led to the decline of organic matter and total nitrogen in the plough horizon.As the RE of JL was higher than those of YJ and CG and the amount of nitrogen taken up by the rice from the plough horizon was less,the amount of nitrogen loss in the plough horizon from JL was lower than those from CG and YJ.