Study On Nitrogen And Phosphor Losses In Riverine Plain Area By Simulated Rainfall Method

Nitrogen and phosphorous losses from farmland have been identified as one of important causative factors for agricultural non-point source pollution. Usually most scientists thought that nitrogen and phosphorus losses caused by fannland surface runoff were the main reason of surface water eutrophication. In view of the hydrological characteristic of long-term flooded condition of riverine plain area in southern China, simulated rainfall experiments were carried out to study the effect of two main loss types, i.e. runoff and infiltration, on the water environment, and the mechanism and process of nitrogen and phosphorus losses at different levels of soil available N and P contents. The situation of soil available nitrogen and phosphorous content of grain and vegetable fields is also analyzed in the paper. Main results are showed as follows:Under the way of runoff, the outflow period is very short and the outflow velocity is quick. When the rainfall is 180mm, the infiltrated water is 90% of the total runoff water, and the infiltration time is 863 minutes which is 5.6 times of the runoff time of 153 minutes. Meanwhile, the outflow period is very long and the outflow velocity is slow under the way of infiltration, which the outflow characteristic of infiltration way could provide a potential pathway for water dissolvable N and P loss. Under the way of runoff, the soil water content becomes lower in soil deeper profile while it is the reverse under the way of infiltration.When the precipitation is 180mm and soil nitrate content level is 55-206 N mg/kg, the nitrogen loss from infiltration is 13-49 times that from runoff. Under the way of infiltration, NO₃-N is the main form of nitrogen loss. In the components of lost total nitrogen (TN) from infiltration process, the ratio of NO3-N, water dissolvable NH₄+-N and water non-dissolvable nitrogen is 80.3%, 0.3% and 19.4% respectively. Under 3 soil NO₃-N content levels of 55, 109 and 206 mg N/kg, the NO₃-N concentrations in the infiltration liquid are 44, 71 and 119 mg N /L respectively, and the TN concentration in the infiltration liquid are 57, 80 and 162 mg N /L respectively. The NO₃-N and TN concentrations in the infiltration liquid are tens to hundreds. times those in runoff liquid, i.e. the NO₃-N and TN concentrations in the infiltration liquid are 238-385 times and 14-64 times those in runoff liquid respectively. Under the way of infiltration, the higher concentration of NO₃-N in the infiltration liquid appears at initial outflow period, then decrease gradually. In the way of runoff, the main lost nitrogen is the non-dissolvable nitrogen, and the ratio of NO₃-N, water dissolvable NH₄+-N and water non-dissolvable nitrogen lost from runoff process is 8.0%, 7.4% and 84.5% respectively.As for the soil phosphorus loss, the total phosphorus (TP) losses are 42.9 and 58.54 mg P respectively in the runoff way while are 8.6 and 34.6 mg P respectively in the infiltration way in the P1 (40 mg P /kg) and P2 (97 mg P /kg) levels of soil Olsen-P content. While in the P3 (156 mg P /kg) level of higher soil Olsen-P content, the TP loss is 102.0 mg P in the infiltration way which is higher than that of 87.3 mg P in the runoff way. The water dissolvable P loss is 45%-54% and 3%-33% of the TP loss respectively in infiltration and runoff ways in three levels of soil Olsen-P content. With the...