Study On The Characteristics Of Nitrogen Migration In Rice Fields In Ningxia Irrigation Region

The farmland return flow is becoming one of the main factors which could influence the safety of water in Ningxia irrigation region, according to the problem of water pollution in Ningxia irrigation area bring about by the farmland return flow which is abounded of nitrogen. We studied the mobilization and transformation of nitrogen in anthropogenic-alluvial soil in Ningxia irrigation area by laboratory simulation experiments and field experiments. We measured the hydraulic parameters and nitrate nitrogen transport parameters on typical soil (anthropogenic-alluvial soil) by indoor experiment. With the help of field experiments, we obtained the mobilization and transformation characteristics of nitrogen in the paddy rice ecosystem, analyzed the water and nitrogen coupling effect on the paddy rice, and calculated the seepage losses of nitrogen with the water balance method. With a comprehensive consideration of the ecological and economical benefits, we provided some more optimized fertilization and irrigation scheme. The major results are as follows:(1) We measured the hydraulic parameters, and analyzed the major influence factors of them and the characteristics of water movement in the farmland. The results showed that the hydraulic parameters of different soil layers were influenced by the clay content, organic matter and bulk density. The rate of water seepage was controlled by the soil layers of 45～60 cm and 90～100 cm, and the lateral water movement happened mostly in the soil layers under the depth of 60 cm.(2) We studied the horizontal and vertical transport of NO₃^--N in anthropogenic-alluvial soil, and analyzed the characteristics of NO₃^--N migration in the farmland. The NO3--N horizontal transport followed very well with the soil moisture movement under unsaturated conditions, and accumulated at the interface of wetting front. The NO₃^--N lost easily in dry field after irrigation, if we could control the depth of ground water level well, the losses of NO₃^--N would be reduced because the up flow will increase. The rate of NO3--N movement was very different in each soil layer, and the layer of 45～60 cm was the restrictive layer.(3) We studied the temporal and spatial dynamics of different forms of nitrogen in the paddy rice field, and analyzed the effect of water-fertilizer coupling to nitrogen migration and transformation. The average concentration of different forms of nitrogen were influenced by the irrigation amount and nitrogen input. The increase of the concentration of NH₄⁺-N,NO₃^--N and T-N last for about one week after each fertilization. The water pollution due to runoff from the paddy rice field would be reduced if we control the irrigation amount well. The average concentration of NH₄⁺-N in the entire period of duration at different layers assumes remarkably related with the nitrogen input, but with the irrigation amount just 30cm was fitted. The significant relationship between the concentration of NO₃^--N in the seepage water and nitrogen input just obtained at 30cm layers, no other remarkably relationship presented. There was a significant positive correlation between T-N average concentration at 30 cm, 60 cm and nitrogen input, but little relationship with amount of irrigation and initial concentration of T-N. Fertilizing and irrigating mostly affect the nitrogen concentration of the lower layers.(4) We studied the effects of water and nitrogen coupling to the growth and nitrogen accumulation of paddy rice. All growth indexes were positively correlated with nitrogen input but not with irrigation amount. If the rice yield is only considered, three irrigating treatments under normal nitrogen input and water saving treatments under 20% reducing of nitrogen input are feasible scheme.(5) We used water balance method to calculate the amount of nitrogen seepage in the paddy rice field in Ningxia irrigation area, and find that the major form of nitrogen loss is NO3--N, which mostly occurs at the tillering stage of paddy. The amount of nitrogen seepage at 120 cm in the paddy rice field of different treatments was about 12.7⁴6.2 kg·hm-2, which was 4.23%¹5.4% of nitrogen fertilizer input for one season of paddy. NO₃^--N was the main form of nitrogen seepage, which could be up to 80%. Irrigation amount was the more important factors for nitrogen seepage at 120 cm than nitrogen application. The seepage of water and nitrogen mostly occurred at the tillering stage of rice, and excessive irrigation was the major cause. If we could reduce the irrigation amount in the tillering stage of rice, the water and nitrogen seepage must be reduced, and the risk of water pollution resulted by paddy field would be reduce too. If we just consider about environmental benefits, then 30% water saving treatments of three nitrogen input conditions would be the better irrigating and fertilizing scheme. If we consider the environmental benefit and economical benefit at the same time, the treatment of N240W1050 was the optimized treatments.