Quantifying Analysis On Non-point Source Nitrogen Pollution In Xiaoqing River Basin Of Shandong Province

Modern agricultural practices are strongly linked to fertilizer application for maintaining optimum yields. However, inefficient fertilizer use has led to a significant portion of the nitrogen (N) applied to farm fields reaching surface or underground water systems. Non-point source pollution caused by Nitrate from cropland is major factor seriously causing water quality degradation with the increasing application of N fertilizers. The scientists from all over the world are committing themselves on improving the utilization of N fertilizer and decreasing the non-point source N pollution from farmland. By combining the field experiment methods with biogeochemical model in this paper, non-point source pollution of nitrogen in farmland was studied on its diffusion and movement mechanisms, releasing routes and transferring characteristics in the typical watershed, Xiaoqing River basin, around Bohai Bay. Observations of N leaching from typical croplands in Xiaoqing River basin were used for the validation and modification of the N cycling process-based model, the DNDC or DeNitrification-DeComposition model. The optimum management practices for different crop systems at site scale were proposed based on analyzing the simulated results in typical croplands. And then coupled with Geographic Information System (GIS), the DNDC model estimated the N leaching load and identified key pollution areas at regional scale with the support of the GIS database, and quantified the distribution and contribution of N pollution, and evaluated impacts of current management practices on N accumulation and loss. At last, some suggestions were further put forward. The main results were as follows:(1) The lysimeter method was used to study the characters of N leaching under summer maize-winter wheat rotation system, winter wheat-green onion rotation field, and vegetables-greenhouse fields in typical farmlands of Xiaoqing River Basin. For summer maize-winter wheat rotation field in Licheng site, the results showed that NO3--N was the dominant N style remained in the soil. The soil NO3--N accumulation for conventional treatment ranged from 90.13 to 426.97 kg N hm^-2 during the winter wheat growth period, and from 67.96 to 204.32 kg N hm^-2 during summer maize growth period. During the whole rotation growth season, the amount of water leaching was 176.5mm, and N leaching caused by water leaching was 38.76 kg N hm^-2 occupying 6.5% of the total N fertilizer input for the conventional treatment, which was over 10 times than that for the control treatment with 3.54 kg N hm^-2. Six different treatments were set up in winter wheat-green onion rotation field in Zhangqiu site. The amount of N leaching during green onion growth season was significantly increased with N fertilizer application increasing. The N leaching was 20.35 kg N hm^-2 for the conventional-fertilizer treatment accounting for 5.2% of the total N fertilizer applied, while it was 2.28 kg hm^-2 for the control treatment. The N leaching amount was 17.49 and 13.46 kg hm^-2 for the optimum-fertilizer and decrease-fertilizer treatment, respectively, they both can reduce N leaching effectively. Moreover, the runoff experiment showed that the maximum amount of N runoff was 0.51 kg N hm^-2 for the conventional-fertilizer treatment, only accounting for 0.01 % of the total N fertilizer applied, it was far less than the amount of N leaching, proving that the N leaching was dominant during green onion growth season. The relative relationship between the N runoff loss and fertilizer applied showed insignificantly. For the greenhouse vegetable field in Shouguang, The results showed that the N leaching amounted to 214.04 kg N hm^-2 for the conventional treatment, accounting for 14.6% of the total N fertilizer applied with the amount of 1465 kg N hm^-2 during one crop growth season, while it was 76.26 kg N hm^-2 for the control treatment. The former amount of N leaching was 2 times than latter.(2) Observations of field N leaching were used for model validation and modification, the results showed that the modified model can simulate the soil water movement and the change of N leaching well in winter wheat-summer maize rotation field, winter wheat-green onion rotation field, and greenhouse vegetables field. Moreover, the model generally had acceptable performances in the model simulations for soil residual NO3--N, NH4+-N in winter wheat-summer maize rotation field. Sensitivity tests showed that the simulated great impacts of soil pH, fertilization, and soil porosity on nitrate leaching were consistent with observations reported by other researchers.(3) The optimum management practices for different crop systems at site scale were proposed based on analyzing the simulated results in typical croplands, which was the decrease of fertilizer application to 60% of conventional fertilizer rate, coupled with increase the maize residue returned rate to 100% for the wheat-summer maize rotation field at Licheng site; the decrease of fertilizer application to 50% of conventional fertilizer rate for the wheat-green onion rotation field at Zhangqiu site; the decrease of fertilizer application to 20% of conventional fertilizer rate, coupled with decrease the irrigation rate to 80% of conventional irrigation rate for greenhouse vegetable field at Shouguang site. Under maintaining the grain yields, the optimum management practices for each site can reduce the N loss by N leaching, N2O emissions, and NH3 volatilization. These results can guide directly for the agricultural production.(4) The DNDC model was used to estimate the N potential leaching load at regional scale with the support of the GIS database which was established by compiling the local climate/soil condition and agricultural census data. The results showed that the N potential leaching load ranged from 10.44×10³ t to 36.86×10³ t, with the average of 23.65×10³ t in Xiaoqing River Basin in 2006. Taking the amount of total N fertilizer applied 222.2×10³ t, the average N leaching accounted for 10.6% of the N fertilizer applied. The spatial distribution of N leaching showed that the cells (i.e. towns) with N leaching more than 80 kg N hm^-2 mainly distributed in Zhangqiu City and Shouguang City; the cells with N leaching less than 10 kg N hm^-2 mainly distribued in Licheng District, Jinan City, and Huangtai county and Guangrao county. The amount of N leaching in other regions mainly concentrated between 20 and 40 kg N hm^-2, and also between 40 and 80 kg N hm^-2.(5) Based the regional DNDC model, the regional optimum management practices for winter wheat summer maize rotation system had been established preliminarily. The management practices can reduce the N fertilizer rate 30% compared with conventional N fertilization with no negative impact on agricultural production, meanwhile, the N leaching can reduced 35%, N2O emission reduced 28%, and NH3 volatilization decreased 34%. However, the effects of N fertilization on crop yields and N leaching varied greatly in different regions. Therefore, consideration of the N pollution control effectiveness, it was not effective or efficient to implement a uniform policy thoughout the whole watershed. Instead, it should be site-specific to make management practices.