Model Simulation On Agricultural Non-point Source Nitrogen Pollution In Xiaoqinghe Watershed

Application of chemical fertilizer substantially increased crop production in the 20th Century. However, overuse of nitrogenous fertilizer has introduced excessive nitrogen (N) into the environment in various forms. During the past decades, livestock industries rapidly grew to meet the increasing demand for dairy and meat products in China that has led to more N discharged into the air, soil and rivers. The excessive N loads formed the agricultural non-point source N pollution in the country. This study was to specify the sources of N pollution by means of a modeling tool. To handle the complexity of N cycling in the agroecosystems, a process-based model, Manure-DNDC, was adopted and modified to quantify N releases from the various agricultural sources in the Xiaoqinghe watershed. The main results were as follows:(1) Validation tests were conducted at site and regional scales to verify the applicability of Manure-DNDC for the domain watershed. The modified Manure-DNDC model is capable of simulating soil climate, crop biomass, N leaching parameters in typical winter wheat/summer maize, winter wheat/scallion and vegetable field. The model can also get good results for excretion N from livestock and ammonia from farm. The modified Manure-DNDC model is also capable of simulating crop product, surface runoff and soil erosion that drive the soil or manure N to move at horizontal dimension. R square for the production of winter wheat and summer maize simulation are higher than 0.8.(2) Agricultural non-point source N pollution evaluation database for every town and agricultural eco-system N balance evaluation system was established in Xiaoqinghe watershed. The database was constructed combined with remote sensing image identifying and Geographical Information System (GIS) spatial analysis as the watershed was flat in middle and downstream. High resolution remote sensing data was used for indentifying main channels and then modify the DEM based on the channels. Then watershed boundary, river network, slope, slope length and longest flow path will be picked up using GIS spatial analysis. Soil erosion parameter was get from lookup table. Soil moisture constant was get from model simulation. Agricultural non-point source N pollution database for Xiaoqinghe watershed in every town will be available combined with measured soil organic matter, crop sown area and management, meteorological data, and livestock data. Agricultural eco-system N balance evaluation system was constructed integrate the database, modified Manure-DNDC, surface runoff and soil erosion.(3) Large amount of N surplus appeared in agricultural planting and livestock breeding in Xiaoqinghe watershed, which caused serious agricultural non-point source N pollution. Based on the baseline simulations for 2008, 259.9 million kg N was added to the agricultural soils in the Xiaoqinghe watershed, including 184.2 million kg N from synthetic fertilizer application and 66.6 million kg N from livestock manure. It means that 282.06 kg N from synthetic fertilizer and 101.95 kg N from manure was added to every hectare cropland. Driven by the rainfalls and irrigation application, 23.8 million kg N was leached from the cropping systems in the year. Driven by surface runoff, 46.6 and 7.1 million kg N were lost from livestock operation and cropping systems, respectively. Average leaching and runoff N lost from cropland were 36.42 kg N/hm~2 and 10.82kg N/hm2, respectively. At the end of year, there was 35.0 million kg N manure left in the livestock operation systems, mainly in form of compost. N lost by mean of runoff was 2.26 times of that lost by means of leaching, with 86.83% runoff lost happened in livestock breeding system. In general, the major agricultural non-point source of N in this watershed were livestock manure runoff losses and fertilizer leaching from the cropland.(4) Main factor affecting N leaching was water and fertilizer management. Regions with high leaching mainly located in south Jinan, central and north Zhangqiu, and Shouguang, with 41 towns higher than 30 kg/hm2. N lost from the cropping system was mainly related to fertilizer application rates. If fertilizer rate was lower than 300 kg/hm2, N leaching for more than 70% towns was lower than 10 kg/hm2. However, 20% of N will be leached if fertilizer rate was higher than 400 kg/hm~2. Precipitation and irrigation are main drivers for N leaching in cropland. About 70% N leaching happened in June and July. Rotations with three vegetable in a year suffered from the most serious N leaching with 10.7 million kg N and 285.32 kg/hm~2. Average leaching intensity for winter wheat and summer maize was 18.72 kg/hm~2, with 6.19 million kg N in total. Intensive grain crop and vegetable planting was main sources for cropland N leaching.(5) Land slope, precipitation and soil property played key roles in determining the intensity of the surface runoff and soil erosion. The N lost from surface runoff and soil erosion was mainly in organic forms. Towns with high intensity of soil erosion mainly locate in upstream of Xiaoqinghe watershed, such as Licheng in Jinan, Zhangqiu and Zouping. N leaching from surface runoff in middle of the watershed was mainly lower than 5 kg/hm2. Winter wheat and summer maize rotation, cotton, wheat, and maize were major planting mode for N runoff lost. N lost from livestock operation systems was heavy all over the watershed with 31 towns higher than 100 kg/hm~2. South Jinan and Zhangqiu was the most serious area because of the large livestock scale.(6) The main reason for agricultural non-point source N pollution in Xiaoqinghe watershed was too much synthetic fertilizer application, too much livestock compared to the cropland, and inappropriate manure operation system. Optional management practices were tested with the modeling approach. The results indicated that the effective measures for mitigating N loading for in the target watershed include optimizing fertilizer application rates for cropping systems, adjusting irrigation intensity for vegetable fields, abounding cultivation on the slop lands, and improving manure management practices. An assessment was conducted to identify the best management practices for mitigating N loads. The results indicated that N leaching loss could be decreased by 41% if synthetic N fertilizer application rate is reduced by 15%, which will maintain the crop yields. Reducing the livestock herd size in the counties with intensified animal farms would significantly reducing N loading rates.