Simulation of potential nitrate leaching in croplands of typical watershed around Bohai Bay using DNDC model

Nitrogen (N) leaching from agricultural systems plays a key role in the water contamination. Quantifying potential nitrogen leaching at watershed scale is important for providing mitigation policies or strategies. Taking the typical small watershed-Xiaoqinghe basin around Bohai Bay as an example, this paper selected the denitrification-decomposition model (DNDC), combined with detailed soil hydrological and biogeochemical processes, to predict nitrogen leaching of croplands in the watershed under the support of GIS database. The results showed that simulations of the DNDC model was reasonable and had good agreement with observations of annual amounts of leached water and rates of nitrate leaching from 3 typical cropping systems in Xiaoqing River basin. According to the tested DNDC model, the potential N leaching loads ranged from 10.44×103 to 36.86×103 t, with an average of 23.65×103 t in the watershed in 2006. Taking the amount of total N fertilizer applied 222.2×103 t in that year, the average N leaching accounted for 10.6% of the total N fertilizer input. The spatial distribution of N leaching among all the towns in the watershed varied greatly due to the differences in climatic conditions, soil properties, as well as farm management practices, which was similar to the distribution of the fertilizer application rate. The amount of N leaching in most regions mainly varied in a range of 20-80 kg/hm-2. These regions mainly distributed along the Xiaoqing River and Shouguang city. The study also concluded that it is necessary to reduce potential N leaching rates according the practical conditions in different regions of the watershed.