A nitrogen cycle model in paddy fields to improve material flow analysis: the Day-Nhue River Basin case study

Abstract

Material flow analysis (MFA) has been applied to assess the environmental impact of human activities on nutrient flows at the river basin scale. This paper reports the enhancement of the MFA model by incorporating a nitrogen cycle model for the paddy process in the Day-Nhue River Basin, Vietnam. Dynamic transport and transformation of nitrogen in the paddy soil were quantified using the results from previous studies. All nitrogen inputs to a paddy were considered. The primary nitrogen inputs were classi- fied as nitrate, ammonium, and organic nitrogen. The modified MFA model was used to quantify nitrogen loads from paddy fields to atmosphere, surface water, and soil/groundwater using the classified nitrogen inputs and corresponding nitrogen loss rates. This restructured paddy process significantly influenced the estimated nitrogen loads to the environment. Nitrogen loads to the air and soil/groundwater increased by 18 and 64 % in spring fields and by 0 and 25 % in summer fields, respectively. Compared with MFA that did not apply the nitrogen cycle, nitrogen load to surface water remarkably increased; it was 39 times higher for spring fields and 29 times higher for summer fields. The estimated runoff loads for the two rice seasons were in the range of measured values reported in previous studies. As a result of these calculations, it was proposed that the application of chemical fertilizer could be reduced by 50 % in both seasons to control environmental impacts without impacting rice production. The inclusion of this detailed paddy nitrogen cycle significantly enhanced the quality of the MFA model.