To evaluate the impacts of agrochemical fertilizer application on the aquatic environment of paddy fields in tropical regions, 7.04 ha of paddy field situated in northern Vietnam were selected as a study area. The fate of nutrient constituents was surveyed through a questionnaire as well as analytical observation. Taking the major environmental components of the paddy field into account, a mass-balance flow regarding fertilizing constituents such as nitrogen and phosphate was built up and used to estimate the contribution of fertilizer to paddy field water pollution. In the mass-flow analysis, the randomness of fertilizer used by local farmers is incorporated within the conventional input-output model. For the control volume of soil in 0–40 cm below paddy plot surface, the estimated average concentrations in soil were 1.8 ppm for inorganic nitrogen and 1.3 ppm for inorganic phosphate on the assumption of 50% non-excessive probability. From the reasonable consistency of the deduced concentrations of nutrients with observed ones, not only the mass balance of fertilizing substances but also the impacts of fertilizer on the paddy field aquatic environment in Vietnam could be understood. On the results from nutrient mass-balance analysis, it was found that 11.3–13.3 kg N/ha would percolate into the underground aquifer that is linked to surface channel flow in the vicinity of the paddy plots and affect the irrigation water quality. The nutrient portion in the materials harvested as grain and straw occupied 58% of total dosed nitrogen and 75.6% of total supplied phosphorus.

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