Paddy farming have two aspects related to water quality, so called water purification function and environmental pollution function, simultaneously. Especially, even though nitrogen is essential element for crop production, it's become major factor for environmental pollution at river and reservoirs when it is run off over agricultural area. So it's important to evaluate which function is mainly operated at paddy farming and how much is their function. For this, we investigated nitrogen balance using estimation of its components separately and assessed water purification function.

Before water purification function is quantified at paddy farming, we suggested some hypothesis that paddy fields regards as digestion reactors for purifying wastes water in the cities. Like the digestion reactors, wastes water inflow to paddy fields, digest waste in paddy fields, and purified water out flow to the river. We established the models to determine water purification function in the base of the hypothesis as equation 1 and 2.

Surface water purification = N_irr. - N_run-off                 Equation 1

Ground water purification = N_irr. - N_drain                          Equation 2

If an amount of nitrogen in water irrigated is larger than an amount of nitrogen in water run off or water drained by infiltration at paddy fields, we could determine that paddy fields have an effect on purification of water. At status of reverse, paddy fields could have a malfunction caused environmental pollution.

It is important that balances of water and nitrogen at paddy fields have to be investigated to compare with amounts of nitrogen of input and output, rationally in practice. We investigated the components of nitrogen balance at paddy farming which were different from sources. 9 items mainly supposed in nitrogen balance, which is divided by Irrigation (N_irr.), Precipitation (N_rain), Fertilizer (N_fert.), Infiltration (N_drain), run-off (N_run-off), Harvest (N_plant), Denitrification and Volatilization (N_air), Nitrogen in soil (N_soil), Nitrogen accumulated. The estimation models for each item were established and analyzed by using survey data, statistic data, literature and so on. Their results are shown as Fig. 1.

 Fig. 1. Diagram of nitrogen balance at paddy field that was estimated their components separately in Korea.

Because we could confirm that nitrogen balance analyzed as Fig. 1. is reasonable to general situation in Korea, assessment of water purification was conducted in the base of equation 1 and 2.

Based on Figure 1, we could find out that the amount of nitrogen in irrigated water was larger than the amount of nitrogen in infiltration water or the amount of nitrogen in run-off water. The fact above means that irrigated water into paddy fields is more polluted than drained water, infiltration and run-off, and nitrogen in irrigated water may be digested in paddy fields during flooding period, particularly nowadays, in Korea.

As a result, it could be determined that paddy fields in Korea have a beneficial function for purifying water in practice as much as N_irr. minus N_drain or N_run-off. The other hands, it could be divided two kinds of purification such as surface water and ground water purifications according to concern about pollutant sites. These models were showed as below for calculating amount of nitrogen purified the water in paddy fields. And results calculated are 34.86 kg ha^-1 yr^-1 and 25.33 kg ha^-1 yr^-1 as surface water and ground, respectively.