Assessment and control of nutrient losses from paddy fields is important to protect water quality of lakes and streams in Korea. A four-year field study was carried out to investigate water management practices and losses of nitrogen (N) and phosphorus (P) in rice paddy irrigation fields in southern Korea. The amount and water quality of rainfall, irrigation, surface drainage, and infiltration were measured and analyzed to estimate inputs and losses of N and P. The observed irrigation amount surpassed consumptive use, and approximately 52 to 69 percent of inflow (precipitation plus irrigation) was lost to surface drainage. Field data showed that significant amounts of irrigation water and rainfall were not effectively used for rice paddy culture. Water quality data indicated that drainage from paddy fields could degrade the recipient water environment. The nutrient balance indicated that significant amounts of nutrients (29.5 percent of total N and 8.6 percent of total P compared to input) were lost through surface drainage. Furthermore, up to half the nutrient losses occurred during nonstorm periods. The study results indicate that inadequate water management influences N and P losses during both storm and nonstorm periods. Proper water management is required to reduce nutrient losses through surface drainage from paddy fields; this includes such measures as minimum irrigation, effective use of rainfall, adoption of proper drainage outlet structures, and minimized forced surface drainage. (KEY TERMS: agriculture; nonpoint source pollution; water quality; irrigation; water conservation; drainage.)

The present study was carried out to evaluate nutrient losses that occur during the course of agricultural activity from rice paddy fields of reclaimed tidal flat. For this study, we chose a salt-affected rice paddy field located in the Saemangeum reclaimed tidal area, which is located on the western South Korean coasts. The plot size was 1,000 m 2 (40 m · 25 m) with three replicates. The soil belonged to the Gwanghwal series, i.e., it was of the coarse silty, mixed, mesic type of Typic Haplaquents (saline alluvial soil). The input quantities of nitrogen and phosphorus (as chemical fertilizer) into the experimental rice paddy field were 200 kg N ha -1 and 51 kg P 2 O 5 ha -1 per annum, and the respective input quantities of each due to precipitation were 9.3-12.9 kg N ha -1 and 0.4-0.7 kg P ha -1 per annum. In terms of irrigation water, these input quantities were 4.5-8.2 kg N ha -1 and 0.3-0.9 kg P ha -1 per annum, respectively. Losses of these nutrients due to surface runoff were 22.5-38.1 kg N ha -1 and 0.7-2.2 kg P ha -1 for the year 2003, and 26.8-29.6 kg N ha -1 and 1.6-1.9 kg P ha -1 for the year 2004, respectively. Losses of these nutrients due to subsurface infiltration during the irrigation period were 0.44-0.67 kg N ha -1 and 0.03-0.04 kg P ha -1 for the year 2003, and 0.15-0.16 kg N ha -1 and 0.05-0.06 kg P ha -1 for 2004. When losses of nitrogen and phosphorus were compared to the amount of nutrients supplied by chemical fertilizers, it was found that 11.3-19.1% of nitrogen and 0.5-1.7% of phosphorus were lost via surface runoff, whereas subsurface losses accounted to 0.2-0.8% for nitrogen and only 0.02-0.04% for phosphorus during the 2-year study period.

The relationships between nitrogen (N) and phosphorus (P) concentrations in surface flooding water and those in the leachate of various soil depths were monitored, and temporal variation of leaching losses of N and P from a paddy plot during rice cultivation was estimated under the conditions of southern Korea. Even flooded conditions nitrification in subsurface soil was identified, but nitrate concentrations in leachate were less than 10 mg/L, the standard drinking water nitrate concentration set by the World Health Organization (WHO). The NO 3 -N and ortho-P concentrations in the leachate were generally higher than those in the surface flooding water. Field data implied that leaching losses would not be accurately estimated under the flooded conditions of the paddy field when using the N and P concentrations of surface flooding water and infiltration depth. The leaching losses of NO 3 -N from paddy fields were high immediately after fertilization. The study results suggested that proper fertilization and irrigation strategies are required to reduce leaching losses of NO 3 -N from paddy fields.

The wide-scale reclamation of tidal flats distributed throughout the western and southern coastal areas in Korea has been completed, in an effort to expand the available arable land. The present studies were conducted in order to characterize the concentrations and compositional patterns of selected PAHs, in order to obtain more information regarding environmental risk assessments for sustainable and environment-friendly agriculture in reclaimed tidelands and tidelands in South Korea. The PAH contents were low to moderate, relative to other urbanized regions of the world. Sigma PAHs ranged from 69.8 to 1,175.2 ng g(-1) in dry weight, with a mean value of 394.4 ng g(-1). Differences were observed in the Sigma PAHs concentrations between industrial complex areas and rural regions. The two dominant PAHs were identified as fluoranthene and pyrene. These compounds constituted 1.4 to 55.0% (mean, 33.4%) and 2.7 to 45.6% (mean, 22.0%) of the Sigma PAHs. Our correlation analysis revealed that the Sigma PAHs contents were associated significantly with the organic carbon content (R(2) = 0.86, P < 0.01) and the cation exchange capacity (CEC; R(2) = 0.89, P < 0.01) in the reclaimed tidelands and tidelands.

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