Increase of the nitrate concentration in watersheds is one of the significant problems which the modern agriculture technology is facing. To evaluate ratio of the contribution of some kinds of nitrogen sources is important to make effective measures to decrease nitrate in watershed. The measurement of nitrogen isotope ratios(¹⁵N/¹⁴N) of nitrate from different sources and soil environments and comparison of these ratios to the nitrogen isotope ratios of water nitrate offer a method for identifying nitrates sources. This study was carried out to investigate nutrient loss to stream of applied nutrients in alpine cropping area, to evaluate effects on water quality in the watershed, and to inquire as to origin of nitrogen by natural abundance of nitrogen ratio method (d¹⁵N).

Research site was located in the middle and north east of Korea was famous for the production of red pepper, chinese cabbage and potato. It was known that serious soil erosion occurs during summer rainy season in that area. In order to identify the effects of applied nutrients, red pepper, chinese cabbage and potato were planted in sloping area with 7-35%, and nitrogen and phosphate were analyzed in leachate and run-off water. The cropping area has the soil textures of sandy loam and loam and their soil series were Jigog and Anryong.

The water flow of the stream was the highest in August, three times higher than that of June. After heavy rainfall, the concentration of nitrogen in the stream became lower, but phosphate concentration became higher than before rainfall. The nitrogen concentration of the run-off was 10 times higher than the concentration of stream before rainfall event, but phosphate concentration became 30 times higher. When we estimated the amount of nutrient loss by run-off under this experimental condition, nitrogen loss was 57 kg and phosphate was 220 kg during cultivation period between June to September.

This result clearly showed the seriousness of nutrient loss as well as soil erosion in alpine cropping system that has frequent heavy rainfall events.

On the other hand the nitrate concentration and d¹⁵N value observed, the type of water samples could be categorized into four groups as shown in Fig.1, such as high d¹⁵N and high nitrate, high d¹⁵N and medium nitrates, low d¹⁵N and low nitrate, and low d¹⁵N and high nitrate, reflecting the main sources of nitrate contamination, such as domestic waste, animal waste and soil organic matter, soil organic matter and natural, and chemical fertilizer.