LOSS CHARACTERISTICS OF NITROGEN AND PHOSPHORUS IN SURFACE RUNOFF WITH DIFFERENT LAND USE TYPES OF A SMALL WATERSHED IN FREEZE-THAW AGRICULTURAL AREA

This study was carried out in the Abujiaohe watershed in Sanjiang Plain, northeast China. This study investigated the characteristics of nitrogen (N) and phosphorus (P) losses by surface runoff during natural rainfall events. The surface runoff on the Abujiaohe watershed was estimated based on the Soil Conservation Service Curve Number (SCS-CN) model. The results showed that N loss in runoff of paddy land was higher than that of dry land, wetland and forest land. The average event mean concentrations in runoff of paddy land were 18.39mg/L for TN, 2.81mg/L for ammonium nitrogen (NH4+-N), 8.64mg/L for nitrate nitrogen (NO3--N). The minimum N loss occurred on the forest land, the average concentrations in surface runoff of which were 11.45mg/L for TN, 1.64 mg/L for NH4+-N, 4.04mg/L for NO3--N, respectively. In the six rainfall events, NO3--N was the major form of N loss, which was 2.11, 3.07, 2.46 and 3.45 times higher than NH4+-N loss for dry land, paddy land, forest land and wetland. However, the average TP concentrations in runoff measured for four land use types showed followed the order: paddy land (0.35mg/L)>dry land (0.27mg/L)>wetland (0.26mg/L)>forest land (0.08mg/L). In addition, the loads of N and P (kg.ha(-1).a(-1)) in runoff were determined by concentrations and runoff volume. The estimated results using SCS-CN model demonstrated that N loads were higher than TP loads in runoff for four land use types. Meanwhile, the peak value of TN load in surface runoff was 21.78 kg.ha(-1)a(-1) for paddy land and the maximum NO3--N load was 17.82kg.ha(-1)a(-1) for wetland. This phenomenon was due to the fact that the nitrate in soil is easily lost by water movement, especially when runoff discharge is low. Dry land, which accounted for 24.1% of the total watershed area, contributed to 29.9% TN loss amounts and 49.2% TP losses, and however, paddy land accounting for 32.0% the total area contributed to 42.5% of TN losses and 29.0% of TP losses. These indicated that agricultural activity had an obvious impact on the N and P losses in the Abujiaohe watershed.