Abstract
In the Makinohara area of Shizuoka Prefecture, by measuring the flow rate and nitrate concentration of a stream throughout the year in 1995, we estimated the nitrogen leaching rate from a tea field in its catchment basin. The nitrate concentration of the stream showed little seasonal variation at each branch, with annual averages of 29.6, 19.0 and 32.5 mgN L^<-1>, respectively. The total annual nitrogen outflow from the stream was 3.34x10^4 kgN y^<-1>. Based on the annual flow rate of the stream, the annual rainfall and the evapo-transpiration data reported for the tea field, we estimated the drainage area of the stream as 137 ha, (mainly composed of a tea field and forest area of 19.4 ha) and estimated the leaching rate from the tea field as 284 kgN ha^<-1> y^<-1>. In the paddy field receiving irrigation water from the small stream above, the nitrogen inflow integrated over the irrigation period of 99 d was 314 kgN ha^<-1> and the nitrogen outflow was 80 kgN ha^<-1> The amount of nitrogen removed in the paddy field reached 234 kgN ha^<-1>. The nitrogen budget and stable nitrogen isotope ratios of nitrate in the inflow and outflow indicate that denitrification was the main contribution to nitrogen removal in the paddy field. Using the leaching rate from the tea field and nitrogen removal rate of the paddy field as obtained above, and agricultural statistical data for the tea and paddy fields in the Makinohara area, we estimated the total amount of nitrogen leached out from the tea field as 1.86x10^6 kgN y^<-1>, and that 3.24x10^5 kgN y^<-1> was removed in the paddy field. In the Makinohara area, the reduction of nitrogen load by the tea and paddy field land system was estimated to be 17.4 % under present conditions.
