Nitrogen in percolation water in paddy fields with a rice/wheat rotation

Abstract

Nitrogen in percolation water was observed in paddy field soil under rice/wheat rotation. Different N-application rates were designed. Porous pipes were installed in triplicate at depths of 30, 60 and 90 cm to collect the water in the period of wheat growth. Suction cups were installed in triplicate at the same depths to collect the water during the period of rice growth. NH₄ ⁺, NO₃ ^- and total N in the water were analysed with a continuous-flow nitrogen analyzer. Results showed that nitrate was the predominant form of nitrogen in percolation water during the period of wheat growth. Nitrate leaching was high in early spring after the `tillering fertilisation'. More than 50 mg l^-1 of nitrate concentration in percolation water was observed for 30 and 60 cm in depth and more than 15 mg l^-1 were observed for 90 cm. The concentration decreased quickly and was very low, less than 2 mg l^-1 usually, in the earring stage of wheat. Nitrate in water was low, less than 1.5 mg l^-1 usually, when the field was flooded during the period of rice growth. Some soluble organic N existed in the water. Nitrate in percolation water increased when the field was drained. The leaching loss of nitrogen during winter wheat growth period was estimated to be about 3.4% of the N-fertiliser applied at the normal application rate of farmers; for the rice growth period it was around 1.8%. Although a reduced N-application decreased N leaching, it caused a marked decrease in crop yield.