Effect of nitrogen fertilization on methane flux in a structured clay soil cultivated with onion in Central Hokkaido, Japan

Abstract

This study was conducted to investigate the effect of N fertilization on the soil CH4 flux during the growing season of onion in a structured clay soil with stagnant water at depths of 70–80 cm and with a peat-mixed subsoil. The following 4 treatments were analyzed over a period of two years: T1) fertilized, onion growing, T2) fertilized, bare field, T3) unfertilized, onion growing, and T4) unfertilized, bare field. In the fertilized T1 and T2 treatments, fertilizers (mixture of 3 : 1 NH4NO3 : (NH4)2SO4) at rates of 322 kg N ha−1 in 1999 and 242 kg N ha−1 in 2000 were applied as basal fertilizers before onion was transplanted. CH4 fluxes among the treatments ranged from −0.06 to 0.12 mg CH4-C m−2 h−1 in 1999, and from −0.03 to 0.01 mg CH4-C m−2 h−1 in 2000, which were high after heavy rain in summer. Cumulative CH4 flux from May to November in the fertilized T1 and T2 treatments was 59 mg CH4-C m−2 for both treatments in 1999, and 3.2 and −0.9 mg CH4-C m−2 in 2000, respectively. On the other hand, in the unfertilized T3 and T4 treatments, the cumulative CH4 flux was 0.2 and −9.2 mg CH4-C m−2 in 1999, and −26 and −20 mg CH4-C m−2 in 2000, respectively. Although the cumulative CH4 flux in each treatment was higher in 1999 than in 2000, the fertilized treatments in both years showed a significantly higher cumulative CH4 flux than the unfertilized treatments. This might be ascribed to the higher level of nitrification in the fertilized treatments, because a high nitrate concentration was observed in the fertilized treatments in the onion growing season. The results also revealed that onion growing did not exert a significant influence on the CH4 flux. The precipitation from May to November was 642 mm in 1999 and 1,008 mm in 2000, and the CH4 emission increased when the precipitation was low. In addition, the CH4 concentration in the soil profile increased with the increase of the depth in summer as the soil was dry. These findings indicated that CH4 diffusion from the soil to the atmosphere was inhibited by rainwater.