EFFECT OF RICE STRAW ON (NH4)2SO4-N BALANCE AND RICE PRODUCTION IN PADDY SOIL

Rice straw and 15N labeled (NH4)2SO4, respectively supplied in four C/N ratios, were basal dressed to grow three consecutive seasons rice crops in pots to investigate the influence of straw materials, not only in the current first season, but also in the second and third seasons on the fate of applied (NH4)2SO4-N and rice production, as well as the residual effects of three sampling stages just after each harvest. As expected, the straw materials might significantly delay the (NH4)2SO4-N uptake by rice and then retard the growth and development of rice crops. These effects were solely observed in the current 1st season and related to the quantity of added straw. In the further 2nd and 3rd seasons, more (NH4)2SO4-N recovery and greater rice responses were obtained in the higher straw supply treatment though both responses were extremely poor. After plantation of 3 seasons rices, no significant difference of the sum total dry weight or the sum recovery of applied N in panicles (except that in the highest straw supply treatment) in 3 seasons between the straw and strawless treatments was presented. The immobilization of (NH4)2SO4-N was apparent in the 1st season and related not only to the soil, but also to the added straw. The applied N residual in soil was rather stable and hardly decomposed to release available N for the further season rice crops, especially those at the time after harvesting 2nd season rice, though the basal dressed (NH)2SO4-N losses would last two seasons in the rice pot experiment.