Nitrogen cycling in rice-based cropping systems

Results at the IRRI [International Rice Research Inst., Los Banos, Laguna, Philippines] showed higher biomass, total N and grain-N uptake of dry season rice when straw incorporated at all N levels, particularly at 90 kg N/ha. The NO3 raised to the negative power-N was higher than NH4 raised to the positive power-N in all soil depths after rice harvest. The same results were noted in the corn crop. The NH4 raised to the positive power-N in 15-cm soil depth was higher during the earlier stage of corn and declined after harvest. The N and straw treatments had no effect in grain and biomass of soybean. The NO3 raised to the negative power-N in soil increased with depth and with each growth stage until after harvest. The NH4 raised to the positive power-N correspondingly decreased with depth. The results of wet season rice indicate that the total biomass was significantly higher in soybean-rice than the other cropping patterns. The NH4 raised to the positive power-N was higher at maximum tillering stage. However, the NO3 raised to the negative power-N increased after rice harvest due to nitrification process in the soil. In the MMSU [Mariano Marcos State Univ., Batac, Ilocos Norte, Philippines] experiments the rice grain yield, biomass and grain N uptake of dry season rice grain yield, biomass and grain N uptake of dry season rice were significantly higher with increasing soil depth particularly at 15-30 cm during maximum tillering stage of rice. The NH4 raised to the positive power-N also increased with addition of N and straw incorporation. On wheat, however, the N and straw treatments did not affect grain yield, biomass and N uptake although NH4 raised to the positive power-N increased. The NO3 raised to the negative power-N in soil increased after wheat harvest. Similarly, grain yield and N uptake in soybean were not affected by the treatments. Nevertheless, the soil NO3 raised to the negative power-N at 15-30 cm depth was higher at the vegetative stage of soybean than after harvest although NH4 raised to the positive power-N was higher, indicating delayed nitrification. The NH4 raised to the positive power-N increased with N and straw addition in the 5-15 cm and 15-30 cm depths. In the wet season rice crop there was higher rice grain yield, N uptake and biomass with N addition and incorporation of soybean stubble. The NO3 raised to the negative power-N and NH4 raised to the positive power-N the soil were higher in the soybean-rice than in other cropping patterns. Higher NH4 raised to the positive power-N was observed at maximum stage than that after rice harvest