Dynamics of soil and applied nitrogen in selected rice-based cropping systems in Northern, Luzon, Philippines.

Results of the study revealed the following: 1) As high as 3.84 t rice grain/ha can be harvested from rice-tomato-corn intensive cropping system under rainfed lowland condition of Ilocos Norte [Philippines] during wet season. Nitrogen uptake ranging from 47 to 82 kg/ha by rice crop due to cropping patterns were recorded in three sites. 2) During the dry season, when the crop was partially irrigated with groundwater, significantly high yield of 20.3 t tomato fruits/ha and 13.6 t sweet pepper fruits/ha can be harvested. Corn and tomato crops can absorb as high as 181 kg and 177 kg N/ha respectively. 3) The surface and subsurface soil layers can accumulate 14 to 15 kg NH4-N/ha each at 15 days after transplanting to maximum tillering in wet season. The clay soil of Paoay had the capacity to accumulate more NH4-N in the surface layer than the other two soils. The total NH4-N accumulation in 0-100 cm soil differed depending upon the cropping patterns. 4) Substantially high nitrate N was present in different soil layers due to drying of soil after 15 to 20 days transplanting and immediately after rice harvest in rainfed lowland rice field. 5) The residual nitrate N from the preceeding dry season crops and after the experiment was monitored in the aerobic phase of the soil. Planting suitable catch crop can utilize this nitrate. 6) During wet season, nitrate N in 0-25 cm layer substantially decreased in all cropping systems at 15 days after rice transplanting to maximum flowering revealing favorable conditions for denitrification due to high WFPS (74 to 104 percent). 7) Leaching of nitrate N from the surface to the subsurface soil layer occurred during maximum tillering up to rice harvest (Oct to Nov) and fallow period in June after continuous rainfall. 8) Dramatic increase of NO3-N in 0-25 cm soil in Jan and Feb was observed. This increase was favored by band application of urea fertilizers, 50 to 70 percent of WFPS for nitrification and soil disturbances during intercultural operations. 9) Quite a large variation of NH4-N was observed in different soil layers during the dry season. Ammonium N was not recorded at all in some soil layers while substantially high amount of this mineral N was recorded in other layers. 10) After one year of rice and non-rice crops, average apparent N loss of 255 kg/ha from MMSU, 153 kg/ha in Magnuang, and 226 kg/ha from Paoay site was observed. As high as 319 kg/ha from rice-sweetpepper-corn at MMSU, 549 kg/ha from rice-sweetpepper at Magnuang, and 324 kg/ha from rice-tomato at Paoay as compared to other cropping systems were recorded. 11) After one year of rice and non-rice crops total soil C slightly increased only in rice-corn-mungbean system in MMSU site, total soil N remained more or less constant and slight fluctuation in C/N was obtained in surface soil.