SEASONAL EFFECTS OF SOIL-MOISTURE ON SOIL N AVAILABILITY, CROP N STATUS, AND YIELD OF VEGETABLES IN A TROPICAL, RICE-BASED LOWLAND

The influence of seasonal variations in soil moisture on soil nitrogen availability, crop nitrogen status, root length distribution, and yie ld was studied in four vegetable crops year-round from 1994 to 1995 at the Asian Vegetable Research and Development Center (AVRDC) in tropic al southern Taiwan. Treatments included two cultivation systems (tradi tional flat beds and permanent high beds) and N-fertilization methods (recommended rate and N-min-method). Nitrification of ammonium fertili zer (ammonium sulfate) was investigated in different seasons. Soil nit rate followed a seasonal pattern of accumulation during the dry season and low contents during the wet season. Alleviation of water stress b y high beds appeared as a prerequisite for effective absorption of min eralized soil nitrogen by vegetables in the rainy season. On more floo d-prone flat beds, N-absorption of vegetables was ineffective in the w et season, and more NO3 was leached below the root zone. Nitrification of ammonium fertilizer was delayed and nitrate disappeared quickly af ter rainfall. Soil nitrate accumulated in the dry season when evaporat ion exceeded precipitation. Soil-organic-matter content was low, and l ack of leaching alone could Plot explain this accumulation. Release of clay-fixed nitrogen may play a significant role. Therefore, fertilize r rates can be reduced in this season. Vegetables with a high capacity to absorb nitrogen should be grown at the end of the dry season becau se nitrate is subject to quick loss with the onset of seasonal rainfal l. Overall, stresses caused by excessive soil moisture in the rainy se ason and deficient soil moisture in the dry season were apparently mor e detrimental to vegetable growth than was the limited availability of soil nitrogen. These results are directly applicable to lowland veget able production in Southeast Asia.