Seasonal Effects of Soil Moisture on Soil N Availability, Crop N Status, and Yield of Vegetables in a Tropical, Rice-Based Lowland

Abstract

The influence of seasonal variations in soil moisture on soil nitrogen availability, crop nitrogen status, root length distribution, and yield was studied in four vegetable crops year-round from 1994 to 1995 at the Asian Vegetable Research and Development Center (AVRDC) in tropical southern Taiwan. Treatments included two cultivation systems (traditional flat beds and permanent high beds) and N-fertilization methods (recommended rate and Nmin-method). Nitrifcation of ammonium fertilizer (ammonium sulfate) was investigated in different seasons.

Soil nitrate followed a seasonal pattern of accumulation during the dry season and low contents during the wet season. Alleviation of water stress by high beds appeared as a prerequisite for effective absorption of mineralized soil nitrogen by vegetables in the rainy season. On more food-prone flat beds, N-absorption of vegetables was ineffective in the wet season, and more NO3 was leached below the root zone. Nitrification of ammonium fertilizer was delayed and nitrate disappeared quickly after rainfall.

Soil nitrate accumulated in the dry season when evaporation exceeded precipitation. Soil-organic-matter content was low, and lack of leaching alone could not explain this accumulation. Release of clay-fixed nitrogen may play a significant role. Therefore, fertilizer 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 because nitrate is subject to quick loss with the onset of seasonal rainfall. Overall, stresses caused by excessive soil
moisture in the rainy season and deficient soil moisture in the dry season were apparently more detrimental to vegetable growth than was the limited availability of soil nitrogen. These results are directly applicable to lowland vegetable production in Southeast Asia.