Soil Nitrate as a Source of Nitrogen for Rainfed and Deep-Flooded Rice

Abstract

Deepwater rice (DWR) is grown under rainfed upland conditions for 2-4 months, followed by deep flooding for 1-5 months. Nitrogen is important for the pre-flood growth of the crop, particularly to promote growth and development before the stress of deep flooding. Nitrate accumulated during transition from the dry to the wet season is a ptential source of nitrogen. We measured soil nitrate before and during flooding in the Central Plain of Thailand on two different soil types, a Thio-gypsic Tropaquept and a Sulfic Tropaquept. Soil samples were collected from depth intervals of 0-20, 20-40 and 40-60 cm from plots without N fertilizer, and extracted with 2M KCI for analysis of N by a cadmium reduction technique. Plant samples were dried and analyses for nitrogen content by Kjedahl analysis. Soil nitrate-N levels were less than 15 kg ha-1, and appear to have contributed little to crop N uptake. Nitrogen was lost from the crop at the beginning of flooding, but subsequently uptake of N contimued, apparently from sources other than the pre-flood N transformations in the soil.