The Effects of Repeated Soil Wetting and Drying on Lowland Rice Yield with System of Rice Intensification (SRI) Methods

In lowland rice farming, water control is the most important management practice that determines the efficacy of other production inputs such as nutrients, herbicides, pesticides, farm machines, microbial activity, mineralization rate, etc. Poor drainage that keeps soil saturated is detrimental to crops and degrades soil quality. In many rice irrigation systems, drainage mechanisms and practices are dysfunctional or inadequate because farmers believe that rice grows best when water is supplied in abundance. Rice fields are therefore kept continuously flooded and are drained only at time of harvest. This practice is not only wasteful, but also leads to leaching of soluble nutrients, blocks soil microbial activities, and slows down mineralization and nutrient release from the soil complexes. The natural nitrogen supply for plants and microorganisms results principally from the mineralization of organic nitrogen compounds. The water management practices proposed for the System of Rice Intensification (SRI), cycles of repeated wetting and drying, were found to be beneficial to rice plant growth through increased nutrient availability leading ultimately to higher grain yields. The phenomenon of having a large flush of nitrogen mineralization occurring after rewetting of dry soil was first reported by Birch in 1958. This intensive pathway of nitrogen mineralization and nitrogen availability has potential to increase lowland rice yields. In a series of lowland rice experiments conducted in a semiarid region of Africa, repeated wetting and drying increased grain yields by up to three hundred percent in comparison with continuous flooding. Introduction Worldwide, nitrogen is considered the most limiting nutrient for rice production; therefore, increased nitrogen availability should translate into yield increases. However, in spite of extensive research and advances in fertilizer management, rice grown in the lowlands generally utilizes less than 40% of applied N, and often as little as 20-30% (Vlek and Craswell, 1979; Schneiders and Scherer, 1998; Kronzucker et al., 1999). The main N losses occur from leaching and denitrification as well as volatilization of NH3 from the floodwaters after it diffuses from the soil-water interface. Nitrogen, usually found as ammonium in anaerobic lowland soils, occurs more generally as nitrate (NO3) in aerobic upland soils. Ammonium ought to be more beneficial as a source of N because metabolizing NH4 requires less energy than does NO3. It has been thought, however, that at optimum pH, nitrate is equally as effective as ammonia as a source of nitrogen for rice (Tanaka et al., 1984). Recent research has found that, actually, N in nitrate form produces 40 to 70% more yield than an equal amount of N as ammonia, with a combination of NH4 and NO3 leading to better yields than provision of either form of N by itself (Kronzucker et al., 1999). SRI methods as discussed below, by repeatedly wetting and drying the soil, would provide N in both forms.