Nitrification in Rice Soils as Affected by Changing the Irrigation Method.

Nitrogen nutrition, which is strongly associated to soil moisture status, is among the several factors contributing to low yield observed in water saving rice cultivation methods such as alternate wetting and drying (AWD) and aerobic cultivation (AC). Nitrification, the conversion of ammonia to nitrate, is a microbiologically mediated soil process that determines the pool size of plant-available inorganic N in unsaturated soils. Ammonia oxidizers (AO) carry out the first step of nitrification. We grew rice in two different irrigation treatments; continuously flooded (CF) and AWD to study the impact of irrigation method on rice – nitrifier interaction. The rice varieties we used were PI312777, Rexmont, and IR80508-B-57-3-B (ApCr). ApCr was grown with three irrigation treatments; CF, AWD and AC. We studied the community composition of AO using terminal restriction fragment length polymorphism and activity of nitrifiers using potential nitrification rate (PNR) in soil at 0-4 cm and 4-8 cm depth at 0 days after seeding (DAS), 50 DAS and 100 DAS. To understand the crop response to available nitrogen form we studied the performance of PI312777, Rexmont, ApCr, IR55423, and PI338046 rice varieties when supply ammonium and nitrate at 0:100, 25:75, 50:50, 75:25 and 100:0 ratios to supplement the nitrogen requirement in a hydroponic medium until 50 DAS.

PNR was highest in the AC treatment and did not vary between CF and AWD. Highest AO activity was observed at 50 DAS across all the irrigation treatments. The community composition of AO bacteria and archaea changed over time but did not differ significantly among rice varieties or irrigation treatments except for AO archaea at 50 DAS in Rexmont grown with CF. All tested rice varieties had higher biomass accumulation when grow in hydroponic medium with both ammonium and nitrate than when only one source is provided. Rexmont behaved differently to varying ammonium:nitrate levels from other varieties. In CF nitrification in the rhizosphere may contribute to high rice yield by facilitating a mixed nitrogen source environment in the root zone. Better understanding the role of AO will help to improve the management of nitrogen fertilizers more efficiently in water saving rice farming which ultimately benefits poor farmers’ pockets.