Water-Saving Rice Farming Affects Nitrifiers In Soil

Abstract

Nitrification is a microbially mediated process wherein bacterial and archaeal nitrifiers oxidize ammonium (NH4+) to nitrite (NO2-) and then to nitrate (NO3-). The activity of nitrifiers largely determines the pool size of plant-available inorganic nitrogen (N) in soils. Nitrifier activity in the rhizosphere is affected by the nature of root exudates, competition with other soil organisms, oxygen availability and plant demand for NH4+-N in soil. Nitrification in flooded paddy rice soils has been studied extensively; however, our understanding of the dynamics of nitrifiers in water-saving rice farming systems is limited. I studied the effects of three water management regimes (continuously flooded - CF; alternating wetting and drying - AWD; and, aerobic cultivation - AC), on nitrifier populations inhabiting the rice rhizosphere. A series of greenhouse experiments was conducted to study plant response to water management and the effect of water management regime on soil nitrifiers at different plant growth stages using five rice varieties. The rhizosphere oxidizing power, the preferred N form (NH4+and/or NO3-) of each rice variety, and the effects of rootderived secondary metabolites on nitrifiers were also studied. The five rice varieties differed significantly in these response variables. The biomass of all five rice varieties was higher when N was supplied in a mixed form, than when N was supplied as either NH4+ or NO3- alone. The physiological N use efficiency (PNUE) of plants was significantly affected by rice variety and the ratio of NH4+ to NO3- in the growth medium. At 0.50 mg ml-1 concentration, root exudates enhanced the activity of Nitrosomonas europaea, an ammonia oxidizing bacterium, but a water extract from crushed rice roots inhibited its activity. Continuous exposure to plant roots for 14 days reduced the nitrifiers' potential activity in soil by 50%. Biomass production was lower and rooting depth and rhizosphere nitrification were higher under AC as compared to CF. Rice variety and water management practice interactively affected the activity and composition of nitrifier communities in the rice soils, which had follow-on effects on biomass production. Understanding these interactions is necessary to help practitioners manage N more effectively in relation to cultivars used and water management strategies adopted.