Effect of Sesbania, Azolla and rice straw incorporation on the kinetics of NH4, K, Fe, Mn, Zn and P in some flooded rice soils

Abstract

In a greenhouse study, with and without rice plants, of five flooded Philippine rice soils whose organic C (OC) content varied from 0.5 to 3.6%, incorporation ofSesbania rostrata, Azolla microphylla and rice straw affected the kinetics of soil solution NH 4 + −N, K+, Fe2+, Mn2+, Zn2+, and P. Sesbania and Azolla increased NH 4 + −N concentration above the control treatment, whereas rice straw depressed it. In all soils Azolla released less NH 4 + −N than Sesbania. The apparent net N release depended on the soil and ranged from 44–81% for Sesbania and 27–52% for Azolla. These effects persisted throughout the growth of IR36. Soil solution and exchangeable NH 4 + −N increased initially but levelled off between 30 to 80 days and between 20 to 40 days after flooding (DF), respectively. With rice, soil solution NH 4 + −N concentration, reached a peak at 15–40 DF and declined to very low levels (<4mg L−1). In the 3 soils of low OC content nitrogen derived from green manure ranged from 34–53% and the apparent revovery of added green manure N varied from 29–67%. Almost all N released from both Azolla and Sesbania were recovered in the rice plant in all soils except Concepcion with only 77%. The concentration of K+, Fe2+, Mn2+ and P in the soil solution were higher with rice straw than Sesbania and Azolla in all soils except Hanggan which showed no change in Fe2+ and Mn2+ but increased K+ and P. In general, rice straw, Sesbania and Azolla decreased Zn2+ concentration in all soils.