No abstract

No 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 of Sesbania rostrata, Azolla microphylla and rice straw affected the kinetics of soil solution NH~--N, K § , Fe 2 § , Mn 2 § , Zn 2+ , and P. Sesbania and Azolla increased NH~--N concentration above the control treatment, whereas rice straw depressed it. In all soils Azolla released less NH~--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~--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~-N concentration, reached a peak at 15-40 DF and declined to very low levels ( < 4 mg L-1 ). In the 3 soils of low OC content nitrogen derived from green manure ranged from 34-53% and the apparent recovery 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 § , Fe 2 § , Mn 2+ 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 Fe 2 § and Mn 2 § but increased K + and P. In general, rice straw, Sesbania and Azolla decreased Zn 2+ concentration in all soils.

No abstract