Nitrous oxide and nitric oxide emissions from lowland rice cultivation with urea deep placement and alternate wetting and drying irrigation.

Urea deep placement (UDP) and the alternate wetting and drying (AWD) irrigation method are two promising rice production technologies. However, studies on the impact of UDP under AWD irrigation on nitrous oxide (N2O) and nitric oxide (NO) emissions are limited. In this study, the effects of UDP with AWD irrigation on these emissions, nitrogen use efficiency (NUE), and rice yields are investigated, compared to conventional broadcast application. N2O and NO emissions from three fertilizer treatments - no nitrogen, UDP, and broadcast application of prilled urea (PU) - were measured. Measurements were taken using an automated gas sampling and analysis system continuously for two consecutive Boro (dry) rice seasons. N2O emission peaks were observed after broadcast application of PU but not after UDP. In contrast, large spikes in N2O emission were observed after UDP, compared to broadcast application, during dry periods. Despite differences in emission peaks, seasonal cumulative N2O emissions from UDP and broadcast treatments were similar. However, NO emissions were minimal and unaffected by UDP or AWD. UDP increased rice yields by 28% and N recovery efficiency by 167%, compared to broadcast urea. This study demonstrates that UDP with AWD irrigation can increase yields and NUE without increasing N2O and NO emissions.

Root-induced solubilization of phosphate in the rhizosphere of lowland rice.

Lowland rice (Oryza sativa L., cv IR74) was grown in cylinders of a P-deficient reduced Ultisol separated into upper and lower cells by a fine nylon mesh so that the roots formed a planar layer above the mesh. This enabled changes in soil P fractions and other root-induced changes in the soil near the root plane to be measured. In both P-fertilized and unfertilized soil, the quantity of readily plant-available P was negligible in comparison with the quantity of P extracted by the plants, and the plants therefore necessarily induced changes in the soil so as to solubilize P. After 6 wk of growth, 90 % of the P taken up was drawn from acid-soluble pools. The remainder was from an alkali-soluble inorganic pool which was on balance depleted, although its concentration profile contained zones of accumulation corresponding to zones of Fe(III) accumulation. There was also a small accumulation of alkali-soluble organic P. There were no changes in the more recalcitrant soil P pools. The zone of P depletion was 4-6 mm wide, increasing with P addition, and coincided with a zone of acidification in which the pH fell from near 6 in the soil bulk to less than 4 near the roots. The acidification was due to H+ generated in oxidation of Fe2+ by root-released O2 , and to H+ released from the roots to balance excess intake of cations over anions. With increasing P deficiency there were increases in the ratio of root: shoot d. wt; the ratio of shoot d. wt to total P in the plant; the excess intake of cations over anions per unit plant d. wt and corresponding release of H+ to the soil; and the quantity of Fe oxidized per unit plant d. wt and corresponding release of H+ to the soil. Independent, in vitro measurements confirmed that acid addition increased the P concentration in the soil solution and the quantity of P that could be desorbed per gram of soil. A mathematical model of the diffusion of acid away from the roots, acid reaction with the soil in solubilizing P, and the diffusion of P back to the absorbing roots showed that, under the conditions of the root-plane experiments, solubilization by acidification accounted for at least 80% of the P taken up in both P-fertilized and unfertilized soil, but that less than 50% of the P solubilized could be taken up by the roots.