QUANTIFICATION OF DENITRIFICATION IN FLOODED SOILS AS AFFECTED BY RICE ESTABLISHMENT METHOD

The recovery of N-15-labeled N2 and N2O evolved from added N-15-labeled fertilizers can reportedly underestimate N loss by denitrification in puddled, flooded soils. Field research was conducted for 2 yr in the Philippines to determine whether the evolution of N gases formed by denitrification is influenced by a rice (Oryza sativa L.) establishment method. Quantification of denitrification was assessed by comparing the cumulative recovery of (N2+N2O)-N-15 evolved from added ''N-labeled nitrate (5 kg N ha-1) with the total N loss by denitrification, which was estimated from the added NO3--N-15 not recovered in the N-15 balance at the conclusion of the 20 day gas collection period. In both years, rice was established by either transplanting (TPR) at 20 x 20 cm spacing between hills with three seedlings per hill (75 plants m-2) or wet broadcast seeding of germinated seed (BSR) with a plant population of 370 m-2 in 1989 and 250 m-2 in 1990. In 1990, wet seeding of germinated seed in 20-cm-wide rows (RSR) with 250 plants m-2 was also included. Nitrate addition and gas collection commenced at 11 days after transplanting (DT) 20 or 2 1 -days-old seedlings and 21 days after wet sowing (DS). In 1989, the recovery of added N as evolved (N2 + N2O)--N-15 (54% for BSR and 47% for TPR) was comparable to total denitrification loss for BSR (53%) but not for TPR (61 %). In 1990, the recovery of evolved (N2 + N2O)-N-15 (32% for TPR, 61% for BSR and 40% for RSR) underestimated denitrification loss (72% for TPR and 73% for BSR and RSR), but to a lesser extent for BSR than for TPR and RSR. The greater recovery of added N as evolved (N2 + N2O)-N-15 when chambers were placed over transplated rice rather than between plants (47 and 40%, respectively) suggested that rice was a conduit for transport of gas from soil. Underestimation of denitrification was attributed to entrapment of N-gases in soil. Results suggest that young BSR, because of high plant density and rapid extension of roots into the soil volume, was a more effective conduit for gas transport than TPR or RSR.