Denitrification coupleD to nitrification in the rhizosphere of rice

The N2 flux method, which has only been used for marine sediments, was adapted to a vegetated submerged soil. Denitrification was measured by the emission of N2 from the rice rhizosphere and the bulk soil in flux chambers with a He+O2 (79/21%) atmosphere. Without addition of N-fertilizer, no N2 emission was detected. However, after the addition of urea a high rate of N2 emission was observed. Mean rate was 34.3±3.8 nmol N h−1 cm−2 (±SE). By the application of the nitrification inhibitor methyl fluoride (1%), the N2 emission decreased by nearly 80%, indicating that nitrification of urea-N to nitrate or nitrite was necessary for denitrification. For the localization of this coupled nitrification–denitrification process rice plants were clipped below the water surface. Clipping resulted in a considerable decrease of N2 emission (3.6±0.3 nmol N h−1 cm−2). Measurements of N2O emission gave similar results (0.350±0.035 nmol N h−1 cm−2 for microcosms with intact plants and 0.034±0.3 nmol N h−1 cm−2 for microcosms with clipped plants). These experiments showed that the aerenchymateous rice plants are important for the transport of O2 and N2 into and from the rhizosphere. The rhizosphere is the major site of coupled nitrification–denitrification in planted rice soil.