Denitrification coupled to nitrification in the rhizosphere of rice

The N-2 flux method, which has only been used for marine sediments, was adapted to a vegetated submerged soil. Denitrification was measured by the emission of N-2 from the rice rhizosphere and the bulk soil in flux chambers with a He + O-2 (79/21%) atmosphere. Without addition of N-fertilizer, no N-2 emission was detected. However, after the addition of urea a high rate of N-2 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 N-2 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 N-2 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 O-2 and N-2 into and from the rhizosphere. The rhizosphere is the major site of coupled nitrification-denitrification in planted rice soil. (C) 1998 Elsevier Science Ltd. All rights reserved.