In a greenhouse experiment potassium nitrate and urea isotopically labeled with 15N were applied to rice with different placements and in planted and unplanted compartments. The objective was to determine whether losses through the nitrification-denitrification sequence are influenced by rhizosphere effects, such as excretion of oxygen or microbial substrates from root surfaces. As expected, substantial losses of potassium nitrate-N were observed, and plant uptake of fertilizer N was very low except with a split-broadcast application. Banded and incorporated applications of urea were more efficient than broadcast. The highest uptake of fertilizer N measured was 50%. Oxidation-reduction potentials stabilized at values 8 to 15 mV lower in planted compartments than in adjacent ones containing no roots. Nitrogen loss data did not support the view that nitrification-denitrification was stimulated in the rhizosphere. The influence of growing plants was to decrease the magnitude of N losses from the system. Losses from broadcast urea application are indicative of some ammonia volatilization, although this was not measured directly.
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