Nitrogen transformations in a flooded soil in the presence and absence of rice plants: 2. Denitrification

Abstract

Denitrification rates (d) in a flooded alkaline clay were measured following addition of either $^{15} NO_3^ - or^{15} NH_4^ +$ to the floodwater, by collecting evolved N₂ + N₂O in an enclosure in the absence or presence of rice plants. Similar estimates of d were obtained in the $^{15} NO_3^ -$ treatment when the isotopic composition of the enclosed atmosphere was determined using arc redistribution or direct mass spectrometric analysis. Approximately 90% of the gaseous products of denitrification were physically trapped in the soil five days after $^{15} NO_3^ -$ addition. Mechanical shaking of the soil-water system was an effective method for releasing entrapped gas. Denitrification showed a marked diurnal variation in both $^{15} NO_3^ -$ and $^{15} NH_4^ +$ treatments planted to rice, with higher rates during the day than at night. Measured rates of denitrification were higher in planted than in unplanted pots for both $^{15} NO_3^ -$and $^{15} NH_4^ +$ treatments for normal gas sampling. However, evidence was obtained that this was not a real effect, but was due to release of entrapped gas. Denitrification losses corrected for gas entrapment were estimated at <5% of applied $^{15} NH_4^ +$. The ¹⁵N mass balance indicated that a much larger amount of applied ammonium (15–25%) was lost by NH₃ volatilisation. The rate of denitrification corrected for gas entrapment was similar to the rate of nitrification estimated by inhibition of ammonium oxidation. Although the inhibitors 2-ethynylpyridine and acetylene prevented denitrification by effectively inhibiting nitrification of $^{15} NH_4^ +$, the total recovery of ¹⁵N in the soil-plant system did not increase. The total recovery of $^{15} NH_4^ +$ was 7–9% higher in the presence than in the absence of rice.