NITROGEN TRANSFORMATIONS IN A FLOODED SOIL IN THE PRESENCE AND ABSENCE OF RICE PLANTS - 2 - DENITRIFICATION

Denitrification rates (d) in a flooded alkaline clay were measured fol lowing addition of either (NO3-)-N-15 or (NH4+)-N-15 to the floodwater , by collecting evolved N-2 + N2O in an enclosure in the absence or pr esence of rice plants. Similar estimates of d were obtained in the (NO 3-)-N-15 treatment when the isotopic composition of the enclosed atmos phere was determined using are redistribution or direct mass spectrome tric analysis. Approximately 90% of the gaseous products of denitrific ation were physically trapped in the soil five days after (NO3-)-N-15 addition. Mechanical shaking of the soil-water system was an effective method for releasing entrapped gas. Denitrification showed a marked d iurnal variation in both (NO3-)-N-15 and (NH4+)-N-15 treatments plante d to rice, with higher rates during the day than at night. Measured ra tes of denitrification were higher in planted than in unplanted pots f or both (NO3-)-N-15 and (NH4+)-N-15 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 (NH4+)-N-15. The N -15 mass balance indicated that a much larger amount of applied ammoni um (15-25%) was lost by NH3 volatilisation. The rate of denitrificatio n corrected for gas entrapment was similar to the rate of nitrificatio n estimated by inhibition of ammonium oxidation. Although the inhibito rs 2-ethynylpyridine and acetylene prevented denitrification by effect ively inhibiting nitrification of' (NH4+)-N-15, the total recovery of N-15 in the soil-plant system did not increase. The total recovery of (NH4+)-N-15 was 7-9% higher in the presence than in the absence of ric e.