DENITRIFICATION IN A CLAY SOIL UNDER PASTURE AND ANNUAL CROP - ESTIMATION OF POTENTIAL LOSSES USING INTACT SOIL CORES

Biological denitrification was found to occur at depth, in the field, beneath a cultivated crop (black gram-Vigna mungo) and a permanent pasture (green panic-Panicum maximum var trichoglume). A laboratory incubation study was conducted on intact soil cores obtained at depths of 5-10, 20-25, 50-55 and 110-115 cm from beneath both plots to establish whether the potential for denitrification could be ascertained in the laboratory. All cores received an addition of N-15-labelled nitrate equivalent to 100 kg N ha-1. Half the cores received an addition of glucose equivalent to 100 mug C g-1 soil. The cores were maintained at 90% water-filled pore space (WFPS) and 25-degrees-C. Average N2O production from the 5 to 10 cm cores, that received NO3- + glucose, ranged from 78.5 to 106.5 mug core-1 at 3 days, after which there was a decrease due to the conversion of N2O to N2. For the 110-115 cm depth under black gram, the average production rate increased from 20.7 mug core-1 at 3 days to 188.5 mug core-1 after 7 days. The principal nitrogen gas being produced was N-15-N2 from both treatments and at both 3 and 7 days and accounted for between 79 and 100% of the total N gas produced. Production of N-15-N2O from the 110 to 115 cm depth beneath black gram was similar to that found at 50-55 cm in a field study at this same site. The total recovery of added N-15-NO3- in soil, water and headspace varied between 47.3 and 94.2% for the soil cores from beneath black gram and 53.8 and 89.3% for the soil cores from under green panic. In the case of the higher recoveries, a large proportion of the N2O evolved was found to be dissolved in the soil water. The potential for biological denitrification to occur in these soils in the field can be established by incubating intact soil cores in the laboratory. However, the denitrification rate is limited by the shortage of C beneath the black gram and both C and NO3- beneath the green panic plot.