The relationship between denitrification and Walkley-Black organic carbon (WBOC) and water soluble organic carbon (WSOC) was investigated for 21 soils using both field moist (<1 cm) and air-dried (<2 mm) surface samples (0-10 am). The soils used were mostly cracking clays and were selected to give a wide range in carbon values.
Denitrification was taken as the disappearance of nitrate-nitrogen (NO3-N) during waterlogged incubation at 30° C for periods of 1 to 8 days. Excess NO3-N substrate was provided by adding 400 ug g-1 NO3-N as KNO3. WBOC levels ranged from 0.68 to 3.85% and initial WSOC from 36 to 166 ug g-1 soil in the <2 mm samples and 25 to 110 in the <1 am samples. Factors affecting the levels of WSOC in soils were examined including age of cultivation, cropping history, and physical sample preparation treatment.
In all soils, the total NO3-N loss increased with time of incubation but the rate of denitrification declined with time of incubation. Under the experimental conditions, denitrification was predominantly described by first order kinetics.
Denitrification was significantly correlated with WBOC with higher correlation coefficients for <1 cm soil than for <2 mm soil e.g. r = 0.68*** after 8 days for <2 mm soil and 0.85*** for <cm soil. There was a highly significant correlation between denitrification and WSOC with r values mostly
above 0.90*** and with little difference between <1 cm and <2 mm soils. The correlations of denitrification rates with WSOC were highly significant and highest during the first 24 hour, with lower values for subsequent periods. Correlations with WSOC and equations relating denitrification to WSOC were similar for all soils or for cracking clays only.
Air drying and subsequent grinding of field moist <1 cm soils to <2 mm resulted in increase of mean WSOC levels, from 47.0 to 73.0 ug g-1 soil, and further grinding to <0.17 mm resulted in a much greater increase to 132.7 ug
g-1 soil. These increases were reflected in higher denitrification capacities, for example after 4 days from mean values of 54.8 for <1 cm soil, to 82.2 and 201.7 ug NO3-N g-1 soil for <2 mm and <0.17 mm soils respectively.
For all soils there was a rapid decline in WSOC levels with time of incubation soils with low initial values being completely exhausted and those with high values reaching a very low figure
after 8 days incubation. The WSOC disappearance trends during waterlogged incubation tend to parallel those for denitrification in periods up to 4 days except for a grey clay, indicating that the WSOC is probably the major source of oxidisable soil carbon for denitrification during short periods of waterlogging but for the 4-8 day period it is a negligible to a minor source.
The results obtained thus show the importance of supply of organic carbon in controlling denitrification following waterlogging under conditions of adequate NO3-N substrate and that analysis of soil for WSOC provides a good index of their capacity
for denitrification of nitrate under these conditions.