Enhanced accumulation of mineral-N following canola

The accumulation of mineral-nitrogen (N) in the top 10 cm of soil during th e summer fallow was measured in 2 replicated field experiments following a range of crops including wheat, oats, canola, peas and lupins. At the first site, mineral-N was measured following harvest and in autumn before sowing subsequent crops across 3 seasons (1994-96). Crop residues were retained o n the surface with intermittent grazing by sheep throughout the summer fall ow and burnt before the autumn measurements. The smallest increase in miner al-N accumulation occurred following the cereals in all 3 seasons (mean inc rease 31 kg/ha). The highest accumulation of mineral-N in all seasons occur red following canola (mean 94 kg/ha), 3 times as much as that following cer eals, and significantly higher than that after the legumes in 2 of the 3 se asons (mean 50 kg/ha). Differences in the amount, N content, or C : N ratio of the surface-retained crop residues are unlikely explanations for the ob served differences in mineral-N accumulation. At a second site, measurement s of the accumulation of mineral-N following canola and wheat were accompan ied by measurements of populations of selected microorganisms involved with N cycling in soil. More mineral-N accumulated after canola than after whea t, however, populations of free-living, N-fixing bacteria, potential Azospi rillim species and NH4+ oxidising bacteria were significantly lower followi ng canola than following wheat, and populations of total bacteria and NO2- oxidising bacteria did not differ. These results suggest that greater miner al-N accumulation following canola does not result from a shift in those mi crobial populations which favour mineral-N accumulation, however, more deta iled studies are required to resolve the exact cause of the differences. A possible explanation is that biocidal compounds released by canola roots du ring decay may cause a general 'biofumigation' and thereby result in a flus h of mineral-N similar to that which accompanies chemical fumigation.