NITRATE AND CHLORIDE LEACHING IN VERTOSOLS FOR DIFFERENT TILLAGE AND STUBBLE PRACTICES IN FALLOW-GRAIN CROPPING

Research conducted in the mid 1980s on a 'long-term fallow management trial', located on a black Vertosol at the Hermitage Research Station, indicated that leaching may have been the cause of low concentrations of nitrate-N within the root-zone of zero-tillage stubble-retained tr eatments. The 'fallow management trial' has 12 management treatments: a factorial combination of zero or conventional tillage x stubble rete ntion or burning x 3 nitrogen fertiliser rates (0, 23, and 69 kg N/ha) . To test the leaching hypothesis, all trial treatments were analysed for nitrate and chloride concentrations to a depth of 5.4 m in order t o assess the relative rates of drainage, solute movement, and nitrate leaching between treatments. Similar analyses were conducted on 2 cult ivated sites and 2 permanently grassed sites on-farm, also on black Ve rtosols, to compare solute movement rates under the continuous winter cereal rotation (trial site) with a winter-summer cropping regime and permanent pasture. Results from the Hermitage trial site showed zero t illage with stubble retention had a chloride concentration peak 2 m de eper down the profile (4.5 m) than all other management treatments, in dicating that drainage rates were greatest in zero tillage-stubble ret ained treatments. Nitrate profiles, however, showed that movement of n itrate-N to below the root-zone was greatest under zero tillage with s tubble burning with 69 kg N/ha applied (Z-B 69N), followed by zero til lage with stubble retention and 69 kg N/ha. The large nitrate loss fro m the root-zone of Z-B 69N (about 30% of applied fertiliser) was consi dered to be a result of high concentrations of nitrate-N in the top 1. 5 m associated with stubble burning and fertilisation. The on-farm cul tivated sites had very little nitrate-N throughout the whole profile, suggesting that either the use of summer as well as winter crops reduc ed residual or 'spared' nitrate-N (through control of root-lesion nema todes) and/or mineralisation rates were lower on these sites.