Nitrate and chloride leaching in Vertosols for different tillage and stubble practices in fallow–grain cropping

Abstract

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 treatments. The ‘fallow management trial’ has 12 management treatments: a factorial combination of zero or conventional tillage×stubble retention or burning×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 to assess the relative rates of drainage, solute movement, and nitrate leaching between treatments. Similar analyses were conducted on 2 cultivated sites and 2 permanently grassed sites on-farm, also on black Vertosols, 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 tillage with stubble retention had a chloride concentration peak 2 m deeper down the profile (4·5 m) than all other management treatments, indicating that drainage rates were greatest in zero tillage–stubble retained treatments. Nitrate profiles, however, showed that movement of nitrate-N to below the root-zone was greatest under zero tillage with stubble burning with 69 kg N/ha applied (Z-B 69N), followed by zero tillage with stubble retention and 69 kg N/ha. The large nitrate loss from the root-zone of Z-B 69N (about 30% of applied fertiliser) was considered 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 cultivated sites had very little nitrate-N throughout the whole profile, suggesting that either the use of summer as well as winter crops reduced residual or ‘spared’ nitrate-N (through control of root-lesion nematodes) and/or mineralisation rates were lower on these sites.