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Nitrate leaching in a black earth after twenty-two years of conventional and zero tillage

J.E.Turpin, J.P. Thompson, S.A. Warin' and J. MacKenzie

Queensland Wheat Research Institute, Toowoomba QLD 4350
University of Queensland, St. Lucia QLD 4072

Under the fallow management trial at QDPI Hermitage Research Station (Warwick), zero tillage with stubble retention has been shown to have less nitrate within its top 150 cm prior to planting and therefore requires greater fertiliser inputs to maintain yield. Evidence of nitrate movement to at least 200 cm, particularly with zero tillage with stubble retention, prompted a deep soil sampling study to quantify leaching of nitrate beyond 200 cm.


Following the 1989 harvest two soil cores per plot were taken with a hydraulically driven rig to a depth of 5.4 m. Three strikes of the same 180 cm tube were used to gain each successive depth; 0-180 cm, 180-360 cm, 360-540 cm, with 180 cm steel pipe extentions of 25 mm diameter bolted between the anvil and sampling tube.

Results and discussion

Although the trial design includes four different fallow treatments, only the two most commonly used on-farm will be examined here, those being zero and conventional tillage both with stubble retention. Examination of nitrate profiles indicates significant differences in distribution patterns between treatments (Fig. 1). Above 150 cm (the approximate rooting depth) differences are not significant whereas below 150 cm zero tillage treatments accumulate more nitrate than conventional tillage.

The nitrate leached depends on the amount of nitrate available and the water flux. Here the zero tillage-stubble retained treatments have a greater quantity of available nitrate in the profile, suggesting that there may be differences in plant uptake and tie-up in organic pools between the treatments. Further work on nitrate availability is being collated in the form of a nitrogen balance sheet. Ignoring the availbility issues, the data indicates that water flux is greatest with zero tillage-stubble retained. Evidence for this is currently being obtained from data on movement of chloride and bromide.

For these two treatments it is clear that zero tillage will leach a greater quantity of soil nitrate due to the combination of nitrate availability and water flux. Thus leaching of nitrate is shown to be a major cause of the need for increased fertiliser nitrogen input under zero tillage.

Figure 1. Nitrate profile for conventional and zero tillage with stubble retention.

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