Nitrate leaching to a depth of 300 cm was investigated under field conditions in a krasnozem at Redland Bay, Queensland. Ammonium nitrate fertilizer was applied at 165 kg N ha-1 in bands 76 cm apart. Plots were either kept fallow or cropped using a horticultural rotation of green beans (Phaseotus vulgaris) followed by an oats (Avena sativa) cover crop. Soil samples were collected at regular intervals away from the band. Nitrate and water movement were related by estimating the leaching fraction from the water balance of the fallow plots.
Of the nitrate initial!/ in the soil, 63% (224 kg N ha-1) remained between the depths of 40 and 120 cm following the bean crop. The oats cover crop grown to the soft dough stage reduced nitrate levels in the soil to a depth of 160 cm to < 2 mg N03-N kg-1 (equivalent to 44 kg N ha-1).
In the bare plots, the nitrate concentration peak was leached to a depth uf 190 cm within 17 months. The nitrate movement per 10 mm of drainage water was 1.9 and 0.4-0.5 cm through the 15-60 and 90-190 cm depth intervals respectively. In each case, movement was less than that estimated by piston flow. Lateral movement of nitrate was gradual. Vertical movement became concentration dependent below a depth of 100 cm. Prior to nitrate analysis, it was necessary to extract soil with a sulphate solution to recover nitrate quantitatively. These results are discussed in relation to the soil volumetric moisture content, direction of water flow, soil structural properties and the adsorption of nitrate by soil colloids.
In the field experiment, approximately equal proportions of leached nitrate were derived from fertilizer and mineralization of soil organic matter. The method utilizing variations in the natural abundance of 15N (Kohl et al., 1971) was used to estimate the relative contribution of the two sources but was found unsatisfactory. The 15N enrichment of mineralized nitrate was little different from that of nitrogen in air. The variability in 15N enrichment of mineralized nitrate between sites was large and the 15N enrichment of nitrate produced in the field could not be estimated quantitatively by laboratory incubations.
Laboratory studies showed that nitrate exclusion occurred in the soil from depths of 0-20 cm. Below 20 cm, nitrate adsorption occurred increasing from 0.16 at 40-90 cm to 0.31 and 0.45 me/100 g in the depth intervals, 120-300 and 360-600 cm respectively when equilibrated with 0.005 M KNO3 . Correlation studies suggested that these changes were related to decreasing organic matter and pH and increasing kaolinite content with depth. The role of sesquioxide fractions could not be assessed using a number of extractants. Adsorption isotherms were determined for soil from the 0-15, 40-90 and 160-300 cm depths. At nitrate concentrations occurring in field soil, 21 to 24% and 29 to 45% of added nitrate was adsorbed by soil from the 40-90 and 160-300 cm intervals respectively. At the highest concentration used (0.5 M KN03), both subsoils adsorbed 1.9 me/100g.
Packed columns of < 2 mm soil from depths 0-15, 40-90 and 160-300 cm were used to investigate the influence of adsorption on nitrate movement with water. Nitrate was added to produce three solution concentrations averaging 20, 90 and 900 mg N03-N ℓ-1 at the peaks. Water and 0.2 M K2SO4 solution were used as leaching agents. In the soil from 0-15 cm, nitrate moved at the same rate as water except at the low concentration where exclusion caused faster nitrate movement. There was no difference between the two leaching solutions. With the two subsoils, the depth of leaching increased with nitrate concentration and the distribution became skewed with the greatest effects being in the deepest soil. In the treatments where sulphate was used, these effects were prevented and nitrate moved at the same rate as the leachate.
Results from the laboratory and column experiments are discussed in relation to the data from the field experiment. It was concluded that adsorption was an important mechanism restricting the rate of nitrate leaching with water through the krasnozem soil.