Interest in the fate of agricultural chemicals in the environment is presently of major concern world wide. The availability of inorganic nitrogen fertilisers (N) has led to their over use and to low N use efficiencies. The loss of N fertilisers not only represents a loss of productivity but a serious threat to environmental health. Therefore the presence substantial quantities of nitrate (NO3) in the lower profile in association with the anion exchange in some north Queensland soils, raises questions of the long term fate of such NO3 accumulations. Unfortunately, knowledge of the mechanisms which control the formation and maintenance of the NO3 accumulations is virtually non-existent.
A series of experiments was performed in order to elucidate a greater understanding of the chemistry of the soil systems with (3 profiles) and without (1 profile) NO3 accumulations. It was found that the NO3 accumulating profiles carried a greater positive charge or anion exchange capacity (AEC), due to the higher proportion of variable charge minerals present. All profiles studied possessed AEC, though no relationship was established between the AEC distribution and the extractable NO3 distribution. Of the other common anions present, chloride (CI) also accumulated in the lower profile, while sulfate (SO4) accumulated in the upper profile. It was concluded that the profile positioning of the different anions was a result of differences in their adsorption mechanisms and exchange selectivity. With the highly selective and strongly adsorbed SO4 accumulating in the upper profile, while weakly adsorbed NO3 and CI are leached below this zone to accumulate in the lower profile.
Examination of selected samples from the four soil profiles showed that they behaved according to variable charge theory, with pH increases giving rise to increased cation exchange capacity (CEC) and lower AEC, while decreases in ionic strength resulted in reductions in both the CEC and AEC. The extent to which the charge density varied differed between profiles and as well as at the different depths studied. As a result of these observations it was theorised that the stability of the NO3 accumulations may depend on the variation in both the soil solution pH and ionic strength. A series of binary competitive anion adsorption experiments was also conducted to determine the effect of anion selectivity on the positioning of the NO3 accumulations. Observations from these experiments indicated a selectivity of SO4 » NO3 ~ CI, and that no changes in the order of selectivity occurred with profile depth.
To relate these static measurements to a more dynamic system, column experiments were conducted on repacked soil profiles, with each segment (0.1 m) of the soil column representing 1 metre of the 11.5 m soil profile. Upon application of an artificial rainwater solution, representing a case of low NO3 addition, the accumulated NO3 was leached from the profiles. Though a chromatographic anion exchange mechanism was originally proposed for the NO3 release, this theory was disproved by the net loss of anionic charge from all profiles. A reduction in the soil solution electrical conductivity (EC) proved critical in assigning a mechanism for the loss of NO3 under rainwater addition. Using the relationship between soil solution ionic strength and EC, and that determined for charge density and ionic strength, a mechanism was formulated whereby equilibration of the soil solution ionic strength with the input low ionic strength rainwater solution resulted in the reduction in positive charge density thus releasing NO3 from the exchange.
The mechanism developed for the release of NO3 was also consistent with the adsorption of NO3 upon addition of N fertiliser, though it did not explain the placement of the accumulations. From the characterisation experiments, SO4 was observed to accumulate in the upper profile, and it was though to restrict the NO3 accumulation to the lower profile. To test this hypothesis, gypsum was applied, resulting in the rapid movement of SO4 though the upper profile, though once the SO4 reached the region of low occupancy, adsorption occurred. Upon a subsequent fertiliser addition, NO3 leached rapidly though the upper profile by-passing the region where SO4 adsorption resulted in low adsorption site availability. Once the NO3 was leached below the SO4 accumulation zone, leaching was retarded by adsorption, thus indicating that the profile distribution of SO4 was controlling the position of the NO3 accumulations. Therefore, accumulation of NO3 is firstly dependent on the presence of substantial AEC, with the distribution of accumulated NO3 dependent on the charge density variation with ionic strength, and secondly on the competitive effects of other anions present in the soil profile.