High rates of ammonium (up to 270 kg-N/ha) have been observed below 1.2 m in a Vertosol soil near Warra in southeastern Queensland. Because nitrogen is one of the most limiting nutrients for cropping, this ammonium would represent a valuable resource if it could be accessed by crops. However, in order to increase the chance of finding other similar areas of ammonium, it is desirable to identify the source of the accumulation. The aims of this study were thus to identify the source of the ammonium at Warra, and subsequently the distribution of subsoil ammonium within SE Queensland.
Preliminary sampling was conducted to a depth of 5 m at the Warra site on adjacent areas of cropping, pasture and native vegetation (Acacia harpohylla), to identify whether subsoil ammonium was an inherent feature of this soil, or had developed after the clearing of native vegetation. This sampling revealed minimal anmionium
concentrations under native vegetation, but significant concentrations below 1.2 m under both cropping and pasture. From this it was concluded that tree clearing was likely to be linked to the elevated anmionium levels. In addition it was proposed that some combination of subsoil acidity, salinity and/or anaerobiosis was inhibiting subsoil nitrification and allowing the ammonium to persist over time.
In order to determine whether the presence/absence of nitrification was in some way linked to the presence/absence of subsoil ammonium, an incubation study was undertaken. This study revealed that nitrification could not be detected over 180 days at Warra, but that nitrification would occur if the soil was inoculated with surface soil microorganisms and the pH increased from 4.5 to 7. Nitrification was also undetectable over 60 days in the subsoil of two other similar sites that did not contain elevated subsoil ammonium. From this data it was concluded that
ammonium has probably been able to accumulate at Warra due to the lack of an active nitrifying population (which are possibly absent/inhibited due to the acidity of the subsoil). However the low nitrification rates at the two other sites examined indicated that the presence/absence of nitrification cannot be used to explain why subsoil ammonium occurs at some sites but not others, and that the answer to this question must lie in the mechanism of ammonium formation.
After the clearing of native vegetation the mineralisation of tree root material, or the mineralisation of leached organic nitrogen, had the potential to contribute to subsoil ammonium concentrations. Incubation experiments were conducted to examine the mineralisation of both in situ material and added organic nitrogen, such as may be present during a leaching event. Results indicated that mineralisation rates in the subsoil were undetectable using the techniques employed, and
that mineralising organisms were limited by subsoil acidity. It was thus concluded that mineralisation was unlikely to have contributed significantly to the subsoil ammonium accumulations.
Nitrate reduction to ammonium was a further pathway that may have been responsible for the ammonium accumulation at Warra. Experiments were conducted to examine the redox status of the subsoil, and to determine whether nitrate ammonification could be observed with the aid of a 15N tracer. Results indicated that the subsoil was not reducing at the time of sampling, and while a small amount of nitrate ammonification was observed during laboratory experiments, this was insufficient to account for all the ammonium present. Consequently, it was concluded that this pathway was also unlikely to be responsible for a large proportion of the ammonium observed at the study site.
Once the Warra site had been cleared and nutrient uptake
from deep within the profile had ceased, the release of fixed ammonium also had the potential to contribute to exchangeable ammonium concentrations. However, quantification of fixed ammonium revealed that only low amounts were present (<30 mg-N/kg). There was also no difference between concentrations on cropping, pasture and native vegetation sites, indicating that significant release of this material into the exchangeable fraction had not occurred. Analysis of clay mineralogy also revealed that the minerals present (smectite, kaolin, quartz, and anatase) had limited ability to fix ammonium.
Leaching of ammonium is not commonly observed in soil environments with appreciable clay contents (such as Warra). However, this pathway was also investigated as a source of ammonium, as leaching would have increased after tree clearing. Quantity/intensity isotherms were used to examine the ammonium exchange capability of the Warra soil, and two other similar soils
that did not contain subsoil ammonium accumulations (given that if leaching is the source of ammonium, there must be some difference in the leaching characteristics of similar sites that do/do not contain subsoil accumulations). The data obtained indicated that Warra was likely to allow less ammonium leaching than the other sites, and that leaching was an unlikely source of the subsoil ammonium.
Although the results of this study failed to identify the source of the subsoil ammonium at Warra, preliminary sampling was conducted throughout SE Queensland to identify whether other areas in the region contained subsoil ammonium accumulations. This sampling revealed that while four of the eight sites examined contained subsoil ammonium, the concentrations observed were much lower than at Warra, and there was no obvious explanation as to why some sites contained ammonium and others did not. The results of this survey did indicate that the ammonium formation process
is one that occurs relatively quickly after the clearing of native vegetation - indicating that organic matter mineralisation may be linked to the ammonium buildup. Consequently, it is possible that future sampling argeting areas of initial high soil fertility may be able to identify further ammonium deposits. However, the sites that have been currently identified as having subsoil ammonium accumulations have fairly low concentrations of ammonium in the rooting depth of viable agricultural species, and have a limited capacity to supply nitrogen for agricultural purposes. Consequently, it is considered that unless future sampling can identify further significant deposits of ammonium, that the investment of resources in the identification of agricultural species to access subsoil ammonium is unlikely to accrue significant economic benefit to the agricultural community.