Assessing soil formation and erosion balances in the top end with ultra-sensitive isotope tracing
Creators: name: Lal, Rajeev Ravikash
Publisher: name: The Australian National University
Year: 2015

description: The traditional Australian agricultural areas in the south are predicted to become drier, and the frequency of extreme events is expected to increase as a result of climate change. In addition food production is also likely to come under increasing pressure because of population growth. To meet the demands of increasing food production it is likely that the largely unexploited river basins of northern Australia (Top End) will be targeted for agricultural development. However because of the highly erodible nature of the soils in these catchments if the agricultural development is to be managed sustainably, then the rates of soil loss and soil formation from the underlying bedrock must be comparable. There are, however, very few measurements of soil erosion and formation rates in northern Australia. In this study the technique of accelerator mass spectroscopy (AMS) has been employed to determine soil erosion and soil formation rates using the analysis of plutonium-239 (Pu-239) and beryllium-10 (Be-10) tracers in soil cores, sediments and surface rocks from the Daly River Basin (a region identified as having large areas of potentially arable soils) in the wet-dry tropics of northern Australia. 239Pu, released in the 1950s and 1960s by atmospheric nuclear weapons tests, was used to obtain a quantitative assessment of recent rates of soil loss. Soil cores 40-50 cm deep were collected from fields broadly classified into forest, grazing and cultivated areas. Cores taken from undisturbed and unburnt areas in open eucalypt woodland forest were used as reference sites. The soil loss rates have been established by comparing the excess or deficiency of the Pu-239 tracer over that of the reference sites. Owing to agricultural practices, modern soil loss rates can be significantly higher than long-term soil formation rates, but establishing soil formation rates has proved to be a difficult problem. At long-term equilibrium, however, soil formation from the underlying rock is balanced by soil loss from the surface. In this study two independent methods were applied for determining soil production rates on the undulating plains of the central Daly River catchment. First, rock erosion rates were determined from measurements of in situ produced cosmogenic Be-10 concentrations in quartz extracted from surface rocks. Second, long term erosion rates were determined from measurements of meteoric Be-10 inventories in soil cores. Results from both methods agree well thus providing evidence that this landscape is in morphological equilibrium over geological timescales and are comparable to rates measured in other parts of monsoonal Northern Australia. The long term rock and soil erosion rates were used as a proxy for soil production rates at steady state. The modern soil loss rates (average: 560 m/Ma or 56 mm per century) resulting as a consequence of current agricultural practices were found to be up to two orders of magnitude higher than these equilibrium soil production rates (average: 10.2 m/Ma or 1 mm per century). This rate of soil loss is clearly unsustainable and therefore soil conservation strategies have to be prioritized in the future development of these catchments.