A fractionation scheme that provided the measurement of a labile pool (particulate organic carbon), a charcoal-carbon pool, and a humic pool by difference was tested as a means of initialising the Rothamsted organic carbon turnover model version 26.3. Equating these 3 fractions with the resistant plant material, inert organic matter, and humic pools of the model, respectively, gave good agreement between measured and modelled data for 2 long-term rotation trials in Australia using a soil depth of 30 cm. At one location, Brigalow Research Station in Queensland, there were 3 distinct soil types, two clays and a duplex soil, in a semi-arid, subtropical climate. At this site, continuous wheat with some sorghum was established after clearing land under brigalow (Acacia harpophylla) and continued for 18 years. The second location was near Tarlee, South Australia, and was established on existing agricultural land. One soil type (red brown earth) with 2 rotations (continuous wheat and wheat–fallow) were available over a period of 8 years.The modelled and measured data were in good agreement for both locations but the level of agreement was substantially improved when the resistant plant material decomposition rate was reduced from 0.3 to 0.15/year. No other modifications were required and the resulting values provided excellent agreement between the modelled and measured data not only for the total soil organic carbon but also for the individual pools. Using this fractionation scheme therefore provides an excellent means of initialising and testing the Rothamsted model, not only in Australia, but also in countries with similar soil types and climate.For the first time, the work reported here demonstrates a methodology linking measured soil carbon pools with a conceptual soil carbon turnover model. This approach has the advantage of allowing the model to be initialised at any point in the landscape without the necessity for historical data or for using the model itself to generate an initial equilibrium pool structure. The correct prediction of the changing total soil organic carbon levels, as well as the pool structure over time, acts as an internal verification and gives confidence that the model is performing as intended.

The Brigalow Catchment Study (BCS) was established to determine the impact on hydrology when brigalow land is cleared for cropping and grazing. The paired catchment study was commenced in 1965 using catchments of approximately 15 ha, with natural vegetation dominated by brigalow scrub (Acacia harpophylla). Three contiguous catchments were selected near Theodore in central Queensland to represent the extensive brigalow bioregion of central and southern Queensland and northern New South Wales (~40 Mha). The hydrology of the 3 catchments was characterised during a 17-year calibration period (1965–81). The catchments were considered hydrologically similar, with sufficient data available for an empirical comparison between catchments. In 1982, two of the catchments were cleared, with one developed for cropping and the other sown to improved pasture. The third catchment was used as an uncleared control. Hydrologic characteristics were then compared for the following 21 years. In their virgin state, the catchments behaved similarly, with average annual runoff being 5% of annual rainfall. Once cleared, total runoff from the cropping catchment increased to 11% of annual rainfall and total runoff from the pasture catchment increased to 9% of annual rainfall; however, timing of the individual runoff events varied between land uses. In order to confirm that changes in hydrology were a function of land use and not just seasonal variability or sampling error, several analytic techniques were used: a simple comparison of runoff totals, comparison of events, comparison of probability of exceedance for daily runoff, and comparison of predicted and observed runoff using a water balance modelling approach.

Abstract. This paper describes a long-term, paired-catchment study, its broad findings, and considerations for future resource management of brigalow lands in north-eastern Australia. The Brigalow Catchment Study (BCS) commenced in 1965 with a pre-clearing calibration phase of 17 years to define the hydrology of 3 adjoining catchments (12-17 ha). After 2 catchments were cleared in 1982, 3 land uses (brigalow forest Acacia harpophylla, cropping, and grazed pasture) were monitored for water balance, resource condition and productivity, providing information for scientific understanding and resource management of the major land uses of the brigalow bioregion.In addition, this paper draws upon several project reviews to highlight the value of the BCS as an 'outdoor laboratory', its data resource, and to reflect on the study's scientific rigor to support present and future value. An assessment of the BCS against national and international attributes of best practice for long-term studies showed the study to rate highly in aspects of design, implementation, monitoring, and data management, and moderately in formal publication, strategic management, and networking. The literature shows that Brigalow Catchment Study is the longest paired-catchment study in Australia, and continues to sample the interactions between climate, soils, water, land use, and management.Finally, this paper provides the context for component-specific papers on changes in hydrology, productivity, and salt balance. Results from the study to date include: a doubling of runoff after clearing, a reduction in wheat yield by more than 60% over 20 years, a halving of pasture availability 3 years after clearing, a decline in cattle liveweight gain of 4 kg/ha.year over an 8-year period with a constant stocking rate, and the leaching of 60% of the root-zone (0-1.5 m) chloride after clearing for cropping. Unanticipated applications of the data from the study include: (i) a crucial set of soil samples for calibration of the RothC soil carbon model used to estimate Australia's soil carbon emissions; and (ii) estimates of deep drainage as a basis for salinity risk assessment in the region.

Productivity of grain crops and grazed pastures inevitably declines without soil nutrient replacement and may eventually make these enterprises unprofitable. We monitored these declines in north-eastern Australia during 23 years after clearing 2 of 3 adjacent brigalow catchments, in order to define the productivity levels of developed brigalow land over time. One catchment (11.7 ha) was used for grain production and another (12.7 ha) for beef production from a sown buffel grass pasture. There was no upward or downward trend in annual rainfall amounts throughout the study period. In the cropped catchment, grain yield from 14 winter crops without added nutrients declined significantly in 20 years from 2.9 to 1.1 t/ha.year on the upper-slope clay soil (92 kg/ha.year) and from 2.4 to 0.6 t/ha.year on the Sodosol (88 kg/ha.year). Crop production per year declined by 20% between 2 successive 10-year periods. Wheat grain protein content also declined with time, falling below the critical value for adequate soil N supply (11.5%) 12 years after clearing on the Sodosol and 16 years after clearing on the clay soil. Such declines in grain quantity and quality without applied fertiliser reduce profitability. The initial pasture dry matter on offer of 8 t/ha had halved 3 years after clearing, and a decline in cattle liveweight gain of 4 kg/ha.year was observed over an 8-year period with constant stocking of 0.59 head/ha. Due to fluctuating stocking rate levels of 0.3–0.7 head/ha over the trial period, liveweight productivity trends are attributed to the multiple effects of stocking rate changes and fertility decline. The amount of nitrogen exported from the cleared catchments was 36.1 kg/ha.year in grain but only 1.6 kg/ha.year in cattle (as liveweight gain). Total soil N at 0–0.3 m declined by 84 kg/ha.year under cropping but there was no significant decline under grazing. The soil nutrients removed during grain and beef production need to be replaced in order to avert productivity decline post-clearing.

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