No abstract

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.

This paper describes the application of mid-infrared (MIR) spectroscopy and partial least-squares (PLS) analysis to predict the concentration of organic carbon fractions present in soil. The PLS calibrations were derived from a standard set of soils that had been analysed for total organic carbon (TOC), particulate organic carbon (POC), and charcoal carbon (char-C) using physical and chemical means. PLS calibration models from this standard set of soils allowed the prediction of TOC, POC, and char-C fractions with a coefficient of determination (R2) of measured v. predicted data ranging between 0.97 and 0.73. For the POC fraction, the coefficient of determination could be improved (R2 = 0.94) through the use of local calibration sets. The capacity to estimate soil fractions such as char-C rapidly and inexpensively makes this approach highly attractive for studies where large numbers of analyses are required. Inclusion of a set of soils from Kenya demonstrated the robustness of the method for total organic carbon and charcoal carbon prediction.

No abstract

Boron toxicity was identified in barley crops grown on a range of soils at 16 widespread locations in South Australia, and also at one site in western Victoria. The soils on which boron toxicity occurred included red-brown earths (Calcic Natrixeralf), calcareous earths (Xerollic Calciorthid and Calcic Paleorthid), and calcareous sands ('Petrocalcixerollic' Xerochrept). At one site the soil was a grey clay (Palexerollic Chromoxerert). The properties of some examples of normal and high-boron soils which were sampled in close proximity are discussed. For individual high-boron soil profiles it was possible to demonstrate statistically significant relationships between extractable boron and ESP, CEC and clay content. However, these relationships did not hold generally for comparisons between normal and high-boron soils. Boron concentrations in affected barley ranged from 56 mg/kg in mature straw to 323 mg/kg in whole tops at Feekes stage 10.1. In control samples the mean boron concentration was 22.8 mg/kg. The concentrations of other nutrient elements (P, K, S, Mg, Cu, Zn, Mn, Mo) were within normal ranges, and did not differ between control samples and plants with toxicity symptoms. Barley plants affected by the toxicity had increased concentrations of Na and Cl, and decreased concentrations of Ca compared with control plants. These effects were small, but statistically significant, and were consistent with the notion that the toxicity was associated with sodic soils. The findings extend our earlier work on boron toxicity at a single site, and demonstrate that the toxicity is widespread in South Australia.