Managing legume leys, residues and fertilisers to enhance the sustainability of wheat cropping systems in Australia: 2. Soil physical fertility and carbon

Soil organic matter (SOM) is considered as a key indicator of sustainability, therefore measurements of SOM changes under various forms of management are needed for the development of sustainable systems. Because the measurement of total SOM is not sensitive enough to monitor short and medium term changes, techniques that measure meaningful fractions of SOM should be used. In this study both total carbon (CT) measured by combustion and labile carbon (CL) determined by oxidation with 333 mM potassium permanganate (KMnO4) were measured. Field trials, consisting of a legume phase followed by three wheat (Triticum aestivum L. cv. Janz) crops, were established on a degraded Ferric Luvisol (Red Earth) soil in New South Wales to investigate the effect of crop residue and fertiliser management on wheat yield, soil physical properties and SOM. Total and labile C increased following a lucerne (Medicago sativa L. cv. Trifecta) phase, however, chickpea (Cicer arietinum L. cv. Amethyst), barrel medic (Medicago truncatula L. cv. Sephi) and fallow leys resulted in no increases in soil C concentrations. During the wheat phase the concentration of CL significantly increased on the treatments with wheat stubble retention. This resulted in the C Management Index (CMI), an index comparing changes in labile and total C fractions relative to an uncropped reference soil, increasing from 19 to 27. The greatest treatment effect on soil physical properties was the retention of wheat stubble on the soil surface over the summer fallow period which increased hydraulic conductivity (K) by more than 65%, relative to the stubble removed treatment. Mean weight diameter (MWD) increased from 799 to 920 μm and a significant relationship was found between hydraulic conductivity and water stable aggregates >500 μm. Soil strength at 15 cm decreased from 2713 in the non-return to 2064 kPa in the stubble retained treatments with both treatments having a similar water content at the time of measurement. Although legume species are widely used as a rotation phase, their use in combination with cereal stubble retention is more likely to improve the overall fertility of the farming system.