Using active fractions of soil organic matter as indicators of the sustainability of Ferrosol farming systems

Chemical and physical degradation of Red Ferrosols in eastern Australia is a major issue necessitating the development of more sustainable cropping sy stems. This paper derives critical concentrations of the active (permangana te-oxidisable) fraction of soil organic matter (C1) which maximise soil wat er recharge and minimise the likelihood of surface runoff in these soils. Ferrosol soils were collected from commercial properties in both north and south Queensland, while additional data were made available from a similar collection of Tasmanian Ferrosols. Sites represented a range of management histories, from grazed and ungrazed grass pastures to continuously cropped soil under various tillage systems. The concentration of both total carbon (C) and C1 varied among regions and farming systems. C1 was the primary factor controlling aggregate breakdown, measured by the percentage of aggregates <0.125 mm (P125) in the surface crust after simula ted rainfall. The rates of change in P125 per unit change in C1 were not si gnificantly different (P < 0.05) for soils from the different localities. H owever, soils from the coastal Burnett (south-east Queensland) always produ ced lower P125 (i.e. less aggregate breakdown) than did soils from the inla nd Burnett and north Queensland locations given the same concentration of C 1. This difference was not associated with a particular land use. The 'critical' concentrations of C1 for each region were taken as the C1 co ncentrations that would allow an infiltration rate greater than or equal to the intensity of a 1 in 1 or 1 in 10 year frequency rainfall event of 30 m in duration. This analysis also provided an indication of the risk associat ed with the concentrations of C1 currently characterising each farming syst em in each rainfall environment. None of the conventionally tilled Queensla nd Ferrosols contained sufficient C1 to cope with rainfall events expected to occur with a 1 in 10 frequency, while in many situations the C1 concentr ation was sufficiently low that runoff events would be expected on an annua l basis. Our data suggest that management practices designed both to maximise C inpu ts and to maintain a high proportion of active C should be seen as essentia l steps towards developing a more sustainable cropping system.