Chemical and physical degradation of Red Ferrosols in eastern Australia is a
major issue necessitating the development of more sustainable cropping
systems. This paper derives critical concentrations of the active
(permanganate-oxidisable) fraction of soil organic matter (C1) which maximise
soil water 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
simulated rainfall. The rates of change in P125 per unit change in C1 were not
significantly different (P < 0·05) for soils
from the different localities. However, soils from the coastal Burnett
(south-east Queensland) always produced lower P125 (i.e. less aggregate
breakdown) than did soils from the inland Burnett and north Queensland
locations given the same concentration of C1. This difference was not
associated with a particular land use.
The ‘critical’ concentrations of C1 for each region were taken as
the C1 concentrations 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 min duration. This analysis also provided an indication of the risk
associated with the concentrations of C1 currently characterising each farming
system in each rainfall environment. None of the conventionally tilled
Queensland Ferrosols contained sufficient C1 to cope with rainfall events
expected to occur with a 1 in 10 frequency, while in many situations the C1
concentration was sufficiently low that runoff events would be expected on an
annual basis.
Our data suggest that management practices designed both to maximise C inputs
and to maintain a high proportion of active C should be seen as essential
steps towards developing a more sustainable cropping system.
Effect of Crop Rotation and Nitrogen Fertilization on the Quality and Quantity of Soil Organic Matter
