The interaction of crop residue and fertiliser management and their effect on soil organic matter, soil physical properties and sustainability

Abstract

The cultivation and cropping of soils across the Australian grain growing regions has resulted in substantial losses of soil organic matter (SOM). These losses have been accompanied by even greater losses in the active or labile components of SOM. Soil organic matter is an important indicator of sustainable farming systems as it has a central role in all aspects of soil fertility. The grain growing areas of northern New South Wales and southern Queensland in Australia are predominantly based on vertisol soils that have high water-holding capacities. This characteristic, along with initially high levels of fertility, has allowed reliable crop production on these soils in the past. Crop production systems in this area have increasingly used reduced tillage and the retention of crop stubble to increase soil water storage during the fallow period. Declining soil fertility has increasingly become evident but higher rates of N application have been successful in overcoming declines in grain yield and protein content. Studying changes in the concentration of soil organic matter due to different management practices requires methods that measure responsive and meaningful soil C fractions against large background concentrations. Changes in labile soil C fractions were measured using oxidation with 333 mM KMn0₄. The relative amounts of labile and nonlabile C (total C - labile C) were compared to a non-cropped reference soil and used to calculate a carbon management index (CMI). This study focused on increasing the understanding of crop residue breakdown and the influence of tillage and rotation practices on soil carbon fractions and the physical fertility of vertisol soils from the northern grains region. In the first study, two long-term trials established in 1981 were used to assess changes in soil fertility under different tillage and rotation management practices. The trials were located on a commercial property, "Gabo" near Croppa Creek, and at the Liverpool Plains Field Station, Breeza managed by NSW Agriculture. The study of changes in soil C fractions and soil physical fertility at the long-term trial at Breeza was complemented by a survey of commercial farming paddocks located on the Liverpool Plains in northern NSW. All the soils in the survey were black vertisol soils similar to the long-term trial site with an uncropped reference site located nearby. The final component of this study was a glasshouse-experiment using dual-isotope labelling with ¹³C and ¹⁴C to measure changes in soil carbon (C) due to residue decomposition and inputs from the growing crop. The influence of tillage and the type of crop residue were also investigated in this study.