Changes in soil carbon sequestration, fractionation and soil fertility in response to sugarcane residue retention are site-specific

Sugarcane crop residues contain substantial quantities of C and plant nutrients, but there have been relatively few studies of how sugarcane residues enrich the soil and contribute to C sequestration, and most studies have been undertaken at only one or a few sites. The purpose of this study was to address these knowledge gaps by determining the magnitude and time scale of changes in soil concentrations of total C, C fractions and plant nutrients following retention of sugarcane residues. C fractions were determined by two different methods. We sampled soils from five experiments, in contrasting environments, where sugarcane residues had been either retained or removed for between 1 and 17 years. Changes in the concentration of both soil C and plant nutrients were highly site-specific and not in proportion to the period that residues were retained: for example, soil C (0–250 mm) decreased by 0.9 g kg−1 and 0.5 g kg−1 at sites where residues had been retained for 1 and 17 years, respectively, but increased by 2.0 g kg−1 at a site with residues retained for 6 years. Soil C composition, defined by the KMnO4 oxidation and particulate organic C-ultraviolet photo-oxidation fractionation (POC-UV) schemes, appeared to be a more sensitive indicator of changes in residue management, indicating that increases in readily-oxidisable C and particulate organic C, respectively, after 1 year of retaining instead of burning residues. The two methods provided different information that was complementary in understanding changes in soil C. The KMnO4 method identified downward movement of C fractions in the profile to 250 mm, while the labile fractions measured by the POC-UV scheme appeared to be more sensitive to early changes in residue management (after 1 year). While recent studies have found that several concentrations of KMnO4 reduced all C fractions by a similar magnitude and thus concentrated on the fraction oxidised by the 333 mM concentration of KMnO4, we found that use of both this and the 33 mM concentration enabled a greater understanding of changes in C pools due to residue management.