The contribution of cotton stubble to the soil organic matter content of
Vertisols under cotton production is not well understood. A 3-year experiment
was conducted at the Australian Cotton Research Institute to study the effects
of burning and incorporating cotton stubble on the recovery of fertiliser
nitrogen (N), lint yield, and organic matter levels. This study reports on the
changes in soil organic matter fractions as affected by burning and
incorporating cotton stubble into the soil. Soil samples collected at the
start and end of the 3-year experiment were analysed for total carbon
(CT), total N (NT), and
δ13C (a measure of
13C/12C isotopic ratios).
Labile carbon (CL) was determined by ease of oxidation
and non-labile carbon (CNL) was calculated as the
difference between CT and CL.
Based on the changes in CT, CL,
and CNL, a carbon management index (CMI) was calculated.
Further analyses were made for total polysaccharides
(PT), labile polysaccharides (PL),
and light fraction C (LF-C). Stubble management did not significantly affect
the NT content of the soil. After 3 years, the
stubble-incorporated plots had a significantly higher content of
CT, CL, and polysaccharides.
Incorporation of stubble into the soil increased the CMI by 41%,
whereas burning decreased the CMI by 6%. The amount of LF-C obtained
after 3 years in the stubble-incorporated soil was almost double that obtained
in the stubble-burnt soil. It was concluded that for sustainable management of
soil organic matter in the Vertisols used for cotton production, stubble
produced in the system should be incorporated instead of burnt.
Alpine grassland soils contain large proportion of labile carbon but indicate long turnover times