Carbon sequestration in irrigated vertisols under cotton‐based farming systems

Agricultural production systems account for approximately 15% of greenhouse gas emissions in Australia. Carbon dioxide accounts for a major proportion of these gases. Reducing or avoiding tillage, avoiding crop residue burning, mulching, sowing cover or rotation crops during fallow periods, minimizing land clearance, and using land clearing methods which minimize soil disturbance are thought to reduce carbon dioxide emission and improve carbon sequestration in soil, although experimental data obtained under Australian conditions are sparse. The effects of minimizing tillage and using cotton (Gossypium hirsutum L.)‐based crop rotations on carbon sequestration in irrigated Vertisols was evaluated from 1993 to 1998 in several experimental sites located in north‐western and central‐western New South Wales, Australia. Carbon sequestration was highest where minimum tillage and rotation with wheat (Triticum aestivum L.) had been practiced for extended periods (>10 years). In the short‐term (<5 years), however, replacing intensive tillage with minimum tillage resulted in a fall in soil carbon sequestration. This was attributed to the low decomposition rate of cotton crop residues. Significant differences were also absent between crop rotations (e.g., cotton‐legume and cotton‐cereal) with respect to short‐term carbon sequestration.