Carbon, nitrogen and phosphorus stoichiometric ratios under cotton cropping systems in Australian Vertisols: a meta-analysis of seven experiments

Enhancing storage of carbon (C) in agricultural soil has been proposed as a partial solution to offset the accelerated release of greenhouse gases associated with global warming. A net loss of soil C is common in most cotton (Gossypium hirsutum L.)-based farming systems in Australian Vertisols. Some authors have suggested that an "ideal" stoichiometric ratio C:N:P:S of 10,000:833:200:143 was required for sustained C sequestration. The objective of this study was to determine the C, N, and P stoichiometric ratios that were present under cotton cropping systems sown in some typical irrigated and dryland Vertisols of Eastern Australia. Measurements were made on archived soil samples from seven experiments conducted between 1993 and 2012. Soil C sequestration was mostly negative in many of these sites or at best neutral. The archived soils were analysed for total P by digestion, and total C and N by dry combustion. The results were grouped and analysed according to four rotation types (continuous cotton systems, and cotton fb. (followed by) cereal, cotton fb. legume and cotton fb. cereal and legume, and two water management practices (irrigated and dryland) using a mixed model approach for meta-analysis of multiple experiments. Although small differences in C:N:P ratios existed among rotation types, values were much lower with irrigation (30:3:1) than in the dryland sites (263:14:1) and the "ideal" ratio (50:4:1). Low nutrient availability may not be the cause of soil C losses under irrigation but it may be one factor that inhibits C sequestration under dryland conditions.