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

Irrigation induced surface carbon flow in a Vertisol under furrow irrigated cotton cropping systems

Pathways of sequestered carbon loss from cotton (Gossypium hirsutum L.) farming systems include the carbon transported off-site in runoff and erosion. There is a lack of field studies that quantify the carbon gains and losses in hydrological pathways in cotton and other irrigated row cropping systems A three-year field investigation was overlaid on a long-term experiment near Narrabri, New South Wales, Australia with the objective to evaluate the effect of tillage practices and crop rotations on carbon loads in irrigation and runoff waters, and their impact on soil carbon balance in an intensive cotton production system. The treatments included maximum or minimum tillage sown with cotton monoculture, cotton-wheat (Triticum aestivum L.) or cotton-maize (Zea mays L.) rotations. Maximum tillage consisted of slashing of cotton plants after harvest, followed by disc-ploughing to incorporate the cotton stalks to 0.2 m, followed by chisel ploughing to 0.3 m, then 1 m bed construction. For minimum tillage, slashing was followed by root cutting, then incorporation of cotton stalks into beds (0.1 m) and followed by bed renovation with a disc-hiller. The minimum-tilled cotton-wheat rotation included similar tillage operations after cotton, however maize or cotton was planted into standing wheat stubble with zero tillage. Irrigation volume, sediment, and total and dissolved organic carbon gains and losses during irrigation were monitored during the 2014–15, 2015–16 and 2016–17 cotton seasons. Runoff from maximum-tilled and minimum-tilled cotton monoculture systems averaged 32% and 40%, respectively, of applied irrigation. Irrigation-induced total organic carbon (TOC) losses in runoff from the cotton field were influenced by tillage during 2015–16 and ranged from 24 to 72 kg ha−1 year-1 across three years. Net TOC enrichment of cotton field soils by irrigation water ranged from 30 to 265 kg TOC ha-1. Overall, the average seasonal net carbon gains in irrigation water were equivalent to mitigating 4.7 to 24% of long term annual soil organic carbon (SOC) decline rate in the same experiment. Storm events intensified the movement of carbon and soil from bed to furrows. These sediments were prone to further erosion during subsequent irrigations. Minimum tillage can minimise carbon losses in runoff when combined with a crop sequence such as cotton-wheat-maize. Consequently, research on soil carbon sequestration in irrigated systems must account for carbon flow during irrigation because it is a significant factor in the carbon balance. Long term monitoring over several years is needed to quantify storminduced carbon losses in semi-arid limited rainfall environments.Funding for this research was provided by the Cotton Research and Development Corporation of Australia (Grant numbers DAN 1503, DAN 1405, DAN 1402) and the New South Wales Department of Primary Industries

The current status of nitrogen fertiliser use efficiency and future research directions for the Australian cotton industry

Fifty years of sustained investment in research and development has left the Australian cotton industry well placed to manage nitrogen (N) fertiliser. The average production in the Australian cotton industry today is greater than two tonnes of lint per hectare due to improved plant genetics and crop management. However, this average yield is well below the yield that would be expected from the amount of N fertiliser used. It is clear from the recent studies that across all growing regions, conversion of fertiliser N into lint is not uniformly occurring at application rates greater than 200–240 kg·hm− 2 of N. This indicates that factors other than N availability are limiting yield, and that the observed nitrogen fertiliser use efficiency (NFUE) values may be caused by subsoil constraints such as sodicity and compaction. There is a need to investigate the impact of subsoil constraints on yield and NFUE. Gains in NFUE will be made through improved N fertiliser application timing, better targeting the amount of fertiliser applied for the expected yield, and improved soil N management. There is also a need to improve the ability and confidence of growers to estimate the contribution of soil N mineralisation to the crop N budget. Many Australian studies including data that could theoretically be collated in a meta-analysis suggest relative NFUE values as a function of irrigation technique; however, with the extensive list of uncontrolled variables and few studies using non-furrow irrigation, this would be a poor substitute for a single field-based study directly measuring their efficacies. In irrigated cotton, a re-examination of optimal NFUE is due because of the availability of new varieties and the potential management and long-term soil resilience implications of the continued removal of mineralised soil N suggested by high NFUE values. NFUE critical limits still need to be derived for dryland systems

Carbon gains and losses with irrigation, runoff and drainage in cotton farming systems

Theoretical estimates of soil carbon sequestration in Australian farming systems often do not coincide with measured values of soil carbon, possibly due to post sequestration carbon losses (Hulugalle et al. 2013b; Nachimuthu and Hulugalle 2016). Most authors have assumed that this was associated with microbial respiration (Huon et al. 2013). However, although largely overlooked in the past, part of this decline is thought to be due to carbon losses through soil erosion (Chappell et al. 2015; Hulugalle et al. 2013b; Kuhn et al. 2012) and mechanisms associated after the erosion event (Lal 2003). Some losses may also be due to leaching of dissolved organic carbon (DOC) with deep drainage (Nachimuthu and Hulugalle 2016). There is a paucity of empirical studies on the links between crop management practices and carbon movement in terrestrial hydrological pathways in Australian cotton and other irrigated row cropping systems. Thus, predicting carbon stocks using current accounting systems do not fully account for all carbon loss pathways. The purpose of this investigation was to study the impact of a range of management practices on rrigation induced carbon flow in intensive cotton production systems

Sowing maize as a rotation crop in irrigated cotton cropping systems in a Vertosol: effects on soil properties, greenhouse gas emissions, black root rot incidence, cotton lint yield and fibre quality

Although sowing winter cereal crops in rotation with irrigated cotton (Gossypium hirsutum L.) is practised by many Australian cotton growers, summer cereals such as maize (Zea mays L.) are sown more frequently than previously. Our objective was to quantify the impact of sowing maize rotation crops on soil properties, greenhouse gas emissions, incidence of black root rot (BRR) disease and crop yields in an ongoing long-term experiment located in a Vertosol in north-western New South Wales. The historical treatments were cotton monoculture (sown after either conventional or minimum tillage) and a minimum-tilled cotton-wheat (Triticum aestivum L.) rotation. The experiment was redesigned in 2011 by splitting all plots and sowing either maize during summer following the previous year's cotton or retaining the historical cropping system as a control. pH and exchangeable cation concentrations were highest, and electrical conductivity (EC1 : 5) lowest during 2012, the season following a flood event, but were unaffected by sowing maize. In subsequent seasons, with the onset of dry conditions, pH and cation concentrations decreased, and EC1 : 5 increased. The upper horizons (0-0.3 m) of plots where maize was sown had higher concentrations of exchangeable Ca and Mg during 2012, and 0.45-1.20 m had higher concentrations of exchangeable Na and exchangeable sodium percentage, but these differences disappeared in subsequent years. Soil organic carbon (SOC) in the surface 0.15 m was higher with maize, with differences becoming evident three years after maize was first sown but without any increases in SOC storage. Soil under maize was less resilient to structural degradation. BRR incidence was lower in maize-sown plots only during 2012. Stepwise linear regression suggested that high concentrations of exchangeable Ca and Mg in the surface 0.15 m played a role in reducing BRR incidence during 2012. Maize rotation introduced into cotton monocultures improved lint yields and reduced greenhouse gas emissions but had little impact in a minimum-tilled cotton-wheat rotation. Maize is a suitable rotation crop for irrigated cotton in a two-crop sequence but is of little advantage in a cotton-wheat-maize sequence.Our research was funded by the Cotton Research & Development Corporation (CRDC) of Australia, Cotton Catchments Communities Co-operative Research Centre and NSW Department of Primary Industries