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.