No abstract

Treated sewage effluent may contain large amounts of nitrogen and phosphorus, and moderate to high amounts of salts. With good management, it can be used as a source of irrigation water and nutrients for a range of crops and soils under different climatic conditions and irrigation systems. However, there are few long-term studies of irrigation with treated sewage effluent in swelling soils such as Vertosols. This study was established in 2000 on a cotton farm near Narrabri, north-western New South Wales, to assess long-term (14-year) changes in soil salinity, sodicity and carbon storage in a self-mulching, medium-fine, grey Vertosol under conservation farming and furrow-irrigated with tertiary-treated sewage effluent and stored rainfall runoff. Experimental treatments in 2000–02 were gypsum applied at a rate of 2.5t/ha in June 2000 and an untreated control. In 2003–13, the gypsum-treated plots received a single pass with a combined AerWay cultivator and sweeps to ~0.15m depth before sowing cotton; in the control plots, wheat stubble was undisturbed. By retaining significant amounts of crop residues on the soil surface, both practices are recognised as conservation farming methods. Parameters for water sampled from the head-ditch during each irrigation included electrical conductivity (ECw), pHw, concentrations of cations potassium (K+), calcium (Ca2+), magnesium (Mg2+) and sodium (Na+), and sodium adsorption ratio (SAR). Parameters for soil sampled to 0.6m depth before sowing cotton were pH (0.01M CaCl2), salinity (EC of 1:5 soil:water suspension), bulk density, soil organic carbon (SOC), exchangeable Ca, Mg, K and Na, exchangeable sodium percentage (ESP) and electrochemical stability index (ESI). SOC storage (‘stocks’) in any one depth was estimated as the product of bulk density, sampling depth interval and SOC concentration. Management system had little or no effect on cotton lint yields and the soil properties measured. Major changes in soil properties were driven by a combination of irrigation water quality and seasonal variations in weather. The cultivated treatment did not degrade soil quality compared with the control and may be an option to control herbicide-resistant weeds or volunteer Roundup-Ready cotton. Irrigation water was alkaline (average pHw 8.9), moderately saline (average ECw 1.0dS/m) and potentially highly dispersive (average SAR 12.1). Long-term irrigation with tertiary-treated sewage effluent resulted in sodification (ESP > 6) at all depths, alkalinisation at 0–0.10 and 0.30–0.60m, and accumulation in the surface 0.10m of Ca and K. Average ESP at 0–0.6m depth increased from 3.8 during 2000 to 13.2 during 2013. Sodification occurred within a few years of applying the effluent. Exchangeable Ca at 0–0.10m depth increased from 19cmolc/kg during 2000 to 22cmolc/kg during 2013, and exchangeable K from 1.5cmolc/kg during 2000 to 2.1cmolc/kg during 2013. Drought conditions caused an increase in salt accumulation, alleviated by a subsequent period of heavy rainfall and flooding. The reduction in salinity was accompanied by a fall in exchangeable Mg concentrations. Salinity and exchangeable Mg concentration were strongly influenced by interactions between seasonal rainfall (i.e. floods and drought) and the quality of the effluent, whereas ESP and exchangeable K concentration were not affected by variations in seasonal rainfall. SOC stocks declined until the flooding events but increased thereafter.

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.

Abstract. Long-term studies of soil organic carbon dynamics in two-and three-crop rotations in irrigated cotton (Gossypium hirsutum L.) based cropping systems under varying stubble management practices in Australian Vertosols are relatively few. Our objective was to quantify soil organic carbon dynamics during a 9-year period in four irrigated, cottonbased cropping systems sown on permanent beds in a Vertosol with restricted subsoil drainage near Narrabri in north-western New South Wales, Australia. The experimental treatments were: cotton-cotton (CC); cotton-vetch (Vicia villosa Roth. in 2002-06, Vicia benghalensis L. in 2007-11) (CV); cotton-wheat (Triticum aestivum L.), where wheat stubble was incorporated (CW); and cotton-wheat-vetch, where wheat stubble was retained as in-situ mulch (CWV). Vetch was terminated during or just before flowering by a combination of mowing and contact herbicides, and the residues were retained as in situ mulch. Estimates of carbon sequestered by above-and below-ground biomass inputs were in the order CWV >> CW = CV > CC. Carbon concentrations in the 0-1.2 m depth and carbon storage in the 0-0.3 and 0-1.2 m depths were similar among all cropping systems. Net carbon sequestration rates did not differ among cropping systems and did not change significantly with time in the 0-0.3 m depth, but net losses occurred in the 0-1.2 m depth. The discrepancy between measured and estimated values of sequestered carbon suggests that either the value of 5% used to estimate carbon sequestration from biomass inputs was an overestimate for this site, or post-sequestration losses may have been high. The latter has not been investigated in Australian Vertosols. Future research efforts should identify the cause and quantify the magnitude of these losses of organic carbon from soil.