Soil Properties and Carbon Storage in a Grey Vertisol Irrigated with Treated Sewage Effluent.

Treated sewage effluent may contain large amounts of N, P and salts. However, long-term studies of irrigation with treated sewage effluent in swelling soils are few. The objective of this study was to assess long-term (14 year) changes in soil properties in a Vertisol under conservation farming and irrigated with tertiary treated sewage effluent. The experiment was established in 2000 on a cotton farm near Narrabri, Australia, which is furrow irrigated with stored rainfall runoff and tertiary treated sewage effluent. From 2000 to 2002 the experimental treatments were gypsum applied at a rate of 2.5 t/ha and an untreated control. From 2003 until 2013 the gypsum-treated plots were subjected to a single pass with a combined aer-way cultivator and sweeps before sowing cotton whereas the wheat stubble in the control plots was undisturbed. Water sampled during each irrigation was analysed for electrical conductivity (EC_w) and pH_w, and K⁺, Ca²⁺, Mg²⁺ and Na⁺, and sodium adsorption ratio, SAR calculated. Soil was sampled to a depth of 0.6 m before sowing cotton and analysed for pH (0.01M CaCl₂), salinity (as electrical conductivity, EC_1:5), bulk density, soil organic carbon (SOC) and exchangeable Ca, Mg, K and Na, and exchangeable sodium percentage (ESP) and electrochemical stability index, ESI, calculated. The two management systems had little or no effect on cotton lint yields and soil properties, whereas irrigation water quality and seasonal variations in weather did. In comparison with the control, cultivation with an aer-way cultivator did not degrade soil quality, and may be an option to control herbicide-resistant weeds or volunteer Roundup-Ready^® cotton. Irrigation water was alkaline (pH_w 8.9), moderately saline (EC_w 1.0 dS/m) and potentially highly dispersive (SAR 12.1). Irrigation with treated sewage effluent resulted in sodification, alkalinisation, and accumulation in the surface 0.10 m of Ca and K. Drought and effluent irrigation caused salt and exchangeable Mg accumulation, and reduction in SOC storage that were alleviated by a subsequent period of heavy rainfall.