Soil properties and carbon stocks in a grey Vertosol irrigated with treated sewage effluent

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.