Nitrate-N losses in drainage water under irrigated vertosols of north-western NSW

Tim Weaver1, Nilantha Hulugalle2, Hossein Ghadiri3, Steven Harden4

1NSW Department of Primary Industries, Locked Bag 1000, Narrabri, NSW 2390, www.dpi.nsw.gov.au, [email protected]

2Fenner School for the Environment and Society, Australian National University, Canberra, ACT 0200

3Griffith School of Environment, Environmental Futures Research Institute, Griffith University, Nathan, Qld 4111

4NSW Department of Primary Industries, Calala, NSW 2340

Abstract

A comparison on the effects of soil and crop management practices in irrigated farming systems on the quality of drainage water in Vertosols has not been reported in the literature. The objective of this study was to quantify nitrate-N in drainage water in the subsoil (0.6, 0.9, 1.2 m) of sodic and non-sodic Vertosols under selected crop rotations, viz. continuous cotton (Gossypium hirsutum L.), cotton–dolichos (Lablab purpureus L.) and cotton–wheat (Triticum aestivum L.). A cotton–wheat rotation was sown at Wee Waa and Myall Vale; wheat stubble was incorporated in the former and retained as in situ mulch in the latter. At Merah North, there were three cropping sequences; viz. continuous cotton, cotton–wheat, and cotton–dolichos sown between 1993 and 2000 in adjacent plots with identical land management histories. The three treatments were sown with cotton during the 2000–2001 and 2002–2003 growing seasons, wheat during the 2001 winter and sorghum during the 2001–2002 growing season with stubble being incorporated. Drainage water was sampled with 50-mm diameter ceramic-cup samplers from depths of 0.6, 0.9 and 1.2m in six sites in each plot and irrigation water from the head ditch after irrigation between mid-October and late February during the cotton-growing seasons of 2000-2001 and 2002–2003. Soil water extracted from the ceramic-cup samplers was analysed for nitrate-N. The nitrate-N concentrations in drainage water varied among sites, and reflected variations in soil properties, fallow length since the preceding crop, fallow rainfall and irrigation water quality.