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Recovering Leached N by Sowing Wheat After Irrigated Cotton in a Vertisol

ABSTRACT Cotton (Gossypium hirsutum L.) production in Australia occurs mainly in self-mulching Vertisols. Comprehensive studies of nutrient and salt leaching in semi-arid Vertisols are few. These studies suggest that although generally slow-draining, under saturated or near saturated conditions when hydraulic conductivity increases exponentially, nutrient and salt leaching can be high with anions such as chlorides, nitrates and sulphates, and cations such as Na dominating the leachate. Given that large amounts of nutrients can be leached out of the crop root zone, the costs to the farmer can be high. For example, nitrate-N leaching can result in costs of the order of Australian $200/year. If the leached N can be recovered, then these costs may be minimized. One agronomic practice whereby leached N can be recovered is by sowing deep-rooted non-leguminous rotation or green manure crops after the primary crop. These crops, usually cereal, are reported to extract N which has leached below the primary crop's root zone. Examples include rye (Secale cereale L.), safflower (Carthamus tinctorius L.) and oats (Avena sativa L.) after cotton and soybean (Glycine max L.). However, in Australia, the crop that is most commonly sown in rotation with cotton is wheat (Triticum aestivum L.). The objective of this study was to determine whether a wheat crop sown in rotation with cotton in a grey Vertisol was able to extract N leached below the root zone of the preceding cotton crop. The experiment was established in 1993 on a commercial cotton farm near Wee Waa, northwestern New South Wales, Australia on a self-mulching grey Vertisol. The experimental treatments were: cotton followed by either “high” (urea at rates of 140 kg N/ha in 1993, 120 kg N/ha in 1995 and 1997, and 145 kg N/ha in 1999) or ‘low” (145 kg N/ha as urea in 1999 only) fertility wheat. Fertilizer was applied to cotton as anhydrous ammonia at a rate of 120 kg N/ha in 1994 and 1996, and 180 kg N/ha in 1998. All crops in the cropping sequences were irrigated by furrow irrigation at a rate of 1 ML/ha when rainfall was insufficient to overcome evaporative demand. Wheat sown after cotton was able to extract N leached out of the cotton root zone during the preceding cotton season. Applying N fertilizer to wheat further increased N extraction from the subsoil, particularly when wet conditions had occurred during the early part of the preceding cotton season. High fertility wheat extracted 112 kg/ha, 87 kg/ha and 131 kg/ha of subsoil N during 1995, 1997 and 1999, respectively. Low fertility wheat extracted 71 kg N/ha in 1995, 50 kg N/ha in 1997 and 105 kg of N/ha in 1999 from the subsoil. N extracted from the subsoil was a major contributor to the variation in wheat yields between high and low fertility wheat. Between 1995 and 1999, average grain yields were 4.7 t/ha with high fertility wheat and 4.0 t/ha with low fertility wheat.