Leaching of dissolved organic carbon and nitrogen under cotton farming systems in a Vertisol

Leaching with deep drainage is one of the loss pathways of carbon (C) and nitrogen (N) in cropping fields. However, field studies in irrigated row cropping systems are sparse. A 3-year investigation on C and N leaching associated with deep drainage was overlaid on a long-term experiment on tillage practices and crop rotations in Australia. The treatments included cotton (Gossypium hirsutum L.) monoculture and cotton-wheat (Triticum aestivum L.) or maize (Zea maize L.) rotations with maximum or minimum tillage. The deep drainage C and N concentrations at 0.6 and 1.2 m depth were measured after furrow irrigation with ceramic cup samplers during the 2014-15, 2015-16 and 2016-17 cotton seasons. Pre-planting dissolved organic carbon (DOC) concentration in soil at 0.6-1.2 m depth during 2016-17 was 64 mg kg(-1) for maximum tilled cotton monoculture, 36 mg kg(-1) for minimum tilled cotton monoculture and 39 mg kg(-1) for cotton-wheat, and in maize and cotton subplots 51 and 41 mg kg(-1), respectively. Post-harvest DOC values in soil were similar in all treatments (average of 32 mg DOC kg(-1)). Total organic carbon (TOC) losses in deep drainage were equal to 2%-30% of TOC gained in irrigation water. Oxidized N losses in deep drainage ranged from 0.7% to 12% of applied N (260 kg ha(-1)). NOx-N concentrations in leachate under maize systems (20 mg L-1) were up to 73% lower than those in cotton systems (75 mg L-1). Maize sown in rotation with cotton can improve cotton yield, reduce N leaching and improve N use efficiency of subsequent cotton.