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.