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In-crop fertiliser application improved nitrogen recovery in irrigated cotton – a multi-year field study with ¹⁵N-labelled urea.

Jonathan Baird , Graeme Schwenke, Greg Constable , Ben Macdonald , Helen Suter

Abstract

Context: Plant recovery of nitrogen (N) fertiliser applied to alkaline soils for irrigated cotton ranges from 20% to 50%. Aligning the application timing to synchronise N availability with plant requirements should lead to greater crop yield and N use efficiency (NUE). Aim: Identify the fate of N fertiliser when applied either pre-sow or in-crop to an irrigated cotton system. Method: Field investigations were conducted using isotope-labelled N fertiliser (¹⁵N-urea), applied either pre-sow (100:0) or in-crop (0:100). Sampling during the crop growth identified the fate of N fertiliser, both within the soil profile and within the plant. Results: Applying N in-crop (0:100) improved N fertiliser recovery by 10% as measured at plant maturity compared to when N fertiliser was applied pre-sow (100:0). Most of the additional N was recovered in the cotton seed. The period of greatest loss for the 0:100 treatment (45%) occurred between N fertiliser application and first square. Irrigation water mobilised the fertiliser-derived N laterally through the planting hill, with N fertiliser recovered between the application point and the non-irrigated furrow. In contrast, the 0:100 treatment had a more uniform distribution of N throughout the planting hill. Inherent soil N (non-¹⁵N) supplied 60–75% of plant N more than that of the applied N fertiliser. Conclusion: Applying N fertiliser in-crop gave greater fertiliser N use efficiency and recovery, reducing N loss to the environment. Implication: The synchronising of N fertiliser delivery with cotton uptake improved efficiency, allowing for reduced application rates while maintaining high yields.

CP24334  Accepted 04 April 2025

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