Nitrogen sources and sinks – Understanding the fate of nitrogen fertiliser when applied at various times and quantifying internally remobilised nitrogen in cotton

Achieving a sustainable cropping system requires the efficient use of resources, particularly nitrogen (N). Nitrogen fertiliser is applied in most irrigated cotton fields to maximise yield potential, but plant fertiliser recovery can be low. Identifying the crucial pathways of fertiliser remobilisation internally within cotton plants will lead to greater awareness of the plants’ ability to match the N demands of the developing fruiting matter. This study investigated the fate of N fertiliser when applied to cotton at various dates, with the goal to improve N fertiliser recovery in a modern transgenic cotton cultivar. 15 N-labelled urea (10 atom%), was applied at multiple times and harvested at four key cotton growth stages (first square, early bolls, cut-out and maturity). Remobilised N was determined as the difference in the proportion of N fertiliser in individual plant components against the fertiliser utilised by the whole plant. The application of fertiliser N at first square resulted in 23 % greater fertiliser N recovery at plant maturity compared to fertiliser N applied 100 % pre-plant ( P < 0.001). The improvement was in-part due to higher N derived from the fertiliser (Ndff%) in the cotton seed (+3%). Conversely, the Ndff% was higher in the stem (+4%), and petioles (+1%) when the fertiliser was applied pre-plant. In total 73 % of plant N, was remobilised to another plant organ, predominantly the seed (67 %). Applying N fertiliser post-planting improved N recovery and lint yield compared to applying all fertiliser pre-plant.