Dressed for success. Are crop N uptake, N loss and lint yield of irrigated cotton affected by how in-crop N fertiliser is applied?

Abstract

Soil mineral nitrogen (N) and mineralisable N need to be supplemented by N fertiliser to optimise the growth and lint yield of modern irrigated cotton varieties. Splitting N fertiliser application between pre- and post-sowing is common, but what is the most efficient means of in-crop N addition? Two field experiments were conducted over two seasons to investigate the effects of (1) in-crop urea application method and (2) water-run (fertigation) N product choice. In one year, sidedressed urea increased cotton growth and N uptake by up to 26% compared with broadcast urea or water-run urea, but did not increase lint yields. In the other year, sidedressing increased plant nodes and bolls, but not lint yield. Sidedressing also minimised the N lost in irrigation runoff or left behind as residual soil mineral N. The addition of a urease inhibitor to broadcast urea did not benefit crop production or reduce N loss. Irrigating 1–2 days after broadcasting obviates the need to inhibit urea hydrolysis. Water-run N application led to substantial N loss in irrigation runoff. The form of N lost (urea, ammonium, nitrate) differed between the products applied but total runoff losses were similar between urea, urea ammonium nitrate and ammonia. Additional N loss via ammonia volatilisation from the water-run ammonia treatment makes this product choice the least efficient. Further research to reduce N in irrigation runoff is warranted, especially when fertigation is used. The lack of cotton lint yield responses points to the minor contribution of fertilizer N to the plant’s needs in these years, and the need to account for the significant contribution of soil mineral N to plant uptake when devising fertilizer rates. Sidedressing with urea was the most N efficient means of in-crop N delivery. • In-crop urea application method did not affect cotton lint yields. • In-crop sidedressed urea increased N uptake and growth, reduced N loss. • Urease inhibitor with topdressed urea had no impact on lint yield or N runoff loss. • N loss via irrigation runoff was high for water-run urea. • Runoff losses and lint yields were similar for water-run UAN, ammonia and urea.