Cotton growth and the fate of N fertilizer as affected by saline water irrigation and N fertigation in a drip-irrigated field

The objective of this two-year field experiment was to study the effects of irrigation amount, N rate, and irrigation water salinity on cotton growth and the fate of N fertilizer. The movement of N through the plant-soil system was traced using ¹⁵N-labeled urea. The study consisted of twelve treatments, including two irrigation amounts (405 and 540?mm, I405 and I540, respectively); two N application rates (240 and 360?kg?N/ha, N240 and N360, respectively); and three irrigation water salinity levels 0.35, 4.61 and 8.04?dS/m, representing fresh water (FW), brackish water (BW), and saline water (SW), respectively]. A randomized complete block design was used with three replications. The results showed that cotton biomass, N uptake, and yield increased as irrigation amount and N amount increased; however, all three variables were significantly less in SW than in FW and BW. Plant ¹⁵N recovery rates were greater (i) in the I540 treatments than in the I405 treatments and (ii) in the N360 treatments than in the N240 treatments. Plant ¹⁵N recovery rates in BW were 7.98% and 30.01% greater than those in FW and SW, respectively. Residual soil ¹⁵N increased as N fertilizer amount increased but declined as irrigation amount increased. Residual soil ¹⁵N in BW and SW was 6.02% and 21.44% greater, respectively, than in FW. Total ¹⁵N recovery was significant greater in BW than in FW and SW. The ¹⁵N leaching losses increased significantly with increases in irrigation amount, irrigation water salinity, and N rate. Our study suggests that if appropriate amounts of irrigation water and N fertilizer are used, then brackish irrigation water (4.61?dS/m) will not affect cotton growth, yield and N recovery. In contrast, saline irrigation water (EC?>?8?dS/m) reduces cotton growth, yield, and N use efficiency.