NITROGEN USE EFFICIENCY OF SPLIT NITROGEN APPLICATIONS IN SOFT WHITE WINTER-WHEAT

Nitrogen use efficiency (grain weight per unit of N supplied from soil or fertilizer) can be reduced by overfertilization, suboptimal yields , and N losses. Nitrogen is typically fall-applied in eastern Washingt on for soft white winter wheat (Triticum aestivum L.) production, and is therefore subject to overwinter losses or accumulation in deep soil layers. Increasing grain protein levels of soft white winter wheat ha ve been attributed in part to excessive N application rates and high r esidual N levels. Spring N applications were evaluated over four site- years as an option to all-fall applied N for reducing N inputs and imp roving N use efficiency, thereby allowing producers to maintain produc tivity while controlling grain protein. Fire to six N rates ranging fr om 0 to 140 kg ha(-1) for the all-fall N applications were compared wi th fall-spring split applications of 84 to 140 kg total N ha(-1). Nitr ogen was applied in the spring by topdressing (TD) or with a spoke-whe el point-injection (PI) system. A N-15 experiment was conducted at two locations during the second year to quantify N uptake from fertilizer and soil N. High preplant residual N conditions resulted in Limited r esponses to added N. Nitrogen use efficiency nas 26 to 44% lower in th e 140 kg ha(-1) fall-applied N treatments than in the zero-N control. Reduced N use was related to N losses from the system and to decreased N utilization efficiency (grain weight produced per unit plant N). Sp ring-applied N, with paint injection or topdressing, maintained or inc reased N use efficiency compared with equivalent: all-fall N rates of 84 and 112 kg N ha(-1). More N-15-labeled fertilizer (7-16% more) was recovered with a split N application than with fall-applied N at the s ame total rate of 112 kg N ha(-1). At maturity, 68% of plant N was fro m soil sources with a split application, compared with 80% with an all -fall application. These results suggest spring-IV applications with p oint injection or topdressing can improve fertilizer N recovery and N use efficiency over preplant applications in dryland winter wheat.