VARIABLE RATE NITROGEN AND SEEDING TO IMPROVE NITROGEN USE EFFICIENCY
Brown, Tabitha Therisa
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Increased nitrogen (N) fertilizer additions to modern agricultural cropping systems will be necessary to feed a growing world population. However, greater nitrogen use efficiency (NUE) is required if agroecosystems are to continue to provide certain ecosystem services (e.g., greenhouse gas emission reductions and water quality goals). The aim of this research was to investigate the role of variable rate N and seeding of winter wheat (Triticum aestivum) for optimizing yield-water-NUE relationships across heterogeneous landscapes. Field plot studies were conducted at the Cook Agronomy Farm (CAF) near Pullman, WA during the 2010, 2011 and 2012 winter wheat harvest years. A randomized complete block split plot N rate x seeding rate experiment with N fertilizer rate as main plot and seeding rate as subplot was employed across three landscape positions. Assessed were evidence of “haying-off," depletion of available water resources, and the link between yield, protein, and NUE response to landscape by N fertilization rate by seeding rate treatment combinations. A performance classification was developed to evaluate wheat performance with regard to N utilization efficiency (Gw/Nt) and N uptake efficiency (Nt/Ns) components of the NUE. Evidence of haying-off in winter wheat was medium to high for drier landscape positions, particularly during low precipitation years and likely occurs in these landscapes most years. Treatment impacts on NUE varied by year and landscape but overall NUE decreased by 14 to 22 kg grain yield per kg N supply as N rate increased from 0 to 160 kg N ha-1 across three landscape positions and two site years (2011 and 2012). Target NUE and maximum anthesis biomass could be achieved with a 34 to 68% reduction in typical seeding rates. The NUE-based performance classification was helpful in identifying environmental or management conditions contributing to low or high NUE indicating potential to be used as an evaluation tool. This research also included a policy fellowship focused on N2O emission reductions and greenhouse gas offset credits that could be generated from adoption of variable rate N for wheat and concluded that offset credits alone would not provide enough incentive for adoption of variable rate N.