Submitted to: Soil Science Society of America Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/17/2014
Publication Date: 5/1/2014
Citation: Forrestal, P.J., Meisinger, J.J., Kratochvil, R.J. 2014. Winter wheat starter nitrogen management: a preplant soil nitrate test and site specific nitrogen loss potential. Soil Science Society of America Journal. 78(3):1021-1034. Interpretive Summary: Managing residual nitrate nitrogen following corn is difficult because it is highly variable, yet important because it can supply starter nitrogen for winter wheat or it can be leached into water resources during the winter water-recharge season in the Humid East. Starter nitrogen recommendations for wheat in humid regions most commonly use a standard rate of starter nitrogen without adjusting for fall residual nitrogen. A series of 65 starter-nitrogen winter wheat yield response studies were conducted in Maryland to estimate the fall soil nitrate concentration that is sufficient for establishing wheat, and that is cost effective. A subset of studies measured soil nitrate loss below two feet during the fall-winter season, and additional studies evaluated over-winter bromide loss as a potential index for nitrate leaching. Results showed that using winter rainfall divided by the site’s available soil water holding capacity, provided a good index to categorize sites at risk for nitrate leaching, with sites having rainfall more than 2.5 times the soil available water holding capacity losing more than two-thirds of their fall nitrate nitrogen. The wheat yield response data showed a significant relationship between the yield response to starter nitrogen and the fall soil nitrate concentration in the top foot of soil, with yield responses decreasing as fall soil nitrate increased. This yield response relationship was combined with economic data to estimate the soil nitrate concentration where the break-even cost of starter fertilizer equaled the value of added yield. Wheat grown on soils with fall nitrate nitrogen concentration below this break-even point are likely to respond to starter nitrogen which would lower the risk of nitrate leaching, while soils above this point are likely to be nitrogen sufficient and at a higher risk for nitrate loss if they are fertilized. These data provide a method for public and private nutrient managers, producers, and policy makers, to improve nitrogen use efficiency for winter wheat by identifying nitrogen sufficient sites and conserving fertilizer nitrogen, thereby improving economic returns and reducing nitrate losses to receiving waters.
Technical Abstract: Managing highly variable residual nitrate-nitrogen (NO3-N) following corn (Zea mays L.) is difficult because it can supply starter nitrogen (N) for winter wheat (Triticum aestivum L.), and/or be leached into water resources during the fall-winter water-recharge season in the Humid East. A series of 65 starter-N response studies were conducted in the Coastal Plain and Piedmont regions of Maryland to estimate the soil NO3-N concentration that was both agronomically sufficient and cost effective for wheat. A subset of studies measured soil NO3-N loss (0-60 cm) during the fall-winter season and additional studies evaluated bromide to index potential NO3-N leaching. A soil exchange frequency index [EFI = rainfall * (available soil water capacity)-1] was used to identify where NO3-N loss was high and wheat response to starter-N was unlikely. A post-planting fall EFI = 2.5 was associated with NO3-N losses of > 65% (0-60 cm). A significant (P<0.001) linear-plateau relationship was found between pre-plant soil NO3-N concentration (0-30 cm) and wheat grain-yield response to starter-N for locations with EFI < 2.5. This agronomic linear-plateau relation was combined with a break-even economic scenario (fertilizer-N cost = grain-response value), to estimate the fall residual NO3-N sufficiency for wheat, which was 7 mg NO3-N kg-1 soil (0-30 cm), which corresponded to 9 mg kg-1 soil (0-15 cm). These findings show that a pre-plant soil NO3-N test for winter wheat can facilitate identification of sites where starter-N will produce economic returns and reduce potential NO3-N losses to water resources.