Impact of Hydraulically Restrictive Horizons on Nitrogen Uptake Efficiency in Winter Wheat Grown in the Inland Northwest.

The Palouse region is often noted for its unique topography and deep soils. Despite its reputation, the Palouse also has many fragipan and argillic horizons that affect hydrology of the region. Compaction limits the growth of wheat roots by reducing root penetration and proliferation in deeper layers. It is therefore expected that dense clay layers would limit root access to and uptake of plant available water and nitrogen deeper in the profile. This could have substantial impacts on nitrogen recommendations for wheat crops especially in variable rate systems. An observational study was established in 6 fields spanning the Palouse and dryland wheat growing regions of the inland northwest in March 2017. This study aimed to evaluate nitrogen uptake efficiency in variable field conditions in the Palouse and dryland regions of eastern Washington. An unsupervised approach was used to identify diverse locations in each of the study fields. A principal components analysis of in-season RapidEye satellite imagery was used to identify variable clusters of the fields. A 25 m² observational site was randomly identified within each cluster and monitored throughout the growing season. Each site was sampled for: i) spring and post-harvest soil ammonium and nitrate concentration to 2 m depth , ii) soil bulk density to 1.3 m depth, iii) grain and biomass yield at crop maturity, iv) grain and biomass total nitrogen concentration at maturity, and v) electromagnetic induction scans to 1 m depth post-harvest. This study aims to assess if restrictive layers should be considered in variable nitrogen management schemes and if sensor-based approaches can help guide restrictive layer-based management approaches in wheat-growing regions of the inland northwest.