Nitrogen Impacts Si Uptake and Partitioning By Wheat and Canola, Soil Si Forms and Soil Crusting.

Soil crusting is an issue in semi-arid, wheat (Triticum aestivum L.) dominated inland Pacific Northwestern U.S. It creates seedling establishment problems with poor seedling establishment of late summer-seeded winter crops during dry, hot conditions. This region is challenged to diversify wheat rotations with canola (Brassica napus L.) in response to diversifying food, feed and fuel global markets. A subsequent shift long-term soil-plant nutrient cycling, including Si balances may occur as a secondary affect in intensively N managed canola-wheat cropping systems, which could impact soil chemical and physical properties such as crusting. Many factors may affect soil crusting including crop residue chemistry and N fertilization. Increased N supply greatly increased water transpiration, total shoot dry matter, hemicellulose and cellulose yields, but only minor increases in shoot Si and lignin yields were observed. In contrast, both crops increased root Si with increased transpiration, and canola root Si increased more than wheat. Soil incubations were conducted to determine the effects of Si, N fertilization and crop residues from wheat and canola on soil mechanical resistance and soil crusting. Ritzville silt loam soils were collected from two fields, one previously cropped in a cereal-fallow for over 50 years and one previously cropped in wheat or canola-fallow for 29 years. Four levels of an aqueous solution of H₄SiO₄ were applied to each soil: 0, 0.02, 0.2, and 2.0 g Si/kg soil. Soil was also amended with wheat or canola residues with and without 200 mg N/kg soil as ammonium phosphate and incubated for 140 days. Silica applications increased soil Si_am, Si_ws, surface resistance, and crust thickness. Electron micrographs showed increased amorphous material, presumably a Si precipitate, on soil particles with increased Si application. Nitrogen fertilization lowered soil pH, soil Si_am, Si_ws, surface resistance, and crust thickness, yet crop residue type had no effect on these parameters. We originally hypothesized that canola integration of wheat rotations might reduce Si recycling to the soil surface since straw canola Si is much lower than wheat, thereby reducing soil surface crusting. While Si seems to be involved with crusting of agricultural soils, lower canola shoot Si concentrations are may be offset by higher root Si concentrations. Yet growers testify increased “mellowness’ of soil following canola production, so other biochemical differences such as fiber composition may play a role.