Chemical changes in cereal straw under two fallow systems and its relationship to nutrient availability
In recent years, conventional mechanical fallow has been partly replaced by herbicide-fallow on the prairies. With the chemical system of weed control, the straw is not mixed with the soil until seeding the following spring; meanwhile the nutrient status of the straw may change during the fallow period. To determine how the nutrient status of the straw may be changing, a site was chosen and divided into a chemical fallow (chem) and mechanical fallow (conv) system. The standing residue was sampled from the chem side and surface trash from the conv side at six week intervals starting in April of 1994. The site was located in the Brown soil zone near Central Butte. The straw being sampled was from a previous wheat crop. The straw on the conv site exhibited increases in nitrogen and phosphorus content while carbon content decreased due to loss of C as CO2 during decomposition. Significant losses of water soluble P occurred from the standing straw over the fallow period. To help understand how straw weathering affects nutrient availability in a subsequent crop, a growth chamber experiment was conducted in which three different straw treatments were applied to the soil in pots and wheat was grown in the soil. Anion-exchange membranes were buried for two hours once a week in each pot to monitor changes in the nutrient status of the soil. Wheat plants were grown for 45 days and then harvested. Dry matter yields and nitrogen and phosphorus uptake by the plants was determined. Yields were reduced when straw was applied to the soil. Nitrogen uptake was reduced with the addition of straw to the soil due to immobilization by both fresh and weathered straw. However, the phosphorus concentration in the plant tissue and also the P uptake was significantly enhanced with straw additions, indicating that leaching of soluble P contained in cereal straw may be an important contributor to P available to a subsequent crop.
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