Wheat Straw Composition and Placement Effects on Decomposition in Dryland Agriculture of the Pacific Northwest

Abstract

AbstractField‐measured characteristics of cereal residue decomposition under semiarid dryland agriculture are needed for systematic management of residue in reduced tillage systems. Straw placement, loading rate, and nutrients are all important characteristics. Straw composition and placement, at a moderate loading rate, were evaluated for their effects on decomposition. Wheat straw with three different N and S contents was contained in fiberglass cloth bags which were placed above, on, and below the soil surface to simulate, respectively, standing stubble, straw matted on the surface, and straw plowed under. Weight loss and changes in N and S content were measured during a 26‐month period in a winter wheat (Triticum aestivum L.)‐pea (Pisum sativum L.) rotation. After 26 months exposure residue losses were 25, 31, and 85% for placements above, on, and in the soil, respectively. Above‐surface and on‐surface straw showed a nearly constant decomposition rate with little response to weather variables; decomposition rate of buried straws responded to both soil moisture and temperature. Mineralization or immobilization of N and S were sensitive to placement and initial nutrient content of the straws. Net N mineralization in buried straw varied from 17 to 2 kg N/ha as straw composition varied from 0.78 to 0.20% N. When placed above or on the surface net N mineralizations ranged from 6 to −4 kg N/ha as straw varied from 0.78 to 0.20% N. Net S mineralization paralleled N mineralization, however, magnitudes were smaller. First‐order rate constants for weight loss successfully characterized the placement and straw composition effects.