Decomposition rate of cereal straw as affected by soil placement

Decomposition rates for crop residues have generally been estimated based on data obtained using buried litter bags. Because of limited soil - residue contact, the litter bag technique may not adequately simulate decomposition when residues are mixed through the soil. In field microplots, decomposition of wheat ( Triticum aestivum) and barley ( Hordeum vulgare) straw ( autumn-incorporated at a rate of 7 t/ ha) mixed through the 0 - 0.20 m soil layer was compared with straw decomposition in fibreglass bags ( 4-mm mesh) buried at a depth of 0.20 m. A surface-placed straw treatment and a no-straw control were included for comparison. Emissions of CO2 were monitored from the incorporated straw treatments and undecomposed straw was recovered after 158 days ( mean soil temperature during the trial period was 8 degrees C at 0.10 m). Emissions of CO2 from the soil-mixed straw treatment were generally greater than from the buried bag treatment in the 2 months following incorporation. Output of CO2-C over the first 73 days averaged 83 g/ m(2) for the soil-mixed straw treatment compared with 61 g/ m 2 for the litter bag treatment and 34 g/ m 2 for the no-straw control. Over the entire trial, CO2-C attributable to straw ( CO2-C output from straw-treated plots minus CO2-C emitted from the control) was 66% greater for soil-mixed straw than for litter bag straw, indicating that within soil placement can have a strong and persistent effect on straw decomposition. Straw type had a small but significant ( P < 0.05) effect on CO2 output ( barley > wheat). Straw mass loss during the trial averaged 66% for soil-mixed straw, 32% for litter bag straw, and 13% for straw placed on the soil surface. The low recovery of soil-mixed straw is partly due to difficulty of extracting small (< 2 mm) residue fragments from the soil; however, such fragments could legitimately be considered part of the soil organic matter. The results confirm that straw that is well distributed through the soil may decompose more rapidly than would be anticipated from litter bag measurements.