Carbon Dioxide Emissions and Transformation of Soil Carbon and Nitrogen during Wheat Straw Decomposition

Carbon exchange between soil and the atmosphere plays an important role in the global C cycle, but partitioning soil CO 2 emissions by source (soil organic matter mineralization, crop residue decomposition, and root respiration) is difficult. Our objective was to determine the contribution of decomposing wheat ( Triticum aestivum L.) straw to CO 2 emissions from a Swinton silt loam (fine‐silty, mixed, mesic Typic Haploboroll) under controlled conditions (constant 20°C). Two types of straw (i.e., fresh straw collected shortly after harvest and standing stubble that had “weathered” in the field for a year) were either incorporated into or placed on the soil surface at a rate equivalent to 2800 kg ha ‐1 . One set of soils was watered every 2 or 3 d to 90% of field capacity and a second set was allowed to dry (from 90% field capacity) to below the permanent wilting point before watering. Emissions of CO 2 were measured every 2 or 3 d with a vented chamber connected to a portable CO 2 analyzer. Within 2 d, incorporation of straw increased CO 2 flux from 0.3 to ≈ 1.5 µmol CO 2 m ‐2 s ‐1 . Surface straw had significantly increased fluxes, but the effect was small compared with incorporated straw. Straw type had little effect on emissions. Total CO 2 ‐C emitted in 77 d from continuously moist soil was 25 g m ‐2 with no added straw, 41 g m ‐2 with surface straw, and 73 g m ‐2 with incorporated straw (values are averages for two straw types). In all, 38% of incorporated‐straw C and 13% of surface straw C was emitted as CO 2 . Soil subjected to moist‐dry cycles emitted from 36 to 62% less CO 2 than continuously moist soil. The Douglas‐Rickman residue‐decomposition model simulated the amount of straw C remaining in the soil as a function of time. In the absence of straw, CO 2 ‐C emitted was similar to the amount by which light‐fraction C decreased, suggesting that this labile fraction of organic matter was a major source of respired C.