CARBON-DIOXIDE EMISSIONS AND TRANSFORMATION OF SOIL CARBON AND NITROGEN DURING WHEAT-STRAW DECOMPOSITION

Carbon exchange between soil and the atmosphere plays an important rol e in the global C cycle, but partitioning soil CO2 emissions by source (soil organic matter mineralization, crop residue decomposition, and root respiration) is difficult. Our objective was to determine the con tribution of decomposing wheat (Triticum aestivum L,) straw to CO2 emi ssions from a Swinton silt loam (fine-silty, mixed, mesic Typic Haplob oroll) under controlled conditions (constant 20 degrees C), Two types of straw (i.e., fresh straw collected shortly after harvest and standi ng 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, Emissi ons of CO2 were measured every 2 or 3 d with a vented chamber connecte d to a portable CO2 analyzer, Within 2 d, incorporation of straw incre ased CO2 flux from 0.3 to approximate to 1.5 mu mol CO2 m(-2) s(-1). S urface straw had significantly increased fluxes, but the effect was sm all compared with incorporated straw. Straw type had little effect on emissions. Total CO2-C emitted in 77 d from continuously moist soil wa s 25 g m(-2) with no added straw, 41g m(-2),vith surface straw, and 73 g m(-2) with incorporated straw (values are averages for two straw ty pes). In all, 38% of incorporated-straw C and 13% of surface straw C w as emitted as CO2, Soil subjected to moist-dry cycles emitted from 36 to 62% less CO2 than continuously moist soil. The Douglas-Rickman resi due-decomposition model simulated the amount of straw C remaining in t he soil as a function of time. In the absence of straw, CO2-C emitted was similar to the amount by which light-fraction C decreased, suggest ing that this labile fraction of organic matter was a major source of respired C.