Soil Respiration in a Winter Wheat Ecosystem

Respiration of a silt loam soil (Udollic Ochraqualfs) was studied under cultivation of winter wheat (Triticum aestivum L.) over a 3-yr period. Evolved CO₂ was measured by the alkali absorption method during 24-h periods at intervals of usually two weeks. Extended estimates of CO₂ evolution were made after extrapolating the measured results, in accord with abiotic influences, to include the longer time intervals. Among the abiotic factors examined in relation to evolution of CO₂ during the annual agricultural cycle, the most significant was temperature and second, soil water content (R² = 0.67 for two factors combined). Better models were obtained when the annual cycle was split into two periods, that with predominating plant activity from October through June, and that when CO₂ arose solely from the activity of heterotrophs from harvest to planting. Although factors affecting CO₂ evolution were the same, predictability increased, and up to 88% of changes in soil respiration during the first period were related to soil temperature and moisture content of the 0-to 10-cm layer. For the second period, air or soil temperature plus moisture of the soil at 20 to 30 cm accounted for 76% of the variability. Using models developed for both periods, average annual respiratory loss of C under wheat in Missouri was approximated at 640 g m⁻². Maximum soil respiration, presumably from decomposition activity on freshly incorporated residues, occurred during a 56-d period after harvest, and accounted for 40% of the total annual amount of CO₂ evolved from soil. Differences in seasonal patterns of soil respiration and photosynthetic uptake of CO₂, indicate that no more than one-fifth of the photosynthesis requirement for CO₂ by winter wheat is produced by soil respiration during the growing season.