Soil Respiration in a Winter Wheat Ecosystem

Abstract Respiration of a silt loam soil (Udollic Ochraqualfs) was studied under cultivation of winter wheat ( Triticum aestivum L.) over a 3‐yr period. Evolved CO 2 was measured by the alkali absorption method during 24‐h periods at intervals of usually two weeks. Extended estimates of CO 2 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 CO 2 during the annual agricultural cycle, the most significant was temperature and second, soil water content ( R 2 = 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 CO 2 arose solely from the activity of heterotrophs from harvest to planting. Although factors affecting CO 2 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 CO 2 evolved from soil. Differences in seasonal patterns of soil respiration and photosynthetic uptake of CO 2 , indicate that no more than one‐fifth of the photosynthesis requirement for CO 2 by winter wheat is produced by soil respiration during the growing season.