Long-term changes in total soil organic C usually occur gradually. These long-term trends might be obscured by smaller, rapid changes in soil C due to seasonal inputs of plant residues, roots, and exudates, or decomposition of such inputs. Yet there is little, if any, data describing the magnitude of seasonal changes in soil C. If seasonal fluctuations in soil C are substantial, then important implications exist for accurate comparison of soil C between sites, between treatments, and even in the same experimental unit over time. Thirty-nine consecutive monthly soil samples were taken from a field experiment planted every year with winter wheat (Triticum aestivum L.) in the Pacific Northwest, United States. The variation in soil organic C was 14 to 16% of the mean over the 39-mo period in the top 250 kg m-2 equivalent mass (~0- to 20-cm depth). Two to eight percent could be identified as a regular seasonal pattern. The no-till management system had the greatest seasonal fluctuation, and the timing of the annual maximum was different from that of the tilled soil management treatments. In the shallower soil layer (~0-7 cm), total soil organic C varied 12 to 29% in which 4 to 13% could be attributed to a 12-mo seasonal pattern. Given the small magnitude of changes in soil C being measured and modeled in many agricultural and natural systems, soil samples taken at a single point in time are likely to encounter substantial but hidden measurement variability. The variability may be compounded by factors of the timing of sampling in relation to natural soil organic matter cycles and differences in the cycle due to treatment and weather. Sampling plans, which account for seasonal fluctuation and the different fluctuation patterns under different soil situations, will improve measurement accuracy.
