It has been hypothesized that Soil Organic Carbon (SOC) associated with fine particles (clay plus silt) has a maximum concentration, that any further SOC accumulation is associated with coarse particles (fine to coarse sand), and that particulate organic carbon (POM, > 53 ìm) cannot accumulate until the soil's capacity for fine particle-associated carbon is exceeded. To test these hypotheses for a Brookston clay loam soil in southwest Ontario, Canada, total SOC contents, and the proportions of carbon associated with fine particles and POM were measured over the 0-30 cm depth under 6 soil management systems. The managements included 3 long-term treatments (1983-2004) consisting of continuous bluegrass sod (BG), no-tillage (NT) and mouldboard plow tillage (MB), plus 3 newer tillage treatments consisting of 8-y MB after 14-y BG, 8-y MB after 14-y NT, and 8-y NT after 14-y MB. All non-BG plots were cropped to a corn-soybean rotation, and SOC was measured during the corn phase. Eight years of MB after BG decreased SOC at 0-5 cm by 59% compared with continuous BG, however, there was no impact on SOC below that depth. Establishing NT after 14-y MB produced a gain of SOC in the top 5 cm, but also a loss in SOC at 10-20 cm relative to long-term MB. Correspondingly, establishing MB after 14-y NT decreased SOC in the surface of 5 cm, but increased SOC at the 10-20 cm depth relative to long-term NT. For the top 5 cm of soil, 22-yr continuous management resulted in different proportions of carbon in POM relative to the total SOC, i.e. 19% POM for BG, 18% POM for NT, and 7% POM for MB. Establishing 8-y MB after 14-y BG resulted in 82% less POM carbon, 42% less silt-associated carbon, and 52% less clay-associated carbon, compared to long-term BG. Establishing 8-y MB after 14-y NT caused 67%, 26% and 21% decreases in the amounts of near-surface (0-5 cm) carbon associated with POM, silt, and clay, respectively, relative to long-term NT. On the other hand, converting 14-y MB to 8-y NT increased the amount of near-surface (0-5 cm) carbon by 100%, 5% and 14% for POM, silt and clay, respectively, relative to long-term MB. Although ploughing BG soil did not change the subsurface carbon distributions, ploughing NT soil did increase the amount of carbon associated with the POM and silt fractions in the 10-20 cm depth. Establishing 8-y NT after 14-y MB decreased the amount of fine particle-associated carbon in the 10-20 cm depth range, especially that in the silt fraction (15% of deduction). In the top 5 cm of Brookston clay loam under continuous BG, carbon associated with fine particles was 71% greater than the maximum estimated by Carter et al. (2003) [Carter, M.R., Angers, D.A., Gregorich, E.G. and Bolinder, M.A. 2003. Can. J. Soil Sci. 83:11–23.]. On the other hand, fine particle-associated carbon in the top 5 cm was 101% of the estimated maximum for 22-y NT, and 87% of the estimated maximum for 8-y NT, indicating carbon saturation in fine particles for long-term NT and a slight potential for further carbon sequestration at the 0-5 cm in short-term NT. The degree of saturation of fine particle-associated carbon in the top 5 cm was about 70% of the estimated maximum for long-term MB, but about 78% of the maximum for 8-y MB after 14-y BG management, implying that further carbon losses from the top 0-5 cm of soil are likely as ploughing continues. The results indicate that for Brookston clay loam soil: i) converting MB tillage to NT affects the SOC distribution but not the total SOC content in the top 20 cm; ii) the hypothesis that SOC associated with fine particles (clay plus silt) has a maximum concentration may be dependent upon soil bioclimatic condition; and iii) POM-carbon can accumulate before the capacity for fine particle-associated carbon is exceeded. Key words: Soil organic carbon, clay plus silt associated C, particulate organic matter, tillage, tillage conversion, Canada.