ORIGINAL PAPER Distribution of organic carbon in physical fractions of soils as affected by agricultural management Sindhu Jagadamma & Rattan Lal Received: 22 October 2009 / Revised: 25 March 2010 / Accepted: 26 March 2010 / Published online: 13 April 2010 # Springer-Verlag 2010 Abstract Soil organic carbon (SOC) is distributed hetero- geneously among different-sized primary particles and aggregates. Further, the SOC associated with different physical fractions respond differently to managements. Therefore, this study was conducted with the objective to quantify the SOC associated with all the three structural levels of SOC (particulate organic matter, soil separates and aggregate-size fractions) as influenced by long-term change in management. The study also aims at reevaluating the concept that the SOC sink capacity of individual size- fractions is limited. Long-term tillage and crop rotation effects on distribution of SOC among fractions were compared with soil from adjacent undisturbed area under native vegetation for the mixed, mesic, Typic Fragiudalf of Wooster, OH. Forty five years of no-till (NT) management resulted in more SOC accumulation in soil surface (0– 7.5 cm) than in chisel tillage and plow tillage (PT) treatments. However, PT at this site resulted in a redistri- bution of SOC from surface to deeper soil layers. The soils under continuous corn accumulated significantly more SOC than those under corn–soybean rotation at 7.5–45 cm depth. Although soil texture was dominated by the silt-sized particles, most of the SOC pool was associated with the clay fraction. Compared to PT, the NT treatment resulted in (i) significantly higher proportion of large macroaggregates (>2,000 μm) and (ii) 1.5–2.8 times higher SOC concen- trations in all aggregate-size classes. A comparative evaluation using radar graphs indicated that among the physical fractions, the SOC associated with sand and silt fractions quickly changed with a land use conversion from native vegetation to agricultural crops. A key finding of this study is the assessment of SOC sink capacity of individual fractions, which revealed that the clay fraction of agricul- tural soils continues to accumulate more SOC, albeit at a slower rate, with progressive increase in total SOC concentration. However, the clay fraction of soil under native woodlot showed an indication for SOC saturation. The data presented in this study from all the three structural levels of SOC would be helpful for refining the conceptual pool definitions of the current soil organic matter prediction models. Keywords Soil organic carbon . Physical fractionation . Aggregates . Particulate organic matter . Tillage . Crop rotation Introduction Carbon sequestration in soils can offset the increasing atmospheric CO 2 concentration. The C sink capacity of world soils, estimated at about 1 Pg C year -1 , can annually offset 0.47 ppm of CO 2 increase in the atmosphere (IPCC 1999; Lal 2005). In addition, several ancillary benefits associated with soil organic carbon (SOC) sequestration are essential to numerous ecosystem services including food production, and soil, water and air quality. If the potential benefits of SOC sequestration are to be realized, then the importance of quantifying the underlying processes that S. Jagadamma (*) Environmental Sciences Division, Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN 37830, USA e-mail: jagadammas@ornl.gov R. Lal Carbon Management and Sequestration Center, The Ohio State University, 2021 Coffey Rd, Columbus, OH 43210, USA Biol Fertil Soils (2010) 46:543–554 DOI 10.1007/s00374-010-0459-7