The purpose of this study is to assess the impact of different crop rotations, carbon inputs and tillage practices on the chemical and physical characteristics of soil organic matter to develop a better understanding of soil C sequestration mechanisms.  The study was conducted at the USDA-ARS Beltsville Farming Systems Project (FSP), a long-term cropping systems trial in Maryland, comprised of organic and conventional cropping systems. The conventional systems are both three-year corn-rye-soybean-wheat/soybean rotations, one under no-till (NT) management and one under chisel till (CT) management.  Neither receives any manure applications.  The organic systems differ in crop rotation length and frequency of manure input (4500-6700 kg ha^-1 application^-1).  A two year corn-rye-soybean-vetch rotation receives poultry litter (PL) every other year; a three year corn-rye-soybean-wheat/vetch rotation receives PL two of every three years; and a six-year corn-rye-soybean-wheat/alflalfa-alfalfa-alfalfa-alfalfa rotation receives PL two of every six years.  The mass of soil C to a depth of 1 m in the three organic systems was greater than in CT and greater or equal to that in NT in 2006, 11 years after plots were established.  The distribution of soil C with depth differed among CT, NT and the three organic systems.  However, total soil C and distribution with depth were essentially identical among the three organic systems despite differences in the quantity and quality of C inputs.  In 2011 we sampled FSP soils to 30 cm depth and divided cores into five depth increments. We isolated particulate organic matter (POM) using a combination of size (53 um) and density fractionation (sodium polytungstate 1.7 g cm^-3).  Free and occluded POM (fPOM and oPOM, respectively) were collected from each depth increment.  We will present data on fPOM and oPOM total C and N and chemical characteristics determined using pyrolysis gas chromatography-mass spectrometry to explore differences in C sequestration mechanisms in FSP cropping systems.