Quantifying the amount of carbon (C) incorporated from decomposing residues into soil organic carbon (CS) requires knowing the rate of C stabilization (humification rate) into different soil organic matter pools. However, the differential humification rates of C derived from belowground and aboveground biomass into CS pools has been poorly quantified. We estimated the contribution of aboveground and belowground biomass to the formation of CS in four agricultural treatments by measuring changes in d13C natural abundance in particulate organic matter (CPOM) associated with manipulations of C3 and C4 biomass. The treatments were (1) continuous corn cropping (C4 plant), (2) continuous soybean cropping (C3), and two stubble exchange treatments (3 and 4) where the aboveground biomass left after the grain harvest was exchanged between corn and soybean plots, allowing the separation of aboveground and belowground C inputs to CS based on the different d13C signatures. After two growing seasons, CPOM was primarily derived from belowground C inputs, even though they represented only ~10% of the total plant C inputs as residues. Belowground biomass contributed from 60% to almost 80% of the total new C present in the CPOM in the top 10 cm of soil. The humification rate of belowground C inputs into CPOM was 24% and 10%, while that of aboveground C inputs was only 0.5% and 1.0% for soybean and corn, respectively. Our results indicate that roots can play a disproportionately important role in the CPOM budget in soils. Keywords Particulate organic matter; root carbon inputs; carbon isotopes; humification rate; corn; soybean.