Soil organic matter (SOM) cycling between soils and the atmosphere affects a wide range of important ecosystem functions. However the key processes controlling this cycle, fine root inputs and heterotrophic respiration, are poorly understood primarily because they are difficult to directly measure in the field. Here we used radiocarbon measurements of density separated SOM, light fraction (LF) characterization with nuclear magnetic resonance (NMR), and root biomass estimates to examine the relative effect of two industrial forestry practices, fertilization and the genetic control of planted seedlings, on SOM cycling.  The study sites were two 10-year old loblolly pine plantation forests in north central Florida that were planted on sandy Spodosols and that received two different levels of fertilization. Our primary hypothesis was that greater aboveground growth would correspond to increased inputs of C to the soil as root biomass, and a greater accumulation of SOM. For the density separated fractions, the A horizon LF (<1.6 g cm-3) was nearly 98% of the SOM in these sandy soils at both research sites, but it was unaffected by fertilization or family deployment despite fine root biomass being on average significantly lower (53%)(p<0.01) in the fertilized forests. In addition, LF radiocarbon values ranged from -9-127‰ and were unaffected by any treatment. The wide-range in radiocarbon carbon values indicated that even in these sandy soil conditions, some components of the LF fraction decomposed slowly.  The chemical composition of the soil was affected by fertilization, but not in consistent ways between sites. Overall, the results suggest that despite a significant reduction in root biomass with increased fertilization, and as observed in a companion study, soil CO2 efflux, the amount of SOM remained unchanged in these forests.