In long-term experimental plantations at La Selva Biological Station, Costa Rica, we investigated the effects of six tree species on soil properties. Species included Pinus patula (the only exotic), Pentaclethra macroloba (N-fixing), Hyeronima alchorneoides, Virola koschnyi, Vochysia ferruginea, and V. guatemalensis. In 1988, trees were established on degraded pasture in a randomized complete block design. Soil organic C (SOC) differed significantly among species by Year 3, and this difference has persisted into Year 15. The change in SOC ranged from a net loss over 15 yrs of 5.6 Mg/ha under Pinus to a net increase of 2.7 Mg/ha under V. ferruginea. To assess the effects of detrital quality on SOC dynamics, we measured biochemical attributes of fine roots, senesced leaves, and soil for each species. These tissues differed in three lignin-derived families of phenols, the cinnamyls, syringyls and vanillins. Soil differed significantly among species in concentrations of cinnamyls and syringyls, and syringyl concentrations were significantly correlated with SOC (P = 0.03). Representing the degree of decomposition, the ratio of carboxylic acid to aldehyde compounds for syringyls was significantly higher under Pinus than under other species. For vanillins, this ratio varied in the sequence Virola < Vochysia guatemalensis, V. ferruginea < Hyeronima, Pentaclethra < Pinus. In laboratory incubations for determination of potential C mineralization, CO₂-C released was lower under Virola, Pinus, and Pentaclethra relative to the others. With the exception of Virola, these data together with biochemical results indicated that SOC under Pinus and Pentaclethra was more decomposed. We modeled with CENTURY to integrate data regarding organic matter inputs, detrital biochemistry, and potential mineralization, and to provide insights into plant-mediated controls over soil carbon sequestration.