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Legumes like Lupinus lepidus and L. latifolius affect soil C and N concentrations and microbial activity in Mount St. Helens’ pyroclastic deposits. Concentrations of total Kjeldahl-N (TKN), total soluble-C (TOC) were measured in soil from under live L. lepidus (LULE), live L. latifolius (LULA) and dead L. lepidus (DEAD), and in bare soil (BARE). Soil microbial biomass-C estimated by substrate-induced respiration (SIR-C), metabolic quotient (qCO(2)) and C and N mineralization from lupine biomass-amended and non-amended soils were also estimated. The greatest concentrations of TKN, TOC and SIR-C were observed near the surface. In surface soil (0-5 cm), LULA soil contained the highest concentrations of TKN, TOC and SIR-C followed by DEAD, LULE and BARE plant-soil types, respectively. The concentrations of H2O-C were relatively constant among different plant-soil types and depths. Changes in qCO(2) with depth varied with plant-soil type. While TOC was strongly and linearly correlated with TKN, the relationship between SIR-C and both TKN and TOC was curvilinear and suggested a carrying capacity for SIR-C of about 400 kg microbial biomass-C ha(-1). In an incubation study, the patterns of cumulative net respiration were similar to those observed for TKN and TOC (i.e. LULA > DEAD > LULE > BARE). Soils amended with L. (atifolius biomass had a higher rate of net N mineralization than soils amended with L. lepidus biomass. Low amounts of TOC, TKN and soluble C, together with zero net N mineralization in some amended soils, suggest that microbial communities in Mount St. Helens’ soils are N limited. Competition for inorganic-N between microorganisms and plants may thus be an important controlling mechanism for succession.