Abstract
Lupines (Lupinus lepidus var. lobbii) are important integrators of above and belowground development of Mount St. Helens (1980) pyroclastic substrates because they increase soil organic matter formation and microbial activity and influence other biotic processes. However, basic information is required to understand the unfolding pattern of soil development and to corroborate evidence for increasing rates of organic matter accumulation suggested by earlier work. Soil properties were measured in bare pyroclastic sites without lupines or other plants, under live and under dead L. lepidus. In 2005, pyroclastic substrates had low cation exchange capacity but appeared able to supply sufficient P for plant growth. Soil under live and dead lupines contained higher concentrations of soluble and total C and N, larger, more active microbial communities, more Bradford reactive soil protein and enzymes, and had higher mineralization potentials than bare soil. Comparatively, lupine soil was less dense and had lower C to N ratios and relative respiration rates. Soil microbial biomass-C, determined by substrate-induced respiration, had not increased under lupines since 1990, and was indistinguishable from hot water soluble-C, suggesting microorganisms were a predominant pool of labile-C in these developing soils. Evidence for an important soil glycoprotein, glomalin, in these pyroclastic substrates suggests it can form early during soil development. Concentrations of total C and N under lupines have risen to nearly 4 and 0.4 g kg−1 respectively since the 1980 eruption, but 2005 data indicate little change since 1997 and imply inputs from lupines into soil are in equilibrium with losses.
Published Version
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