Soil carbon dynamics across a windthrow disturbance sequence in southeast alaska

Few studies have examined the influence of natural disturbances, such as windthrow, on soil organic matter formation, stabilization, and loss in soils. In shallow (<1 m) mountain forest soils, windthrow activity may result in the redistribution and mixing of mineral and organic soil horizons down to bedrock. We studied the patterns of soil carbon, the dominant constituent of soil organic matter, in watersheds along a windthrow disturbance sequence in a mountainous temperate rain forest in southeast Alaska. Our objectives were (1) to evaluate the influence of windthrow and illuviation on the accumulation of soil organic carbon in mineral horizons and (2) to compare the forms of soil organic matter that have accumulated. Soils were described, and the thickness of the major organic and mineral horizons was measured, every 5 m along transects in three watersheds with contrasting windthrow histories. A subset of the soil description sites was randomly selected and then sampled to determine the quantity and quality of soil carbon. Mineral soil samples were physically fractionated based on particle density. Total carbon (C) and nitrogen (N), natural ¹⁵N and ¹³C abundance, micromorphologic analysis (using scanning electron microscopy), and solid‐state ¹³C nuclear magnetic resonance (NMR) were used to compare soil organic matter pools in the three watersheds. The Bh horizon heavy‐fraction (>1.65 g/cm³) C pools decreased from 25 Mg/ha in the least disturbed watershed to 3 Mg/ ha in the most disturbed watershed; whereas light‐fraction C pools were similar (4–6 Mg/ ha) across all watersheds. Soil C stocks decreased from 216 Mg/ha in the least disturbed watershed to 157 Mg/ha in the most disturbed watersheds. Evidence was found that mobile organic carbon transported in soil water accumulated in mineral horizons principally through sorption to mineral particles. Strong association with mineral particles was greater in the thicker, presumably older, illuvial horizons. We found light amorphous material in some of these thicker illuvial horizons, suggesting that mobile organic carbon may have been immobilized through flocculation of metal‐bearing organic acids. Watersheds that experienced more intense soil mixing from windthrow had lower levels of strongly humified soil organic matter, and more in a partially decomposed particulate form.