Vertical Distribution of Organic Carbon in Boreal Forest Soils across Alaska.

Organic carbon accumulation and distribution patterns in soil profiles found in boreal forest ecosystems across Alaska are presented. Study sites were selected to represent different climatic regions and soil parent materials. Soil pits were excavated to one meter or more or to either a lithic or paralithic contact. Soil morphological properties were described and genetic horizons sampled according to the USDA Soil Survey Manual. In well drained, upland soils with uniform parent materials such as loess, over 50% of the SOC is stored in the organic horizons and, then, decreases gradually with depth. In well drained soils, formed in loess or alluvium over outwash, SOC accumulation in mineral soils occurs mainly in the finer-textured upper horizons followed by an abrupt decrease in the lower substrate. Volcanic ash is the dominant soil forming parent material in Southcentral Alaska and SOC distribution has an irregular pattern downward for more than a meter due to the intermittent nature of volcanic ash deposition. In permafrost-affected soils, considerable quantities of SOC are sequestered in the upper permafrost that is typically more than 40 cm below the mineral surface. Natural disturbances, such as wildfire, windthrow, and erosion and deposition, and management practices especially land clearing affect the carbon stored deeper in the soil profile. These disturbances can alter the surface drainage or the soil temperature regime and thus expose the deeper carbon to accelerated decomposition. In assessing SOC status and formulating carbon models in northern boreal forest ecosystems, carbon below 20 cm cannot be ignored.