Monday, November 13, 2006
110-4
Sensitivity of Soil Organic Carbon Dynamics to Long-Term Throughfall Exclusion in Interior Alaska.
Sarah Runck1, David Valentine2, and John Yarie1. (1) Dept of Forest Sciences, Univ of Alaska, PO Box 757200, Fairbanks, AK 99775-7200, (2) University of Alaska, U of AK Forest Science, PO Box 757200, Fairbanks, AK 99775-7200
Despite increased precipitation trends during the past century, warm and dry summers in interior Alaska have induced moisture stress and inhibited growth of Alaskan tree species. In a long-term (1989-present) experiment excluding summer throughfall from upland and floodplain mid-successional forests near Fairbanks, Alaska, stand-level aboveground net primary production (ANPP) has declined only in upland throughfall exclusions, even though surface soil moisture levels declined in all sites. Our study addresses the influence of throughfall exclusion on key components of soil organic C storage, particularly in surface soils where soil moisture content closely tracks weather and a substantial proportion of soil organic matter is stored. We hypothesized that dryer surface soils have (1) relocated rhizodeposited C downward in the soil profile and (2) slowed decomposition in surface soils. After 16 years of throughfall exclusion, there has been no detectable change in O horizon mass, depth, or bulk density or mean annual aboveground litter input. However, O horizon live coarse root (2-5 mm diameter) biomass declined in uplands by 50% and in floodplains by 80%. To determine the effect of throughfall exclusion on decomposition, we decomposed a common substrate, wooden tongue depressors, in the O horizon and upper 15.2 cm mineral soil for one year. Throughfall exclusion had a strong negative effect on the decomposition environment of surface soil, reducing common substrate mass loss in the O horizon of uplands by 82% and floodplains by 86% compared to controls. Reduction in common substrate mass loss in mineral soil at either landscape position was marginal. These results suggest that reduced soil moisture may increase mean residence time of C in mid-successional floodplain boreal forests, whereas in drier upland forests, reduced C turnover in surface soils may be offset by reduced ANPP.