| Summary: | 2010 Fall. Includes bibliographical references. Warming has been linked to changes in Arctic soil carbon cycling. Cold temperatures and anoxic conditions in the Arctic diminish microbial activity. As a result mineralization rates are low and the system is nitrogen-limited, further reducing biological activity. Reducing this constraint on nutrient availability has resulted in a vegetation shift and loss of soil carbon; however, the mechanisms behind soil carbon loss are not well understood. The focus of this study was on the active mineral layer directly below the organic horizon. Soils were collected during the 2007 growing season from a long-term nutrient addition experiment in which soils had been fertilized with additional N and P since 1996 and 1989 at the Arctic LTER site at Toolik Lake, on the Alaskan North Slope. Roots were separated from the soil to estimate biomass. Soils were separated into four size classes of water-stable aggregates (Large and small macroaggregates, microaggregates, and silt+clay). Small macroaggregates were separated into three sub-fractions (coarse particulate organic matter (POM), occluded microaggregates, and silt+clay). Density floatation was used to separate light fraction (LF) organic matter from heavy fraction in small macroaggregates and microaggregates. Intra-aggregate POM (iPOM) content was determined in small macroaggregates and microaggregates. Differences in aggregate size distribution, C and N allocation, and C:N in each fraction were analyzed. Small Macroaggregates were the dominant aggregate fraction in all treatments. Mid-season declines in large macroaggregate abundance from soils with nutrient addition differed statistically from the control, though both comprised <10% of the whole soil. The ratio of free:occluded microaggregates rose over the growing season, which indicated that microaggregates occluded within small macroaggregates were released upon macroaggregate disruption. Occluded microaggregates tended to possess higher carbon and nitrogen contents ...
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