Warming has been linked to changes in carbon cycling of Arctic soils. 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. Removing this constraint on nutrient availability results in a vegetation shift and loss of soil carbon; however, the mechanisms behind soil carbon loss are not well understood. For this study, our focus 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 nitrogen and phosphorus since 1996 and 1988 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). Density floatation was used to separate light fraction (LF) organic matter from heavy fraction in small macroaggregates and microaggregates. We analyzed differences in aggregate size distribution, carbon and nitrogen allocation, and C:N in each fraction.
Results/Conclusions
Small Macroaggregates were the dominant aggregate fraction in all treatments. Mid-season declines in large macroaggregate abundance statistically differed from the control, though both comprised <10% of the whole soil. Small macroaggregate carbon content did not differ with fertilization, but in fertilized soils, a higher percentage of carbon was allocated to LF than control soils. LF nitrogen concentrations were greater in soils fertilized since 1988 than other soils. More nitrogen was allocated to LF in fertilized soils than control. Fertilized soils had greater root biomass earlier in the season than control, but while control root biomass stayed constant, fertilized root biomass declined by the end of the season, suggesting an increase in decomposition. Small macroaggregate LF had a lower C:N than microaggregate LF in all treatments. The C:N dropped over the growing season in all small macroaggregate fractions with soils fertilized since 1988 having the lowest C:N. Our results suggest that nutrient addition results in changes in SOM dynamics during the growing season. A decrease in root biomass over the growing season may indicate increased root decomposition with fertilization. Reallocation of SOM from physically protected aggregates to LF with fertilization may result in shifts in SOM stability in these soils.
