Print PageCold temperatures and anoxic conditions in the Arctic diminish microbial activity. As a result mineralization rates are low and the system is nitrogen-limited. Reducing these constraints may result in a net loss of Arctic soil C. However, SOM decomposition is mediated through the formation and stabilization of water-stable aggregates. In particular, small macroaggregates (250-2000µm) physically protect SOM and further reduce SOM loss through inducing SOM incorporation into microaggregates occluded within macroaggregates. During the 2007 growing season, we collected soils on the active mineral soil layer directly below the organic horizon in a long-term nutrient addition experiment at the Arctic LTER site at Toolik Lake, on the Alaskan North Slope. Soils had been fertilized with additional nitrogen and phosphorus since 1996 and 1989. Soils were initially separated into four size classes of water-stable aggregates (Large and small macroaggregates, free microaggregates, and silt+clay). A subsample of the small macroaggregate fraction was then physically disrupted and separated into three fractions (coarse sand and particulate organic matter, occluded microaggregates, and silt+clay). We analyzed differences in aggregate size distribution, C and N allocation, and C:N in each fraction and sub-fraction. We will present a comparison of free and occluded microaggregates.
Results/Conclusions
Occluded microaggregates comprised, on the average, approximately 50% of small macroaggregate mass. Occluded microaggregate abundance was not affected by nutrient addition except in late June, when control soils contained significantly more occluded microaggregates than fertilized soils. Occluded microaggregate C and N concentrations were not significantly affected by nutrient addition. Free microaggregates in soils with nutrient addition since 1989 contained more N on a whole soil basis, and a lower C:N than the control, but otherwise did not differ with nutrient addition. However, occluded microaggregates contained on average, more C and N than free microaggregates. The ratio of the free:occluded microaggregates remained between 0.73 and 0.84 through the first three sampling dates in the control, then increased to 1.29 by August, while in fertilized soils, this ratio was >1 by mid-June. The ratio of free:occluded C:N approached one by the August sampling date, which indicates that SOM quality (lower C:N) was initially greater in occluded than free microaggregates, but that macroaggregate turnover may have led to a homogenization of the free and occluded microaggregate pools. Our results suggest that macroaggregate turnover rate is greater in soils under nutrient addition than control soils, which may result in increased microbial degradation of SOM.
