a b s t r a c tThe long-term impact of earthworm presence on soil carbon (C) dynamics of previously uninhabited northeastern forests is still largely unknown. Currently, earthworm presence is understood to both enhance soil respiration and create stable microaggregates, processes assumed to have conflicting effects on long term C storage. To date, studies investigating earthworm-created microaggregates and occluded C have rarely been done in undisturbed forest soils. A paired mesocosm study (n ¼ 5) was conducted investigating the impact of the endogeic earthworm species Aporrectodea tuberculata on the physical proportion of microaggregates and the associated mineral soil C of a minimally disturbed forest soil. Pairs analyzed after 4 weeks of incubation demonstrated no significant aggregate effects. At 4 months, paired cores with earthworms (WW) showed a 67% increase in large macroaggregates (>2000 mm diameter, lgMA), compared to cores without earthworms (NW). While distribution shifted among various microaggregate pools (free and occluded within macroaggregates), the net proportion of microaggregates in the soil (dry weight basis) was unaltered. After 4 months, the mineral soil of WW cores had an average of 60% more C than the NW cores due to the relocation of the forest floor. The C associated with the microaggregate fractions increased an average of 56%. Of this increase in C, 95% was accounted for by the microaggregates occluded within the lgMA fraction, a fraction that was almost 4 times greater in the WW cores. Over 50% of the C relocated into the mineral soil was associated with the physically protected microaggregate fractions, indicating that though this species of earthworm did not alter the proportion of microaggregates in these soils, they occluded a substantial proportion of C within those physical fractions. In this particular forest soil, the actions of Aporrectodea tuberculata increased the physically protected C pool through microaggregate restructuring and C enrichment.
Harvesting activities are known to decrease forest floor carbon pools, but the response varies with harvest intensity. We examined partial harvesting (33–55% of basal area removed) effects on the forest floor at four northern hardwood sites in Vermont, USA. Six baseline quantitative samples were taken at each site and 9–36 new locations were sampled 1.5–2.6 years after harvesting. Forest soil disturbance was estimated, and basal area was tallied pre- and post-harvest. The forest floor consisted primarily of Oi and Oe horizons. The pre-harvest site means in carbon stock ranged from 6.8 to 12.3 Mg ha-1 and were not significantly different after harvesting. The pre-harvest site means in depth ranged from 2.8 to 4.5 cm and, post-harvest, there was significantly decreased thickness at one site and significantly greater density at two sites postharvest. This compaction was also visually observed in the field. Partial harvesting, which included single-tree and group selection, created highly variable conditions that challenged our experimental design. However, the two sites with the higher number of resampling locations (35–36) had relatively low variability in forest floor metrics and showed significant responses in thickness and density. Continued monitoring is needed to determine long-term trends.
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