Earthworm presence in northern forests: Impact on distribution of soil carbon within aggregate fractions.

摘要

Growing concerns over climate change is driving research aimed at determining ways of retaining soil carbon (C) within managed northeastern forests. Earthworms are exotic to the state of Vermont and the current extent of earthworm community presence in the state's forests, as well as the long term impact these communities will have on soil C storage, is still unknown. Current research suggests that earthworms have conflicting effects on the C cycle of soils, simultaneously enhancing mineralization through soil mixing, while protecting C through the stabilization of microaggregate (mA) structures. The mA soil fraction represents a pool of physically stable structures capable of maintaining occluded C for long periods of time. To date, studies investigating earthworm effects on mA formation and occluded C have rarely been done in undisturbed forest soils.;Earthworms were found in 10 of 18 forest sites utilized in a statewide Vermont earthworm survey, and community presence correlated with thinner forest floor depths. For 8 sites, the impact of earthworm presence on the quantity of C within water stable mA was investigated. Earthworm presence correlated with greater total C in the top 20 cm of mineral soil, highlighting the relocation of the forest floor noted in all 18 sites. A small, but significant, decrease was noted in the proportion of bulk soil mA, however through C enrichment from the forest floor, there was a significant increase in the pool of mA-associated C. A paired mesocosm study was also conducted, utilizing the endogeic earthworm species Aporrectodea tuberculata, placed in an earthworm-free, undisturbed forest soil. Findings from this study corroborated the correlations noted in the field with significant, though small, decreases in the proportion of bulk soil mA. The larger macroaggregate fraction was increased by about 4 times under earthworm influence. The C enrichment of mA structures occluded within the macroaggregate fraction accounted for approximately 95% of the total increase in mA-associated C, and 50% of the total C integrated into the mineral soil. It can be assumed that the C preferentially occluded within the mA structures by earthworm ingestion will experience longer mean residence time relative to bulk soil C.;We conclude that, for the forest soils investigated, earthworm communities decreased the proportion of mA slightly but that the pool of physically stabilized C was increased through mA turnover. Forest soils usually experience low soil mixing and therefore typically contain high proportions of mA, though the quantity of C within these structures varies. Due to mA restructuring within the earthworm gut, it is unlikely that earthworm community expansions will alter the proportion of mA in forest soils, however the quantity of C present within these structures is likely to increase. The individual site investigated in the controlled study was particularly low in mineral soil C, and therefore the long-term presence of earthworms would likely result in an increase to mineral C storage. However, this result may not be applicable for forests with high levels of mineral soil C prior to earthworm invasion.