Print PageNon-native earthworms are invading many northern forests, profoundly altering soil properties including the dynamics of carbon (C) and nitrogen (N). We examined mechanisms of earthworm effects on forest soil C and N by double-labeling sugar maple saplings with 13C and 15N and following the movement of the isotope label from leaf and root litter into soils and soil-aggregate fractions for two years. We applied labeled litter to plots with different earthworm community composition, three plots dominated by Lumbricus terrestris (T), three by L. rubellus (R), and no-worm (NW) controls. We also examined flow of the label from roots of the labeled trees to soil in the presence of earthworms. Soil aggregates are defined on the basis of size, composition, and stability. The overall question is how earthworm activity influences the stabilization of organic C and N in forest soils.
Results/Conclusions
We recovered 13C in soil to 20 cm depth, and after one year 16% to 35% of 13C released from litter was recovered in soil with significantly higher values for R and T than NW plots. In earthworm plots, enrichment of 13C and 15N derived from leaf litter was highest in the macroaggregate fraction, especially in fine particulate organic matter and microaggregates held within the macroaggregates. In contrast, in the no-worm plots highest isotope enrichment was associated with silt and clay fractions, reflecting adsorption of dissolved organic matter. One year after root labeling high enrichment of 13C was observed in rhizosphere soil – an estimated 10% of net 13C assimilation. After two years the distribution of isotope among aggregate fractions was similar to the litter plots with earthworms, i.e. high enrichment of the fine particulate organic matter within macroaggregates. These results indicate that through their feeding and mixing activity earthworms facilitate the rapid conversion of C and N in litter and root detritus into stabilized fractions in mineral soils of northern hardwood forests.
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