Aggregate formation and carbon sequestration by earthworms in soil from a temperate forest exposed to elevated atmospheric CO2: A microcosm experiment

Abstract

The role of soils in mitigating increases in atmospheric carbon dioxide (CO2) levels is uncertain, in part for the complex biotic and abiotic interactions determining soil carbon change. Earthworms, in particular, interact with the physical and chemical protection mechanisms of organic matter, major determinants of carbon storage capacity of soils. Studies of a Liquidambar styraciflua forest plantation located at the Oak Ridge National Laboratory (ORNL), which was exposed to elevated CO2 using free air CO2 enrichment (FACE), show that a key mechanism facilitating soil carbon gain was the protection from rapid decomposition afforded by soil aggregates. To evaluate the effects of site earthworms with different feeding behaviors on soil aggregate formation and the sources of organic matter stabilizing these aggregates, we conducted a 26-day laboratory incubation experiment using plant and soil materials with differential dual isotopic compositions obtained from different CO2 and 15N-labeling treatments at the ORNL-FACE site. We used crushed and sieved (<250 μm) unlabeled soil (δ13C = −25.5‰; δ15N = 5.1‰) to create four treatments: (I) soil only; (II) soil and plant material; (III) soil, plant material, and the native, endogeic earthworm Diplocardia sp.; (IV) soil, plant material, and the European, epi-endogeic earthworm Lumbricus rubellus. Added plant materials consisted of both sweetgum (L. styraciflua) leaf (δ13C = −34.2‰; δ15N = 4755.4‰) and root (δ13C = −38.7‰; δ15N = 44.7‰) litter. Overall, earthworms increased the mass of newly formed soil macroaggregates >250 μm (p < 0.001). Most of the carbon within macroaggregates was soil-derived. Leaf- and root-derived carbon was found only in the treatment with L. rubellus. Hence, the source of carbon within macroaggregates paralleled earthworm feeding ecologies, with endogeic earthworms (Diplocardia sp.) feeding mostly on soil organic matter and epi-endogeic earthworms (L. rubellus) feeding on both plant residues and soil organic matter. Our results suggest that earthworms at the ORNL-FACE site directly contribute to the formation of soil aggregates and, could be an important factor contributing to the soil stabilization of increased recent carbon inputs resulting from atmospheric CO2 enrichment at this site.