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

Highlights

• Diplocardia doubled the macroaggregates when compared to treatments without earthworms.

• Native endogeic Diplocardia increased the flow of soil C from other soil fractions.

• Lumbricus rubellus was effective at transferring root litter into aggregates.

• Earthworm casts are an intrinsic part of carbon stabilization at this ORNL-FACE site.

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.

Introduction

Studying mechanisms of soil carbon storage has become relevant in the context of rising atmospheric CO₂ concentrations and projected climatic changes (Schlesinger, 1997, Srivastava et al., 2012). The physical protection of carbon in soil aggregates is a key mechanism affecting soil carbon dynamics (e.g. Tisdall and Oades, 1982, Six et al., 2004, Blanco-Canqui and Lal, 2004, Jastrow et al., 2007). Stabilization of soil organic carbon (SOC) in aggregates decreases its accessibility to microbes and soil fauna, thereby protecting SOC from rapid mineralization and increasing its residence time (Sollins et al., 1996, Marinissen and Hillenaar, 1997, Jastrow et al., 2007). Aggregate turnover is a function of physical processes (e.g., wetting and drying, freezing and thawing, tillage), root and mycorrhizal fungi growth, and bioturbation facilitated by the activities of the soil fauna (Six et al., 2004, Jastrow et al., 2007).

Earthworms can impact soil processes disproportionately to their density or biomass when compared to other organisms (Lavelle et al., 2006, Brussaard et al., 2007). The most direct effect of earthworm activity on SOC dynamics is through their casting, burrowing, and feeding behavior (Blair et al., 1995, Edwards and Bohlen, 1996, Sollins et al., 1996, Lavelle et al., 1998, Curry and Schmidt, 2007). As such, earthworms can promote SOC stabilization in macroaggregates (>250 μm) and microaggregates (53–250 μm) formed in their casts (Marinissen and Hillenaar, 1997, Bossuyt et al., 2004, Bossuyt et al., 2005, Bossuyt et al., 2006, Pulleman et al., 2005). Differences in feeding behavior between earthworm species can alter characteristics of their castings, potentially leading to differential impacts on soil aggregate dynamics and long term SOC storage (Bossuyt et al., 2006).

The Oak Ridge National Laboratory (ORNL) FACE site is one of the few FACE sites where a measurable increase in soil carbon has been demonstrated (Jastrow et al., 2005). The principal mechanism identified for soil carbon accrual at this FACE site was the stabilization of enhanced root carbon inputs in soil aggregates, particularly microaggregates (53–250 μm) (Jastrow et al., 2005). In addition, native Diplocardia earthworms were the most abundant species at the ORNL-FACE site (Sánchez-de León et al. unpublished data). As earthworms are important agents of soil aggregation (e.g., Lavelle and Spain, 2001, Blanco-Canqui and Lal, 2004, Six et al., 2004), investigating the role of earthworms as agents of soil aggregate formation in these soils is therefore needed to better understand the mechanisms facilitating SOC stabilization and accrual at this site.

We used a microcosm experiment to measure aggregate formation mediated by two earthworm species found at the ORNL-FACE site, Diplocardia sp., endogeic (i.e., feeding in mineral soil; Kalisz and Wood, 1995), and Lumbricus rubellus Hoffmeister, epi-endogeic (i.e., feeding between the litter and lower soil layers; (Hendrix and Bohlen, 2002). We used isotopically unlabeled soil (δ¹³C = −25.5‰; δ¹⁵N = 5.1‰) labeled (C-depleted, N-enriched) leaf (δ¹³C = −34.2‰; δ¹⁵N = 4755.4‰) and root (δ¹³C = −38.7‰; δ¹⁵N = 44.7‰) material from the current and elevated CO₂ treatments at the ORNL FACE site to distinguish the organic matter source in soil aggregates formed during the experiment. We hypothesized that Diplocardia sp. would be more effective at producing aggregates than L. rubellus due to the different soil feeding behavior of Diplocardia. We also hypothesized that the organic matter source within earthworm-formed aggregates would be consistent with their feeding ecology: soil organic matter for Diplocardia sp. and a mixture of plant litter and soil material for L. rubellus.