Protection of soil carbon by microaggregates within earthworm casts Heleen Bossuyt a, * , Johan Six b , Paul F. Hendrix a,c a Institute of Ecology, University of Georgia, Athens, GA 30602, USA b Department of Agronomy and Range Science, University of California, Davis, CA 95616, USA c Department of Crop and Soil Sciences, University of Georgia, Athens, GA 30602, USA Received 2 January 2003; received in revised form 18 February 2004; accepted 10 July 2004 Abstract Earthworms are known to play a role in aggregate formation and soil organic matter (SOM) protection. However, it is still unclear at what scale and how quickly earthworms manage to protect SOM. We investigated the effects of Aporrectodea caliginosa on aggregation and aggregate-associated C pools using 13 C-labeled sorghum (Sorghum bicolor (L.) Moench) leaf residue. Two incubations were set up. The first incubation consisted of soil samples crushed !250 mm to break up all macroaggregates with three treatments: (i) control soil; (ii) soilC 13 C- labeled residue and (iii) soilC 13 C-labeled residueCearthworms. Earthworms were added after 8 d and 12 d (days) later, aggregate size distribution was measured together with total C and 13 C in each aggregate fraction. A second incubation was made to assay protected versus unprotected total C and 13 C from 21-d laboratory incubations of intact and crushed large (O2000 mm) and small (250–2000 mm) macroaggregates and microaggregates (53–250 mm). Eight different pools of aggregate-associated C were quantified: (1) and (2) unprotected C pools in large and small macroaggregates, (3) unprotected C pools in microaggregates, (4) and (5) protected C pools in large and small macroaggregates, (6) protected C pool in microaggregates, and (7) and (8) protected C pools in microaggregates within large and small macroaggregates. In the presence of earthworms, a higher proportion of large macroaggregates was newly formed and these aggregates contained more C and 13 C compared to bulk soil. There were no significant differences between the samples with or without earthworms in the C pool-sizes protected by macroaggregates, microaggregates or microaggregates within small macroaggregates. However, in the presence of earthworms, the C protected by microaggregates within large macroaggregates was a significant pool and 22% of this C pool was newly added C. In conclusion, these results clearly indicate the direct involvement of earthworms in providing protection of soil C in microaggregates within large macroaggregates leading to a possible long-term stabilization of soil C. q 2004 Elsevier Ltd. All rights reserved. Keywords: Aggregation; Microaggregates; Carbon; Earthworms; Carbon protection 1. Introduction Soil aggregation has a great influence on the physical characteristics of the soil. Well-aggregated soils possess a larger pore space, a higher infiltration rate and better gaseous exchange between soil and atmosphere than poorly- aggregated soils, leading to enhanced microbial activity (Lynch and Bragg, 1985). Soil aggregation and soil organic matter (SOM) dynamics are closely linked. Aggregates are thought to play an important role in the physical protection of SOM and at the same time, SOM binds with mineral particles to form aggregates of different sizes (Tisdall and Oades, 1982). The preservation of SOM is desirable for land use since SOM is widely recognized as a key component in nutrient cycling. Furthermore, the retention of organic C in soil is becoming more important since the rise in atmospheric CO 2 and global warming are recent concerns (Schlesinger, 1997). Earthworms are considered to improve soil aggregation and they are known to promote the cycling of nutrients (Lee and Foster, 1991; Edwards and Bohlen, 1996). They play a crucial role in the removal of plant litter and other organic materials from the soil surface and the incorporation of these organic materials into soil aggregates (Martin, 1991). Earthworms ingest organic matter and mix it with inorganic 0038-0717/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2004.07.035 Soil Biology & Biochemistry 37 (2005) 251–258 www.elsevier.com/locate/soilbio * Corresponding author. Address: Laboratory for Soil and Water Management, Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, B-3001 Heverlee, Belgium. Tel.: C32 16 32 96 76; fax: C32 16 32 19 97. E-mail address: heleen.bossuyt@agr.kuleuven.ac.be (H. Bossuyt).