Structure of earthworm burrows related to organic matter of a constructed Technosol

Literature considering earthworms as a biological agent of Technosol functioning, especially those constructed in order to reclaim degraded areas (e.g. industrial wasteland), is scarce. The main objectives of the following work is: (i) to describe the structure of burrows produced by two different eco-morphological groups of earthworms in a constructed Technosol and (ii) to link their burrowing activity to Technosol organic matter transfers and carbon content in one-species and two-species combinations. For these purposes, the contributions of Lumbricus terrestris as an epi-anecic earthworm and Aporrectodea caliginosa as an endogeic one to such functioning aspects of a constructed Technosol, resulting from soil engineering processes (e.g. choice of proportions, parent materials) were assessed using laboratory microcosm experiments. The Technosol studied was composed of green waste compost, treated industrial soil and paper mill sludge. Earthworms were inoculated separately and together in the constructed Technosol over a period of 75 days. Ultra-structural analysis of randomly selected burrows of the one-species treatments and the soil of the control treatment were sampled to describe their structure. Functional consequences on the organic matter in the Technosol were assessed by studying remaining surface litter mass, transfer of surface organic matter to depth and by measuring soil carbon content. At the ultrastructural scale, the burrowing activity of the two eco-morphological groups of earthworms locally modified the organo-mineral associations of the Technosol. Burrows presented a similar structure for both species, with a looser internal cutan including some organic elements and microbial activity tracks (closed to the lumen) and a compacted external mineral cutan (distant from the lumen). However, structural differences were observed between species. L. terrestris burrows contained a visible organo-mineral interface, resulting from the interface between the internal cutan, which was much more organic than for the A. caliginosa one, and the external mineral cutan. For L. terrestris, mucus was present as a large stratum visible between the interface and the external mineral cutan, whereas the mucus was scattered for A. caliginosa. Aggregation also differed between species. Even though both burrows presented some organo-mineral aggregates, plant organic matter particularly contributed to the aggregates for L. terrestris, whereas aggregation was essentially bacterial for A. caliginosa. As is the case for “natural soils”, these results confirmed the notions of i) “ecosystem engineers” in constructed Technosols, by demonstrating that earthworms create organo-mineral structures with a similar structure and ii) eco-morphological groups by distinguishing differences between these structures. At a larger scale (microcosm scale), L. terrestris buried significantly more surface organic matter into depth than A. caliginosa. However, almost no effect of either of the two earthworms (one-species and two-species treatments) on soil carbon content was noticeable. It is suggested that not enough soil carbon measures were made given the temporal and spatial scales of this experiment or earthworm effects may have been masked in the man-made soil built with materials with high initial carbon content. Although sometimes slight at the microcosm scale, differences suggested that earthworm combinations of dissimilar eco-morphological groups led to different effects on organic transfers and carbon content of the constructed Technosol. Developing knowledge about effects of soil fauna diversity in constructed Technosols using reliable tools (e.g. trait-based approaches) is required. It would better predict the effects of biological agents such as earthworms in soil engineering and in turn improve the ecological restoration of such Technosols.