Soil structure turnover driven by soil fauna revealed by particle-pore relationships

The activity of macro- and mesofauna is a key driver for soil structure turnover, thus affecting various soil functions such as soil carbon storage and nutrient cycling. However, there are still few quantitative studies on the rates and efficiency of soil structure turnover induced by different soil fauna groups. In mesocosm experiments, we examined how earthworms (Eisenia fetida, Lumbricus terrestris, Aporrectodea caliginosa), enchytraeids (Enchytraeus albidus), and ants (Lasius niger) alter the spatial arrangement of minerals, particulate organic matter (POM), and pores by using X-ray μCT imaging. Soil structure turnover was quantified by structure labelling and tracking the randomisation of garnet particles. The ratio between particle-pore distance and soil matrix-pore distance was used as an indicator for soil structure turnover¹, and the POM-pore distance as an indicator for organic matter incorporation. Faunal species were placed in cylindrical mesocosms and incubated for 22-24 days. Subsamples were then analysed to quantify the spatial relationships among POM, garnet particles, pore space, and the soil matrix. The predominantly anecic species Lumbricus terrestris mainly elevated POM incorporation into the soil matrix but contributed less to soil structure turnover because of limited burrowing activity. In contrast, the endogeic species Aporrectodea caliginosa promoted structure turnover but with limited POM incorporation because of greater, but predominantly horizontal burrowing activity. Structural impacts of the epigeic species Eisenia fetida were insignificant and largely restricted to surface cast formation. The much smaller enchytraeid worm, Enchytraeus albidus, caused only minor changes in particle-pore relationships, with no new pore formation, and its activity was mainly limited to the soil surface. Preliminary results from experiments with Lasius niger suggest that nest and burrow formation occurred, but there was limited particle transport and no evidence of cast formation. We found that soil structure turnover at the pore scale followed species-specific patterns: Earthworms re-shaped the soil structure, and thus, the particle-pore relationships, ants mainly modified the existing pore architecture, while enchytraeids had only minor effects as they mainly inhabited the existing pore space and soil surface. Overall, our study demonstrated that particle-pore relationships provide a robust way to assess how groups of soil fauna drive soil structure turnover and influence carbon availability.

¹ Schlüter, Steffen; Vogel, Hans-Jörg (2016): Analysis of Soil Structure Turnover with Garnet Particles and X-Ray Microtomography. PLoS One 11 (7), e0159948. DOI: 10.1371/journal.pone.0159948.