The role of earthworms in the organic matter cycling of forest floors in temperate forests – A mesocosm experiment with labeled beech litter

Earthworms may act as double-edged swords for soil organic matter (SOM). While they can enhance organic matter (OM) mineralization via increased microbial activity they can also elevate OM stabilization in aggregates as particulate or mineral-associated OM. In this study, we are testing this potentially opposing impact in beech-dominated (Fagus sylvatica L.) mixed forests on limestone, a forest ecosystem with particularly high earthworm activity. A specific focus lies on OM transformation along the continuum from the forest floor (O horizons) to mineral soil (A horizons). The forest floor can represent a substantial OM-pool which is an important source for SOM formation via bioturbation or leaching but can be vulnerable to alterations due to climate change. In a lab mesocosm experiment, we are incubating local earthworm species in soil columns consisting of O and A horizons from two contrasting beech forest sites from 600 and 1250 m elevation in the Swiss Jura Mountain range. Both sites have a mull-type forest floor with the high-elevation site exhibiting an Of horizon present throughout the year while an Of horizon is not present all year at the low-elevation site. We established four earthworm treatments for each site all including the respective mineral soil and forest floor: (1) no earthworms, (2) two Octolasion cyaneum S., (3) one Lumbricus terrestris L., and (4) two O. cyaneum together with one L. terrestris. In this setup, the Ol horizon was replaced with beech litter highly enriched with ¹³C, ¹⁵N, and ²H. Soil respiration (CO₂) and leaching (C, N, and H in dissolved OM) are repeatedly measured. Total respiration (¹²C and ¹³C) is measured weekly for the first four months and biweekly afterward. Every two months fluxes from A and O horizons are measured separately. After approximately 4 and 10 months each, a set of mesocosms is harvested to investigate isotope enrichment in earthworm biomass, cast, physical soil fractions, PLFAs, and microbial necromass. We find first indications for stabilization of new litter input as, under similar total CO₂ fluxes, the litter-derived fraction is higher for treatments without worms. However, if both earthworm species are present, the cumulative heterotrophic respiration is elevated compared to the treatments involving only one earthworm species and the no-earthworm treatment. This is presumably due to higher earthworm density and, therefore, increased bioturbation. In contrast, we find no differences in the amount of dissolved organic matter leached out of the mesocosms between the treatments so far. X-ray CT scans will inform us about earthworm behavior within the mesocosms. This will help us understand how their activity translates into the vertical distribution of the isotopic label.