Earthworms are known to stimulate soil greenhouse gas (GHG) emissions, but the majority of previous studies have used simplified model systems or lacked continuous high-frequency measurements. To address this, we conducted a two-year study using large lysimeters (5 m² area and 1.5 m soil depth) in an ecotron facility, continuously measuring ecosystem-level CO₂, N₂O, and H₂O fluxes. We investigated the impact of endogeic and anecic earthworms on GHG emissions and ecosystem water use efficiency (WUE) in a simulated agricultural setting. Although we observed transient stimulations of carbon fluxes in the presence of earthworms, cumulative fluxes over the study indicated no significant increase in CO₂ emissions. Endogeic earthworms reduced N₂O emissions during the wheat culture (-44.6%), but this effect was not sustained throughout the experiment. No consistent effects on ecosystem evapotranspiration or WUE were found. Our study suggests that earthworms do not significantly contribute to GHG emissions over a two-year period in experimental conditions that mimic an agricultural setting. These findings highlight the need for realistic experiments and continuous GHG measurements.

The CO₂ concentration was measured at the inlet and outlet of each dome (every 12 min) using a multiplexer system coupled with two LI-7000 CO₂/H₂O analysers (LI-COR Biosciences, Lincoln, NE, USA). The Reichstein et al. (2005) C flux partitioning algorithm was used to estimate the daytime ecosystem respiration based on an exponential regression model (Lloyd et al., 1994). This allowed for the estimation of ecosystem respiration over 24 h (Reco = Reco_night + Reco_day) and gross primary production (GPP = NEE_day – Reco_day).

Ecosystem-level N₂O fluxes were measured continuously as an open system using a TILDAS Compact Single analyser (N₂O Aerodyne Research, Inc., USA). The analyzer was coupled to a multiplexer system allowing N₂O fluxes measurement every 72 min for each Macrocosm. Evapotranspiration (ET) was computed as the lysimeter weight difference between two consecutive days. Four shear beam load cells per lysimeter (CMI-C3, Precia-Molen, Privas CEDEx France), with an accuracy of ±200 g, were used to measure the changes in mass. Ecosystem WUE was estimated as the ratio of GPP to ET derived from measurements by lysimeter weight changes over 24h.

Earthworms were sampled by utilizing the non-invasive octet electric method (Schmidt et al., 2001), which enabled sampling of a 1 m² surface area for a duration of 50 minutes using a customized version of the octet device manufactured by Electrotechnik Schuller (Darmstadt, Germany). To achieve coverage of the entire 5 m² surface area, five devices were simultaneously deployed.

References:

Reichstein, M. et al. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. Glob. Change Biol. 11, 1424–1439 (2005).

Lloyd, J. & Taylor, J. A. On the Temperature Dependence of Soil Respiration. Funct. Ecol. 8, 315–323 (1994).

Schmidt, O. Appraisal of the electrical octet method for estimating earthworm populations in arable land. Ann. Appl. Biol. 138, 231–241 (2001).