No evidence that earthworms increase soil greenhouse gas emissions (CO 2 and N 2 O) in the presence of plants and soil moisture fluctuations

Abstract

Earthworms can stimulate plant productivity, but their impact on soil greenhouse gases (GHG) is still debated. Methodological challenges of measuring GHG in experiments with plants are presumably contributing to the status quo, with the majority of studies being conducted without plants. Here we report the effect of earthworms (without, anecic, endogeic and their combination) and plants (with and without) on GHG (CO2 and N2O) emissions in an experiment. N2O emissions were also 34.6 and 44.8% lower when both earthworm species and only endogeic species were present, respectively, while plants reduced the cumulative N2O emissions by 19.8%. No effects on CO2 were found. Estimates of soil macroporosity measured by X-ray tomography show that the GHG emissions were mediated by their burrowing activity affecting the soil aeration and water status. Both GHG emissions decreased with the macropore volume in the top soil, presumably due to reduced moisture and microbial activity. N2O emissions also decreased with macropore volume in the deepest layer, likely caused by a reduction in anaerobic microsites. Our results indicate that, under experimental conditions allowing for plant and earthworm engineering effects on soil moisture, earthworms do not increase GHG emissions and that endogeic earthworms may even reduce N2O emissions.

Earthworms do not increase greenhouse gas emissions (CO2 and N2O) in an ecotron experiment simulating a realistic three-crop rotation system

“…In a more recent eld study (upland rice), John et al 39 showed that earthworms did not affect CO 2 emissions over the whole rice growing season but that they did have an effect depending on the rice growth stage. Last, but not least, the results are in line with the ndings of Ganault et al 15 , performed on the same soil and using a subset of the same earthworm species in a mesocosm experiment with plants, which showed no signi cant effect of earthworms on soil CO 2 emissions.…”

“…Moreover, the presence endogeic earthworms let to a statistically marginal reduction of N 2 O emissions in the wheat (-48.6%) and intercrop_1 period (i.e., between wheat and mustard) and a similar trend was noted for the mustard and maize crops. These ndings are in agreement with Ganault et al 15 study, which reported a signi cant reduction in N 2 O emissions in mesocosm experiments with the same soil type when the endogeic species A. icterica was present. Similar to Ganault et al 15 , we also identi ed transient N 2 O stimulation during some periods of the experiment, but overall, these effects were not signi cant when integrating over the entire duration of the experiment.…”

“…On the other hand, plants compete with microbes for nitrogen acquisition 27 , decrease soil water content through transpiration, and modify soil porosity through root growth, which can alter the dominant processes that produce N 2 O emissions (nitri cation, denitri cation, and nitrate ammoni cation) 24,28 . This is in line with the results from a mesocosm experiment with plants and earthworms 15 , which found that the presence of plants lowered N 2 O emissions by 19.8%, in correlation with a 43% and 20% decrease in nitrate and ammonium respectively.…”

“…Other common drawbacks of many experiments include limited duration and the use of only point measurements of uxes, which may fail to capture the full range of phenomena and processes in the system. These conjectures are supported by experimental results conducted in eld conditions or based on laboratory setups trying to mimic natural conditions and in which two main naturally occurring factors were found to mitigate earthworm mediated CO 2 and N 2 O emissions, namely: (i) growing plants and (ii) soil water content uctuations due to drainage or drying and rewetting cycles [13][14][15][16][17] . Plants, as primary producers, play a crucial role in biogeochemical cycles because they determine the amount and quality of carbon that enters the soil system 25 .…”

“…They can be divided into three ecological categories based on their feeding and burrowing habits: (1) anecic species that feed on fresh litter from the soil surface and create mainly permanent burrows, (2) epigeic species that live on the soil surface and feed on surface litter without creating permanent burrows, and (3) endogeic species that live and feed on mineral soil and associated organic matter below the surface, and that create non-permanent burrows without preferential orientation 12 . However, some eld and laboratory studies included in the Lubbers et al 6 meta-analysis as well as more recent ones [13][14][15][16][17][18][19] have reported results that are not in line with the general conclusion that earthworms increase greenhouse gas emissions.…”