Are Earthworms Good or Bad for the Soil Greenhouse Gas Balance?.

The soils of the world are a major source of greenhouse gases (GHG’s). Terrestrial ecosystems have contributed as much as half of the anthropogenic carbon dioxide (CO₂) emission over the past two centuries and soils are by far the dominant source of nitrous oxide (N₂O) emissions. Yet, soils can also be a GHG sink through C-sequestration and biological reduction of N₂O. The GHG balance of the soil therefore depends on soil management. Land use conversion from conventional tillage to reduced or no-tillage systems stimulates activity of soil engineers such as earthworms. It has been suggested that earthworm activity can result in stabilization of fresh organic C in soil organic matter. However, several studies reported detrimental effects of earthworms on the soil GHG balance: increasing emissions of N₂O and CO₂. Here, we report on a meta-analysis and an experiment to study these effects. The meta-analysis quantified earthworm effects on the soil GHG balance, including C-stabilization and emissions of N₂O and CO₂. Overall, earthworm activity significantly increases N₂O and CO₂ emissions. No proof was found that they increase soil organic matter. The available data was fragmentary and measurements were often indirect, of short duration, and under controlled laboratory conditions. Knowledge gaps that need to be addressed are (i) direct measurement of C-stabilization over longer time periods, and (ii) field-based studies on N₂O effects. To address the first knowledge gap, we installed a 2.5-year mesocosm experiment in which ¹⁵N-labeled maize (Zea mays L.) residue was regularly applied to the soil in the presence or absence of earthworms. We quantified N₂O and CO₂ emissions as well as C stabilization. After 1.5 years, cumulative emissions of both N₂O and CO₂ increased in the presence of earthworms, and C stabilization decreased. Our preliminary results therefore suggest a detrimental role of earthworms on the soil GHG balance.