Does the Effect of Earthworms on the Soil Greenhouse Gas Balance Differ Between Farming Systems?.

Earthworms are among the most important soil dwelling invertebrates and their ability to increase plant production has long been recognized. Their effect on the soil greenhouse gas (GHG) balance, however, is less clear. On the one hand, earthworm activity can stimulate carbon storage in soil through increased litter- and root production and through stabilizing fresh residue into stable soil organic carbon (SOC) fractions. On the other hand, studies have reported earthworm-induced GHG emissions. In two meta-analyses, we determined the effect of earthworm presence in agroecosystems on GHG emissions and plant productivity. Subsequently, we analysed whether the effect of earthworms on the soil GHG balance is likely to differ between farming systems. In the first meta-analysis we analysed 237 observations from 57 studies and concluded that earthworms significantly increase both CO₂ (+33%) and N₂O emissions (+42%), whereas they do not significantly affect SOC stocks. In the second meta-analysis, we analysed 462 data points from 58 studies and concluded that earthworm presence increases crop yield with 26% and aboveground biomass with 24%. The magnitude of these effects depends on type and rate of fertilization and crop residue management, as well as on earthworm density. In conventional farming systems, the role of earthworms is unlikely to be important as both yield and GHG emissions are dominated by high N application rates. Earthworm activity might be most beneficial in tropical subsistence farming systems, where yield effects are likely to be high and GHG effects low. Finally, for temperate organic farming systems there appears to be a tension between a potentially large benefit to crop yield on the one hand, and considerably increased GHG emissions on the other. We conclude that more research on the role of earthworms in these systems is most urgently needed.