Recent studies show that earthworms can considerably increase emissions of the greenhouse gas nitrous oxide (N2O). However, the extent of this effect varies widely with earthworm ecological group, and exact pathways remain unclear. Here we determined to what extent the earthworm-induced N2O effect is related to the incorporation depth of residue by different ecological groups. We hypothesized that (i) N2O emissions reduce with incorporation depth in the absence of earthworms, as more N2O is reduced to N2 while diffusing upwards; (ii) effects of a deep-burrowing (anecic) earthworms that are confined to various burrowing depths show a similar pattern; and (iii) the effects of an anecic and a topsoil-dwelling (epigeic) earthworm species are therefore identical when they are both confined to the topsoil. In a mesocosm (50 cm depth, sandy soil) study we measured cumulative N2O emissions. In experiment A, maize residue was manually incorporated at a depth of 0, 10, 30 and 50 cm. In experiment B, 13C-labeled maize residue was applied on top, but individuals of the anecic species Lumbricus terrestris (L.) were confined to 10, 30 or 50 cm with a nylon mesh, and individuals of the epigeic species Lumbricus rubellus (Hoffmeister) to 10 cm.
Results/Conclusions
After 83 days, L. terrestris biomass was decreased with 25.3%, without a significant effect of confinement depth. L. rubellus had lost significantly more weight (48.0%; p=0.015). In experiment A, N2O emissions decreased with incorporation depth (p<0.001) from 4.91 to 2.69 mg N2O-N kg-1 soil. In experiment B, N2O emissions differed with earthworm treatment (p<0.001). Highest emissions were observed for the L. rubellus treatment at 10 cm confinement (5.05 mg N2O-N kg-1 soil). Although confining L. terrestris to 10 cm resulted in lower emissions (3.85 mg N2O-N kg-1 soil), this was not significantly different from L. rubellus. Confinement depth significantly affected emissions of L. terrestris (p<0.001), but lowest emissions were found for 30 rather than 50 cm incorporation depth. With regard to our hypotheses, we postulate that (i) N2O emission decreases as expected with residue incorporation depth in the absence of earthworms; (ii) in the presence of earthworms this effect was obfuscated by changed gas diffusion properties through burrowing activity; and (iii) no significant differences were observed between an epigeic and anecic earthworm that were both confined to the topsoil. We conclude that earthworm-induced N2O emissions are indeed related to residue incorporation depth, albeit in more intricate ways than previously assumed.