Residue incorporation depth is a controlling factor of earthworm-induced nitrous oxide emissions

Summary

Earthworms can increase nitrous oxide (N2O) emissions, particularly in no-tillage systems where earthworms are abundant. Here, we study the effect of residue incorporation depth on earthworm-induced N2O emissions. We hypothesized that cumulative N2O emissions decrease with residue incorporation depth, because (i) increased water filled pore space (WFPS) in deeper soil layers leads to higher denitrification rates as well as more complete denitrification; and (ii) the longer upward diffusion path increases N2O reduction to N2. Two 84-day laboratory mesocosm experiments were conducted. First, we manually incorporated maize (Zea maysL.) residue at different soil depths (incorporation experiment). Second, 13C-enriched maize residue was applied to the soil surface and anecic species Lumbricus terrestris (L.) and epigeic species Lumbricus rubellus (Hoffmeister) were confined to different soil depths (earthworm experiment). Residue incorporation depth affected cumulative N2O emissions in both experiments (P <0.001). In the incorporation experiment, N2O emissions decreased from 4.91 mg N2O–N kg-1 soil (surface application) to 2.71 mg N2O–N kg-1 soil (40–50 cm incorporation). In the earthworm experiment, N2O emissions from L. terrestris decreased from 3.87 mg N2O–N kg-1 soil (confined to 0–10 cm) to 2.01 mg N2O–N kg-1 soil (confined to 0–30 cm). Both experimental setups resulted in dissimilar WFPS profiles that affected N2O dynamics. We also found significant differences in residue C recovery in soil organic matter between L. terrestris (28–41%) and L. rubellus (56%). We conclude that (i) N2O emissions decrease with residue incorporation depth, although this effect was complicated by dissimilar WFPS profiles; and (ii) larger residue C incorporation by L. rubellus than L. terrestris indicates that earthworm species differ in their C stabilization potential. Our findings underline the importance of studying earthworm diversity in the context of greenhouse gas emissions from agro-ecosystems.