Thursday, August 5, 2010 - 2:50 PM

COS 93-5: Interactions between earthworm ecological strategy and microbial activity affect N2O dynamics

Ingrid M. Lubbers, Lucas Nebert, Georgios Giannopoulos, Lijbert Brussaard, and Jan Willem Van Groeningen. Wageningen University

Background/Question/Methods

Agro-ecosystems are a major contributor to global emissions of the greenhouse gas nitrous oxide (N2O). It is known that earthworm activity may raise N2O emissions from these systems, depending on the earthworm species present. Rather than emitting N2O themselves, earthworms are thought to enhance soil microbial activity (nitrification, denitrification, and nitrifier denitrification) by changing physico-chemical properties, excreting mucus, and increasing available carbon. Here we focus on the link between earthworm activity and microbial N2O emission pathways in different crop residue incorporation systems. Our hypotheses are that earthworm-induced N2O emissions i) reflect earthworm ecological strategy; and ii) can be traced to changes in microbial activity. We initiated two 90-day mesocosm studies with combinations of the soil-dwelling Aporrectodea caliginosa (Savigny; endogeic) and the topsoil-dwelling Lumbricus rubellus (Hoffmeister; epigeic), all at realistic densities for Dutch agro-ecosystems. Crop residue was placed on top or incorporated in both experiments. Mesocosms in experiment 1 contained a loamy soil; in experiment 2 a sandy soil. In both experiments N2O fluxes and soil N dynamics were measured throughout the incubation. In experiment 2, microbial biomass and denitrification enzyme activity (DEA) were measured, as well as gene copy number for the amoA, nirK, nirS and nosZ enzymes (using qPCR).

Results/Conclusions

Experiment 1: when residue was incorporated, only A. caliginosa significantly increased cumulative N2O emissions (from 1350 to 2223 µg N2O-N kg-1 soil; p<0.05). When residue was applied on top, L. rubellus increased N2O emissions from 524 to 929 µg N2O-N kg-1 soil (p<0.001), and a significant interaction between both earthworm species increased emissions to 1397 µg N2O-N kg-1 soil (p<0.05). Experiment 2: cumulative N2O emissions increased only when L rubellus was present and residue was applied on top (from 64 to 128 µg N2O-N kg-1 soil; p=0.047). When residue was applied on top, DEA significantly increased in the presence of L. rubellus (from 27.1 to 45.0 µg N2O-N hr-1 kg-1 soil; p<0.001), and a trend towards higher nosZ gene copy numbers was observed. This coincided with a positive correlation between DEA and N2O fluxes (p=0.007). We conclude that i) earthworm-induced N2O emissions follow earthworm ecological strategy: epigeic earthworms can increase N2O emissions when residue is applied on top; endogeic earthworms when residue is incorporated into the soil by other earthworms species or humans, and ii) increased earthworm-induced microbial (denitrification) activity results in increasing N2O emissions. So far, this effect can only be associated with epigeic earthworm species.