Earthworms did not increase long-term nitrous oxide fluxes in perennial forage and riparian buffer ecosystems

Abstract

Many laboratory and mesocosm studies have demonstrated that earthworms influence nitrogen (N) cycling reactions and produce nitrous oxide (N2O) in well-aerated soils, but whether earthworms can stimulate N2O fluxes in realistic field conditions remains to be determined. We conducted two field experiments, in perennial forage agroecosystems for 2 yr and agriculture riparian buffers for 1 yr, to compare N2O fluxes from enclosures with ambient and artificially elevated earthworm populations. Despite a short-term (< 3 month) increase in mean N2O fluxes from the perennial forage enclosures with artificially elevated earthworm populations, this effect disappeared within 1 yr, with no significant difference (p> 0.05) in mean N2O flux from enclosures in either field experiment. The elevated earthworm populations declined and stabilized at the same level as the ambient earthworm populations within 1–2 yr after the field experiments began. The homeostatic regulation of earthworm populations under field conditions could be due to inter- and intra-specific competition, related to limitation in the food supply and habitat preferred by earthworms. Mean N2O fluxes in the perennial forage fields were negatively correlated with soil moisture, but not related to earthworm populations. In the riparian buffers, the average N2O flux was negatively correlated with vegetation cover, and positively correlated with soil moisture and the size of the earthworm population at the end of the study. Our results suggest that the effects of earthworm addition on N2O emissions in laboratory studies can not necessarily be extrapolated to field settings. Earthworm field experiments that continue in the longer-term and in a variety of ecosystems should improve our understanding of the seasonal and environmental variability in earthworm activity and N2O production under field conditions.