Influence of Lumbricus terrestris and Folsomia candida on N sub(2)O formation pathways in two different soils - with particular focus on N sub(2) emissions

Rationale The gaseous N losses mediated by soil denitrifiers are generally inferred by measuring N sub(2)O fluxes, but should include associated N sub(2) emissions, which may be affected by abiotic soil characteristics and biotic interactions. Soil fauna, particularly anecic earthworms and euedaphic collembola, alter the activity of denitrifiers, creating hotspots for denitrification. These soil fauna are abundant in perennial agroecosystems intended to contribute to more sustainable production of bioenergy. Methods Two microcosm experiments were designed to evaluate gaseous N emissions from a silty loam and a sandy soil, both provided with litter from the bioenergy crop Silphium perfoliatum (cup-plant) and inoculated with an anecic earthworm (Lumbricus terrestris), which was added alone or together with an euedaphic collembola ( Folsomia candida ). In experiment 1, litter-derived N flux was determined by adding super(15)N-labelled litter, followed by mass spectrometric analysis of N sub(2) and N sub(2)O isotopologues. In experiment 2, the delta super(18)O values and super(15)N site preference of N sub(2)O were determined by isotope ratio mass spectrometry to reveal underlying N sub(2)O formation pathways. Results Lumbricus terrestris significantly increased litter-derived N sub(2) emissions in the loamy soil, from 174.5 to 1019.3 mu g N sub(2)-N k g super(-1) soil, but not in the sandy soil (non-significant change from 944.7 to 1054.7 mu g N sub(2)-N k g super(-1) soil). Earthworm feeding on plant litter resulted in elevated N sub(2)O emissions in both soils, derived mainly from turnover of the soil mineral N pool during denitrification. Folsomia candida did not affect N losses but showed a tendency to redirect N sub(2)O formation pathways from fungal to bacterial denitrification. The N sub(2)O/(N sub(2) + N sub(2)O) product ratio was predominantly affected by abiotic soil characteristics (loamy soil: 0.14, sandy soil: 0.26). Conclusions When feeding on S. perfoliatum litter, the anecic L. terrestris , but not the euedaphic F. candida , has the potential to cause substantial N losses. Biotic interactions between the species are not influential, but abiotic soil characteristics have an effect. The coarse-textured sandy soil had lower gaseous N losses attributable to anecic earthworms.