Understanding the fate and linkage of N and S in earthworm-engineered peat soil by coupling stable isotopes and nano-scale secondary ion mass spectrometry

Earthworm burrow-lining is a biophysical interface where the relationship between the spatial location of active soil microorganisms and the actual physical structure of the soil has a major influence on geochemical processes and nutrient cycling. Isotopic tracers and NanoSIMS were combined to highlight, at the nanometric scale, N and S transfers from earthworms to this non-destructured biophysical interface. Peat mesocosms were inoculated with double labelled earthworms (15N and 34S; Lumbricus rubellus H.) and then sampled for NanoSIMS quantitative measurements. The maximum enrichments of 15N and 34S detected in peat after 24 days were 1.6 and 5.6 APE (Atom % excess), respectively. NanoSIMS analyses, performed along a transect in peat at increasing distances from contact between the epidermis of an earthworm and its burrow-lining, revealed rapid and brief transfers of biogenic N and S into peat with two contrasted distributions i.e., decreasing 15N and patchy 34S. The sphere of influence of L. rubellus was clearly delimited after 24 days at 1 mm around the burrow, thus specifying the functional traits of this epi-anecic earthworm. The observed N inputs into peat were probably derived from oxidation of the labile products of earthworm metabolism, such as urea and/or ammonium, as shown by the strong linkage of 15N with oxygen measured as 16O−. The transfers of labile S products from earthworm to the surrounding soil are reported here for the first time and could be derived from mucoprotein secretion. The interesting spatially inverse relationship between 15N and 34S enriched-spots at increasing distances from the burrow-lining highlighted different fates of the biogenic N and S compounds excreted by earthworms and could reveal the activities of syntrophic partnerships in N and S cycling. The strategy of combining isotopic tracers with NanoSIMS demonstrated high potential for determining the fate of biogenic nutrient inputs occurring in a complex matrix such as soil. It opens up considerable opportunities to link the biological traits of specific soil engineers with geochemical processes operating at the microbial scale in the spatial structure created by earthworms.