Between Mosses and Microbes: Stable Isotopes (15N/14N, 13C/12C) Clarify the Trophic Niches of Oribatid Mites on a High-Arctic Permafrost Archipelago (Svalbard, Norway).

Tundra biomes are strongly affected by globally changing temperatures. Rising temperatures will increase soil biota activity and result in higher CO2 release from tundra soils and 'greening' of this biome, which in turn will affect carbon cycling and soil food webs. It is therefore important to better understand those high-latitude soil food webs in tundra biomes. Here, we investigated adult and juvenile soil living oribatid mites (Acari) in eight different microhabitats (i.e., patches of vegetation from Bistorta vivipara, Carex sp./Luzula sp., Cassiope tetragona, Dryas octopetala, Salix spp. and Saxifraga spp., mosses and grasses) using stable isotope ratios (15N/14N, 13C/12C) in the Svalbard archipelago, a challenging habitat for soil biota with permafrost soils being entirely frozen for about 9 months of the year. Ten oribatid mite taxa were sufficiently abundant for stable isotope analyses, that is, Camisia dictyna, Ceratoppia sphaerica, Diapterobates notatus, Dissorhina ornata, Fuscozetes setiger, Mycobates sarekensis, Oppiidae, Oribatula tibialis, Scutozetes arcticus and Tectocepheus sarekensis. Four trophic groups of oribatid mites were identified (phytophagous-likely feeding on mosses, secondary decomposers, predators/scavengers and lichen feeders); primary decomposers were missing, pointing to low litter quality. The "detrital shift" in Δ13C was generally low pointing to the dominance of young carbon resources including mosses. Most oribatid mite taxa (Camisia dictyna, Ceratoppia sphaerica, Diapterobates notatus, Fuscozetes setiger, Mycobates sarekensis and Scutozetes arcticus) were boreal/tundra habitat specialists. Juveniles and adults typically occupied the same trophic niche despite smaller body size and mouthparts in juveniles than in adults. Oribatid mite trophic niches did not differ between microhabitats indicating that they are not specifically adapted to certain plant species or microhabitats. Overall, the stable trophic ecology of oribatid mites across microhabitats allows predictions of Arctic soil food web changes under continued warming.