Effects of multi-level warming on soil nematodes and microarthropods in tundra heath mesocosms

Soil fauna, including nematodes and microarthropods (mites and springtails), regulate nutrient cycling and decomposition, yet their responses to warming remain unclear in temperature-sensitive tundra ecosystems. We experimentally warmed intact tundra heath monoliths from northern Sweden under five temperature treatments (ambient to +9 °C) for 20 weeks. Monoliths were dominated by either Betula nana or Empetrum nigrum subsp. hermaphroditum. After incubation, we quantified nematode and microarthropod densities and community composition. Warming altered both nematode and microarthropod communities, with responses mediated by vegetation type, trophic group, and taxon. Microarthropods densities were generally more sensitive to warming, especially under the dominance of E. nigrum, whereas nematode communities showed stronger compositional shifts. Overall nematode densities were stable across warming levels, except for fungivorous nematodes, which decreased linearly with warming. In contrast, total and carnivorous microarthropod densities increased linearly with warming, while springtails and fungivorous microarthropods displayed non-linear, U-shaped responses. Together, these results demonstrate that soil fauna responses to warming range from negligible to linear and non-linear, underscoring the importance of experimental designs that span multiple warming levels. The strong dependence of these responses on vegetation context and trophic identity emphasizes the need to account for local environmental heterogeneity when predicting tundra ecosystem responses to climate change.