Experimental warming and permafrost thaw decrease soil nematode abundance in northern palsa peatlands

Abstract. Global warming is profoundly impacting northern ecosystems, particularly those underlain by permafrost. Permafrost-affected peat plateaus, called palsa peatlands, consist of mounds with peat and mineral soil covering ice lenses. When permafrost thaws, palsas can collapse and undergo significant hydrological changes to form wet mires. This affects the physical structure of the soil and as a result, the communities of soil-dwelling organisms, such as nematodes. Although the role of nematodes in carbon cycling is not fully understood, they can influence greenhouse gas emissions through interactions with plants and microbes. This study examined the effects of palsa degradation and experimental warming on nematode feeding guilds (bacterivores, fungivores, root feeders, and omni-carnivores) in northern Norway, where permafrost is rapidly thawing. Our findings showed that intact, vegetated palsas supported higher abundances of all nematode feeding guilds. With warming, bacterivorous and omni-carnivorous nematodes were negatively affected. Additionally, we observed a shift in dominance of bacterivores to fungivores over the summer, suggesting a temporal shift in the predominant decomposition pathway. No direct relationships were found between changes in any of the guild abundances and measured CO2 and CH4 fluxes. This study highlights the fact that expected warming and the degradation of palsas may have varied but had predominantly negative impacts on different nematode feeding guilds. Given the role of soil nematodes in nutrient cycling and other soil processes, their decline under warmer conditions could have ecosystem-level consequences in these palsa peatlands.