Long-term warming reduced microbial biomass but increased recent plant-derived C in microbes of a subarctic grassland

Long-term soil warming and nitrogen (N) availability have been shown to affect microbial biomass and community composition. Altered assimilation patterns of recent plant-derived C and changes in soil C stocks following warming as well as increased N availability are critical in mediating the direction and magnitude of these community shifts. A ¹³C pulse labelling experiment was done on a warming gradient in an Icelandic grassland (Sigurdsson et al., 2016), to investigate the role of recent plant-derived C and warming on the microbial community structure and size. We observed an overall increase of microbial ¹³C (e.g., root-exudate) uptake, while warming led to significant microbial biomass loss in all microbial groups. The increase of microbial ¹³C uptake with warming differed between microbial groups: an increase was only observed in the general and Gram-positive bacterial phospholipid fatty acid (PLFA) markers and in the PLFA and neutral lipid fatty acid (NLFA) markers of arbuscular mycorrhizal fungi (AMF). Nitrogen addition of 50 kg ha⁻¹ y⁻¹ for two years had no effect on the microbial uptake, microbial biomass or community composition, indicating that microbes were not N limited, and no plant-mediated N addition effects occurred. Additionally, we show that both warming and soil C depletion were responsible for the microbial biomass loss. Soil warming caused stronger loss in microbial groups with higher ¹³C uptake. In our experiment, warming caused a general reduction of microbial biomass, despite a relative increase in microbial ¹³C uptake, and altered microbial community composition. The warming effects on microbial biomass and community composition were partly mediated through soil C depletion with warming and changes in recent plant-derived C uptake patterns of the microbial community.