| Summary: | The release of respiratory CO₂ from polar tundra soils depends on the interactions between soil organic carbon and microbial communities, a topic of increasing importance due to global climate change. We experimentally amended samples of organic-rich tundra soil and mineral-rich temperate soil with dissolved organic carbon (DOC) and examined the following (at the same temperature): (1) the effects of DOC enrichment on bacterial and eukaryotic microbial biomasses, respiration, and acid phosphatase activities in both soil types, (2) whether relationships between microbial biomass, respiration, and acid phosphatase activities were similar across soil types, and (3) to what extent DOC enrichment altered the microbial food web in both soil types. Both soil types showed immediate, temporary increases in respiration following C enrichment, though the C source (amino acids, citric acid, or glucose) was an important source of variation. Additionally, a consistent relationship between respiration rates and acid phosphatase activities suggested that C and P cycling were linked in similar ways in the two soil types. The relationships of respiration rate and acid phosphatase activity to bacterial biomass were also consistent across soil types, excepting some amended tundra soil samples, perhaps due to greater protist bacterivory in those samples. In the tundra samples only, changes in predator biomass were accompanied by elevation of bacterial biomass-specific respiration rates. Our findings highlight the potential importance of protist bacterivory on microbial processes in tundra soils, but further research is needed to assess whether microbial food webs differ consistently among soils that vary in organic content.
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