Changes in the microbial control of C and N cycling in future subarctic tundra soils

Arctic climate warming will affect microbially-controlled nutrient cycling through elevated nutrient availability and changes in vegetation productivity and composition. Plant-derived C inputs will serve as a microbial energy and C source, while the inorganic N released from soil organic matter (SOM) could . Albeit controlled by different mechanisms, increased C and N availability may each stimulate microbes to degrade more SOM, creating positive climate change feedbacks. Simultaneously, microbes provide the main pathway to sequester and stabilize C and N in SOM through growth and subsequent necromass formation, termed the “microbial pump”, generating a negative climate change feedback. Combining estimates of microbial growth, biomass, and biogeochemical rates allows for the calculation of Carbon Use Efficiency (CUE) and Nitrogen Use Efficiency (NUE). Analyzing NUE relative to CUE may elucidate microbial resource limitation. Resultantly, NUE exceeding CUE indicates microbial N-limitation, and suggests that microbes sequester N in SOM. Conversely, CUE exceeding NUE suggests microbial C-limitation and points towards microbial C sequestration in SOM. In addition, the strength of the microbial C and N pumps can be estimated by assessing microbial growth along with microbial C and N retention times, providing insights into how long resources will be retained in the microbial biomass. These tools contribute to a comprehensive understanding of whether resources will be liberated through decomposition or sequestered via the microbial pump.

Here, we investigated microbial responses to changes in resource availability associated with future climate change in a subarctic tundra heath. We used additions of mineral N and litter in the field to mimic the effects of elevated nutrient availability and shrubification on microbial growth rates (radio-isotope tracing), C and gross N mineralisation rates (gas chromatography and ¹⁵N pool dilution methods, respectively), and microbial community size was estimated with phospholipid fatty acids.

We found that field N-fertilization generally decreased microbial NUE, and that the resulting NUE/CUE ratio was close to 1, thereby pointing towards alleviated microbial N-limitation. Field N-fertilization also accelerated N-cycling but had no significant effects on C retention times. Conversely, litter addition in the field led to NUE exceeding CUE, implying the induction of microbial N-limitation, and it slowed C turnover times, but had no significant effect on N turnover times. When N and litter were applied together, similar CUE and NUE values, but accelerated C and N turnover times were observed. Overall, fungal contribution to resource cycling diminished across all field treatments, evident from a reduced fungal-to-bacterial growth ratio compared to the control treatment.

These findings highlight that changes in nutrient availability impact microbial C and N cycling independently and emphasize that the microbial resource limitation may be altered by the substrate stoichiometry. Additionally, our results suggest that while microbial N cycling is likely to accelerate, thus weakening the microbial N pump, microbial C cycling may be impeded and microbial C pump strengthened. Overall, these observations align with projections of more fertile and productive subarctic ecosystems in the future, and underscore the potential for microbial C sequestration even under altered resource availability.