Changes in microbial physiology and community structure along seasonal and altitudinal gradients: Implications for soil respiration

Background/Question/Methods

Soil microbial respiration is a critical component of the global carbon cycle, but it is uncertain how microbial community composition affects this process. Two microbial community properties that could have particular importance to ecosystem C budgets are the temperature responses of the community (such as the Q₁₀ of respiration) and the biomass-specific respiration rate (CO₂ produced per unit biomass per unit time). Previous studies have noted a thermodynamic trade-off between the rate and efficiency of growth in heterotrophic organisms. Growth rate and yield determine the biomass-specific respiration rate of growing microbial populations, but these traits have not previously been used to scale from microbial communities to ecosystems. Here we report seasonal variations in Q₁₀ of substrate-induced respiration for soils collected in high elevation alpine and subalpine soils of the Front Range of the Colorado Rocky Mountains, and relate these temperature responses to differences in the bacterial community structure (as described by 16S rRNA gene sequences). Additionally, we measured seasonal changes in microbial growth kinetics and specific respiration rates in a subalpine coniferous forest soil, related them to measurements made on cultured and uncultured soil bacteria and fungi, and modeled the effects of shifting growth kinetics on soil heterotrophic respiration (R_h).  

Results/Conclusions

The lower elevation, subalpine forest soil exhibited large seasonal variations in Q₁₀, which were highest in summer and higher than alpine Q₁₀ in all seasons except winter. Q₁₀ in alpine soils were consistently low throughout the year. Alpine and subalpine bacterial communities both varied seasonally, and were markedly distinct from each other. Subalpine communities from colder times of year were the most similar to the alpine communities. A statistical analysis showed that bacterial community structure and Q₁₀ were correlated. Our analysis of seasonal changes in growth kinetics in the subalpine forest showed that subnivean soil microbial communities had higher growth rates and lower growth yields than the summer and fall communities, causing higher biomass-specific respiration rates during the winter. Based on experiments using specific growth inhibitors, bacteria had higher growth rates and lower yields than fungi, overall, suggesting a more important role for bacteria in determining R_h. Modeled R_h was sensitive to microbial kinetics and Q₁₀: a six-fold lower annual R_h resulted from using kinetic parameters from summer versus winter communities. Under the most realistic scenario using seasonally changing communities, the model’s estimate of R_h (22.67 mol m^-2 yr^-1), agreed well with other published models and measurements for this ecosystem.