PS 22-11 - Biophysical controls on soil carbon fluxes & storage at Hopkins Memorial Forest

Background/Question/Methods

Soil respiration (Rs), or the sum of belowground autotrophic and heterotrophic metabolism, is a major component of the global carbon cycle and the largest flux of carbon dioxide (CO2) from the land surface to the atmosphere. The magnitude of Rs is highly variable, and sensitive to changes in soil temperature, moisture, production, and substrate availability to decomposer microorganisms. To evaluate environmental and biophysical controls on soil respiration at Hopkins Memorial Forest (Williams College; Northwestern Massachusetts; 42.7235oN/73.2227oW), we made biweekly measurements of field-based Rs throughout the growing season in 48 plots, divided between two sites of differing elevation (High Elevation 250 m; Low Elevation 700 m), soil pH, and parent material. We characterized soil carbon (C), nitrogen (N) and chemistry in each plot, and measured soil temperature and moisture throughout the measurement period. To assess the size of the microbially-accessible C pool and compare the rate of heterotrophic C emission across sites, we incubated soil subsamples at 20oC and regularly measured respiration for seven months after sampling. Finally, we compared the capacity of the soil microbial community at each site to degrade substrates of varying chemical complexity (glucose, vanillin, and lignin) using a substrate addition experiment.

Results/Conclusions

At Hopkins Memorial Forest, the higher elevation site has reduced soil pH, greater soil C content, and lower concentrations of polyvalent cations in comparison to the lower site. Across one growing season, upper site R_s also showed higher seasonal temperature sensitivity, while modeled cumulative growing season R_s was approximately equal (~5000 g C m^-2 ) across sites. We found that cross-site variation in Q₁₀ values was explained by exchangeable K⁺ concentration in the surface soil. Following nine months of laboratory incubation, we also found that a larger fraction of the soil C pool was inaccessible to microbial decomposers at the higher elevation site. There was broad similarity in substrate use capacity across sites. Immediately following addition, glucose stimulated respiration by four- to six-fold, while vanillin and lignin both caused an approximately two-fold increase in CO₂ efflux. Two days following the additions, vanillin-amended samples maintained significantly elevated respiration rates. These results suggest that microbial capacity to degrade substrates of varying chemical complexity is consistent across sites, and physical and mineral protection of organic matter is more likely to explain cross-site variation in soil C pool size.