LB 7-58 - The role of belowground processes in the respiration dynamics of an ombrotrophic bog

Background/Question/Methods:

Northern peatlands are globally significant carbon sinks that vary from year-to-year depending on environmental and biogeochemical conditions. These variations are due to changes in primary production and autotrophic respiration (AR; respiration by plant parts). Respiration dynamics are complex, where some plants act to mediate belowground decomposition and support heterotrophic respiration through the presence of mycorrhizal fungi, especially in sedge dominated peatlands. Currently, the role of belowground processes in peatlands, especially those that pertain to the rhizosphere, are not well documented. The objective of our study was to explore how belowground processes control respiration dynamics in an ombrotrophic bog in eastern Ontario (Mer Bleue). Flux measurements of CO₂ were obtained in plots covering the dominant shrubs and sparse sedges at the site, and manipulations were applied to some of the plots (clipping) to determine heterotrophic respiration (HR; respiration by microbial bacteria, mycorrhizae, etc.) and respiration from the vascular plants without the presence of the mosses. Root exudate and nutrient analyses were conducted to determine the effect of ecosystem nutrition on respiration, and carbon isotope analyses of CO₂ were conducted to determine the source of respired carbon.

Results/Conclusions:

Our study reveals the importance of belowground processes to our understanding of peatland respiration dynamics. We found that the respiration response of the shrubs and sedges at the site to changes in climate is influenced by their different mechanisms of obtaining water and nutrient resources. The sedges showing increased respiration fluxes with increases of phosphate concentrations exuded from their roots. The shrubs showed increased respiration fluxes with an increase in organic acids like lactate and citric acid. The isotope analyses show that the shrubs seem to be more intimately associated with the mosses than are the sedges. The clipped plots show a sign of ‘old’ carbon, but don’t show a sign of enriched δ¹³C. The difference between the clipped plots and the vegetated plots suggests HR is being fueled by carbon that is a few decades old but is essentially the same as the plants in terms of δ¹³C, whereas AR is releasing carbon that is fixed within the past few years. We also see enriched δ¹³C at depth though, which implies that the clipped plots are releasing C respired at depth. So, there must be a distinct carbon pool at depth that we don’t see contributing to HR.