Peatland ecosystems cover two percent of earth’s land surface, but store one third of global terrestrial carbon (C). High water tables (WTs), cool temperatures, and recalcitrant tissues in the dominant plants, (Sphagnum mosses) contribute to low rates of decomposition currently outweighed by C inputs from Sphagnum photosynthesis. Rising global temperatures increase decomposition rates, may lower WTs as evapotranspiration increases, and may directly influence Sphagnum photosynthesis. Thus, rising temperatures threaten to transform peatlands from C sinks into C sources. This study investigated the interactive effects of temperature and WT on Sphagnum photosynthesis as a proxy for peatland C inputs. Our study focused on mountain peatlands, understudied unique peatlands that face different C sequestration challenges than their Northern counterparts. We predicted that high temperatures and low WT would negatively impact Sphagnum photosynthesis, and that temperature and WT would interact to reduce photosynthesis. We employed a 38-day laboratory factorial design study that included four treatment groups: ambient temperature/low WT, ambient temperature/high WT, high temperature/high WT, and high temperature/low WT. To examine photosynthetic capacity throughout the study, we measured chlorophyll fluorescence (Fv/Fm), and we employed gas exchange measurements at the conclusion of the study to assess C fixation under different temperature and WT conditions.
Results/Conclusions
At the beginning of the study, both high temperature treatment groups had 8.67% higher Fv/Fm than ambient temperature groups, but as the experiment progressed Fv/Fm in the high temperature groups declined. At the conclusion, Fv/Fm in the high temperature groups was 14.37% lower than that of the ambient temperature groups. Temperature and WT also interacted to reduce Fv/Fm. Overall, photosynthetic capacity measurements revealed that temperature largely affects potential C fixation in Sphagnum mosses; thus, as global temperatures rise, C sequestration in peatlands may decrease. C fixation at the completion of the experiment exhibited large inter-treatment variability, and while it was not significantly influenced by WT or temperature, did trend towards negative C balance in the high temperature/low WT treatment. C fixation positively correlated with moss relative water content, indicating that Sphagnum desiccation resulting from water table drawdown may also impede C fixation. Overall, these results supported our initial hypothesis that prolonged exposure to high temperatures and low WT impedes Sphagnum C sequestration, and indicate that these climate factors may interact to eventually transform mountain peatlands from C sinks to C sources.