Globally, peatlands store an estimated 270-490 Pg of carbon (C) and account for as much 33% of soil C reserves, making them an important C reservoir that can influence long-term biosphere-atmosphere C balance in the face of environmental change. Peatlands are dominated by a diversity of Sphagnum mosses that drive C inputs, many of which form symbiotic associations with nitrogen (N) – fixing cyanobacteria, which may represent an important source of N to these ecosystems. Because of the fundamental link between N availability and C fixation in photosynthetic organisms, it is critical that N and C inputs be considered in parallel in determining peatland ecosystem resilience to environmental change. Using a laboratory manipulation experiment, we examined how water table variability influenced net C inputs over a 24-week period in three common Sphagnum mosses that exhibit a vertical taxonomic stratification in hummock and hollow microclimates. We also examined how N fixation from cyanobacterial symbionts varied as a function of water table variability (continuously wet, low variability, high variability, and continuously dry) and Sphagnum species, with the ultimate aim of linking peatland C and N cycling under an array of possible hydrological scenarios.
Results/Conclusions
Carbon inputs varied as a function of water table treatment, where continuously wet samples and high water table variability resulted in the greatest C gains, low water table variability resulted in reductions up to 10% from these levels, and the continuously dry treatment resulted in >80% reductions or C losses. The hummock species, Sphagnum capillifolium exhibited the greatest C gains on a per shoot and per area basis, compared to the hollow species S. angustifolium and the intermediate generalist species S. magellanicum. S. capillifolium has a very dense growth form that resists water loss to the environment, and our results indicate this may be one factor that enhances resiliency to fluctuations in environmental water availability. Cyanobacteria were present in all taxa, but were most common in the S. angustifolium, where between 5 and 80 cells were recorded on individual leaves. N fixation rates up to 3 kg N ha-1 yr-1 were recorded at the onset of treatments in S. angustifolium, but water all water table treatments caused declines up to 90% in N fixation rates, suggesting N fixation may be more sensitive to hydrological variability than C fixation in peatland ecosystems.