Summary 1.We separated the effects of plant species controls on decomposition rates from environmental controls in northern peatlands using a full factorial, reciprocal transplant experiment of eight dominant bryophytes in four distinct peatland types in boreal Alberta, Canada. Standard fractionation techniques as well as compound-specific pyrolysis molecular beam mass spectrometry were used to identify a biochemical mechanism underlying any interspecific differences in decomposition rates. 2. We found that over a 3-year field incubation, individual moss species and not micro-environmental conditions controlled early stages of decomposition. Across species, Sphagnum mosses exhibited a trade-off in resource partitioning into metabolic and structural carbohydrates, a pattern that served as a strong predictor of litter decomposition. 3. Decomposition rates showed a negative co-variation between species and their microtopographic position, as species that live in hummocks decomposed slowly but hummock microhabitats themselves corresponded to rapid decomposition rates. By forming litter that degrades slowly, hummock mosses appear to promote the maintenance of macropore structure in surface peat hummocks that aid in water retention. 4. Synthesis . Many northern regions are experiencing rapid climate warming that is expected to accelerate the decomposition of large soil carbon pools stored within peatlands. However, our results suggest that some common peatland moss species form tissue that resists decomposition across a range of peatland environments, suggesting that moss resource allocation could stabilize peatland carbon losses under a changing climate.

Symbiotic relationships between N 2 -fixing prokaryotes and their autotrophic hosts are essential in nitrogen (N)-limited ecosystems, yet the importance of this association in pristine boreal peatlands, which store 25 % of the world's soil (C), has been overlooked. External inputs of N to bogs are predominantly atmospheric, and given that regions of boreal Canada anchor some of the lowest rates found globally (*1 kg N ha -1 year -1), biomass production is thought to be limited primarily by N. Despite historically low N deposition, we show that boreal bogs have accumulated approximately 12-25 times more N than can be explained by atmospheric inputs.Here we demonstrate high rates of biological N 2 -fixation in prokaryotes associated with Sphagnum mosses that can fully account for the missing input of N needed to sustain high rates of C sequestration. Additionally, N amendment experiments in the field did not increase Sphagnum production, indicating that mosses are not limited by N. Lastly, by examining the composition and abundance of N 2 -fixing prokaryotes by quantifying gene expression of 16S rRNA and nitrogenase-encoding nifH, we show that rates of N 2 -fixation are driven by the substantial contribution from methanotrophs, and not from cyanobacteria. We conclude biological N 2 -fixation drives high sequestration of C in pristine peatlands, and may play an important role in moderating fluxes of methane, one of the most important greenhouse gases produced in peatlands. Understanding the mechanistic controls on biological N 2 -fixation is crucial for assessing the fate Responsible Editor: Matthew Wallenstein.Electronic supplementary material The online version of this article (doi:10.1007/s10533-014-0019-6) contains supplementary material, which is available to authorized users. Biogeochemistry (2014) 121:317-328 DOI 10.1007 of peatland carbon stocks under scenarios of climate change and enhanced anthropogenic N deposition.

Moisture seasonality may control forest floor decomposition rates in tropical forest. We used a mass balance model and 5 yr (December 1986 through December 1990) of weekly litterfall and monthly forest floor mass measurements from control and dry season irrigated plots to test this hypothesis on Barro Colorado Island, Panama. Litterfall and forest floor mass were greater in the dry season that in the wet season. Irrigation affected neither the timing nor the quantity of litterfall. In contrast, dry season irrigation reduced forest floor mass throughout the year, not just during the dry season. Forest floor decomposition during the dry season was enhanced by irrigation. During the dry season, net decomposition (in grams per square metre per day) and exponential decay coefficients (per day) averaged 48 and 42% greater, respectively, in irrigated plots than in controls plots. As a consequence, seasonal differences in decomposition rates were more pronounced in the control plots than in the irrigated plots. Net decomposition rates, for examples, a averaged 105 and 22% greater during the wet season than during the dry season on control and irrigated plots, respectively. Net decomposition was positively correlated with rainfall in the control plots, but not in the irrigated plots. These results support the hypothesis that moisture seasonality controls forest floor decomposition in tropical moist forests.

Boreal peatlands in Canada have harbored relict permafrost since the Little Ice Age due to the strong insulating properties of peat. Ongoing climate change has triggered widespread degradation of localized permafrost in peatlands across continental Canada. Here, we explore the influence of differing permafrost regimes (bogs with no surface permafrost, localized permafrost features with surface permafrost, and internal lawns representing areas of permafrost degradation) on rates of peat accumulation at the southernmost limit of permafrost in continental Canada. Net organic matter accumulation generally was greater in unfrozen bogs and internal lawns than in the permafrost landforms, suggesting that surface permafrost inhibits peat accumulation and that degradation of surface permafrost stimulates net carbon storage in peatlands. To determine whether differences in substrate quality across permafrost regimes control trace gas emissions to the atmosphere, we used a reciprocal transplant study to experimentally evaluate environmental versus substrate controls on carbon emissions from bog, internal lawn, and permafrost peat. Emissions of CO 2 were highest from peat incubated in the localized permafrost feature, suggesting that slow organic matter accumulation rates are due, at least in part, to rapid decomposition in surface permafrost peat. Emissions of CH 4 were greatest from peat incubated in the internal lawn, regardless of peat type. Localized permafrost features in peatlands represent relict surface permafrost in disequilibrium with the current climate of boreal North America, and therefore are extremely sensitive to ongoing and future climate change. Our results suggest that the loss of surface permafrost in peatlands increases net carbon storage as peat, though in terms of radiative forcing, increased CH 4 emissions to the atmosphere will partially or even completely offset this enhanced peatland carbon sink for at least 70 years following permafrost degradation.

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