The goal of this project was to investigate changes in the structure of dissolved and solid phase organic matter, the production of CO2 and CH4, and the composition of decomposer microbial communities in response to the climatic forcing of environmental processes that determine the balance between carbon gas production versus storage and sequestration in peatlands. Cutting-edge analytical chemistry and next generation sequencing of microbial genes were been applied to habitats at the Marcell Experimental Forest (MEF), where the US DOE's Oak Ridge National Laboratory and the USDA Forest Service are constructing a large-scale ecosystem study entitled, "Spruce and Peatland Responses Under Climatic and Environmental Change"(SPRUCE). Our study represented a comprehensive characterization of the sources, transformation, and decomposition of organic matter in the S1 bog at MEF. Multiple lines of evidence point to distinct, vertical zones of organic matter transformation: 1) the acrotelm consisting of living mosses, root material, and newly formed litter (0-30 cm), 2) the mesotelm, a mid-depth transition zone (30-75 cm) characterized by labile organic C compounds and intense decomposition, and 3) the underlying catotelm (below 75cm) characterized by refractory organic compounds as well as relatively low decomposition rates. These zones are in part defined by physical changes in hydraulic conductivity and water table depth. O-alkyl-C, which represents the carbohydrate fraction in the peat, was shown to be an excellent proxy for soil decomposition rates. The carbon cycle in deep peat was shown to be fueled by modern carbon sources further indicating that hydrology and surface vegetation play a role in belowground carbon cycling. We provide the first metagenomic study of an ombrotrophic peat bog, with novel insights into microbial specialization and functions in this unique terrestrial ecosystem. Vertical structuring of microbial communities closely paralleled the chemical evolution of peat, with large shifts in microbial populations occurring in the biogeochemical hotspot, the mesotelm, where the highest rates of decomposition were detected. Stable isotope geochemistry and potential rates of methane production paralleled vertical changes in methanogen community composition to indicate a predominance of acetoclastic methanogenesis mediated by the Methanosarcinales in the mesotelm, while hydrogen-utilizing methanogens dominated in the deeper catotelm. Evidence pointed to the availability of phosphorus as well as nitrogen limiting the microbially-mediated turnover of organic carbon at MEF. Prior to initiation of the experimental treatments, our study provided key baseline data for the SPRUCE site on the vertical stratification of peat decomposition, key enzymatic pathways, and microbial taxa containing these pathways. The sensitivity of soil carbon turnover to climate change is strongly linked to recalcitrant carbon stocks and the temperature sensitivity of decomposition is thought to increase with increasing molecular complexity of carbon substrates. This project delivered results on how climate change perturbations impact the microbially-mediated turnover of recalcitrant organic matter in peatland forest soils, both under controlled conditions in the laboratory and at the ecosystem-scale in the field. This project revisited the concept of "recalcitrance" in the regulation of soil carbon turnover using a combination of natural abundance radiocarbon and optical spectroscopic measurements on bulk DOM, and high resolution molecular characterization of DOM. The project elucidated how organic matter reactivity and decomposition will respond to climate change in a both a qualitative (organic matter lability) and quantitiative (increased rates) manner. An Aromaticity Index was developed to represent a more direct and accurate parameter for modeling of DOM reactivity in peatlands. The abundance and community composition o ...

This publication provides extended abstracts of papers presented at a workshop on forested peatland carbon dynamics. Topics of papers include peat accumulation, carbon flux measurements, peatlands of the western boreal forest and the Mackenzie Valley, the carbon chemistry of peat, the effect of temperature on microbial decomposition, modelling carbon accumulation, peatland hydrology modelling, peatland fire & impacts on carbon dynamics, soil carbon dynamics in the boreal forest, and how peat fits into the Kyoto Protocol.

As global climate continues to change, it becomes more important to understand possible feedbacks from soils to the climate system. This dissertation focuses on soil microbial community responses to climate change factors in northern hardwood forests. Two soil warming experiments at Harvard Forest in Massachusetts, and a climate change manipulation experiment with both elevated temperature and increased moisture inputs in Michigan were sampled. The hyphal in-growth bag method was to understand how soil fungal biomass and respiration respond to climate change factors. Our results from phospholipid fatty acid (PLFA) analyses suggest that the hyphal in-growth bag method allows relatively pure samples of fungal hyphae to be partitioned from bacteria in the soil. The contribution of fungal hyphal respiration to soil respiration was examined in climate change manipulation experiments in Massachusetts and Michigan. The Harvard Forest soil warming experiments in Massachusetts are long-term studies with 8 and 18 years of +5 °C warming treatment. Hyphal respiration and biomass production tended to decrease with soil warming at Harvard Forest. This suggests that fungal hyphae adjust to higher temperatures by decreasing the amount of carbon respired and the amount of carbon stored in biomass. The Ford Forestry Center experiment in Michigan has a 2 x 2 fully factorial design with warming (+4-5 °C) and moisture addition (+30% average ambient growing season precipitation). This experiment was used to examine hyphal growth and respiration of arbuscular mycorrhizal fungi (AMF), soil enzymatic capacity, microbial biomass and microbial community structure in the soil over two years of experimental treatment. Results from the hyphal in-growth bag study indicate that AMF hyphal growth and respiration respond negatively to drought. Soil enzyme activities tend to be higher in heated versus unheated soils. There were significant temporal variations in enzyme activity and microbial biomass estimates. When microbial biomass was estimated using chloroform fumigation extractions there were no differences between experimental treatments and the control. When PLFA analyses were used to estimate microbial biomass we found that biomass responds negatively to higher temperatures and positively to moisture addition. This pattern was present for both bacteria and fungi. More information on the quality and composition of the organic matter and nutrients in soils from climate change manipulation experiments will allow us to gain a more thorough understanding of the mechanisms driving the patterns reported here. The information presented here will improve current soil carbon and nitrogen cycling models.

"Peatlands only cover ~3% of the globe's terrestrial and fresh water surface and yet they store up to 30% of the global soil carbon (C). As such, the C storage and ecological importance of peatlands are proportionally much greater than their area might suggest. Microorganisms drive organic matter decomposition, and thus C and nutrient mobilization, though the controls on microbial activity remain poorly understood. The overall objective of this thesis was to elucidate the microbial role in organic matter decomposition in temperate peatlands. I explored substrate and microbial-ecological controls, with particular emphasis on extracellular enzyme activity, under both natural climate variability and increased nutrient deposition.Dissolved organic carbon (DOC) serves numerous ecological and chemical functions including acting as a microbial substrate. Through laboratory experiments, the concentration, biodegradability, and intrinsic properties of DOC leached from peat, fresh material, and litter from nine species of ombrotrophic bog vegetation were characterized. Initial DOC concentration and the fraction of biodegradable DOC differed significantly among species and were significantly higher in fresh material than either litter or peat extracts. Vegetation species and degree of decomposition of the parent organic matter are significant controls on microbial substrate quality.Through laboratory and field measurements, temperature and pH were determined to be the common environmental controls on the enzyme-latch mechanism across four biogeochemically different peatland types though site-specific factors such as nutrient availability led to deviations from these patterns. Overall, enzyme activity decreased significantly with depth and showed significant variation over the course of the growing season with a minimum in the spring and a maximum in the summer and fall highlighting the vulnerability of the peatland soil organic C stock to anthropogenic climate warming.Laboratory analysis of samples from a long-term fertilization experiment found significant changes in peat and dissolved matter nutrient stoichiometry and extracellular enzyme activity relative to control following 9 - 14 years of nitrogen (N), phosphorus (P), and potassium (K) fertilization. Results suggest concurrent and counteracting responses to long-term nutrient fertilization with complex interactions among surface vegetation, microbes, and nutrient availability. In particular, the N-inhibition of phenol oxidase activity may partially counteract the effect of enhanced microbial substrate quality on C and nutrient mobilization.Research in this thesis contributes new understanding of the controls on microbial decomposition of organic matter, and ultimately greenhouse gas production and DOC export, in northern peatlands. This work, however, also shows that microbial-ecological relationships are complex and site-specific factors pose problems for incorporating them in ecological models." --

Simulations of increased CO2 also confirmed positive growth response in the short term. The response of soil carbon was similar, however predicted to be less than the increase of biomass. Nitrogen availability and negative feedback mechanisms of the plant soil system were critical to the results, indicating that nitrogen progressively limited the growth response.

Forests are increasingly being managed for their carbon sequestration potential. As such, an understanding of the factors controlling carbon dynamics across and within sites is becoming increasingly important for guiding carbon management strategies. Given that much of a forest's carbon is stored in soils, identifying the factors that control how much carbon is stored in soils is critical. This study used detailed vegetation and soil measurements across a rich northern hardwood forest in Corinth, Vermont to identify factors that drive soil carbon storage in a northern hardwood forest, a common type in New England, and investigated how multiple non-native species might impact these factors. These forests have a large component of white ash (Fraxinus americana), a species threatened by the invasive emerald ash borer (Agrilus planipennis), creating an urgency to assess how ash trees influence soil organic carbon sequestration, as well as how their mortality may impact future carbon dynamics. Furthermore, non-native earthworms, which have a large impact of forest floor and soil carbon, are impacting these systems. This work quantified how these stressors are affecting carbon storage and tree regeneration. Analysis of organic litter material and mineral soil samples from these areas indicate both earthworms and overstory ash basal area significantly impact leaf litter nitrogen content and leaf litter carbon to nitrogen ratio (C:N); however, there was no interaction between the two factors. Earthworms also significantly decreased soil pH, however it is difficult to disentangle if earthworms are drawn to higher pH areas or if they create these conditions. Conversely, basal area of white ash had a significant, increasing effect on leaf litter pH. Soil pH was the best predictor of soil carbon in the upper soil horizons, and carbon to nitrogen ratio (C:N) in the forest floor was best explained by the interaction of litter pH and earthworm prevalence. Collectively, these results suggest both earthworm and emerald ash borer may alter soil carbon and nutrient dynamics in rich northern hardwood forests and the pathways by which carbon is stored.

Northern peatlands store approximately 30% of the world's soil carbon, and are also responsible for contemporary fluxes of the greenhouse gases carbon dioxide (CO2) and methane (CH4), making them important players in the global carbon (C) cycle. These greenhouse gas emissions are mediated by peat-dwelling microbes; however, the environmental factors governing the structures and functions of peat microbial communities are still poorly understood. In order to better understand these dynamics, I examined the effects of two forms of environmental change on peatland microbial communities. Firstly, to gain fundamental knowledge of the drivers of microbial community shift due to natural peatland succession, I examined the effects of long-term peat transplantation from a rich fen to a late-successional poor fen. This allowed me to evaluate the relative effects of solid phase chemistry and substrate (largely determined by the parent material/vegetation) versus aqueous chemistry (influenced more by groundwater or precipitation sources), on peat microbial communities. My results suggest that solid phase chemistry, particularly total nitrogen (TN) and C:N, may be important in determining the makeup of peatland bacterial communities. Secondly, I examined the effects of soil warming simulating projected climate change in a poor and an intermediate fen on peat microbial respiration and CH4 production as the preliminary stage of a multi-year, large-scale field experiment. Soil warming did not lead to any effects on CO2 production or CH4 flux during peat incubation.