Marcell Experimental Forest Research Articles

Microbial Metabolic Potential for Carbon Degradation and Nutrient (Nitrogen and Phosphorus) Acquisition in an Ombrotrophic Peatland

This study integrated metagenomic and nuclear magnetic resonance (NMR) spectroscopic approaches to investigate microbial metabolic potential for organic matter decomposition and nitrogen (N) and phosphorus (P) acquisition in soils of an ombrotrophic peatland in the Marcell Experimental Forest (MEF), Minnesota, USA. This analysis revealed vertical stratification in key enzymatic pathways and taxa containing these pathways. Metagenomic analyses revealed that genes encoding laccases and dioxygenases, involved in aromatic compound degradation, declined in relative abundance with depth, while the relative abundance of genes encoding metabolism of amino sugars and all four saccharide groups increased with depth in parallel with a 50% reduction in carbohydrate content. Most Cu-oxidases were closely related to genes from Proteobacteria and Acidobacteria, and type 4 laccase-like Cu-oxidase genes were >8 times more abundant than type 3 genes, suggesting an important and overlooked role for type 4 Cu-oxidase in phenolic compound degradation. Genes associated with sulfate reduction and methanogenesis were the most abundant anaerobic respiration genes in these systems, with low levels of detection observed for genes of denitrification and Fe(III) reduction. Fermentation genes increased in relative abundance with depth and were largely affiliated with Syntrophobacter. Methylocystaceae-like small-subunit (SSU) rRNA genes, pmoA, and mmoX genes were more abundant among methanotrophs. Genes encoding N2 fixation, P uptake, and P regulons were significantly enriched in the surface peat and in comparison to other ecosystems, indicating N and P limitation. Persistence of inorganic orthophosphate throughout the peat profile in this P-limiting environment indicates that P may be bound to recalcitrant organic compounds, thus limiting P bioavailability in the subsurface. Comparative metagenomic analysis revealed a high metabolic potential for P transport and starvation, N2 fixation, and oligosaccharide degradation at MEF relative to other wetland and soil environments, consistent with the nutrient-poor and carbohydrate-rich conditions found in this Sphagnum-dominated boreal peatland.

Long‐Term Soil Moisture Patterns in a Northern Minnesota Forest

Forest hydrological and biogeochemical processes are highly dependent on soil water. At the Marcell Experimental Forest, seasonal patterns of soil moisture have been monitored at three forested locations since 1966. This unique, long‐term data set was used to analyze seasonal trends in soil moisture as well as the influence of time‐lagged precipitation and modified Thornthwaite modeled potential evapotranspiration (PET) on seasonal soil moisture at three depths (0–15, 76–107, and 198–229 cm). Despite no change in precipitation during the 45‐yr record, mean annual soil moisture from 0 to 228.6 cm has declined (p < 0.05). Precipitation minus PET was found to account for >50% of the variability in seasonal soil moisture (p < 0.001). Our findings suggest that further increases in mean annual temperature and evapotranspiration may lead to decreases in soil moisture. These decreases could negatively impact forested ecosystems in northern Minnesota.

Whole-climate experiments for peatlands

The US Department of Agriculture Forest Service Marcell Experimental Forest covers 890 hectares in northern Minnesota, less than 100 miles south of the Canadian border. The forest’s peatland ecosystem straddles another border: It inhabits the southern margin of a boreal biome suspected …

Interannual, seasonal, and retrospective analysis of the methane and carbon dioxide budgets of a temperate peatland

AbstractThree years (2009–2011) of near‐continuous methane (CH4) and carbon dioxide (CO2) fluxes were measured with the eddy covariance (EC) technique at a temperate peatland located within the Marcell Experimental Forest, in northern Minnesota, USA. The peatland was a net source of CH4 and a net sink of CO2 in each year with annual carbon budgets of −26.8 (±18.7), −15.5 (±14.8), and −14.6 (±21.5) g C m−2 yr−1 for 2009–2011, respectively. Differences in the seasonal hydrometeorological conditions among the three study years were most pronounced during 2011, which was considerably warmer (+1.3°C) and wetter (+40 mm) than the 30 year average. The annual CH4 budget was +11.8 (±3.1), +12.2 (±3.0), and +24.9 (±5.6) g C m−2 yr−1 for the respective years and accounted for 23%–39% of the annual carbon budget. The larger CH4 emission in 2011 is attributed to significant warming of the peat column coupled with a high water table position throughout the entire growing season. Historical (1991–2011) CH4 emissions were estimated based on long‐term hydrometeorological records and functional relationships derived from our 3 year field study. The predicted historical annual CH4 budget ranged from +7.8 to +15.2 (±2.7) g CH4‐C m−2 yr−1. Recent (2007–2011) increases in temperature, precipitation, and rising water table at this site suggest that CH4 emissions have been increasing, but were generally greater from 1991 to 1999 when average soil temperature and precipitation were higher than in recent years. The global warming potential (considering CO2 and CH4) for this peatland was calculated based on a 100 year time horizon. In all three study years, the peatland had a net positive radiative forcing on climate and was greatest (+187 g C m−2) in 2011. The interannual variability in CH4 exchange at this site suggests high sensitivity to variations in hydrometeorological conditions.

Inter-annual and spatial variability in hillslope runoff and mercury flux during spring snowmelt

Spring snowmelt is an important period of mercury (Hg) export from watersheds. Limited research has investigated the potential effects of climate variability on hydrologic and Hg fluxes during spring snowmelt. The purpose of this research was to assess the potential impacts of inter-annual climate variability on Hg mobility in forested uplands, as well as spatial variability in hillslope hydrology and Hg fluxes. We compared hydrological flows, Hg and solute mobility from three adjacent hillslopes in the S7 watershed of the Marcell Experimental Forest, Minnesota during two very different spring snowmelt periods: one following a winter (2009-2010) with severely diminished snow accumulation (snow water equivalent (SWE) = 48 mm) with an early melt, and a second (2010-2011) with significantly greater winter snow accumulation (SWE = 98 mm) with average to late melt timing. Observed inter-annual differences in total Hg (THg) and dissolved organic carbon (DOC) yields were predominantly flow-driven, as the proportion by which solute yields increased was the same as the increase in runoff. Accounting for inter-annual differences in flow, there was no significant difference in THg and DOC export between the two snowmelt periods. The spring 2010 snowmelt highlighted the important contribution of melting soil frost in the timing of a considerable portion of THg exported from the hillslope, accounting for nearly 30% of the THg mobilized. Differences in slope morphology and soil depths to the confining till layer were important in controlling the large observed spatial variability in hydrological flowpaths, transmissivity feedback responses, and Hg flux trends across the adjacent hillslopes.

Simulating the Effect of Sulfate Addition on Methylmercury Output from a Wetland

The watershed analysis risk management framework (WARMF) model was applied to Wetland S6 of the Marcell Experimental Forest, using the data from a field experiment, conducted to investigate the effect of sulfate additions on mercury methylation in the wetland. The wetland was modeled as interconnected land catchments. Actual meteorology data and mercury and sulfate concentrations of precipitation were input to the model. To simulate the sulfate sprinkling, the experimental section of the bog was irrigated with sulfate water on the actual dates of sulfate additions. The model simulated wetland outflows that matched the measured outflows with an R -square of 0.856. WARMF also simulated other phenomena observed in the experiment: higher sulfate and MeHg levels at the wetland outlet after every sulfate addition, and higher sulfate and MeHg levels in the pore water of the bog after only the May addition, not the July and September additions. According to WARMF, the low groundwater table in May allowed the spri...

Carbon pools and productivity in a 1-km 2 heterogeneous forest and peatland mosaic in Minnesota, USA

Determining the magnitude of carbon (C) storage in forests and peatlands is an important step towards predicting how regional carbon balance will respond to climate change. However, spatial heterogeneity of dominant forest and peatland cover types can inhibit accurate C storage estimates. We evaluated ecosystem C pools and productivity in the Marcell Experimental Forest (MEF), in northern Minnesota, USA, using a network of plots that were evenly spaced across a heterogeneous 1-km 2 mosaic composed of a mix of upland forests and peatlands. Using a nested plot design, we estimated the standing C stock of vegetation, coarse detrital wood and soil pools. We also estimated aboveground net primary production (ANPP) as well as coarse root production. Additionally we evaluated how vegetation cover types within the study area differed in C storage. The total ecosystem C pool did not vary significantly among upland areas dominated by aspen (160 ± 13 Mg C ha −1), mixed hardwoods (153 ± 19 Mg C ha −1), and conifers (197 ± 23 Mg C ha −1). Live vegetation accounted for approximately 50% of the total ecosystem C pool in these upland areas, and soil (including forest floor) accounted for another 35–40%, with remaining C stored as detrital wood. Compared to upland areas, total C stored in peatlands was much greater, 1286 ± 125 Mg C ha −1, with 90–99% of that C found in peat soils that ranged from 1 to 5 m in depth. Forested areas ranged from 2.6 to 2.9 Mg C ha −1 in ANPP, which was highest in conifer-dominated upland areas. In alder-dominated and black spruce-dominated peatland areas, ANPP averaged 2.8 Mg C ha −1, and in open peatlands, ANPP averaged 1.5 Mg C ha −1. In treed areas of forest and peatlands, our estimates of coarse root production ranged from 0.1 to 0.2 Mg C ha −1. Despite the lower production in open peatlands, all peatlands have acted as long-term C sinks over hundreds to thousands of years and store significantly more C per unit area than is stored in uplands. Despite occupying only 13% of our study area, peatlands store almost 50% of the C contained within it. Because C storage in peatlands depends largely on climatic drivers, the impact of climate changes on peatlands may have important ramifications for C budgets of the western Great Lakes region.

Mercury inputs and outputs at a small lake in northern Minnesota

Storages and cycling of total mercury (HgT), methylmercury (MeHg), and Hg0 are described for Spring Lake, a small bog lake in the Marcell Experimental Forest in north-central Minnesota. We quantified photoredox transformations, MeHg photolysis, burial to the sediments, and internal and external loadings of HgT and MeHg. Atmospheric deposition was the main input of HgT; MeHg was supplied by a combination of atmospheric, near-shore wetland, and biotic (methylation) sources. HgT outputs were dominated by burial (67%), and Hg0 evasion accounted for 26% of HgT outputs. The watershed of Spring Lake is small (3.7× lake surface area), and accordingly, bog and upland runoff were minor contributors to both HgT and MeHg inputs. Wet deposition was ∼9% of total MeHg input, and other external inputs (runoff, sediment porewater) provided only an additional 7%, indicating that internal production of MeHg was occurring in the lake. Photolysis of MeHg, measured in the field and laboratory, removed ∼3× the lake mass of MeHg (20 mg) annually, and was the dominant sink for MeHg. Residence times of MeHg and HgT in the lake were 48 and 61 days, respectively, during the open-water season, compared with only 8 days for the residence time of MeHg on settling particles (seston). Photoreduction of Hg2+ to Hg0 was greater than the reverse reaction (Hg0 photooxidation), and the residence time of Hg0 in the photic zone was short (hours). Data from this study show active cycling of all the measured species of mercury (HgT, MeHg, and Hg0) and the importance of MeHg photolysis and photo-redox processes.

Sulfate Addition Increases Methylmercury Production in an Experimental Wetland

Atmospheric mercury is the dominant Hg source to fish in northern Minnesota and elsewhere. However, atmospherically derived Hg must be methylated prior to accumulating in fish. Sulfate-reducing bacteria are thought to be the primary methylators of Hg in the environment. Previous laboratory and field mesocosm studies have demonstrated an increase in methylmercury (MeHg) levels in sediment and peatland porewaters following additions of sulfate. In the current ecosystem-scale study, sulfate was added to half of an experimental wetland at the Marcell Experimental Forest located in northeastern Minnesota, increasing annual sulfate load by approximately four times relative to the control half of the wetland. Sulfate was added on four separate occasions during 2002 and delivered via a sprinkler system constructed on the southeast half (1.0 ha) of the S6 experimental wetland. MeHg levels were monitored in porewater and in outflow from the wetland. Prior to the first sulfate addition, MeHg concentrations (filtered, 0.7 microm) were not statistically different between the control (0.47 +/- 0.10 ng L(-1), n = 12; mean +/- one standard error) and experimental 0.52 +/- 0.05 ng L(-1), n = 18) halves. Following the first addition in May 2002, MeHg porewater concentrations increased to 1.63 +/- 0.27 ng L(-1) two weeks after the addition, a 3-fold increase. Subsequent additions in July and September 2002 did not raise porewater MeHg, but the applied sulfate was not observed in porewaters 24 h after addition. MeHg concentrations in outflow from the wetland also increased leading to an estimated 2.4x increase of MeHg flux from the wetland. Our results demonstrate enhanced methylation and increased MeHg concentrations within the wetland and in outflow from the wetland suggesting that decreasing sulfate deposition rates would lower MeHg export from wetlands.

Sediment and Porewater Profiles and Fluxes of Mercury and Methylmercury in a Small Seepage Lake in Northern Minnesota

Mercury (HgT) and methylmercury (MeHg) were measured at 1-2 cm resolution in sediment porewater and sediment cores from Spring Lake in the Marcell Experimental Forest of northern Minnesota. Recent sediment accumulation of HgT was 21.4 microg m(-2) yr(-1) (1990-2000), 2 orders of magnitude greater than the accumulation of MeHg (0.20 microg m(-2) yr(-1)). The highest solid phase concentrations of MeHg were observed persistently at the sediment surface and declined sharply with depth. Porewater profiles showed a small diffusive flux of MeHg from sediment to water (5 ng m(-2) month(-1)). Springtime porewater concentrations of MeHg were relatively low (approximately 0.5 ng L(-1)) and increased by late summer to early fall (1.5-2.2 ng L(-1)), showing distinct peaks roughly correlated with maxima in sulfate reducing activity at 5 and 15 cm. Advective transport carrying MeHg deeper into the sediment was evident in summer and fall. The percent of HgT present as MeHg was highest in the water column above the sediment (10%) and decreased with sediment depth in both the solid and porewater phases. Sediments at this study site are a net sink for MeHg, although diagenetic processes of demethylation and methylation are evident within the lake-sediment environment.

Stream flow and ground water recharge from small forested watersheds in north central Minnesota

In hydrologic studies of forested watersheds, the component of the water balance most likely to be poorly defined or neglected is deep seepage. In the complex glaciated terrain of the northern Lake States, subsurface water movement can be substantial. On the Marcell experimental forest (MEF) in north-central Minnesota, ground water table elevations measured in observation wells in recharge areas were used to calculate rates of ground water recharge. In northern Minnesota winters, precipitation is stored on the surface as snow and ground water recharge ceases. Water table elevations in recharge areas decline over winter at calculable rates. Deviations from these rates during other times of the year are due to ground water recharge. On 10–50ha watersheds on the MEF, ground water recharge varies among watersheds but constitutes about 40% of the total water yield. Annual ground water recharge amounts were found to vary linearly with precipitation. Even in high precipitation years, the infiltration capacity of the watersheds was not exceeded. Regression equations were developed relating yearly ground water recharge, stream flow, and total water yield, to seasonal precipitation amounts, summer and autumn precipitation during the previous year, and non-winter air temperature.

Retention and mobility of cations in a small peatland: trends and mechanisms

Inputs and net storage or release of cations can have a large impact on the acid-base balance of any ecosystem. Because of the absence of mineral soils, ion exchange and accretion in biomass are the major internal processes governing cation accumulation in peatlands. It is widely believed that peatlands have a large capacity to store cations, and thus to affect the acidity of surface waters. A long-term record of hydrologic and chemical inputs and outputs to the S-2 peatland in the Marcell Experimental Forest was used to construct annual and monthly mass balances for the four major cations. Bulk deposition and throughfall measurements suggest that deposition of Na, Ca and Mg has increased over the period 1971-1984; wet-only and dry bucket measurements do not show such a change. Seasonal and annual variations in cation inputs are not visible in the cation export from the peatland because the large cation reservoir on exchange sites in peat buffers the surface waters and prevents them from responding rapidly to changing inputs. Nevertheless, ion exchange represents a relatively minor source of acidity to this peatland, and accumulation of cations in woody biomass accounts for the majority of the cation retention. The major mechanism of retention in the peatland is different for each cation; accumulation in woody biomass is most important for Ca, storage on ion exchange sites in peat is most important for Mg, green plant tissues are the dominant site of K storage, and Na is bound by unknown mechanisms in the peat. Retention efficiencies show large annual variations but average 50% for Ca, 22% for Mg, 29-44% for Na, and 21% for K.

Environmental factors controlling methane emissions from peatlands in northern Minnesota

Controls on methane emission from peatlands in northern Minnesota were investigated by correlation to environmental variables and by field manipulation. From September 1988 through September 1990, methane flux measurements were made at weekly to monthly intervals at six sites in the Marcell Experimental Forest, northern Minnesota (two open bog sites, two forested bog sites, a poor fen, and a fen lagg). Flux was related to water table position and peat temperature with simple correlations at individual sites and multiple regression on all sites together. The effect of water table was also investigated experimentally in “bog corrals” (open‐ended metal enclosures set in the peat) in which water table was artificially raised to the surface in the driest peatland. Temperature largely controlled variation in flux within individual ecosystems at Marcell, but hydrology distinguished between‐site variation. Water table position, peat temperature, and degree of peat humification explained 91% of the variance in log CH4 flux, predicted annual methane emission from individual wetlands successfully, and predicted the change in flux due to the water table manipulation. Raising the water table in the bog corrals by an average of 6 cm in autumn 1989 and 10 cm in summer 1990 increased emission by 2.5x and 2.2x, respectively. Just as expanding the scale of investigation from a single habitat in a wetland to several wetlands necessitates incorporation of additional variables to explain flux (water table, peat characteristics), modeling flux from several wetland regions, if possible, will require the addition of climate parameters.

Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands

The stable carbon isotopic composition of methane (δ13C) emitted from two peatland sites in the Marcell Experimental Forest in northern Minnesota was investigated during the snow‐free season of 1989–1990. A seasonal range in δ13C values of 13‰ was seen for a forested bog with heavier (13C enriched) methane emitted during the wanner summer months. This shift was correlated with water table level suggesting control by microbial oxidation. Methane from a nearby poor fen transitional to bog dominated byCarex oligospermashowed a similar temporal trend but with a much smaller range of 5‰ during the same time period and with no water table level correlation. The methane emitted from the fen was consistently heavier than that emitted by the bog.

Methane flux from Minnesota Peatlands

Northern (>40°N) wetlands have been suggested as the largest natural source of methane (CH4) to the troposphere. To refine our estimates of source strengths from this region and to investigate climatic controls on the process, fluxes were measured from a variety of Minnesota peatlands during May, June, and August 1986. Sites included forested and unforested ombrotrophic bogs and minerotrophic fens in and near the U.S. Department of Agriculture Marcell Experimental Forest and the Red Lake peatlands. Late spring and summer fluxes ranged from 11 to 866 mg CH4 m−2 d−1, averaging 207 mg CH4 m−2 d−1 overall. At Marcell Forest, forested bogs and fen sites had lower fluxes (averages of 77 ± 21 mg CH4 m−2 d−1 and 142 ± 19 mg CH4 m−2 d−1) than open bogs (average of 294 ± 30 mg CH4 m−2 d−1). In the Red Lake peatland, circumneutral fens, with standing water above the peat surface, produced more methane than acid bog sites in which the water table was beneath the moss surface (325 ± 31 and 102 ± 13 mg CH4 m−2 d−1, respectively). Peat temperature was an important control. Methane flux increased in response to increasing soil temperature. For example, the open bog in the Marcell Forest with the highest CH4 flux exhibited a 74‐fold increase in flux over a three‐fold increase in temperature. We estimate that the methane flux from all peatlands north of 40° may be on the order of 70 to 90 Tg/yr though estimates of this sort are plagued by uncertainties in the areal extent of peatlands, length of the CH4 producing season, and the spatial and temporal variability of the flux.

Precipitation chemistry at the marcell experimental forest in north central Minnesota

Concentration (milligrams and microequivalents per liter) values are presented for major ions occurring in rain and snow from July 1978 to July 1980. Ammonium, sodium, and calcium are the largest equivalent weight cations which are largely balanced by sulfate and nitrate anions. Hydrogen is the fourth most abundant cation; but high combined concentrations of other cations in 27% of the sample events mask positive correlations with high hydrogen concentrations. Field‐measured pH averaged 4.3, but the range (3.5 to 6.5) fluctuated with a combination of earth and air mass conditions. When high cation (and anion) data are deleted, low pH correlates well with high nitrate. Sulfate concentrations are similar to those measured in 1936 and do not correlate with pH. Calcium and, to a lesser extent, ammonium tend to increase pH values. High nitrate values also correlate positively with high excess acidity, while high sodium and calcium values correlate with high excess alkalinity.

Peat Temperature Regime of a Minnesota Bog and the Effect of Canopy Removal

Peat bogs occupy approximately 6 000 000 ha in the cool sub-humid northern Lake States of the U.S.A. These peatlands are intimately associated with the regional water resources, often serving as the source of streamfiow. They also represent unique wetland ecosystems. Temperature is an important characteristic both of the bog environment and the quality of streamflow water. Alteration of peat temperature could result in changes in the surface vegetation and changes in microbial activity, so affecting the amount of organic matter accumulated (Clymo 1965; Lather & Cragg 1967; Moore & Bellamy 1974) and the quality of streamfiow water. Therefore, it is necessary to determine the temperature regimes of peat bogs in both undisturbed and disturbed situations prior to any large-scale alterations of management and energy input. Accordingly, temperature regimes were determined in a strip-cut black spruce bog to evaluate the effect of canopy removal. METHODS The study bog, located in northern Minnesota on the Marcell Experimental Forest (c. 47032'N, 93028'W), is approximately 560 m long and 140 m wide with the long axis oriented north and south. The peat profile of this lake-filled, forested bog has been described by Boelter (1964). The bog is isolated from the regional groundwater system. Precipitation and run-off from the adjacent uplands comprise the water input to the bog. Outflow is primarily as intermittent surface streamflow from the south end of the bog. In the winter of 1968-69, 30-m-wide strips were clear-cut, in an east-west direction, leaving 45-m-wide strips of sixty-two-year-old black spruce (Picea mariana (Mill.) B.S.P.). The crown closure of the forested strips was& approximately 7500. The strips were logged when snow cover was ample, so that disturbance of the surface vegetation was negligible, and a luxuriant sedge cover rapidly established in the following summer (Brown 1972). Eight temperature stations were located at the south end of the bog and eight at the north end. Two stations were located in the centre of clear-cut strips, two were in the centre of the forested strips and four were on the boundaries between strips. Peat depth at the temperature stations ranged from 2 to 2-8 m. Each temperature station consisted of a series of copper-constantan thermocouples, located at depths of 0-025, 0 3, 1 and 2 m below the interface of the living sphagnum moss and the organic soil material. The thermocouples were inserted through a wooden stake 2-5 x 2-5 cm and 2 m long; the stake was pushed into the peat to the desired depth. Air temperatures were measured at two locations in the clear-cut strips and two locations in the non-cut strips by shaded thermocouples 1 m above the bog surface. Temperatures were read with a portable potentiometer. A battery-operated resistance heating unit was