Comparisons of soil nitrogen mass balances for an ombrotrophic bog and a minerotrophic fen in northern Minnesota

Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater

Changes in sulfate (SO₄^2-) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO₄^2- deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO₄^2- export from a peatland-dominated catchment as a function of changing SO₄^2- deposition in a long-term (1998-2011), whole-ecosystem, control-impact experiment. Annual SO₄^2- deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO₄^2- additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO₄^2- addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO₄^2- inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO₄^2- deposition declines.

Carbon and Nutrient Stoichiometric Relationships in the Soil–Plant Systems of Disturbed Boreal Forest Peatlands within Athabasca Oil Sands Region, Canada

Peatlands store carbon (C), nitrogen (N), and phosphorus (P), and the stoichiometric relationship among them may be modified by ecosystem disturbances, with major implications for boreal peatland ecosystem functions. To understand the potential impact of landscape fragmentation on peatland nutrient stoichiometry, we characterize the stoichiometric ratios of C, N and P in the soil–plant systems of disturbed boreal forest peatlands and also assessed relationships among site conditions, nutrient availability, stoichiometric ratios (C:N:P) and C storage in four sites that represent the forms of disturbed peatlands in the Athabasca oil sands region. Our results showed that nutrient stoichiometric balance differed across and within these peatlands, among plants, peat, and groundwater. Ratios of C:N and C:P in peat is a function of nutrient and moisture conditions, increasing from nutrient-rich (C:N = 28; C:P = 86) to nutrient-poor fens (C:N = 82; C:P = 1061), and were lower in moist hollows relative to drier hummock microforms. In groundwater, the drier nutrient-rich fen had higher N:P ratios relative to the nutrient-poor fen, reflecting interactions between dominant hydrologic conditions and stoichiometric relationships. The N:P ratio of plants was more similar to those of peat than groundwater pools, especially in the most recently disturbed nutrient-poor fen, where plant C:N:P ratios were greater compared to older disturbed sites in the region. These findings suggest that disturbances that modify moisture and nutrient regimes could potentially upset the C:N:P stoichiometric balance of boreal forest peatlands. It also provides valuable insights and essential baseline data to inform our understanding of how peatland C:N:P stoichiometry would respond to disturbance and restoration interventions in a boreal forest region at the tipping point of environmental change.

Impacts of historical ditching on peat volume and carbon in northern Minnesota USA peatlands

Peatlands play a critical role in terrestrial carbon (C) storage, containing an estimated 30% of global soil C, despite occupying only 3% of global land area. Historic management of peatlands has led to widespread degradation and loss of important ecosystem services, including C sequestration. Legacy drainage features in the peatlands of northern Minnesota, USA were studied to assess the volume of peat and the amount of C lost in the ~100 years since drainage. Using high-resolution Light Detection and Ranging (LiDAR) data, we measured elevation changes adjacent to legacy ditches to model pre-ditch surface elevations, which were used to calculate peat volume loss. We established relationships between volume loss and site characteristics from existing geographic information systems datasets and used those relationships to scale volume loss to all mapped peatland ditches in northern Minnesota (USA). We estimated that 0.165 ± 0.009 km³ of peat have been lost along almost 4000 km of peatland ditches. Peat loss upslope of ditches was significantly less than downslope (P < 0.001). Mean width of the entire ditch-effect zone was 333 ± 8.32 m. Using our volume loss estimates, literature estimates of oxidation, and mean bulk density and peat C% values from Minnesota peatlands, we calculate a total historic loss 3.847 ± 0.364 Tg C. Assuming a constant oxidation rate during the 100 years since drainage, euic and dysic peatlands within the ditch effect zone have lost 0.26 ± 0.08 and 0.40 ± 0.13 Mg C ha^-1 yr^-1, respectively, comparable to IPCC estimates. Our spatially-explicit peat loss estimates could be incorporated into decision support tools to inform management decisions regarding peatland C and other ecosystem services.

Chronic atmospheric reactive N deposition has breached the N sink capacity of a northern ombrotrophic peatbog increasing the gaseous and fluvial N losses

Peatlands play an important role in modulating the climate, mainly through sequestration of carbon dioxide into peat carbon, which depends on the availability of reactive nitrogen (Nr) to mosses. Atmospheric Nr deposition in the UK has been above the critical load for functional and structural changes to peatland mosses, thus threatening to accelerate their succession by vascular plants and increasing the possibility of Nr export to downstream ecosystems. The N balance of peatlands has received comparatively little attention, mainly due to the difficulty in measuring gaseous N losses as well as the Nr inputs due to biological nitrogen fixation (BNF). In this study we have estimated the mean annual N balance of an ombrotrophic bog (Migneint, North Wales) by measuring in situ N₂ + N₂O gaseous fluxes and also BNF in peat and mosses. Fluvial N export was monitored through a continuous record of DON flux, while atmospheric N deposition was modelled on a 5 × 5 km grid. The mean annual N mass balance was slightly positive (0.7 ± 4.1 kg N ha^-1 y^-1) and varied interannually indicating the fragile status of this bog ecosystem that has reached N saturation and is prone to becoming a net N source. Gaseous N losses were a major N output term accounting for 70% of the N inputs, mainly in the form of the inert N₂ gas, thus providing partial mitigation to the adverse effects of chronic Nr enrichment. BNF was suppressed by 69%, compared to rates in pristine bogs, but was still active, contributing ~2% of the N inputs. The long-term peat N storage rate (8.4 ± 0.8 kg N ha^-1 y^-1) cannot be met by the measured N mass balance, showing that the bog catchment is losing more N than it can store due its saturated status.

Unprocessed Atmospheric Nitrate in Waters of the Northern Forest Region in the U.S. and Canada

Little is known about the regional extent and variability of nitrate from atmospheric deposition that is transported to streams without biological processing in forests. We measured water chemistry and isotopic tracers (δ¹⁸O and δ¹⁵N) of nitrate sources across the Northern Forest Region of the U.S. and Canada and reanalyzed data from other studies to determine when, where, and how unprocessed atmospheric nitrate was transported in catchments. These inputs were more widespread and numerous than commonly recognized, but with high spatial and temporal variability. Only 6 of 32 streams had high fractions (>20%) of unprocessed atmospheric nitrate during baseflow. Seventeen had high fractions during stormflow or snowmelt, which corresponded to large fractions in near-surface soil waters or groundwaters, but not deep groundwater. The remaining 10 streams occasionally had some (<20%) unprocessed atmospheric nitrate during stormflow or baseflow. Large, sporadic events may continue to be cryptic due to atmospheric deposition variation among storms and a near complete lack of monitoring for these events. A general lack of observance may bias perceptions of occurrence; sustained monitoring of chronic nitrogen pollution effects on forests with nitrate source apportionments may offer insights needed to advance the science as well as assess regulatory and management schemes.

Natural iron fertilization of the coastal ocean by "blackwater rivers"

The present study elucidates the role of natural iron fertilization of the coastal ocean by so-called "blackwater rivers". Areas of marsh, fen, peatland, boreal forest etc. are characterized by organic-rich soils. From those soils, humic substances (humic and fulvic acids) are leached to the aquatic system resulting in river water that is low in pH and dark-brown in color. The point is that "blackwater rivers" tend to be rich in dissolved iron due to the unique chelating properties of humic and fulvic acids which bind Fe(III) and keep it in solution. We performed algal physiological (growth rate) experiments under conditions of iron deficiency with the marine unicellular phytoplankton algae Chlorella salina and Diacronema lutheri in 0.2 μm cut-off filtered mixtures of natural "blackwater river" water and synthetic seawater. Our results demonstrate that the iron naturally present in "blackwater rivers" is readily bioavailable to both marine algal species. Furthermore, the humic and fulvic acids exert an additional stimulatory effect on the marine algae. Both algae thrive much better in the presence of natural humic and fulvic acids as compared to a medium where EDTA is used as an iron-chelating agent. Our results indicate that "blackwater rivers", in sharp contrast to other types of rivers, are excellent sources of bioavailable iron to marine phytoplankton. This natural iron fertilization may give rise to photosynthesis-driven sequestration of CO₂ from the atmosphere to the sea, as can be seen from the visualization of CO₂ surface concentrations by NASA (NASA GEOS-5 model) which shows the global sources and sinks of CO₂ localized in time and space. The results by NASA suggest that strong marine CO₂ sinks in coastal waters tend to occur close to "blackwater river" estuaries. It is thus evident that "blackwater rivers" act as important sources of a limiting nutrient (iron) to the ocean.

Contemporary Mobilization of Legacy Pb Stores by DOM in a Boreal Peatland

We examined how different landscape areas in a catchment containing a northern ombrotrophic peatland and upland mineral soils responded to dramatic decreases in atmospheric deposition of lead (Pb). Pb concentrations in the outflow stream from the peatland measured from 2009-2015 indicated continued mobilization and export of Pb derived from historic inputs to the bog. In contrast, Pb concentrations in surface peat and runoff from upland mineral soils have declined in response to reductions in atmospheric deposition. Relative to the early 1980s, Pb concentrations in the streamflow decreased only ∼50%, while Pb in surface peat and upland subsurface runoff decreased by more than 90%. Water level fluctuations in the slow-accumulating peat have allowed dissolved organic matter (DOM) to continue mobilizing Pb deposited in the peatland decades earlier. Strong correlations between dissolved organic carbon (DOC) and Pb concentrations in outflow from the peatland and in bog porewaters demonstrate Pb mobility related to DOM production. Peat stores of Pb in 2016 were less than or equal to those reported in the early 1980s despite the dry mass inventory increasing by 60-80%. Much of the loss in Pb stored in peat can be accounted for by stream runoff from the peatland.

Effects of airborne ammonium and nitrate pollution strongly differ in peat bogs, but symbiotic nitrogen fixation remains unaffected

Pristine bogs, peatlands in which vegetation is exclusively fed by rainwater (ombrotrophic), typically have a low atmospheric deposition of reactive nitrogen (N) (<0.5kgha^-1y^-1). An important additional N source is N₂ fixation by symbiotic microorganisms (diazotrophs) in peat and mosses. Although the effects of increased total airborne N by anthropogenic emissions on bog vegetation are well documented, the important question remains how different N forms (ammonium, NH₄⁺, versus nitrate, NO₃^-) affect N cycling, as their relative contribution to the total load strongly varies among regions globally. Here, we studied the effects of 11years of experimentally increased deposition (32 versus 8kgNha^-1y^-1) of either NH₄⁺ or NO₃^- on N accumulation in three moss and one lichen species (Sphagnum capillifolium, S. papillosum, Pleurozium schreberi and Cladonia portentosa), N₂ fixation rates of their symbionts, and potential N losses to peat soil and atmosphere, in a bog in Scotland. Increased input of both N forms led to 15-90% increase in N content for all moss species, without affecting their cover. The keystone species S. capillifolium showed 4 times higher N allocation into free amino acids, indicating N stress, but only in response to increased NH₄⁺. In contrast, NO₃^- addition resulted in enhanced peat N mineralization linked to microbial NO₃^- reduction, increasing soil pH, N concentrations and N losses via denitrification. Unexpectedly, increased deposition from 8 to 32kgha^-1y^-1 in both N forms did not affect N₂ fixation rates for any of the moss species and corresponded to an additional input of 5kgNha^-1y^-1 with a 100% S. capillifolium cover. Since both N forms clearly show differential effects on living Sphagnum and biogeochemical processes in the underlying peat, N form should be included in the assessment of the effects of N pollution on peatlands.

Molybdenum-Based Diazotrophy in a Sphagnum Peatland in Northern Minnesota

Microbial N₂ fixation (diazotrophy) represents an important nitrogen source to oligotrophic peatland ecosystems, which are important sinks for atmospheric CO₂ and are susceptible to the changing climate. The objectives of this study were (i) to determine the active microbial group and type of nitrogenase mediating diazotrophy in an ombrotrophic Sphagnum-dominated peat bog (the S1 peat bog, Marcell Experimental Forest, Minnesota, USA); and (ii) to determine the effect of environmental parameters (light, O₂, CO₂, and CH₄) on potential rates of diazotrophy measured by acetylene (C₂H₂) reduction and ¹⁵N₂ incorporation. A molecular analysis of metabolically active microbial communities suggested that diazotrophy in surface peat was primarily mediated by Alphaproteobacteria (Bradyrhizobiaceae and Beijerinckiaceae). Despite higher concentrations of dissolved vanadium ([V] 11 nM) than molybdenum ([Mo] 3 nM) in surface peat, a combination of metagenomic, amplicon sequencing, and activity measurements indicated that Mo-containing nitrogenases dominate over the V-containing form. Acetylene reduction was only detected in surface peat exposed to light, with the highest rates observed in peat collected from hollows with the highest water contents. Incorporation of ¹⁵N₂ was suppressed 90% by O₂ and 55% by C₂H₂ and was unaffected by CH₄ and CO₂ amendments. These results suggest that peatland diazotrophy is mediated by a combination of C₂H₂-sensitive and C₂H₂-insensitive microbes that are more active at low concentrations of O₂ and show similar activity at high and low concentrations of CH₄ IMPORTANCE Previous studies indicate that diazotrophy provides an important nitrogen source and is linked to methanotrophy in Sphagnum-dominated peatlands. However, the environmental controls and enzymatic pathways of peatland diazotrophy, as well as the metabolically active microbial populations that catalyze this process, remain in question. Our findings indicate that oxygen levels and photosynthetic activity override low nutrient availability in limiting diazotrophy and that members of the Alphaproteobacteria (Rhizobiales) catalyze this process at the bog surface using the molybdenum-based form of the nitrogenase enzyme.

Carbon accumulation rates recorded in the last 150years in tropical high mountain peatlands of the Atlantic Rainforest, SE - Brazil

Peatlands are environmental matrices that store large amounts of organic carbon (TOC) and work as records of environmental changes. Recent record of organic carbon accumulated were assessed in two Forest National Parks, Itatiaia and Serra dos Órgãos in the Southeastern of Brazil. Based on organic and inorganic characterization, the cores from peatlands presented a predominance of organic material in an advanced stage of decomposition and those soils were classified as typical Haplosaprists Histosols. The combination of favorable topographic and climatic conditions led to rapid C accumulation across coastal mountain in the tropical peatlands studied, presenting an average accumulation rate of C, in the last century, of 194gCm^-2yr^-1 about 7 higher times than the rate found in boreal and subarctic peatlands, those higher values may be related to changes in the hydrological cycle occurred since 1950s.