Impact of experimental drought and rewetting on redox transformations and methanogenesis in mesocosms of a northern fen soil

The impact of climate change on the greenhouse gas balance of peatlands is debated as they function both as sinks of carbon and significant sources of methane. To study redox transformations influencing methane production, we incubated two intact soil monoliths from a northern temperate fen and compared a permanently wet treatment to a treatment undergoing an experimentally induced drought for 50 days. Net turnover of dissolved inorganic carbon (DIC), methane (CH₄) and electron acceptors in the saturated zone was calculated using a mass balance approach, and sulfate gross reduction rates were determined using a ³⁵S radiotracer. Thermodynamic energy yield of different electron accepting processes was calculated and related to the observed respiration patterns. Permanently wet conditions lead to a depletion of electron acceptors within 50 days and onset of methanogenic conditions. During drought, electron acceptors were renewed and methanogenesis was temporarily suppressed in most of the peat for another 20-50 days after rewetting. Methanogenesis began, however, apparently locally before electron acceptors were fully depleted in the remainder of the peat, and iron and sulfate reduction occurred simultaneously. Anaerobic production of DIC could mostly but not fully be explained by reduction of nitrate, sulfate and ferric iron. Sulfate gross reduction rates of up to ∼450 nmol cm⁻³ d⁻¹ determined with ³⁵S-SO₄ and potentially explained the surplus of 50-60 mmol m⁻² of DIC production in one treatment; however, the sulfate pools were too small to sustain such rates beyond some hours to days. Furthermore, anaerobic DIC production proceeded at constant rates after depletion of dissolved inorganic electron acceptors, although not being balanced by methane production. An unknown electron acceptor was thus consumed, and sulfate and potentially other electron acceptors recycled, either by humic substances, by aerenchymatic oxygen transport, or by oxygen in the capillary fringe at low levels of air filled porosity.     

Framework for designing and applying peak runoff control structures for peatland forestry conditions

Drainage-induced diffuse pollution and erosion are key water quality problems in peatland forestry. A major part of the pollutant load is transported during peak runoff periods after snowmelt or intense rainfall. This study investigated possibilities to increase retention time of runoff waters in drained peatland catchments on purpose to diminish peak runoff and improve settling conditions of suspended solids (SS). To create retention, a peak runoff control (PRC) structure was developed and its functioning, dimensioning and practical applications were studied in five partly or completely ditch-drained catchments in Central Finland. The method reduced runoff peaks by 10–73% or 5.07–57.63 l⁻¹ s⁻¹ km⁻², and functioned especially well during largest runoff peaks. The effectiveness of the PRC method depended on (i) catchment topography (slope) and available detention volume, (ii) dimensioning and location of the PRC structure, and (iii) runoff rates. The PRC structure is cheap and can easily be created with forest drainage machinery during the ditching and ditch network maintenance operations. Different issues relating to the structural design, water quality benefits, and impacts on forestry are discussed.     

Hydrological effects on the diversity of phenolic degrading bacteria in a peatland: implications for carbon cycling

Northern peatlands store ca. 1/3 of the world's soil organic carbon and this is attributed to low decomposition rates as a result of waterlogged, anaerobic conditions and high levels of phenolic substances. Climate change models predict both an increase in summer droughts and increased rainfall, depending on region, but information on the effect of these changes on the microbial population that mediate phenolic degradation is sparse. Temporal temperature gradient gel electrophoresis (TTGGE) was therefore used to assess the effect of simulated summer drought and increased rainfall on the diversity of phenolic degrading bacteria in a northern peatland using the gene XylE, encoding for the enzyme Catechol 2,3-dioxygenase (C23O), as an indicator. Under simulated drought, a greater diversity (129.4%, P<0.05) and abundance of phenolic catabolising bacterial species was found. Concurrent increased total phenol oxidase activities (83.3%) and β-glucosidase activities (157.6%, P<0.01) were found with consistently lower concentrations of phenolic compounds, DOC and increased CO₂ fluxes. This increased mineralisation is likely to lower carbon storage capacity and increase climate forcing. Conversely, the increased rainfall simulation suppressed diversity (62.2%, P<0.05), abundance and phenol oxidase activities (103.3%, P<0.001), giving increased phenolic compound (424.8%, P<0.1 only) and DOC concentrations (201.3%, P<0.001), along with increased anaerobic trace gas fluxes. These hugely increased aquatic carbon concentrations available for export are of serious concern due to their deleterious effect on drinking water quality.     

Observations of a seasonally shifting thermal optimum in peatland carbon-cycling processes; implications for the global carbon cycle and soil enzyme methodologies

Thermal gradient apparatus has been used to study enzyme activity and carbon cycling in peat collected seasonally from a Northern upland peatland. A thermal optimum was observed in the peat where maximum carbon-cycling enzyme activities (phenol oxidase and β-glucosidase), phenolic compound concentrations, dissolved organic carbon (DOC) concentrations and microbial respiration (CO₂ efflux) were all found in a given season. The thermal optimum for these carbon-cycling processes coincided with the highest ambient soil temperature recorded at the time of peat collection, suggesting microbial acclimation to the external conditions. Under the waterlogged conditions of this experiment, phenol oxidase activites correlated positively with phenolic compounds (winter 0.96, P<0.01; spring 0.92, P<0.001; summer 0.94, P<0.001; autumn 0.88, P<0.001) and β-glucosidase activities with DOC (winter 0.91, P<0.01; spring 0.85, P<0.01; summer 0.92, P<0.001; autumn 0.72, P<0.05). We propose, therefore, that the relative activities of these enzymes is crucial in mobilising DOC from the peat matrix, with implications for carbon exports to the receiving waters (magnitude and molecular weight distribution) and CO₂ efflux to the atmosphere. The pronounced seasonality in carbon processing found here, must be taken into account when modelling carbon flux in and from these systems, if responses to climate change are to be predicted satisfactorily. Furthermore, because the optimum activity of these carbon-cycling enzymes shifted with seasonal changes in temperature, it is essential to perform enzyme assays in soil ecological investigations at field temperatures (rather than standardised temperatures), when information on natural process rates is required.