Christian Blodau

Experimental response of peatland carbon dynamics to a water table fluctuation

Abstract. Water table fluctuations influence the carbon balance of wetlands, but the effects are difficult to isolate and quantify in field investigations. Thus, we compared C mineralization in a peatland mesocosm exposed to a water table fluctuation (from 5 to 67 cm beneath the surface) with that in mesocosms with a stable high water table (2 to 6 cm depth) and with production rates obtained from flask incubations. Net turnover rates were calculated from concentration data by diffusive-advective mass-balances. Under stable, high water table conditions, net production of CO2 (6.1 mmol m–2 d–1), CH4 (2.1 mmol m–2 d–1) and DOC (15.4 mmol m–2 d–1) were vertically stratified and production and fluxes equilibrated. Lowering and raising the water table from 5 to 67 cm resulted in complex patterns of net CH4 and CO2 production. Response and eq uilibration times of processes upon drainage and flooding ranged from days (aerobic CO2 production, CH4 oxidation, fluxes under unsaturated conditions) to months (CH4 production, fluxes under saturated conditions). Averaged over the water table fluctuation, net production of CH4 decreased to 0.36 mmol m–2 d–1 and that of CO2 increased to 140 mmol m–2 d–1. Physical disturbance through the incubation of peat strongly increased production rates of CO2, CH4 and DOC compared to in situ, steady state rates. The decoupling of production and fluxes to the atmosphere under conditions with variable water table depths potentially explains part of the frequently reported lack of correlation between environmental variables and trace gas fluxes in field investigations, and questions the applicability of predictions of gas flux based on empirical relationships established under stable average conditions.

Belowground carbon turnover in a temperate ombrotrophic bog

with Q10 =5 .0� 6.4. In the saturated zone, concentrations of 0.1 � 2.5 mmol L � 1 (CO2), 0 to 0.6 mmol L � 1 (CH4), and <10 � 120 mg L � 1 (DOC) were recorded. Turnover was dominated by DOC unrelated to respiration, which ranged from <0.5 to 7 mmol m � 2 d � 1 and amounted on average to < 1% of ER. Peat decomposition constants kd were 0.060 yr � 1 to 0.034 yr � 1 in the unsaturated and <0.002 yr � 1 in the saturated zone. Monthly averages of CH4 fluxes ranged from 0 to 1.6 mmol m � 2 d � 1 and were higher than modeled diffusive fluxes when threshold concentrations for CH4 ebullition were recorded closer to the peatland surface. Our results suggest that the saturated zone is of little relevance to ER in this dry temperate bog and that mobilization of DOC is a potentially more relevant process. Temperature is a more important control on ER than water table position because most of the ER is generated close to the peatland surface. Concurrent, moderate increases in temperature and soil moisture are thus likely to increase losses of CO2 from ER and of CH4 from this type of peatland.

MACROPOROSITY AFFECTS WATER MOVEMENT AND PORE WATER SAMPLING IN PEAT SOILS

The measurement of chemical concentration profiles in pore water is a starting point for the analysis of biogeochemical processes in waterlogged peat soils. Concentration patterns may be obscured when macroporosity causes preferential flow in column experiments and when pore water is retrieved from the peat by suction. To investigate the magnitude of such effects, we used LiBr as a tracer in peat columns at outflow rates of 0, 2–3, and 8 mm d−1. The results were compared with modeled advective-diffusive migration rates. Twenty to fifty percent of the tracer was recovered from depths at which the tracer would have been absent if preferential flow had not occurred. At the high flow rate, the preferential flow was stronger, and the retrieved pore-water was probably in disequilibrium with the matrix. When pore water was retrieved by suction, linear concentration gradients decreased by about 30% through the recovery of water from different depths, and the quality of fitted linear gradients decreased from R2 = 0.99 to R2 = 0.82. When flow rates are low (<3 mm d−1) and pore water concentration values from samplers are aggregated or regressed, the obtained concentration profiles seem to represent the vertical distribution of chemical species reasonably well. The use and interpretation of pore water profiles in peat soils is problematic if flow rates are higher and if vertical gradients are based on individual or few data points that have been obtained by suction samplers.

Support for an anaerobic sulfur cycle in two Canadian peatland soils

concentrations of dissolved sulfate and H2S adjusted to 5–20mmol L � 1 and 0–9mmol L � 1 , respectively, whereas concentrations of CO2 ,C H4, and DOC reached millimolar levels. CO2 production was not explained by methanogenesis and net reduction of inorganic electron acceptors. In the shallow peat, H2S was produced and 34 S in sulfate enriched by 3.6 to 6%, indicating occurrence of BSR. Sulfate reducers also accounted for much of the metabolic activity. Addition of molybdate suppressed CO2 production by 20 to 50%. Deeper into the peat, the sulfate pool was apparently replenished from the peat matrix as sulfate became enriched in 32 S, likely stemming from TRIS or organic sulfur in the peat. Sulfur was thus anaerobically cycled between oxidized and reduced pools. An electron acceptor capable of driving this cycle could not be conclusively identified. Regardless of this uncertainty, the results suggest that anaerobic S cycling can maintain BSR and potentially contribute to low methane production in soils of ombrotrophic bogs.

Groundwater derived arsenic in high carbonate wetland soils: Sources, sinks, and mobility

Wetlands and organic soils have been recognized as important sinks for arsenic in the environment, yet sources and immobilization mechanisms of As are often unclear. To begin rectifying this deficiency, we investigated As retention and binding mechanisms at a degraded, minerotrophic wetland site in contact with groundwater rich in As and Fe. Arsenic occurred in high dissolved concentrations of up to 467 microg L(-1) in the groundwater, but dropped to values below 10 microg L(-1) towards the surface. The solid phase As content instead was high in the topsoil with up to 3400 mg kg(-1) and decreased with depth to 15 mg kg(-1). A similar pattern was observed with respect to Fe. Amorphous and crystalline iron precipitates were the main sorbents for arsenic in the soil horizons according to results from wet chemical sequential extractions. Arsenic was apparently not associated with inorganic carbon phases, but a substantial portion of up to 31% of As(tot) could be mobilized by dispersion of soil organic matter. Ratios of dissolved As(III)/As(V) decreased from the deeper As(III) dominated groundwater to the As(V) dominated soil porewaters, where As was apparently immobilized in its oxidized form. Concentrations of the organic species DMA and MMA were negligible. According to the results of simple one-dimensional estimates the vertical arsenic transport from the source in the groundwater to the topsoil was slow given an extrapolation of current conditions. These results suggest that As accumulation started before the beginning of drainage in the now degraded peatland soils and the degradation and mass loss of organic matter under oxic conditions caused the very high As concentrations found in the topsoil horizon today.

Thermodynamics and organic matter: constraints on neutralization processes in sediments of highly acidic waters

Abstract The controls on the internal neutralization of low productivity, highly acidified waters by sulfide accumulation in sediments are yet poorly understood. It is demonstrated that the neutralization process is constrained by organic matter quality and thermodynamic effects which control the relative rates of SO4 and Fe reduction, and the fate of the reduced Fe and S in the sediments. The investigated sediments were rich in dissolved Fe(II) (0.005–12 mmol l−1) and SO4 (1.3–22 mmol l−1). The pH ranged from 3.0 to 6.8. Contents of reduced inorganic S (0.1–9.5%), molar C/N ratios of the organic matter (12–80) and metabolic turnover rates (1–110 μeq cm−3 a−1) varied strongly. Substantial amounts of Fe sulfides were only found at a simultaneous partial thermodynamic and solubility equilibrium of the involved biogeochemical processes. Sulfide oxidation was apparently inhibited, and SO4 and Fe reduction coexisted. In this type of sediment increases in C availability cause enhanced neutralization rates. In the absence of a partial equilibrium, the sediments were in a sulfide oxidizing and Fe reducing state, and did not accumulate Fe sulfides. The latter type of sediment will increase neutralization rates in response to decreasing deposition of reactive Fe oxides but not necessarily in response to increases in lake productivity by e.g. fertilization measures.

Organic Matter Diagenesis in Acidic Mine Lakes

The significance of organic matter origin for carbon oxidation via sulfate and iron reduction in the sediments of three acid mine lakes is analyzed. Carbon reactivity was estimated by fitting first-order expressions to measured rates. Carbon oxidation rates via sulfate and ferric iron reduction ranged from 3.4 to 4.7 mmol m 2 d -1 and resembled those reported for freshwater lakes. The estimated reaction constants increased from about 10 -3 a -1 at the interface to the former mine grounds to 0.05 to 0.2 a -1 at the current sediment-water interface. Aquatic organic matter accounted for an estimated 45…75% of total carbon oxidation rates while it amounted only to about 5…14% of the total organic matter that had been deposited. The results of this study suggest that in highly acidic mine lakes the reactivity of the deposited organic matter can rapidly increase after flooding, enhancing carbon oxidation and internal neutralization rates in the sediments.

Effects of short-term drying and irrigation on electron flow in mesocosms of a northern bog and an alpine fen.

Methane emissions and element mobility in wetlands are controlled by soil moisture and redox conditions. We manipulated soil moisture by weekly drying and irrigation of mesocosms of peat from a bog and iron and sulfur rich fen. Water table changed more strongly in the decomposed fen peat ( approximately 11 cm) than in the fibric bog peat ( approximately 5 cm), where impacts on redox processes were larger due to larger change in air filled porosity. Methanogenesis was partly decoupled from acetogenesis and acetate accumulated up to 5.6 mmol L(-1) in the fen peat after sulfate was depleted. Irrigation and drying led to rapid redox-cycles with sulfate, hydrogen sulfide, nitrate, and methane being produced and consumed on the scale of days, contributing substantially to the total electron flow and suggesting short-term resilience of the microbial community to intermittent aeration. Anaerobic CO2 production was partly balanced by methanogenesis (0-34%), acetate fermentation (0-86%), and sulfate reduction (1-30%) in the bog peat. In the fen peat unknown electron acceptors and aerenchymatic oxygen influx apparently drove respiration. The results suggest that regular rainfall and subsequent drying may lead to local oxidation-reduction cycles that substantially influence electron flow in electron acceptor poor wetlands.

Evidence for a hydrologically controlled iron cycle in acidic and iron rich sediments

Abstract.Retention of ferrous iron at the interface between ground- and surface water is crucial for the acidity balance of lakes influenced by acid mine drainage. Iron budgets were developed for two sediments in areas of differential groundwater inflow (ca. 1 and 10 L m–2 d–1). In both areas iron was sedimented as schwertmannite (Fe8O8(OH)x(SO4)y , 8 – x = 2y, 1,0 < x < 1,75) at rates of 5.5–5.9 mmol m–2 d–1 leading to iron(III) enriched sediments (3.9–6.2 mmol g–1 dry weight). Compared to the surface water, the inflowing groundwater had higher pH (4.5 vs. 3), ferrous iron (6–20 mmol L–1 vs. 0.8–2.0 mmol L–1) and sulfate (5–60 mmol L–1 vs. 8–13 mmol L–1) concentrations. The inflow of the groundwater caused a change in sediment pore water chemistry and an increase in pH to above 5.5. The pH increase was probably mostly due to decreased transformation rates of schwertmannite to goethite (0.27 mmol m–2 d–1 vs. 5.6 mmol m–2 d–1), also decreasing the production of H+ in the sediment. Compared to the control, in the area with groundwater inflow solid phase iron sulfide (0.011 mmol m–2 d–1vs. 0.0019 mmol m–2 d–1) and carbonate were formed at a higher rate, and more sulfate was reduced in incubation experiments. This finding can be explained by saturation indexes of siderite and by sulfate reduction becoming thermodynamically more competitive by about 40 kJ eq–1 compared to iron reduction. However, only a small fraction of the reduced ferrous iron and sulfide was retained in the sediment, emphasizing the importance of re-oxidation processes. The study demonstrates the existence of biogeochemical patterns in lake sediments due to variations in hydrologic boundary conditions in the adjacent aquifer.

Experimental burial inhibits methanogenesis and anaerobic decomposition in water-saturated peats.

A mechanistic understanding of carbon (C) sequestration and methane (CH(4)) production is of great interest due to the importance of these processes for the global C budget. Here we demonstrate experimentally, by means of column experiments, that burial of water saturated, anoxic bog peat leads to inactivation of anaerobic respiration and methanogenesis. This effect can be related to the slowness of diffusive transport of solutes and evolving energetic constraints on anaerobic respiration. Burial lowered decomposition constants in homogenized peat sand mixtures from about 10(-5) to 10(-7) yr(-1), which is considerably slower than previously assumed, and methanogenesis slowed down in a similar manner. The latter effect could be related to acetoclastic methanogenesis approaching a minimum energy quantum of -25 kJ mol(-1) (CH(4)). Given the robustness of hydraulic properties that locate the oxic-anoxic boundary near the peatland surface and constrain solute transport deeper into the peat, this effect has likely been critical for building the peatland C store and will continue supporting long-term C sequestration in northern peatlands even under moderately changing climatic conditions.