Impact of extreme hydrological conditions on belowground carbon cycling and redox dynamics in peat soils from a northern temperate fen
Peatlands have an important role in the global carbon cycle and constitute the largest pool of carbon stored in terrestrial ecosystems due to their disproportionally high areal soil carbon density. This globally relevant carbon stock is the result of a process mostly initiated after the last glaciation period. A key factor for this long term carbon accumulation is the relative low decomposition of organic matter in these predominantly water logged ecosystems. Hydrological conditions play thus a fundamental role in peatlands and the feedback of carbon cycling in these ecosystems in response to climate change is under debate. Peatlands are important CO2 sinks but also constitute global sources of CH4. The atmospheric exchange and production rates of these greenhouse gases are strongly influenced by the hydrological regime. An increased frequency of extreme meteorological conditions resulting in drying and flooding events is predicted to occur in the future. The major issue regarding the climate change debate at the global scale is how rapid these greenhouse gases are being released to the atmosphere. Despite the general consensus regarding the broad effects of drying and flooding on CO2 and CH4 exchange, belowground processes producing such greenhouse gases and their response to water table dynamics is underrepresented and usually simplified or overgeneralized. Temperature, moisture, oxygen content and nutrient content are among the major environmental controls for organic matter decomposition rates in peat soils. Another important and intrinsic control is peat quality or humification degree of organic matter. The interrelation and relevance of all these factors vary among sites and with hydrological condition in a temporal and spatial scale. This work presents investigations focusing on belowground redox processes aiming to witness the dynamic interrelation of soil physical and chemical (soil gas and pore water chemistry) variables, and evaluates the relevance of some controls of organic matter decomposition during a wide range of hydrological conditions. Most of this work shows information under in situ conditions and complementary laboratory experiments were performed minding the in situ observations. The findings contribute to general knowledge by providing raw data in fen peats under fluctuating and contrasting water table conditions in a relatively high spatiotemporal resolved scale. Dryings led to increased air filled porosity, O2 intrusion, CO2 degassing, inhibition of methanogenesis and renewal of electron acceptors. The opposite trend occurred upon rewetting with pulses of iron and sulphate reduction and delayed methane production to a variable extent. Upon flooding, continued anaerobic conditions stimulated the accumulation of reduced products, methanogenic precursors (acetate and hydrogen) and CH4. The general assumption that the water table directly controls the oxygen content in peat was relativized. This work shows that such relation is greatly influenced by peat physical properties, which partially control the changes in moisture. Based on these findings, the mineral content and the degree of compaction in organic soils can be implemented to more accurately represent the dynamics of aeration in peats upon water table changes. Another general assumption is that drying events, i.e. temporary decline of water table below mean position, lead to increased CO2 production and emission from peat soils to the atmosphere. Such statement was also relativized and must account for the depth distribution of respiration rates in relation to the mean water table of the peat deposit. Based on these findings, the high relative contribution of upper peat layers already exposed above the water table mask the effects of increased CO2 production in deeper peat upon water table drop. Additionally, the role of moisture was shown to be little for aerobic respiration. This work also evaluates the importance of drought severity by accounting for the post drought effects on methane production. More intense and prolonged drying events led to a greater regeneration of electron acceptors in peat soil, which broadly suppressed or limited methane production upon rewetting. This relation was not simple and several factors such as water table position, post drought water table fluctuations, temperature and organic matter content contributed to the recovery of methane production after drying. The provision of substrates by fermentation processes limited peat respiration during shallow water table and drying. In contrast, accumulation of acetate and hydrogen was observed during flooding indicating a decoupling of fermentation from terminal metabolism and favouring the co-occurrence of iron reduction, sulphate reduction and methanogenesis.
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