The restoration of degraded iron-rich fens
20 September 2017
Campus Middelheim, A.143 - Middelheimlaan 1 - 2020 Antwerpen (route: UAntwerpen, Campus Middelheim
Organization / co-organization:
Department of Biology
Ruurd Van Diggelen
Public defence of the PhD thesis of Mr. Willem-Jan Emsens - Faculty of Science - Department of Biology
Fens are mesotrophic wetlands that are influenced by year-round discharge of base-rich and often iron-rich water. Fens typically harbor many endangered plant species, and they can actively sequester carbon (as peat). Unfortunately they are also amongst the most threatened habitat types in the Northern hemisphere.
We investigated if we can restore typical fen plant communities, organic matter accumulation, and nutrient limitation in degraded fens, and we identified various biogeochemical factors that may contribute to successful restoration. Since many fens are characterized by high iron (Fe) contents we specifically focused on interactions between iron chemistry and the biogeochemical environment.
We found that Fe plays an ambiguous role in fens: the capacity of Fe-compounds to bind phosphorus (P) results in an overall increase in P availability to plants in Fe-rich fens, as P is being “trapped” within the fen. Moreover, this Fe-bound P appears available for plant uptake. Next, experimental rewetting of drained Fe-rich fens correlated with a high mobilization of carbon and ammonium in the pore water of peat, whereas such mass mobilization was not observed when drained Fe-poor fens were rewetted. Since these compounds are products of anaerobic decomposition of organic matter, this provides evidence for the “iron-redox-wheel”, in which anaerobic decomposition is coupled with iron(III)reduction. The overall result is that nutrient availability is often high in Fe-rich fens, which stimulates productivity of the herb layer. A highly productive herb layer however is detrimental to typical endangered fen species, which are then outcompeted by fast-growing species.
Shifts in nutrient availability should also impact the potential of fens to accumulate organic matter. We provided evidence for interactive and contrasting effects of fen eutrophication on litter accumulation, litter quality and decomposition. Eutrophication altered fresh litter production (“productivity shift”), litter stoichiometry within the same species (“intraspecific shift”), overall litter stoichiometry of the vegetation under the prediction that low-competitive species are outcompeted by fast-growing competitors (“interspecific shift”), and litter decomposition rates due to an altered external environment (e.g., shifts in microbial activity; “exogenous shift”). The cumulative effect of these interactions will ultimately co-determine whether peatlands remain sinks of carbon under eutrophic conditions.
Finally, removal of a degraded top peat layer was shown to improve conditions for rich fen restoration as it typically resulted in lower nutrient levels, higher groundwater levels, and an increased availability of base cations and light. Such conditions are generally beneficial for endangered rich fen communities.