Abstract
Rewetted, previously drained fens often remain sources rather than sinks for carbon and nutrients. To date, it is poorly understood which soil characteristics stimulate carbon and nutrient mobilization upon rewetting. Here, we assess the hypothesis that a large pool of iron in the soil negatively affects fen restoration success, as flooding-induced iron reduction (Fe3+ to Fe2+) causes a disproportionate breakdown of organic matter that is coupled with a release of inorganic compounds. We collected intact soil cores in two iron-poor and two iron-rich drained fens, half of which were subjected to a rewetting treatment while the other half was kept drained. Prolonged drainage led to the mobilization of nitrate (NO3-, > 1 mmol L-1) in all cores, regardless of soil iron content. In the rewetted iron-rich cores, a sharp increase in pore water iron (Fe) concentrations correlated with concentrations of inorganic carbon (TIC, > 13 mmol L-1) and dissolved organic carbon (DOC, > 16 mmol L-1). Additionally, ammonium (NH4+) accumulated up to phytotoxic concentrations of 1 mmol L-1 in the pore water of the rewetted iron-rich cores. Disproportionate mobilization of Fe, TIC, DOC and NH4+ was absent in the rewetted iron-poor cores, indicating a strong interaction between waterlogging and iron-mediated breakdown of organic matter. Concentrations of dissolved phosphorus (P) rose slightly in all cores upon rewetting, but remained low throughout the experiment. Our results suggest that large pools of iron in the top soil of drained fens can hamper the restoration of the fen’s sink-service for ammonium and carbon upon rewetting. We argue that negative effects of iron should be most apparent in fens with fluctuating water levels, as temporary oxygenation allows frequent regeneration of Fe3+. We conclude that rewetting of iron-poor fens may be more feasible for restoration.
Highlights
Widespread drainage is compromising the ability of the world’s groundwater-fed peatlands (“fens” hereinafter) to serve as sinks for nutrients and carbon [1,2]
Due to differences in geochemistry of the aquifers feeding the fens, the incoming groundwater of locations ES and Zwarte Beek (ZB) is relatively ironenriched whereas the groundwater of Bennekomse Meent (BM) and Leijer Hooilanden (LH) is relatively iron-poor, which explains the observed differences in total soil iron pools
For the variables Fe, Total inorganic carbon (TIC), dissolved organic carbon (DOC), and NH4+, we found strong positive interactions between water regime and soil iron content: concentrations of Fe, TIC, DOC and NH4+ increased in response to rewetting, but the magnitude of this effect was much stronger in the iron-rich soil cores
Summary
Widespread drainage is compromising the ability of the world’s groundwater-fed peatlands (“fens” hereinafter) to serve as sinks for nutrients and carbon [1,2]. Rising concentrations of dissolved organic carbon (DOC) in surface waters adjacent to degraded peatlands have been related to the destabilization of carbon pools within the peat [5,6]. These high DOC loads have become subject to growing international concern as they cause ample environmental problems [7,8]. Reports of excess mobilization of dissolved organic and inorganic carbon, ammonium (NH4+), and phosphate (PO43-) into pore and surface water upon rewetting indicate that successful fen restoration is not always guaranteed at least in short-term [3,9]. All mechanisms and chemical characteristics that affect compound mobilization in drained peat soils are not well understood, which often results in an arbitrary or even random selection of fens that are listed as “suitable” for rewetting
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