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Peat formation is the key process responsible for carbon sequestration in peatlands. In rich fens, peat is formed by brown mosses and belowground biomass of vascular plants. However, the impact of ecohydrological settings on the contribution of mosses and belowground biomass to peat formation remains an open question. We established seven transects in well-preserved fens in NE Poland along an ecohydrological gradient from mesotrophic sedge-moss communities with stable water levels, to more eutrophic tall sedge communities with higher water level fluctuations. In each transect, we measured the production of brown mosses (using the plug method), aboveground vascular plant biomass (one year after cutting) and belowground biomass (using ingrowth cores). Decomposition rates of all biomass fractions were assessed using litter bags. The first-year surplus of potentially peat-forming fractions, i.e., mosses and belowground biomass, decreased with increasing water level fluctuations and along a vegetation gradient from sedge-moss to tall sedge communities. Moss production was highest in the sedge-moss fen with a stable water level at the ground surface. We did not detect any difference in belowground biomass production across the gradient but found it to be consistently higher in the upper 0-5 cm than in the deeper layers. The decomposition rate also showed no response to the gradient, but differed between biomass types, with aboveground biomass of vascular plants decomposing 2.5 times faster than belowground biomass and mosses. Pattern of peat formation potential along the ecohydrological gradient in rich fen was strongly driven by brown moss production. Sedge-moss fens with a stable water level at the ground surface have the highest peat formation capacity compared to other vegetation types. In the part of the gradient that is poorer in nutrients, vascular plants invest in belowground production, and mosses dominate the aboveground layer., Competing Interests: Declaration of competing interest The authors declare that they have no competing interests., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Ground- and surface-water-fed peatlands (i.e., fens) of temperate Europe face high anthropogenic nutrient loads from atmospheric deposition, agricultural catchment areas, and from peat decomposition, if drained. As a result, nitrogen loads may exceed a fen's natural nutrient removal capacity, leading to increased eutrophication of adjacent water bodies. Therefore, it is important to address possible means to decrease a fen's nutrient load, including nutrient uptake by fen plants. To assess how much fen plants can contribute to nutrient removal by uptake, nutrient stocks of above- and below-ground biomass need to be quantified. Therefore, we investigated nitrogen, phosphorous, and potassium uptake capacities of sedges (Carex species), which are common dominants in fen plant communities. We grew specimens of five Carex species with varying preferences in nutrient availability under controlled, different nutrient levels. We show that Carex above-ground biomass harvest can remove up to one third of a system's total nitrogen even at high loads of about 40 g nitrogen m -2 . Species-specific differences in biomass production, rather than preferences in nutrient availability under natural conditions, were drivers of standing nutrient stocks: Highly productive species, i.e., C. acutiformis and C. rostrata, had highest nutrient standing stocks across all nutrient levels. Amounts of nutrients stored in shoots increased almost linearly with increasing nutrient levels, whereas below-ground nutrient stocks species-specifically increased, saturated, or decreased, with increasing nutrient levels. As a rough estimate, depending on the species, 6-16 cycles of annual above-ground harvest would suffice to decrease nitrogen concentrations from the highest to the lowest level used in this study. Overall, our results indicate that Carex biomass harvest can be an efficient means to counteract anthropogenic nitrogen eutrophication in fens., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)