Soil depth and nutrient status are stronger drivers for short-term production and decomposition in temperate fens than water regime in a climatically dry year

Abstract. This study investigated whether rewetting of drained temperate, groundwater-fed fens results in consistent shifts of above- and belowground production and litter mass loss of vascular plants, as compared to undrained fens, and how these processes relate to abiotic and biotic factors. We tested the hypotheses that (1) due to higher availability nutrient rewetted and drained fens exhibit higher production, (2) due to higher decomposability of plant biomass rewetted fens show higher plant biomass decomposition than undrained, and (3) in rewetted fens, less peat is potentially formed than in undrained fens. We analysed the effects of hydrological status (undrained, drained, rewetted) on plant production and litter mass loss during one year in 39 peatland sites across temperate Europe. Above- and belowground productivity, as well as mass loss of autochthonous vascular plant litter and Rooibos tea were measured above- and belowground. Aboveground vascular plant production was higher in rewetted fens than in undrained fens, in line with our first hypothesis. This difference was linked to the proportional graminoid cover and N content in aboveground biomass. In contrast to our first hypothesis, belowground production did not differ between fens of different hydrological status. It was highest in the 0–5 cm horizon, where soil temperature, and microbial biomass were also highest. Likewise, and in contrast to our second hypothesis, decomposition of above- and belowground vascular plant biomass was not affected by hydrological status. Only Rooibos tea mass loss showed small, ecologically irrelevant differences between differing hydrological statuses. Decomposition of aboveground biomass at soil surface was higher with higher nitrogen and phosphor content of the biomass, higher soil temperature, and higher cover proportion of herbs in the vegetation. Belowground biomass loss was positively correlated with phosphor content in soil porewater and aboveground biomass. One-third of the overall belowground biomass production took place in the 0–5 cm horizon, while decomposition in this horizon was lower than in the subsoil, irrespective of hydrological status. Our third hypothesis was also not confirmed because belowground production, decomposition and peat formation potential did not differ between the hydrological status. Although aboveground vascular plant production was higher in rewetted fens higher than in undrained, this difference will not result in a higher peat formation potential in rewetted fens due to the strong decomposition in of the easily degradable aboveground vascular plant biomass on a long time. Rather than hydrological status, soil depth, and nitrogen and phosphor availability had stronger impact on short-term vascular plant production and mass loss in the temperate fens studied. All these results were obtained during an extremely dry and warm growing season throughout Europe and causing deep drop downs of the water level at most sites. These meteorological conditions may have impacted the observed pattern of productivity and decomposition in unknown way.