Above and below-ground nutrient cycling: a criteria for assessing the biogeochemical functioning of a constructed fen

• Revegetation facilitates fen productivity and below-ground nutrients cycling.
• Supply of substrates in revegetated plots increased microbial potential activity.
• Establishment of a diverse plant community could be hampered by P limitations.
• Build-up of microbial biomass in the constructed fen appears to be NO3− limited.
• N mineralization and P availability are potential indicators of the fen’s recovery.

Recent peatland restoration studies have highlighted the need to modify the conventional approach of monitoring the recovery of biogeochemical functions through above-ground processes by integrating below-ground components, which are potentially sensitive indicators of reclamation trajectory in long-term monitoring programs. In this study, the recovery of nutrient cycling processes were assessed from both above and below-ground perspectives in a fen constructed on a post-mining landscape in the Athabasca oil sands region, Canada. The goal of this study was to understand if and how different revegetation strategies (seedlings (SDL); moss layer transfers (MLT); seedlings with moss transfer (SMLT) and a control treatment (CTRL)) influence the evolution of biogeochemical functions in the constructed fen relative to a natural reference (REF). The treatments were replicated 6 times and monitored over 2 growing seasons. Our results showed that revegetation facilitated both above-ground productivity and the cycling of below-ground nutrients, especially in the species-rich SMLT plots. Supply of labile substrates in the re-vegetated plots increased microbial potential activity. This was reflected in higher rates of respiration (c.7.8 g CO2 m−2 day−2), nutrient acquisition (net immobilization) and productivity (c. −16.3 g CO2 m−2 day−2) among revegetated plots relative to the CTRL. Nutrient dynamics within the constructed fen suggest that phosphorus limitation (N:P ratio > 20) could hamper the establishment of a diverse plant community, whereas the build-up of microbial biomass appears to be NO3− limited. Our results also emphasize the need to track the evolution of nutrients cycling processes through long-term monitoring programs, and identify the potential use of ammonification, nitrogen mineralization and phosphorus availability as functional indicators of a fen’s recovery trajectory towards conditions present in natural fens.