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Background Wetlands store a substantial amount of carbon (C) in deep soil organic matter deposits, and play an important role in global fluxes of carbon dioxide and methane. Fine roots (i.e., ephemeral roots that are active in water and nutrient uptake) are recognized as important components of biogeochemical cycles in nutrient-limited wetland ecosystems.However, quantification of fine-root dynamics in wetlands has generally been limited to destructive approaches, possibly because of methodological difficulties associated with the unique environmental, soil, and plant community characteristics of these systems. Non-destructive minirhizotron technology has rarely been used in wetland ecosystems. Scope Our goal was to develop a consensus on, and a methodological framework for, the appropriate installation and use of minirhizotron technology in wetland ecosystems. Here, we discuss a number of potential solutions for the challenges associated with the deployment of minirhizotron technology in wetlands, including minirhizotron installation and anchorage, capture and analysis of minirhizotron images, and upscaling of minirhizotron data for analysis of biogeochemical pools and parameterization of land surface models. Conclusions The appropriate use of minirhizotron technology to examine relatively understudied fineroot dynamics in wetlands will advance our knowledge of ecosystem C and nutrient cycling in these globally important ecosystems.

Fine root production and its relationships to aboveground plant components and environmental drivers such as water table have been poorly quantified in peatland ecosystems, despite being the primary input of labile carbon to peat soils. We studied the relationship between fine root (< 1 mm) production, aboveground biomass and growing season water table within an ombrotrophic peatland in eastern Ontario. We installed 80 in-growth bags (10 cm diameter) to measure fine root production over the full range of 40 cm in water table depth. The point-intersect method was used to estimate peak aboveground biomass components (total, leaf and stem) for the 0.36 m 2 area surrounding each in-growth bag. Mean fine root production was 108±71 g m -2 y -1 and was strongly related to both aboveground biomass and water table. Linear regression analysis showed strong allometric relationships between fine root production and aboveground biomass for shrubs (r 2 = 0.61, p<0.001), suggesting that fine root production estimates can be approximated using aboveground biomass data. Water table had a significant effect on the allocation of biomass to fine roots, leaves and stems with a deeper water table significantly increasing both fine root production at depth and at each depth increment. Shrub biomass allocation to leaves and stems similarly shifted, with greater investment in stems relative to leaves with a deeper water table. As a result, greater fine root biomass was produced per unit leaf biomass in areas with a deeper water table, illustrating an important tradeoff between leaf and fine root tissues in drier conditions. Our results indicate that any drop in water table will likely increase aboveground biomass stocks and the influx of labile carbon to peat soils via fine roots and leaves.

In wetland ecosystems, little is known about the relationships between above- and below-ground plant biomass and water table, a primary driver of their distribution in these systems. These relationships can provide a means for estimating belowground stocks over large areas with variable biomass and predicting vascular plant responses to changing water tables resulting from climate change. We measured above- and below-ground vascular plant biomass across species and microtopography (i.e., hummocks and lawns) in a bog in eastern Ontario. We examined the relationships between above- and below-ground vascular plant biomass their variation with water table and species richness. We took 56 cores during a growing season, separating above- and below-ground biomass by species and plant part (small and coarse root, leaf, stem). Hummocks had greater above- and below-ground biomass, and significantly greater aboveground:belowground ratios than lawns. Lawns had a more even distribution of biomass across species than hummocks aboveground, indicating that only a few species (e.g., Vaccinium myrtilloides Michx. and Chamaedaphne calyculata Moench) are able to thrive in the driest bog conditions. Additionally, fewer species contributed to root biomass at depth, suggesting possible resource partitioning among species. Lower water tables lead to greater belowground biomass. Total above- to below-ground plant biomass relationships were strongest when separated by plant species.

Plant resorption of multiple nutrients during leaf senescence has been established but stoichiometric changes among N, P and K during resorption and after fertilization are poorly understood. We anticipated that increased N supply would lead to further P limitation or co-limitation with N or K [i.e. P-(co)limitation], decrease N resorption and increase P and K resorption, while P and K addition would decrease P and K resorption and increase N resorption. Furthermore, Ca would accumulate while Mg would be resorbed during leaf senescence, irrespective of fertilization. We investigated the effect of N, P and K addition on resorption in two evergreen shrubs (Chamaedaphne calyculata and Rhododendron groenlandicum) in a long-term fertilization experiment at Mer Bleue bog, Ontario, Canada. In general, N addition caused further P-(co)limitation, increased P and K resorption efficiency but did not affect N resorption. P and K addition did not shift the system to N limitation and affect K resorption, but reduced P resorption proficiency. C. calyculata resorbed both Ca and Mg while R. groenlandicum resorbed neither. C. calyculata showed a higher resorption than R. groenlandicum, suggesting it is better adapted to nutrient deficiency than R. groenlandicum. Resorption during leaf senescence decreased N:P, N:K and K:P ratios. The limited response of N and K and the response of P resorption to fertilization reflect the stoichiometric coupling of nutrient cycling, which varies among the two shrub species; changes in species composition may affect nutrient cycling in bogs.

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Information about the impact of nitrogen (N) deposition on the fate of deposited N in peatland ecosystems is lacking. Thus we investigated the fate of experimentally added 15 N in long-term N-fertilized treatments in a Sphagnum-dominated ombrotrophic bog. Fertilization significantly stimulated vascular plant and suppressed Sphagnum and Polytrichum moss growth. N content in peat, mosses, and vascular plants was raised by the fertilizer addition and reached a maximum at 3.2 g m -2 N input level with phosphorus (P) and potassium (K) addition. Most of N was retained in the vegetation and upper 10 cm of the peat. When N deposition equalled 1.6 g m -2 and less, or 3.2 g m -2 N with P and K addition, no inorganic N leaching was observed on the plots. This result indicates that co-fertilization with P and K raised the N retention capacity and that critical N loads with respect to N saturation depend on P and K availability. Most of the deposited 15 N was recovered in the bulk peat, which may be related to a rapid immobilization of inorganic N by microorganisms and mycorrhizal assimilation. Increase of N, P, and K fertilization increased the contribution of vascular plants to N retention significantly and reduced those of mosses. The increase was mainly related to enhanced productivity, vascular biomass and N content in tissues; the reduced retention by mosses resulted from both reduced moss biomass and assimilation. The study shows that the N filter function of ombrotrophic bogs will be influenced by interactions with other nutrients and shifts in plant community structure.