Litter Decomposition and Nutrient Dynamics of Four Macrophytes in Intact, Restored, and Constructed Freshwater Marshes of Canada

The restoration and construction of wetlands offer opportunities to rewet soils, inhibit decomposition, and enhance nutrient retention in decomposing litters. Here, we report the decomposition rates and nutrient dynamics of macrophyte litters in intact, restored, and constructed wetlands. A 2.1-year litterbag experiment of four common freshwater macrophytes (Phalaris arundinacea, Phragmites australis, Scirpus cyperinus, and Typha latifolia) was conducted in eight freshwater marshes (three intact, four restored, and one constructed) within three sites in Manitoba and Ontario, Canada, which varied in restoration age, inundation periods, and surrounding land uses. Litter mass loss and N and P dynamics were measured. Litter decomposition rates (k) followed the order of P. arundinacea (0.42 ± 0.03 year⁻¹) > T. latifolia (0.31 ± 0.03 year⁻¹) > P. australis (0.19 ± 0.01 year⁻¹) > S. cyperinus (0.13 ± 0.01 year⁻¹) in most wetlands and were positively correlated to the initial litter N concentration. Litters decomposed fastest under seasonally inundated conditions rather than permanent inundation. N and P retention in litters were significantly affected by both initial litter N and P concentration and wetland surrounding land uses. After 2.1 years of decomposition, the N:P ratio of all litters converged to 20 to 28:1, regardless of the initial litter N:P ratio or N or P concentrations. The effectiveness of wetland restoration in slowing decomposition and enhancing nutrient accumulation depends on the quality of the input litters and wetland characteristics, including inundated periods and surrounding anthropogenic disturbances.

Implications for Practice

• At the early stage of restoration, irregular drying-and-wetting dynamics in wetlands should be avoided, as it could likely accelerate the litter decomposition rates.
• The surrounding human activities around wetlands (e.g. fertilization and sewage inputs) should be considered when conducting wetland restoration and construction projects on regaining wetland biogeochemical functions.
• Choice of plant species is critical for both slowing the decomposition process and promoting nutrient retention in restored and constructed wetlands.