Carbon, nitrogen and phosphorus retention and dynamics in plant litter, surface and deep soil of intact, restored and drained inland freshwater marshes

In response to widespread wetland degradation in the past century, multiple restoration programs are ongoing throughout Canada, aiming to regain important wetland functions. After decades of restoration practices, despite rewetted and revegetated, it is under debate if their key biogeochemical functions and features (e.g., microbial activity, the carbon and nutrients retention in sediment) have recovered successfully. This thesis investigated intact and short and long term restored inland freshwater marshes and their carbon, nutrient retention and dynamics in the ecosystem. I especially focused on the decomposing litter, microbial biomass and enzyme activities in surface soil, and the carbon and nutrients distribution and stoichiometric patterns in the deep soil profile.Across three wetland restoration sites in Manitoba and Ontario, litter decomposition rate was controlled by litter species and wetland characteristics especially the hydrologic regimes and surrounding land uses, not the intact or restored classes. The nutrients dynamic in litters was determined by both initial litter quality and surrounding human disturbances (e.g., sewage inputs to marshes), and achieved a steady stoichiometric balance for all sites at the end of the decomposition experiment. In a reserve site in Ontario, comparison of an intact and a restored marsh showed consistent spatial variability in microbial biomass and enzyme activities over the landscape transect encompassing the permanently inundated centre, intermittently inundated wet meadow, and never inundated upland. For both marshes, the degree of inundation and dominant plant species mainly determined the surface soil microbial biomass and extracellular enzyme activities, rather than land management or soil properties. Within the soil profiles of thirty-three intact, drained and restored marshes from Manitoba and Alberta, I found the cultivation history and surrounding land uses strongly influenced sediment carbon, nitrogen and phosphorus concentration and stoichiometric features both in surface soils (0-30 cm) and subsoils (30-100 cm). The vertical distribution of elements was highly variable among individual restored marshes and was not distinguished among different restoration age ranges from 2-23 years. Soil stoichiometric features of restored marshes were generally more similar to drained than intact wetlands, indicating a strong cultivation imprint on restored wetland soils.These findings provided science-grounded evidence on the success or failure of duplication of ecosystem carbon and nutrients dynamics and retention in decadal restored freshwater marshes to the adjacent intact conditions. They indicate similar environmental controllers on carbon and nutrients dynamic between intact and restored marsh systems in the surface litter and soil where biological processes dominated, however, alterations in deep sediments related to historical land conversions and physiochemical processes might be unrecoverable. Chronosequence (simply grouping wetlands by restoration age) might not be an adequate approach to delineate the recovery states of wetland functions. Instead, wetland structure (e.g., hydrologic regimes, adapted plant species) and interactions with surrounding or onsite anthropogenic activities (e.g., agriculture) are important on the efficacy of marsh biogeochemical functioning recovery, which should be considered in model development

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