Effects of aboveground biomass and litter on biogeochemical cycling in stands of the invasive wetland plant, Phragmites australis

Background/Question/Methods

Wetlands are important ecosystems for the biogeochemical cycling of many elements, e.g. carbon (C) and nitrogen (N), due to their high productivity, low decomposition, and variable soil redox status.  The introduction of invasive plant species can affect wetland C and N cycling by introducing plant traits that influence ecosystem functioning, such as litter quality and quantity, as well as the environment in which decomposition occurs.  Phragmites australis, common reed, is a clonal, high biomass wetland species found worldwide with invasive genotypes found in all 48 conterminous states.  We hypothesized that Phragmites invasion exerts both direct and indirect influences on biogeochemistry; direct effects include plant nutrient uptake into live biomass and indirect effects include changes in the soil microenvironment and in litter input quality and quantity.  To test this we manipulated aboveground biomass and litter in plots within three wetlands dominated by Phragmites over three years.  Over the course of the study we monitored light levels, soil temperature at multiple depths, porewater chemistry, and water table position. During the last year of the study we determined rates of N mineralization and nitrification, litter decomposition, and potential denitrification.  

Results/Conclusions

Removing Phragmites litter or biomass had the hypothesized effects of increasing light levels at the soil surface and increasing soil temperature, with aboveground biomass or litter removal affecting light levels and soil temperature equally. The highest increase in light levels and soil temperature occurred in total removal plots where both litter and aboveground biomass had been removed.  Biomass removal also affected porewater ion concentrations; Na⁺, Cl^-, Ca²⁺ and Mg²⁺ were lower (reduced evaporative concentration by evapotranspiration) and NO₃^- higher (direct uptake) in biomass removal plots.  There were no treatment effects on litter bag decomposition, in situ N mineralization, or potential denitrification rates.  Though not initially hypothesized, all process rate data (litter bag decomposition, in situ N mineralization and potential denitrification) showed strong site effects, which were likely influenced by different hydrologic conditions among sites.  Hydrology ranged from constantly flooded to a water table position usually below the soil surface demonstrating the overriding importance hydrology can have within wetlands dominated by the same species.  The results of this study show that Phragmites can affect abiotic conditions (light levels, soil temperature, and ion concentrations) but that variation in hydrology may be more important than the presence/absence of Phragmites for determining wetland function.