Can we enhance nutrient removal in wetland ...


For wetland buffer zones (WBZ) to effectively remove nutrients from through-flowing water, nutrient sink mechanisms (sedimentation, peat accumulation and plant uptake) should be complemented with nutrient export mechanisms. The latter incorporate biological processes, such as nitrification, denitrification and export through terrestrial food-chains, which canbe supplemented by biomass harvesting and removal by man. However, while the effectiveness of microbial processes has been well documented and quantified from a number of localities throughout the world, the amount of nutrients that can be removed from wetland buffer zones by harvesting above-ground biomass is site-specific and may depend on a number of ecological and human-related factors, so a local calibration is needed to assess importance of plant uptake and harvesting in regional scenarios of WBZ development. Another important question is how much of the nutrients are built into more stable (structural) compounds, and what part may become easily leached from (dead) plant biomass in contact with groundwater. Finally, in the context of riparian restoration projects, it is interesting to compare effectiveness of nutrient capture by vegetation in restored and undisturbed (reference) wetlands. We addressthese questions by comparing productivity, nutrient concentrations and their solubility in vegetation of(1) relatively undisturbed riverine wetlands in the Narew catchment in Poland and (2) Danish riverine wetlands in the catchment of Odense A, which were restored 15-20 years ago after several decades of arable farming. Aboveground biomass was cut from quadrats of 0,14 m2 localised in all typical vegetation types and distributed across the stream valleys. Around each sampling plot, vegetation composition was described in a 2x2 m quadrate. The harvested biomass was oven-dried, weighted and analysed for the total content of nitrogen (N), phosphorous (P) and carbon (C). Another subsamples were used for a nutrient leaching experiment, in which dried plant litter was cut in 5 cm pieces and incubated for 24h in dark bottles with a 0.02%-solution of NaCl placed onshaking tables in dark climatic chambers at 25°C. Afterwards the leaching solution was filtered and analysed for dissolved N, PO4-P and organic C. While biomass production and nutrient contents varied a lot depending on the ecological type of WBZ and plant community, there was surprisingly little difference between restored and undisturbed sites. However, the leaching experiment showed a much higher fraction of easily soluble nitrogen in vegetation samples from restored Danish sites as compared to undisturbed Polish sites. We attribute these differences mainly to (a) higher concentration of plant-available nitrogen in the intensively-fertilised Danish landscape and (b) different plant composition in restored (dominance of grasses) and undisturbed (dominanceby sedges or reed) sites. We conclude, that biomass harvesting can be an important mechanism of nutrient removal in WBZ, but its effectiveness is significantly affected by dominant plant species.

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