Nitrogen uptake and cycling in Phragmites australis in a lake-receiving nutrient-rich mine water: a 15N tracer study

Uptake and cycling of nitrogen (N) in the littoral zone of a lake-receiving nutrient-rich mine water located in Boliden, northern Sweden, was investigated. Stable isotope tracer solutions of N-15 as NH4+ (NAM mesocosm) or NO3- (NOX mesocosm) were added to mesocosms enclosing plants of common reed (Phraginites australis). The N-15 abundance in various plant parts was measured at pre-defined time intervals over an experimental period of 22 days. During the course of the experiment, plant parts from the NAM mesocosms were significantly more enriched in N-15 than plant parts from the NOX mesocosms. On day 13, 4615N values of the fine roots from the NAM mesocosms had reached +8220 %, while the maximum 4615N value in NOX roots was considerably lower at +4430 %o. Using N-15 values in macrophyte tissues present at the end of the experiment enabled calculations of uptake rates and % of tracer N recovered in the plant (%tracerNrecov). Maximum tracer uptake rates were higher for the NAM mesocosms (1.4 Kg g-1 min-1 or 48 mg N m(-2) d(-1)) compared to the NOX mesocosms (0.23 Kg g(-1) min(-1) or 8.5 mg N m(-2) d(-1)). Calculations of %tracerNrecov indicated that 1-8 and 25-44 % of added N was assimilated by plants in the NOX and NAM mesocosms, respectively. Hence, P. australis was more effective in assimilating NH4, and a larger portion of the tracer N accumulated in the roots compared to the other plant parts. Consequently, macrophyte N removal is most effective for cold-climate aquatic systems receiving mine water dominated by NH4. For permanent removal of N, the whole plant (including the roots) should be harvested.