Nitrogen uptake and cycling in Phragmites australis in a lake-receiving nutrient-rich mine water: a 15N tracer study
2015
Chlot, Sara | Widerlund, Anders | Öhlander, Björn
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 ¹⁵N as NH₄ ⁺ (NAM mesocosm) or NO₃ ⁻ (NOX mesocosm) were added to mesocosms enclosing plants of common reed (Phragmites australis). The ¹⁵N 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 ¹⁵N than plant parts from the NOX mesocosms. On day 13, Δδ¹⁵N values of the fine roots from the NAM mesocosms had reached +8220 ‰, while the maximum Δδ¹⁵N value in NOX roots was considerably lower at +4430 ‰. Using ¹⁵N 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 µg g⁻¹ min⁻¹ or 48 mg N m⁻² d⁻¹) compared to the NOX mesocosms (0.23 µg 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.
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