Seasonal Photochemical Transformations of Nitrogen Species in a Forest Stream and Lake

The photochemical release of inorganic nitrogen from dissolved organic matter is an important source of bio-available nitrogen (N) in N-limited aquatic ecosystems. We conducted photochemical experiments and used mathematical models based on pseudo-first-order reaction kinetics to quantify the photochemical transformations of individual N species and their seasonal effects on N cycling in a mountain forest stream and lake (Plešné Lake, Czech Republic). Results from laboratory experiments on photochemical changes in N speciation were compared to measured lake N budgets. Concentrations of organic nitrogen (N(org); 40–58 µmol L(−1)) decreased from 3 to 26% during 48-hour laboratory irradiation (an equivalent of 4–5 days of natural solar insolation) due to photochemical mineralization to ammonium (NH(4) (+)) and other N forms (N(x); possibly N oxides and N(2)). In addition to N(org) mineralization, N(x) also originated from photochemical nitrate (NO(3) (−)) reduction. Laboratory exposure of a first-order forest stream water samples showed a high amount of seasonality, with the maximum rates of N(org) mineralization and NH(4) (+) production in winter and spring, and the maximum NO(3) (−) reduction occurring in summer. These photochemical changes could have an ecologically significant effect on NH(4) (+) concentrations in streams (doubling their terrestrial fluxes from soils) and on concentrations of dissolved N(org) in the lake. In contrast, photochemical reactions reduced NO(3) (−) fluxes by a negligible (<1%) amount and had a negligible effect on the aquatic cycle of this N form.