Seasonal photochemical transformations of nitrogen species in a forest stream and lake.

Petr Porcal,Jiří Kopáček,Iva Tomková,Steven Arthur Loiselle

doi:10.1371/journal.pone.0116364

Abstract

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 (Norg; 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 (NH4 +) and other N forms (Nx; possibly N oxides and N2). In addition to Norg mineralization, Nx also originated from photochemical nitrate (NO3 −) reduction. Laboratory exposure of a first-order forest stream water samples showed a high amount of seasonality, with the maximum rates of Norg mineralization and NH4 + production in winter and spring, and the maximum NO3 − reduction occurring in summer. These photochemical changes could have an ecologically significant effect on NH4 + concentrations in streams (doubling their terrestrial fluxes from soils) and on concentrations of dissolved Norg in the lake. In contrast, photochemical reactions reduced NO3 − fluxes by a negligible (<1%) amount and had a negligible effect on the aquatic cycle of this N form.

Highlights

The photochemical properties of dissolved organic matter (DOM) have been thoroughly studied during last two decades, mostly focusing on the fate of organic carbon
This study focused on the Plesne Lake and its major surface tributary (PL-I) (Fig. 1)
The light pathway was short and the shading effect was probably small (15% according to the calculation: (1-e2a3506path length)/ (a3506path length), where a350 is the absorption coefficient and path length is the average distance light traveled through the solution52.5 cm [32])

Summary

Introduction

The photochemical properties of dissolved organic matter (DOM) have been thoroughly studied during last two decades, mostly focusing on the fate of organic carbon. N transformations are the most complex, and may include photochemical cleaving of dissolved organic nitrogen (Norg; a common constituent of DOM), resulting in the production of inorganic N forms, the oxidation of ammonium (NH4+), as well as the reduction of oxidized N forms (NO22, NO32) e.g., [4]. Photodegradation of Norg produces certain forms of bio-available nitrogen, mainly NH4+ (a process called ‘photoammonification’) and amino acids [6]. Photoproduction of ammonium has been observed from humic and fulvic acids in the Suwannee River [8], and in Baltic seawater [9]. Reactions of amino groups with hydroxyl radicals (HO?) are the dominant mechanisms of ammonium photoproduction e.g., [4] often enhanced by the presence of a catalyst (e.g., Fe or Ti oxides [10])

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