Abstract
During the last decades, atmospheric nitrogen loading in mountain ranges of the Northern Hemisphere has increased substantially, resulting in high nitrate concentrations in many lakes. Yet, how increased nitrogen has affected denitrification, a key process for nitrogen removal, is poorly understood. We measured actual and potential (nitrate and carbon amended) denitrification rates in sediments of several lake types and habitats in the Pyrenees during the ice-free season. Actual denitrification rates ranged from 0 to 9 μmol N2O m−2 h−1 (mean, 1.5 ± 1.6 SD), whereas potential rates were about 10-times higher. The highest actual rates occurred in warmer sediments with more nitrate available in the overlying water. Consequently, littoral habitats showed, on average, 3-fold higher rates than the deep zone. The highest denitrification potentials were found in more productive lakes located at relatively low altitude and small catchments, with warmer sediments, high relative abundance of denitrification nitrite reductase genes, and sulphate-rich waters. We conclude that increased nitrogen deposition has resulted in elevated denitrification rates, but not sufficiently to compensate for the atmospheric nitrogen loading in most of the highly oligotrophic lakes. However, there is potential for high rates, especially in the more productive lakes and landscape features largely govern this.
Highlights
During the last decades, atmospheric nitrogen loading in mountain ranges of the Northern Hemisphere has increased substantially, resulting in high nitrate concentrations in many lakes
The denitrification rates measured after addition of a high nitrate concentration (i.e. >300 μM) were higher in the corresponding habitat and lake and ranged from 11 to 186 μmol N2O m−2 h−1
Our results indicate that denitrification in mountain lake sediments is mostly nitrate limited
Summary
Atmospheric nitrogen loading in mountain ranges of the Northern Hemisphere has increased substantially, resulting in high nitrate concentrations in many lakes. Many mountain areas of the Northern Hemisphere have received large atmospheric loadings of Nr during the last decades[7,8,9,10], resulting in elevated nitrate concentrations in mountain streams and lakes[7] These waters are deficient in phosphorus (P), and the supply of N usually exceeds the assimilation capacity by algae[7]. The ultimate objectives were to (i) identify factors explaining denitrification rates in mountain lake sediments and (ii) estimate if actual or potential denitrification rates can cope with the current atmospheric N load from deposition. We used a recently described protocol for measuring actual and potential denitrification rates in intact cores[40] This approach avoids modification of substrate diffusion from the overlying water column, providing more realistic estimates of in situ and potential denitrification rates in the sediments
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