Denitrification Temperature Dependence in Remote, Cold, and N‐Poor Lake Sediments

Abstract

The reservoir size and pathway rates of the nitrogen (N) cycle have been deeply modified by the human enhancement of N fixation, atmospheric emissions, and climate warming. Denitrification (DEN) transforms nitrate into nitrogenous gas and thus removes reactive nitrogen (Nr) back to the atmospheric reservoir. There is still a rather limited knowledge of the denitrification rates and their temperature dependence across ecosystems; particularly, for the abundant cold and N‐poor freshwater systems (e.g., Arctic and mountain lakes). We experimentally investigated the denitrification rates of mountain lake sediments by manipulating nitrate concentration and temperature on field collected cores. DEN rates were nitrate limited in field conditions and showed a large potential for an immediate DEN increase with both warming and higher Nr load. The estimated activation energy (Ea) for denitrification at nitrate saturation was 46 ± 7 kJ mol−1 (Q10 1.7 ± 0.4). The apparent Ea increased with nitrate (μM) limitation as Ea = 46 + 419 [ NO3–]−1. Accordingly, we suggest that climate warming may have a synergistic effect with N emission reduction to readjusting the N cycle. Changes of nitrate availability might be more relevant than direct temperature effects on denitrification.

Denitrification rates in lake sediments of mountains affected by high atmospheric nitrogen deposition

“…Nitrate levels were more important in predicting actual rates compared to temperature, which has been observed in other studies 16,60 . In littoral habitats, higher nitrate availability through enhanced diffusion by wave action and inputs via runoff and groundwater flows, in combination with warmer temperatures give rise to conditions that are more favourable to denitrification, which can explain the higher actual rates.…”

“…This method inhibits the reduction of N 2 O to N 2 45,46 and enables reliable estimates of denitrification rates of at least 0.4-1 μmol N 2 O m −2 h −1 40 . This is the most used method to measure denitrification and has been used in all studies of denitrification in mountain lakes [16][17][18][19][20] , allowing comparisons across studies. However, a potential drawback is that incomplete inhibition of N 2 O reduction can occur or nitrification can be partially inhibited, which would underestimate the rates 47,48 .…”

Bacterial Diversity in a Dynamic and Extreme Sub-Arctic Watercourse (Pasvik River, Norwegian Arctic)

“…Members in the phylum Chloroflexi are capable of anoxygenic photosynthesis, as well as nitrogen transformation [33]. Their occurrence suggests that denitrification may occur within the Pasvik River sediments, even if the denitrification rate is expected to be low in cold and oligotrophic environments [34]. Acidobacteria are among the most dominant bacterial groups in soil, but their ecophysiology remains largely unknown [35,36].…”

The DNRA-Denitrification Dichotomy Differentiates Nitrogen Transformation Pathways in Mountain Lake Benthic Habitats

“…Alpine lakes of the Northern hemisphere and subarctic regions are examples of remote ecosystems that have been exposed to increased N r deposition during the last decades (Holtgrieve et al, 2011; Camarero, 2017), triggering a nutrient imbalance in these freshwater systems which are otherwise known to have low nutrient availability (Catalan et al, 2006). While alpine and subarctic lakes are often considered important sensors of global change (Smol, 2012), there is minimal understanding of how increased N r availability affects microbially-driven N-cycle pathways in these ecosystems (McCrackin and Elser, 2010; Palacin-Lizarbe et al, 2018).…”