The role of organic matter and microbial community controlling nitrate reduction under elevated ferrous iron concentrations in boreal lake sediments

Abstract

The nitrogen availability, that affects the greenhouse gas emission and the trophic level of lakes, is controlled mainly by microbial processes. We measured in a boreal nitrate and iron rich lake how the rates of potential denitrification and dissimilatory nitrate reduction to ammonia (DNRA) are affected by degradability of organic matter and availability of aqueous ferrous iron. We also investigated the microbial community by using 16S rRNA gene and shotgun metagenomic sequencing approach, which allows taxonomic analyses and detection of metagenome-assembled genomes (MAGs) containing genes for both nitrate reduction and iron oxidation. The results show that truncated denitrification, leading to release of nitrous oxide, is favored over dinitrogen production in conditions where the degradability of the organic matter is low. DNRA rates were always minor compared to denitrification and appeared to be independent of the degradability of organic carbon. Reduced iron stimulated nitrate reducing processes, although consistently only DNRA. However, the proportion of MAGs containing DNRA genes was low suggesting chemistry driven stimulation by reduced iron. Nevertheless, the metagenomic analyses revealed unique taxa genetically capable of oxidizing iron and reducing nitrate simultaneously. Overall, the results highlight the spatial variability in microbial community and nitrous oxide emissions in boreal lake sediments.

Saturated, Suffocated, and Salty: Human Legacies Produce Hot Spots of Nitrogen in Riparian Zones

“…Beyond the concentration ratios (e.g., DOC:nitrate‐N), the DNRA‐denitrification partitioning is also likely influenced by the quality/lability of DOC (Jantti et al., 2022), which needs additional evaluation. The quality of DOC provided by historic, relict buried horizons (e.g., Gurwick, Groffman, et al., 2008; Gurwick, McCorkle, et al., 2008) would likely be less labile and more humic (Myneni, 2019) compared to that from contemporary, surficial organic horizons and could differentially affect the DNRA‐denitrification competition (e.g., Jantti et al., 2022). Similarly, the forms, speciation, and bioavailability of Fe could differ with sediment depth in the reducing soil conditions associated with milldams (Lu et al., 2022) and needs further investigating with regard to N cycling (Li et al., 2012).…”

“…Sediment size distribution could also directly alter/affect the microbial habitat and communities (e.g., Chen et al., 2021; Li et al., 2020) and should be investigated with its consequences for the DNRA‐denitrification dichotomy. Beyond the concentration ratios (e.g., DOC:nitrate‐N), the DNRA‐denitrification partitioning is also likely influenced by the quality/lability of DOC (Jantti et al., 2022), which needs additional evaluation. The quality of DOC provided by historic, relict buried horizons (e.g., Gurwick, Groffman, et al., 2008; Gurwick, McCorkle, et al., 2008) would likely be less labile and more humic (Myneni, 2019) compared to that from contemporary, surficial organic horizons and could differentially affect the DNRA‐denitrification competition (e.g., Jantti et al., 2022).…”

“…Respiration of this buried organic C also likely contributed to anoxic hot spots favorable for N reduction (Wallace & Soltanian, 2021). We hypothesize that the thickness, degradability (e.g., Gurwick, Groffman, et al., 2008; Gurwick, McCorkle, et al., 2008; Jantti et al., 2022), and spatial distribution (vertical, longitudinal, and transverse) of the buried organic C determined ammonification and the balance of the N reductive processes along the riparian transect (conceptual model Figure 6). High sediment organic C concentrations closer to the riparian‐stream edge likely favored greater accumulation of ammonium‐N via ammonification and DNRA (elaborated on below in the section on electron donors).…”

Organic matter lability modifies the vertical structure of methane-related microbial communities in lake sediments

“…S1 ). The hypolimnetic NO 3 − concentration, measured 2–5 cm above the sediment surface, is typically 46–75 µmol L −1 ( 34 , 35 ). The large catchment area of Lake Pääjärvi (244 km 2 ) is dominated by forests and agriculture.…”

Seasonal Contrasts in Dissolved Selenium Dynamics in Subarctic Thaw Lakes

“…NO 3concentration maxima occur in oxygenated zones, above the NO 2maxima which are 30 times lower than those of NO 3 -(Figures S4 to S7). NO 2serves as a transition species in the denitrification process, and its presence indicates an active microbial nitrogen cycle 15 , which has previously been proposed for the SAS sites based on potential metabolic pathways 6 . Genes responsible for reduction of nitrite to ammonia were found in summer communities in SAS ponds, with nitrite reductase genes being the most abundant 6 , suggesting that dissimilatory nitrate reduction to ammonia (DNRA) (NO 3to NH 4 + via NO 2 -) is a preferred pathway.…”