Browning and mining increase the nitrous oxide production in sediments of large boreal lakes during winter

There is limited knowledge on the N (nitrogen) cycling in winter, on the role of organic matter quality on N cycling, and on the microbes involved. We studied Lake Viinijärvi and Lake Höytiäinen, large boreal lakes in Finland, each lake with clear-water and brown-water sides. Viinijärvi has an additional side affected by mining activities in the catchment showing higher nitrate and sulphate levels. During winter of 2021 we sampled 5 sites at the beginning and at the end of the ice-covered period. Using the Isotope Pairing Technique we incubated sediment cores with 15NO3- and quantified the products of 1) complete denitrification (N2), 2) truncated denitrification (nitrous oxide, N2O), and 3) dissimilatory nitrate reduction to ammonium (DNRA, NH4+) to infer the process rates. We characterized the DOM using FT-ICR MS. We explore the genetic potential (DNA) of the sediment microbiome by using several sequencing techniques. During winter the sediment-water interface is an active compartment. The top sediment microbiome has heterotrophic bacteria with flexible metabolism, breaking-down OM during winter despite most of the DOM is recalcitrant. Impacts of browning and mining with major differences between sites. The genetic potential of the sediment microbiome indicates more DNRA and N2O consumption in clear-waters, while in the mining-impacted site and brown-water sites the dominant pathway depends on the sediment layer with truncated denitrification in top layer, and methanogenesis and N-fixation in sub-top layer. The N2O production (d14), that fits the genetic potential, is highest in the mining-impacted site (35-43 µmol N/m2/d), followed by the brown-water sediments (6-11 µmol N/m2/d), with the lowest rates in the clear-water sediments (0-1 µmol N/m2/d).