Understanding the biogeochemical controls on the partitioning between nitrogen (N) removal through denitrification and anaerobic ammonium oxidation (anammox), and N recycling via dissimilatory nitrate (NO₃ ^-) reduction to ammonium (DNRA) is crucial for constraining lacustrine N budgets. Besides organic carbon, inorganic compounds may serve as electron donors for NO₃ ^- reduction, yet the significance of lithotrophic NO₃ ^- reduction in the environment is still poorly understood. Conducting incubation experiments with additions of ¹⁵N-labeled compounds and reduced inorganic substrates (H₂S, Fe²⁺, Mn²⁺), we assessed the role of alternative electron donors in regulating the partitioning between the different NO₃ ^--reducing processes in ferruginous surface sediments of Lake Lugano, Switzerland. In sediment slurry incubations without added inorganic substrates, denitrification and DNRA were the dominant NO₃ ^--reducing pathways, with DNRA contributing between 31 and 46% to the total NO₃ ^- reduction. The contribution of anammox was less than 1%. Denitrification rates were stimulated by low to moderate additions of ferrous iron (Fe²⁺ ≤ 258 μM) but almost completely suppressed at higher levels (≥1300 μM). Conversely, DNRA was stimulated only at higher Fe²⁺ concentrations. Dissolved sulfide (H₂S, i.e., sum of H₂S, HS^- and S^2-) concentrations up to ∼80 μM, strongly stimulated denitrification, but did not affect DNRA significantly. At higher H₂S levels (≥125 μM), both processes were inhibited. We were unable to find clear evidence for Mn²⁺-supported lithotrophic NO₃ ^- reduction. However, at high concentrations (∼500 μM), Mn²⁺ additions inhibited NO₃ ^- reduction, while it did not affect the balance between the two NO₃ ^- reduction pathways. Our results provide experimental evidence for chemolithotrophic denitrification or DNRA with Fe²⁺ and H₂S in the Lake Lugano sediments, and demonstrate that all tested potential electron donors, despite the beneficial effect at low concentrations of some of them, can inhibit NO₃ ^- reduction at high concentration levels. Our findings thus imply that the concentration of inorganic electron donors in lake sediments can act as an important regulator of both benthic denitrification and DNRA rates, and suggest that they can exert an important control on the relative partitioning between microbial N removal and N retention in lakes.

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