Abstract
Sulfur-based denitrification may be a key biogeochemical nitrate (NO3−) removal process in sulfide-rich regions, but it is still poorly understood in natural terrestrial ecosystems. We examined sulfur-driven NO3− reduction using streambank soils in a headwater catchment underlain by marine sedimentary rock in Akita, Japan. In a catchment exhibiting higher sulfide content in streambed sediment, we sampled two adjacent streambank soils of streambank I (two layers) and of streambank II (eight layers). Anaerobic long-term incubation experiments (40 days, using soils of streambank I) and short-term incubation experiments (5 days, using soils of streambank II) were conducted to evaluate variations of N solutes (NO3−, NO2−, and NH4+), N gases (NO, N2O), and the bacterial flora. In both experiments, two treatment solutions containing NO3− (N treatment), and NO3− and S2O32− (N + S treatment) were prepared. In the N + S treatment of the long-term experiment, NO3− concentrations gradually decreased by 98%, with increases in the SO42−, NO2−, NO, and N2O concentrations and with not increase in the NH4+, indicating denitrification had occurred with a high probability. Temporal accumulation of NO2− was observed in the N + S treatment. The stoichiometric ratio of SO42− production and NO3− depletion rates indicated that denitrification using reduced sulfur occurred even without additional S, indicating inherent S also served as an electron donor for denitrification. In the short-term incubation experiment, S addition was significantly decreased NO3− concentrations and increased NO2−, NO, and N2O concentrations, especially in some subsoils with higher sulfide contents. Many denitrifying sulfur-oxidizing bacteria (Thiobacillus denitrificans and Sulfuricella denitrificans) were detected in both streambank I and II, which dominated up to 5% of the entire microbial population, suggesting that these bacteria are widespread in sulfide-rich soil layers in the catchment. We concluded that the catchment with abundant sulfides in the subsoil possessed the potential for sulfur-driven NO3− reduction, which could widely influence N cycling in and NO3− export from the headwater catchment.
Highlights
Human activities have dramatically increased the amount of reactive nitrogen (N) in global ecosystems and have increased food production; input of reactive N beyond appropriate uses can lead to eutrophication of surface water, causing degradation of aquatic ecosystems and problems such as toxic algal blooms, loss of dissolved oxygen, depletion of fish populations, and biodiversity loss (Vitousek, 1997; Galloway and Cowling, 2002)
To obtain evidence of sulfur-based denitrification in natural soils and sediments in freshwater ecosystems, it may be important to detect the various signs (NO3− reduction accompanied by SO42− production, stoichiometric reactions inferred from the ΔSO42−/ ΔNO3− ratio, accumulation of NO2− and gaseous forms of N, and elements of the microbial community) of sulfur-based denitrification
High total sulfur contents were observed in subsoil samples I2, II4, and II8, and easily oxidizable S (EOS) levels were high in those soil layers
Summary
Human activities have dramatically increased the amount of reactive nitrogen (N) in global ecosystems and have increased food production; input of reactive N beyond appropriate uses can lead to eutrophication of surface water, causing degradation of aquatic ecosystems and problems such as toxic algal blooms, loss of dissolved oxygen, depletion of fish populations, and biodiversity loss (Vitousek, 1997; Galloway and Cowling, 2002). Typical signs of sulfur-based denitrification using sulfide as an electron donor are decreasing NO3− accompanied by increasing SO42− and NO2− (Postma et al, 1991; Jørgensen et al, 2009; Torrentó et al, 2010; Chung et al, 2014) and the microbial stoichiometric reaction ratios of SO42− production and NO3− depletion rates (ΔSO42−/ΔNO3−) for sulfur-based denitrification (Hayakawa et al, 2013; Vaclavkova et al, 2014). To obtain evidence of sulfur-based denitrification in natural soils and sediments in freshwater ecosystems, it may be important to detect the various signs (NO3− reduction accompanied by SO42− production, stoichiometric reactions inferred from the ΔSO42−/ ΔNO3− ratio, accumulation of NO2− and gaseous forms of N, and elements of the microbial community) of sulfur-based denitrification
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