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Massive anthropogenic acceleration of the global nitrogen (N) cycle has stimulated interest in understanding the fate of excess N loading to aquatic ecosystems. Nitrate (NO3 −) is traditionally thought to be removed mainly by microbial respiratory denitrification coupled to carbon (C) oxidation, or through biomass assimilation. Alternatively, chemolithoautotrophic bacterial metabolism may remove NO3 − by coupling its reduction with the oxidation of sulfide to sulfate (SO4 2−). The NO3 − may be reduced to N2 or to NH4 +, a form of dissimilatory nitrate reduction to ammonium (DNRA). The objectives of this study were to investigate the importance of S oxidation as a NO3 − removal process across diverse freshwater streams, lakes, and wetlands in southwestern Michigan (USA). Simultaneous NO3 − removal and SO4 2− production were observed in situ using modified “push-pull” methods in nine streams, nine wetlands, and three lakes. The measured SO4 2− production can account for a significant fraction (25–40%) of the overall NO3 − removal. Addition of 15NO3 − and measurement of 15NH4 + production using the push–pull method revealed that DNRA was a potentially important process of NO3 − removal, particularly in wetland sediments. Enrichment cultures suggest that Thiomicrospira denitrificans may be one of the organisms responsible for this metabolism. These results indicate that NO3 −-driven SO4 2− production could be widespread and biogeochemically important in freshwater sediments. Removal of NO3 − by DNRA may not ameliorate problems such as eutrophication because the N remains bio-available. Additionally, if sulfur (S) pollution enhances NO3 − removal in freshwaters, then controls on N processing in landscapes subject to S and N pollution are more complex than previously appreciated. [ABSTRACT FROM AUTHOR]

N and P budgets quantify inputs and outputs of nutrients at the catchment scale to allow evaluation of inputs and outputs as well as inferences about transport and processing based on unaccounted-for nutrients. N and P budgets were constructed for two catchments in southeastern Michigan with markedly different numbers of impoundments, over two years, to evaluate the influence of impoundments on nutrient fluxes from each catchment. The Huron, with 88 impoundments >10 ha, stored 156 kg P km−2 y−1, while the Raisin (with 14 impoundments) had a net export of 102 kg P km−2 y−1. The Huron catchment also stored and denitrified more N than the Raisin catchment – 2,418 kg N km−2 y−1 compared to 1,538 kg N km−2 y−1. Riverine export of N and P also varied markedly between the catchments, with the Huron River exporting 288 kg N and 7 kg P km−2 y−1 and the Raisin River exporting 1,268 kg N and 34 kg P km−2 y−1. We then re-calculated budget results from previous studies using the approach of the present study, altering input and outputs fluxes as well as system boundaries to obtain comparable budgets. For these comparable budgets, annual P outputs on average accounted for 77% of inputs whereas N outputs accounted for only 39% of N inputs. Across catchments, the percent of inputs exported by the river averaged 16% for N and 5% for P, indicating more effective retention of P than N. [ABSTRACT FROM AUTHOR]