Direct and indirect effects of organic matter sources on denitrification in Florida rivers

Background/Question/Methods

Denitrification, the reduction of nitrate (NO₃^-) to N₂ gas coupled to the metabolic oxidation of organic matter (OM) or other electron donors, is a major pathway for the removal of reactive nitrogen (N) from ecosystems. In streams and rivers, OM that supports denitrification may originate from terrestrial sources or from in situ primary production, but the relative importance of these sources, particularly in larger rivers, is poorly understood. Terrestrially-derived OM, which includes humic and tannic acids, phenolic compounds, and other complex forms, 1) is often less labile than autochthonous OM, 2) may also inhibit denitrification directly via biochemical mechanisms, and 3) may indirectly inhibit denitrification by reducing light availability to  ̶  and thus exudation by  ̶  aquatic primary producers.
We addressed these hypotheses along a natural dissolved OM gradient in rivers of northern Florida, USA, spanning from spring-fed rivers with clear water and productive aquatic plant communities to blackwater rivers with high concentrations of colored, terrestrially-derived DOM. We assessed rates and limitation of denitrification along this gradient using factorial amendments of NO₃^- and dextrose, and directly measured resulting increases in N₂ using Membrane Inlet Mass Spectrometry (MIMS). We used humic acid dosing experiments and cross-incubations of sediments and water to investigate direct inhibition by blackwater DOC. 

Results/Conclusions

Across all site types (spring-fed, blackwater, and intermediate), sediment organic matter content was a major driver of denitrification, explaining between 38% (dextrose-only treatment) and 65% (dextrose-and-nitrate treatment) of the variation in measured rates. We found strong evidence for nitrate limitation in 15 of the 20 sites; response ratios of nitrate-amended treatments to controls were negatively related to background nitrate concentration. In addition, the response ratio of dextrose-and-nitrate to nitrate-only treatments increased as a function of long-term total organic carbon (r²: 0.45), indicating a transition from NO₃^- limitation to co-limitation by NO₃^- and labile organic carbon along the spring-to-blackwater gradient. Humic acid amendments had neither a significant stimulatory nor inhibitory effect on denitrification rates in three spring-fed rivers (p-values: 0.28, 0.41, 0.97). Together, these results suggest that terrestrially-derived DOC is of lower quality for denitrifiers, and indirectly inhibits denitrification by limiting production of DOC by in-stream autotrophs.