Direct and Indirect Effects of Dissolved Organic Matter Source and Concentration on Denitrification in Northern Florida Rivers
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Using a natural gradient of dissolved organic carbon (DOC) source and concentration in rivers of northern Florida, we investigated how terrestrially-derived DOC affects denitrification rates in river sediments. Specifically, we examined if the higher concentrations of DOC in blackwater rivers stimulate denitrification, or whether such terrestrially-derived DOC supports lower denitrification rates because (1) it is less labile than DOC from aquatic primary production; whether (2) terrestrial DOC directly inhibits denitrification via biochemical mechanisms; and/or whether (3) terrestrial DOC indirectly inhibits denitrification via reduced light availability to—and thus DOC exudation by—aquatic primary producers. We differentiated among these mechanisms using laboratory denitrification assays that subjected river sediments to factorial amendments of NO3 − and dextrose, humic acid dosing, and cross-incubations of sediments and water from different river sources. DOC from terrestrial sources neither depressed nor stimulated denitrification rates, indicating low lability of this DOC but no direct inhibition; humic acid additions similarly did not affect denitrification rates. However, responses to addition of labile C increased with long-term average DOC concentration, which supports the hypothesis that terrestrial DOC indirectly inhibits denitrification via decreased autochthonous production. Observed and future changes in DOC concentration may therefore reduce the ability of inland waterways to remove reactive nitrogen.
KeywordsDOC light limitation coupled biogeochemical cycles nitrogen cycle primary production browning humic substances
Thank you to Catherine Bravo, Kelsey Reider, Jenn Sweatman, Evan Rehm, Ewan Isherwood, Sean Koester, Anna Schnarnagl, Nate Lemoine, and Sabrina Jamil for help in the field, and to Ewan Isherwood, Gabriel Sone, and Sean Koester for help in the lab. Yuying Zhang and Nate Lemoine helped with statistical analyses. Joe Boyer, Jim Fourqurean, Jennie Richards, Ewan Isherwood, and two anonymous reviewers gave valuable feedback on former versions of this manuscript. Comments from members of the Bernhardt and Heffernan labs as well as subject matter editor Dr. Stuart Bunn and two anonymous reviewers also helped improve the manuscript. We also thank the SERC Nutrient Analysis Laboratory for analyzing water samples, and Bill Anderson and Todd Kana for support with the MIMS. Permits for access to state and federal lands used as field sites were awarded by the Florida Department of Environmental Protection, U.S. Forest Service, and Florida Forest Service. We also offer special thanks to Ginger Morgan at Ichetucknee Springs State Park for logistical assistance with field work, and to Wekiva Island, King’s Landing, and Canoe Outpost for providing boat ramp access and parking. This research was conducted under grants from Sigma Xi awarded to Megan Fork, and the National Science Foundation (EAR #0838390) awarded to Dr. Jim Heffernan.
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