The biogeochemical influences of nitrate, dissolved oxygen, and dissolved organic carbon on stream nitrate uptake

Streams are important hotspots for the retention and removal of nitrogen (N), an element that contributes to eutrophication and threatens the stability of coastal ecosystems. Nitrate (NO3-) is the most mobile form of N, and understanding the causal mechanisms that foster optimal NO3- retention and removal in stream systems is critical from both predictive and conservation standpoints. Dissolved organic carbon (DOC) is hypothesized to be a major control of instream NO 3- concentrations, but dissolved oxygen (DO) is also an important control of NO3- removal processes. Assessing the individual impacts of NO3-, DO, and DOC concentrations on stream NO3- removal is difficult due to the natural interdependencies of these nutrients in the carbon and nitrogen cycles. This study took an experimental approach to quantifying the influences of NO3-, DOC, and DO on NO3 - transport within two headwater streams of the Ipswich and Parker River watersheds, MA, with contrasting levels of DOC and DO. In a first set of experiments we added increasing levels of NO3- to address how uptake kinetics differed in a low DO/high DOC stream (Cedar Swamp Creek) versus a high DO/low DOC stream (Cart Creek). In a second set of experiments, we manipulated for the first time at the reach scale both DO and DOC in a factorial experiment. DO was added to the low DO stream by injecting oxygen, and removed from the high DO stream by adding sodium sulfite. DOC was added both alone and in combination with the DO manipulations. Results from the NO3- enrichments suggest NO3 - concentration is an important control of NO3- vertical velocity. Results from the DOC and DO manipulations suggest that DO determines whether a stream has net nitrate uptake or production, and that DOC magnifies these processes. Addition of DOC by itself did not lead to increased nitrate uptake, suggesting that inverse relationships between nitrate and DOC may arise from complex interactions among DOC, DO and nitrate concentrations and how they influence dominant stream processes. In addition to these findings, we also observed organic matter priming effects (Kuzyakov et al. 2000) not previously reported in stream systems

The biogeochemical influence of nitrate, dissolved oxygen, and dissolved organic carbon on stream nitrate uptake

Streams are potential hotspots for retention and removal of NO3−, and understanding the mechanisms that enhance NO3− reactivity in stream systems is critical for predicting and preventing eutrophication. Both dissolved organic C (DOC) and dissolved O2 (DO) influence NO3− removal processes. Assessing the individual impacts of NO3−, DO, and DOC concentrations on stream NO3− removal is difficult because these factors covary and are coupled through the C and N cycles. We used an experimental approach to quantify the influences of NO3−, DOC, and DO on NO3− transport in headwater streams of the Ipswich and Parker River watersheds (Massachusetts, USA) with contrasting levels of DOC and DO. In a 1st set of experiments, we added NO3− to address how uptake kinetics differed between a low-DO/high-DOC stream (Cedar Swamp Creek) and a high-DO/low-DOC stream (Cart Creek). In a 2nd set of experiments, we manipulated, for the first time at the reach scale, both DO and DOC in a factorial experiment. DO was added to the low-DO stream by injecting O2 and was removed from the high-DO stream by adding sodium sulfite. DOC was added both alone and in combination with the DO manipulations. NO3− concentration was an important control of NO3− uptake velocity in our study streams, consistent with previous findings. The results of the DOC and DO manipulations suggested that DO determines whether a stream has net NO3− uptake or production and that the presence of DOC magnifies the DO response processes. Addition of DOC by itself did not lead to increased NO3− uptake. In addition, we observed organic matter priming effects, wherein the addition of labile organic matter resulted in accelerated metabolism of naturally occurring DOC in the water column. Priming effects have not been reported previously in stream systems. Results from our experiments suggest that NO3− uptake in streams might arise from complex interactions among DOC, DO, and NO3−, and ultimately, from the influence of DO on dominant stream processes