Recognizing both denitrification and nitrogen consumption improves performance of stream diel N₂ flux models

Abstract

There is keen interest to enhance denitrification within intervening aquatic habitats between agricultural areas and downstream aquatic ecosystems to reduce nitrogen (N) loading impacts to receiving ecosystems. We conducted a series of measurements to examine whole system in situ diel denitrification estimates in experimental ditch and stream environments using a Bayesian one-station diel N 2 flux model. Model estimates revealed complex patterns that indicate fluxes may be controlled by the balance of both N 2 production via denitrification and consumption driven by physical or biological processes associated with strong diel patterns in environmental conditions. We investigated potential improvements in model fits to observed data associated with the addition of a N 2 consumption term to represent biological (N 2 fixation) or physical (bubble formation and N 2 scavenging) mechanisms associated with daytime photosynthesis. We also expanded the current one-station diel flux model to a two-station model to estimate denitrification in discrete reaches. Our modified diel N 2 flux models improved model fit significantly across three metrics (Nash-Sutcliffe efficiency, root mean square ratio, and percent bias) increasing their utility in shallow, open canopy, lotic systems. While more studies are needed to understand specific mechanisms associated with N 2 consumption processes in small agricultural drainages as well as environmental conditions affecting their relative importance, these results improve estimates of N 2 flux where dynamic conditions and heterogeneity of habitats create severe diel patterns in factors controlling dissolved gas concentrations and prohibit accurate estimates of N 2 flux using existing models.

Vegetated Ditch Habitats Provide Net Nitrogen Sink and Phosphorus Storage Capacity in Agricultural Drainage Networks Despite Senescent Plant Leaching

“…Integration of modelled rates across time during our simulated runoff events indicated that approximately 40% of measured N retention was associate with denitrification in cutgrass mesocosms (Table 5). Our estimated denitrification rates (564-620 mg m −2 ) were similar to previously reported daily rates based on sediment core incubations (142-1049 mg m −2 day −1 ) [18,19] and daily diel N 2 -N flux estimates (700-1000 mg m −2 day −1 ) from experimental ditches vegetated with rice cutgrass [37]. Estimates from this study were also similar to reported denitrification rates (220-1176 mg m −2 day −1 ) from vegetated ditches in Italy [11,17,20] and the Czech Republic [14].…”

“…Soana et al [11] also demonstrated that sediment core incubations can account for as little as half of whole reach open-channel estimates of denitrification rates. Open-channel approaches to measuring denitrification potentially provide better estimates by integrating spatial and temporal heterogeneity of factors influencing denitrification at the reach scale but are highly dependent on factors that control the rates of gas transfer at the surface water interface [17,37,45]. While all methods of measuring denitrification in vegetated ditch environments are prone to different biases, our results, coupled with previous observations, provide strong support for denitrification serving as a dominant N removal pathway in vegetated ditches [11,[17][18][19].…”

“…Emergent vegetation plays a key role in reducing N export from agricultural drainage networks by enhancing conditions that promote denitrification [11,18,37]. Integration of modelled rates across time during our simulated runoff events indicated that approximately 40% of measured N retention was associate with denitrification in cutgrass mesocosms (Table 5).…”

Vegetation and Residence Time Interact to Influence Metabolism and Net Nutrient Uptake in Experimental Agricultural Drainage Systems

“…Reach-scale metabolism rates were estimated based on changes in dissolved oxygen (O 2 ) through time with a two-station method [46,47]. The two-station method was appropriate due to expected low reaeration coefficients (K) and a short distance to source water which can confound dissolved gas flux estimates if experimental systems are not treated discretely [48,49]. We used 2 HOBO ® (U26-001, Onset ® Computer Corporation, Bourne, ME, USA) loggers placed in the middle of each ditch at upstream and downstream ends to measure dissolved O 2 and temperature at 15 min intervals during each 24 h nutrient loading event.…”

“…Using a polyvinyl chloride sampler, exetainer vials were overfilled 3x, preserved with zinc chloride (ZnCl 2 ), capped, and stored until analysis. We measured dissolved N 2 :Ar in dissolved gas samples with a Membrane Inlet Mass Spectrometer (MIMS) (Bay Instruments, Easton, MD, USA) [49,55] [43]. We developed and applied a correction factor to eliminate instrument specific O 2 interference with N 2 :Ar measurements which may have contributed to below equilibrium N 2 :Ar ratios observed during daytime hours [43,58].…”