Denitrification removes fixed nitrogen (N) from the biosphere, thereby restricting the availability of this key limiting nutrient for terrestrial plant productivity. This microbially driven process has been exceedingly difficult to measure, however, given the large background of nitrogen gas (N2) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO3−) to examine dentrification rates across six forest sites in southern China and central Japan, which span temperate to tropical climates, as well as various stand ages and N deposition regimes. Our multiple stable isotope approach across soil to watershed scales shows that traditional techniques underestimate terrestrial denitrification fluxes by up to 98%, with annual losses of 5.6–30.1 kg of N per hectare via this gaseous pathway. These N export fluxes are up to sixfold higher than NO3− leaching, pointing to widespread dominance of denitrification in removing NO3− from forest ecosystems across a range of conditions. Further, we report that the loss of NO3− to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.

https://www.pnas.org/content/pnas/112/5/1470.full.pdf

Microbial denitrification dominates nitrate losses from forest ecosystems

Although increasing atmospheric nitrous oxide (N2O) has been linked to nitrogen loading, predicting emissions remains difficult, in part due to challenges in disentangling diverse N2O production pathways. As coastal ecosystems are especially impacted by elevated nitrogen, we investigated controls on N2O production mechanisms in intertidal sediments using novel isotopic approaches and microsensors in flow-through incubations. Here we show that during incubations with elevated nitrate, increased N2O fluxes are not mediated by direct bacterial activity, but instead are largely catalysed by fungal denitrification and/or abiotic reactions (e.g., chemodenitrification). Results of these incubations shed new light on nitrogen cycling complexity and possible factors underlying variability of N2O fluxes, driven in part by fungal respiration and/or iron redox cycling. As both processes exhibit N2O yields typically far greater than direct bacterial production, these results emphasize their possibly substantial, yet widely overlooked, role in N2O fluxes, especially in redox-dynamic sediments of coastal ecosystems.

/pdf/evidence-for-fungal-and-chemodenitrification-based-n2o-flux-35q0fjl6g6.pdf

Evidence for fungal and chemodenitrification based N2O flux from nitrogen impacted coastal sediments

https://espace.library.uq.edu.au/view/UQ:681928/UQ681928_OA.pdf

Quantifying nitrous oxide production pathways in wastewater treatment systems using isotope technology - A critical review.

Nitrate pollution in surface water has become an environmental problem of global concern. The effective way for controlling the nitrate pollution of surface water is to identify the pollution source and reduce the input of nitrate. In recent decades, nitrogen (δ15N) and oxygen (δ18O) isotopes of nitrate has been used as an effective approach for identifying the source and fate of nitrate pollution in surface water. However, owing to the complexity of nitrate pollution source and the influence of isotopic fractionation, the application of this method has some limitations. In this work, we systematically discussed the fundamental principle of using nitrogen and oxygen isotopes to trace the nitrate source, the fate identification of nitrate, and the major testing techniques. Subsequently, the applications of nitrogen and oxygen isotopes for source identification of surface water were illustrated. However, there are still significant gaps in the application of the source identification and transformation mechanisms to nitrate and many research questions on these topics need to be addressed.

/pdf/application-of-nitrogen-and-oxygen-isotopes-for-source-and-2r0zjyv2oy.pdf

Application of Nitrogen and Oxygen Isotopes for Source and Fate Identification of Nitrate Pollution in Surface Water: A Review

This chapter introduces the fundamentals of environmental isotope tracers in hydrologic studies, provides an overview of the most common applications of light isotopes for understanding the hydrologic cycle and tracing solutes in aquatic systems, and summarizes recent advances in these fields. Isotope tracers have been extremely useful in providing new insights into hydrologic processes because they integrate small-scale variability to give an effective indication of catchment-scale processes. The main purpose of this chapter is to provide an overview of recent research into the use of naturally occurring stable isotopes (and associated radioisotopes of the same elements) to track the movement of water and solutes in hydrological systems where the waters are relatively fresh: soils, surface waters, and shallow groundwaters.

7.9 – Environmental Isotope Applications in Hydrologic Studies

. The stable isotopic compositions of nitrate, including the 17O anomalies (Δ17O), were determined twice in 1 yr (June and August 2007) in the oligotrophic water column of Lake Mashu, Japan. These data were then used to quantify the geochemical dynamics of nitrate in the lake, by using the deposition rate of the atmospheric nitrate onto the entire catchment area of the lake. The total amount of nitrate in the lake water decreased from 4.2 to 2.1 Mmol during the period between the observations, while the average Δ17O values remained uniform at +2.5‰. The Δ17O values corresponded to an small and uniform mixing ratio of atmospheric nitrate to total nitrate of 9.7 ± 0.8%. These results indicate that 0.52 ± 0.34 Mmol of the remineralized nitrate was fed into the water column through nitrification, while 2.6 ± 0.4 Mmol of nitrate was simultaneously removed from the water column by assimilation, during the period between the observations. The lake water dissolved nitrate was characterized by rapid removal through assimilation during summer until it was almost completely removed from the euphotic layer, as well as continuous feeding into the lake through nitrification (3.2 ± 0.3 Mmol a−1) and deposition (0.35 ± 0.2 Mmol a−1), regardless of the seasons. The 15N-depleted nitrogen isotopic compositions of nitrate were as low as −6.5‰ in June, which also indicates that in-lake nitrification is the major source of nitrate in the lake and suggests that there is low potential for denitrification in and around the lake. Atmospheric nitrate deposited into the lake will be assimilated quickly, having a mean residence time of 1.2 ± 0.1 yr. In addition, more than 90% of the assimilated nitrate will be remineralized to nitrate and re-assimilated via active nitrogen cycling in the lake.

/pdf/quantifying-nitrate-dynamics-in-an-oligotrophic-lake-using-2cy8zz2917.pdf

Quantifying nitrate dynamics in an oligotrophic lake using Δ17O

/pdf/sources-fate-and-geochemical-dynamics-of-nitrate-in-an-3g6x8ib9wb.pdf

Sources, fate and geochemical dynamics of nitrate in an oligotrophic lake

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Quantifying nitrate dynamics in an oligotrophic lake using Δ17O

Sources, fate and geochemical dynamics of nitrate in an oligotrophic lake