Triple Oxygen Isotope Analysis of Nitrate Using the Denitrifier Method and Thermal Decomposition of N 2 O

Nitrogen stable isotope ratio (δ 15 N) in Greenland snow nitrate and in North American remote lake sediments has decreased gradually beginning as early as ∼1850 Christian Era. This decrease was attributed to increasing atmospheric deposition of anthropogenic nitrate, reflecting an anthropogenic impact on the global nitrogen cycle, and the impact was thought to be amplified ∼1970. However, our subannually resolved ice core records of δ 15 N and major ions (e.g., NO − 3 , SO 2− 4 ) over the last ∼200 y show that the decrease in δ 15 N is not always associated with increasing NO − 3 concentrations, and the decreasing trend actually leveled off ∼1970. Correlation of δ 15 N with H + , NO − 3 , and HNO 3 concentrations, combined with nitrogen isotope fractionation models, suggests that the δ 15 N decrease from ∼1850-1970 was mainly caused by an anthropogenic-driven increase in atmospheric acidity through alteration of the gas−particle partitioning of atmospheric nitrate. The concentrations of NO − 3 and SO 2− 4 also leveled off ∼1970, reflecting the effect of air pollution mitigation strategies in North America on anthropogenic NO x and SO 2 emissions. The consequent atmospheric acidity change, as reflected in the ice core record of H + concentrations, is likely responsible for the leveling off of δ 15 N ∼1970, which, together with the leveling off of NO − 3 concentrations, suggests a regional mitigation of anthropogenic impact on the nitrogen cycle. Our results highlight the importance of atmospheric processes in controlling δ 15 N of nitrate and should be considered when using δ 15 N as a source indicator to study atmospheric flux of nitrate to land surface/ecosystems.fossil fuel emissions | proxy | industrial | acid deposition | clean air act

Section: Methodsmentioning

confidence: 98%

Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change

Geng

Alexander²,

Cole‐Dai

et al. 2014

“…The denitrifier method (33,34) was used to determine δ 15 N and δ 18 O of NO 3 − in all samples, excluding stream waters collected from JFL forests in 2012 and soil extracts from JFL and DHS forest soil profiles collected in 2013, which were determined by the chemical conversion method (35). The Δ 17 O of NO 3 − of selected precipitation and stream water samples was determined by combining bacterial reduction (i.e., denitrifier method) and the thermal decomposition method (36). Subsamples of soil were measured for concentrations of carbon (C), N, and δ 15 N. N inputs and N leaching losses were determined by multiplying N concentrations by water fluxes.…”

Section: Methodsmentioning

confidence: 99%

Microbial denitrification dominates nitrate losses from forest ecosystems

Fang

Koba

Makabe

et al. 2015

Self Cite

196

125

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 (N 2 ) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO 3 − ) 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 NO 3 − leaching, pointing to widespread dominance of denitrification in removing NO 3 − from forest ecosystems across a range of conditions. Further, we report that the loss of NO 3 − to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.denitrification | nitrate isotopes | nitrogen cycling | forested watersheds

“…Nitrogen and oxygen isotope ratios were measured using an automated denitrifier method as described by Morin et al (27). This technique uses Pseudomonas aureofaciens bacteria to convert nitrate to N 2 O, which is then analyzed for its isotopic composition after thermal decomposition to O 2 and N 2 in a gold tube (46). The analytical procedure used for this study is strictly identical to the description given by Morin et al (27).…”

Section: Methodsmentioning

confidence: 99%

Isotopic composition of atmospheric nitrate in a tropical marine boundary layer

Savarino

Morin

Erbland

et al. 2013

Long-term observations of the reactive chemical composition of the tropical marine boundary layer (MBL) are rare, despite its crucial role for the chemical stability of the atmosphere. Recent observations of reactive bromine species in the tropical MBL showed unexpectedly high levels that could potentially have an impact on the ozone budget. Uncertainties in the ozone budget are amplified by our poor understanding of the fate of NO x (= NO + NO 2 ), particularly the importance of nighttime chemical NO x sinks. Here, we present year-round observations of the multiisotopic composition of atmospheric nitrate in the tropical MBL at the Cape Verde Atmospheric Observatory. We show that the observed oxygen isotope ratios of nitrate are compatible with nitrate formation chemistry, which includes the BrNO 3 sink at a level of ca. 20 ± 10% of nitrate formation pathways. The results also suggest that the N 2 O 5 pathway is a negligible NO x sink in this environment. Observations further indicate a possible link between the NO 2 /NO x ratio and the nitrogen isotopic content of nitrate in this low NO x environment, possibly reflecting the seasonal change in the photochemical equilibrium among NO x species. This study demonstrates the relevance of using the stable isotopes of oxygen and nitrogen of atmospheric nitrate in association with concentration measurements to identify and constrain chemical processes occurring in the MBL.monitoring | oxidation | tropic | bromine chemistry | aerosols T he tropical marine lower troposphere is one of the most photochemically active compartments of the global atmosphere. The year-round high solar radiation, temperature, and humidity render this region the second largest contributor to methane removal via the OH sink (1). A large proportion of tropospheric ozone loss also occurs in the tropical marine boundary layer (MBL), where the concentrations of precursors, such as NO x (= NO + NO 2 ) and volatile organic carbons (VOCs) (2), are generally too low to keep the ozone production rate above its destruction rate. The destruction of ozone is further highlighted by the recently discovered role of halogen chemistry at low latitudes (3), which is known to destroy ozone catalytically (4). The subtle oxidation chemistry coupling NO x , halogens, and VOCs with ozone production and destruction in the MBL is still not fully understood, as demonstrated by the historically overlooked bromine chemistry (3) or the role of heterogeneous N 2 O 5 hydrolysis as a NO x sink (5, 6).Being the end product of the NO x /O 3 /VOC interaction, atmospheric nitrate is particularly well suited to probe such chemistry, especially through its stable isotope composition (7). Indeed, isotopic measurements have proven to be instrumental in identifying and quantifying sources, processes affecting the formation of atmospheric nitrate [particulate NO 3 − plus gas-phase nitric acid (HNO 3 ); hereafter defined as NO 3 − ], and the role of its precursors (i.e., the complex chemical interactions between NO x , halogens, and ozone...