The mechanism of oxygen isotope fractionation during N2O production by denitrification

The isotopic composition of soil-derived N2O can help differentiate between N2O production pathways and estimate the fraction of N2O reduced to N2. Until now, δ18O of N2O has been rarely used in the interpretation of N2O isotopic signatures because of the rather complex oxygen isotope fractionations during N2O production by denitrification. The latter process involves nitrate reduction mediated through the following three enzymes: nitrate reductase (NAR), nitrite reductase (NIR) and nitric oxide reductase (NOR). Each step removes one oxygen atom as water (H2O), which gives rise to a branching isotope effect. Moreover, denitrification intermediates may partially or fully exchange oxygen isotopes with ambient water, which is associated with an exchange isotope effect. The main objective of this study was to decipher the mechanism of oxygen isotope fractionation during N2O production by denitrification and, in particular, to investigate the relationship between the extent of oxygen isotope exchange with soil water and the δ18O values of the produced N2O. We performed several soil incubation experiments. For the first time, ∆17 O isotope tracing was applied to simultaneously determine the extent of oxygen isotope exchange and any associated oxygen isotope effect. We found bacterial denitrification to be typically associated with almost complete oxygen isotope exchange and a stable difference in δ18O between soil water and the produced N2O of δ18O(N2O / H2O) = (17.5±1.2) ‰. However, some experimental setups yielded oxygen isotope exchange as low as 56 % and a higher δ18O(N2O / H2O) of up to 37‰. The extent of isotope exchange and δ18O(N2O / H2O) showed a very significant correlation (R2 = 0.70, p < 0.00001). We hypothesise that this observation was due to the contribution of N2O from another production process, most probably fungal denitrification. An oxygen isotope fractionation model was used to test various scenarios with different magnitudes of branching isotope effects at different steps in the reduction process. The results suggest that during denitrification the isotope exchange occurs prior to the isotope branching and that the mechanism of this exchange is mostly associated with the enzymatic nitrite reduction mediated by NIR. For bacterial denitrification, the branching isotope effect can be surprisingly low, about (0.0±0.9) ‰; in contrast to fungal denitrification where higher values of up to 30‰ have been reported previously. This suggests that δ18O might be used as a tracer for differentiation between bacte- 5 rial and fungal denitrification, due to their different magnitudes of branching isotope effect

Oxygen isotope fractionation during N2O production by soil denitrification

The isotopic composition of soil-derived N₂O can help differentiate between N₂O production pathways and estimate the fraction of N₂O reduced to N₂. Until now, <i>δ</i>¹⁸O of N₂O has been rarely used in the interpretation of N₂O isotopic signatures because of the rather complex oxygen isotope fractionations during N₂O production by denitrification. The latter process involves nitrate reduction mediated through the following three enzymes: nitrate reductase (NAR), nitrite reductase (NIR) and nitric oxide reductase (NOR). Each step removes one oxygen atom as water (H₂O), which gives rise to a branching isotope effect. Moreover, denitrification intermediates may partially or fully exchange oxygen isotopes with ambient water, which is associated with an exchange isotope effect. The main objective of this study was to decipher the mechanism of oxygen isotope fractionation during N₂O production by soil denitrification and, in particular, to investigate the relationship between the extent of oxygen isotope exchange with soil water and the <i>δ</i>¹⁸O values of the produced N₂O. <br><br> In our soil incubation experiments Δ¹⁷O isotope tracing was applied for the first time to simultaneously determine the extent of oxygen isotope exchange and any associated oxygen isotope effect. We found that N₂O formation in static anoxic incubation experiments was typically associated with oxygen isotope exchange close to 100 % and a stable difference between the ¹⁸O ∕ ¹⁶O ratio of soil water and the N₂O product of <i>δ</i>¹⁸O(N₂O ∕ H₂O)  =  (17.5 ± 1.2) ‰. However, flow-through experiments gave lower oxygen isotope exchange down to 56 % and a higher <i>δ</i>¹⁸O(N₂O ∕ H₂O) of up to 37 ‰. The extent of isotope exchange and <i>δ</i>¹⁸O(N₂O ∕ H₂O) showed a significant correlation (<i>R</i>² = 0.70, <i>p</i> &lt;  0.00001). We hypothesize that this observation was due to the contribution of N₂O from another production process, most probably fungal denitrification. <br><br> An oxygen isotope fractionation model was used to test various scenarios with different magnitudes of branching isotope effects at different steps in the reduction process. The results suggest that during denitrification, isotope exchange occurs prior to isotope branching and that this exchange is mostly associated with the enzymatic nitrite reduction mediated by NIR. For bacterial denitrification, the branching isotope effect can be surprisingly low, about (0.0 ± 0.9) ‰, in contrast to fungal denitrification where higher values of up to 30 ‰ have been reported previously. This suggests that <i>δ</i>¹⁸O might be used as a tracer for differentiation between bacterial and fungal denitrification, due to their different magnitudes of branching isotope effects

El impuesto predial y la recaudación de impuestos en la Municipalidad Distrital de Shupluy, 2020

El objetivo principal del presente estudio fue: analizar el impuesto predial y los factores que dañan las recaudaciones de impuestos en la Municipalidad distrital de Shupluy, 2020. El tipo de estudio fue descriptivo y de diseño no experimental de corte transversal fue el diseño, en el cual la población estuvo conformado por 30 personas colaboradoras y para nuestra muestra se consideraron igual que la población a las 30 personas colaboradoras que son los propietarios de los predios o contribuyentes del impuesto. La técnica empleada fue la encuesta y su instrumento el cuestionario que fue empleado. Los resultados obtenidos con este estudio han sido muy útiles por que resolvió las dificultades y problemas en las recaudaciones de los impuestos prediales en el municipio distrital de Shupluy. Las causas que afectan las recaudaciones de impuesto en el municipio distrital de Shupluy, 2020, podemos concluir que se ve afectado la recaudación de impuestos debido a un alto porcentaje de contribuyentes morosos en los pagos de los impuestos prediales, como se puede apreciar en nuestros resultados de la tabla N°3 donde se muestra que el 70% manifestó que el nivel de mora de los pagos de los impuestos prediales es mayor a los demás impuestos municipales y según nuestro objetivo 2 la situación actual respecto a la recaudación de impuestos según nuestros resultados en la tabla N° 17 de los encuestados un 70% de los encuestados manifestaron que la población no conoce acerca del impuesto de alcabala por ellos los contribuyentes no realizan los pagos debidos.Tesi

Lacustrine stromatolites as multi-scale recorders of climate change: Insights from the Miocene Ebro Basin

Sedimentological, δ13C and δ18O data from stromatolites in a lower and middle Miocene sequence from the Ebro Basin (N Spain) are used to assess the potential of ancient lacustrine stromatolite lamination as an archive of palaeoenvironmental and palaeoclimatic change. The isotopic evolution through the studied sequence supports a general trend toward less saline conditions with time. Stromatolites and muddy-grainy laminated limestones developed in lake water that underwent little renewal, compared with other carbonate facies. The palaeoclimatic value of the stable-isotope changes and concurrent textural variations in calcite stromatolite lamination is studied at different orders of cyclicity. Stromatolite lamination consists of simple laminae (dark dense, light dense and light porous) grouped into alternating composite light and dark laminae. δ13C and δ18O analyses in consecutive composite laminae (bulk sampling) yielded a cyclic pattern that mimics textural variations. Light laminae, with lower δ13C and δ18O values, reflect higher precipitation/evaporation ratio (P/E) and more influence of biogenic 12C. Dark laminae, with higher δ13C and δ18O values, reflect drier conditions, more complete atmospheric CO2 exchange with water and photosynthetic 12CO2 uptake. Textural features of laminae support these results: the dark laminae are related to higher calcite saturation in lake water during drier periods. Isotopic values from high-resolution sampling through a 2.1-cm thick stromatolite reveal palaeoclimate variations at different temporal scales. Isotopic variation in 3rd order cyclicity of alternating light/dark simple laminae is recording seasonal P/E variations. Light and dark composite laminae (2nd order cyclicity) correspond to pluriannual dominantly-humid or -dry conditions, respectively. A gradual succession from light to dark composite laminae forms the 1st order cycles driven by decreasing P/E through longer pluriannual periods, resulting in lake level lowering. The stromatolites are thus recording lake level changes of centennial to millennial scale

Spatial variations in snowpack chemistry and isotopic composition of NO3- along a nitrogen deposition gradient in West Greenland

Snowpack chemistry, nitrate stable isotopes and net deposition fluxes for the largest ice-free region in Greenland were investigated to determine whether there are spatial gradients from the ice sheet margin to the coast linked to a gradient in precipitation. Late-season snowpack was sampled in March 2011 at 8 locations within 3 lake catchments in each of 3 regions (ice sheet margin in the east, central area near Kelly Ville and the coastal zone to the west). At the coast, snowpack accumulation averaged 181 mm snow water equivalent (SWE), compared with 36 mm SWE by the ice sheet. Coastal snowpack showed significantly greater concentrations of marine salts (Na+, Cl−, other major cations), ammonium (regional means 1.4–2.7 µmol L−1), total and non-sea salt sulfate (total 1.8–7.7, non-sea salt 1.0–1.8 µmol L−1) than the two inland regions. Nitrate (1.5–2.4 µmol L−1) showed significantly lower concentrations at the coast. Despite lower concentrations, higher precipitation at the coast results in a strong deposition gradient for NO3− as well as NH4+ and non-sea salt sulfate (nss-SO42−) increasing from the inland regions to the coast (lowest at Kelly Ville 6, 4 and 3; highest at coast 9, 17 and 11 mol ha−1 yr−1 of NO3−, NH4+ and nss-SO42− respectively). The δ(15N) of snowpack NO3− shows a significant decrease from the ice sheet margin (−7.5 ‰) to the coast (−11.3 ‰). We attribute the spatial gradient of δ(15N) in SW Greenland to post-deposition processing rather than differing sources because of (1) the climatic gradient from ice sheet margin to coast, (2) within-catchment isotopic differences between terrestrial snowpack and lake-ice snowpack, and (3) similarities between fresh snow (rather than accumulated snowpack) at Kelly Ville and the coast. Hence the δ(15N) of coastal snowpack is most representative of snowfall in SW Greenland, but after deposition the effects of photolysis, volatilization and sublimation lead to enrichment of the remaining snowpack with the greatest effect in inland areas of low precipitation and high sublimation losses

Local-scale environmental gradients in ‘snail-shell’ stable isotopes from Holocene Jordanian archaeological sites

Reconstructing environments around archaeological sites is complicated by past land management practices and regional-scale climate proxies that can be contradictory and are often located at a distance from the sites themselves. Here we explore environmental information from fossil snail shells which, even when few in number on an archaeological site, may prove invaluable in constructing site-specific data. The palaeoecology of fossil snails and the stable isotopic composition of their shell carbonate can provide context-specific information on vegetation, water availability, and relative humidity during the occupation of a site. We studied terrestrial and aquatic snails from two later Neolithic archaeological sites in the Jordanian badia, Wadi al-Qattafi and Wisad Pools. At specific archaeological site-scale our study highlights the importance of aquatic snails in the reconstruction of semi-arid environments. At Wisad pools rare aquatic snails in contexts dating between ~8.0 and ~7.6 ka demonstrate episodes of wetness; moreover, their shell isotopic compositions indicate that local watercourses were well established, corroborating previous findings that during this period the immediate environs of Wisad Pools were host to C3 plant species more typical of the Mediterranean zone. Moreover, the δ18O signal in these snail shells allow tentative reconstruction of rainwater isotopic compositions and identify the effects of evaporation. Such fine-grained environmental information is much less evident from the terrestrial snail shell data alone, showing that an ensemble of snail shell data can be highly sensitive to environmental differentials across an archaeological site. Finally, at a regional palaeoclimate-scale our Wisad Pools snail shell stable isotope data are consistent with a sustained, Rapid Climate Change (RCC)-driven wetness between 8.6 and 7.6 ka concurrent with cold and wet conditions in the wider Levant

Different flavours of oxygen help quantify seasonal variations of the biological carbon pump in the Celtic Sea

Shelf seas represent only 10% of the World’s Ocean by area but support up to 30% of its primary production. There are few measurements of biological production at high spatial and temporal resolution in these physically and biologically dynamic systems. Here, we use dissolved oxygen to-argon (O2/Ar) ratios and oxygen triple isotopes in O2 (16O, 17O, 18O) to estimate net community production, N(O2/Ar), and gross O2 production, G(17O), in summer and autumn 2014 and spring and summer 2015 in the Celtic Sea, as part of the UK Shelf-Sea Biogeochemistry Programme. Surface O2/Ar concentration ratios were measured continuously using a shipboard membrane inlet mass spectrometer. Additional depth profiles of O2/Ar concentration ratios, δ(17O) and δ(18O) were measured in discrete water samples from hydrocasts. The data were combined with wind-speed based gas exchange parameterisations to calculate biological air-sea oxygen fluxes. These fluxes were corrected for diapycnal diffusion, entrainment, production below the mixed layer, and changes over time to derive N(O2/Ar) and G(17O). The Celtic Sea showed the highest G(17O) in summer 2014 (825 mmol m–2 d–1) and lowest during autumn 2014 (153 mmol m–2 d–1). N(O2/Ar) was highest in spring 2015 (43 mmol m–2 d–1), followed by summer 2014 (42 mmol m–2 d–1), with a minimum in autumn 2014 (–24 mmol m–2 d–1). Dividing the survey region into three hydrographically distinct areas (Celtic Deep, Central Celtic Sea and Shelf Edge), we found that Celtic Deep and Shelf Edge had higher N(O2/Ar) in summer (71 and 63 mmol m–2 d–1, respectively) than in spring (49 and 22 mmol m–2 d–1). This study shows regional differences in the metabolic balance within the same season, as well as higher net community production in summer than in spring in some areas and years. The seasonal patterns in biological production rates and the export efficiency (f-ratio) identified the importance of biology for supporting the Celtic Sea’s ability to act as a net CO2 sink. Our measurements thus help improve our understanding of the biological carbon pump in temperate shelf seas

Seasonal temperatures from δ18O in Recent Spanish tufa stromatolites: Equilibrium redux!

This study focuses on recent debate over the value of stable-isotope-based environmental proxies recorded in riverine tufa-stromatolites. We recovered a 12-year record (1999-2012) of river-bed tufa stromatolites in the River Piedra (NE Spain) along with a partly overlapping 15-year record (1994 to 2009) of accumulations in a drainage pipe: both deposits formed in water with near identical physico/chemical parameters. Measured water temperature (Tw) data and near constant δ18Owater composition allowed selection of an ‘equilibrium’ palaeotemperature equation that best replicated actual temperatures. We found, as other have, that both the Epstein et al. (1951) and Kim & O’Neil (1997) formulas for Tw calculation from equilibrium calcite δ18O compositions were appropriate for the River Piedra where tufa deposition rates are high, means between 5.6 and 10.8 mm in six months. δ18Ocalcite in both the river and the pipe deposits record essentially the full actual seasonal Tw range. Only the coldest times (Tw < 10ºC), when calcite precipitation mass decreased to minimum, are likely to be unrepresented, an effect most noticeable in the pipe where depositional masses are smaller and below sample resolution. While we cannot rule out kinetic effects on δ18Ocalcite-based calculated Tw, the good fit between measured Tw and δ18Ocalcite-calculated Tw indicates that temperature is the principal control. Textural and deposition rate variability between the river and pipe settings are caused by differences in flow velocity and illumination. In the river, calcification of growing cyanobacterial mat occurred throughout the year, producing composite dense and porous laminae, whereas in the pipe, discontinuous cyanobacterial growth in winter promoted more abiogenic calcification. High-resolution δ18Ocalcite data from synchronous pipe and river laminae show that reversals in Tw occur within laminae, not at lamina boundaries, a pattern consistent with progressive increase in calcite precipitation rate as cyanobacterial growth re-established in spring

Holocene stable isotope record of insolation and rapid climate change in a stalagmite from the Zagros of Iran

We explore Holocene climatic changes recorded by geochemical proxies in a single, well-dated, stalagmite from the northern Zagros Mountains of Iran, a region where stalagmite records have so far only provided short glimpses of Holocene climatic changes. Stalagmite KT-3 from Katalekhor Cave began growing ~9.5 ka under wet early Holocene conditions coincident with the timing of Sapropel 1. At this time d18O values were at or below 9.0‰, stalagmite growth diameter was at its maximum, 234U/238U0 activity values were low and trace element contents indicate flushing of the epikarst. Progressive reduction in winter precipitation amount after 7.0 ka, is recorded by increasing d18O and 234U/238U0 activity values and reduction in trace element contents and growth diameter until ~2.0 ka. These trends follow the reduction in summer insolation and agree with model-simulated changes in total rainfall and rainwater isotopes. Sub-centennial-scale variability is not a feature of the d18O or trace element records; this suggests a stable winter recharge regime, a feature that might militate against KT-3 d18O recording changes in the seasonality of rainfall. KT-3 d13C compositions are enriched relative to lower altitude stalagmites in the Levant, implying low soil CO2 contribution (thin montane soils). However, a broadly decreasing millennial-scale trend in d13C suggests that soil carbon contributions increased with time despite the progressive reduction in winter precipitation amount. The d13C changes probably reflect decreasing summer temperatures and less extreme wintertime coldness, driven by reducing summer and increasing winter-insolation respectively. These combined effects decreased effective evaporation and improved soil-moisture availability favouring vegetation development during the growth season. The d13C values also show ~1.5‰ centennial-scale variability with higher d13C values between 9.0 and 8.7 ka, 8.3 - 7.7 ka, 6.5 - 5.5 ka, 5.4 - 4.5 ka and ~4.3- 2.0 ka: three of these correspond with Rapid Climate Change (RCC) events based on non-seasalt potassium (Kþ) in Greenland ice cores. Higher d13C values indicate poor soil development caused by aridity. The centennial-scale d13C anomaly (8.3-7.7 ka) is in part overprinted by the ~160 year-long, 8.2 ka cold/dry event, but culmination ~7.7 ka corresponds with other records suggesting an intensified Siberian High Pressure system affecting regional climate. The centennial-scale d13C anomaly between 4.3 and 2.0 ka overlaps the 2.65 to 2.50 ka ‘Assyrian megadrought’ evident in stalagmite stable isotope records in northern Iraq. The KT-3 record is key in better understanding Holocene climate change in the central Zagros region, representative of montane ‘fertile crescent’ environments

Hydrochemical and dual-isotope approach to the identification of denitrification in arable field drainage in the Wensum catchment, eastern England

The global pool of reactive nitrogen has doubled in the last century in response to the need to increase food production with the consequent increase in fertiliser-derived reactive nitrogen detrimentally affecting aquatic ecosystems. This study investigates the spatial distribution and significance of denitrification in the lowland, agriculturally-impacted River Wensum catchment in eastern England as a natural attenuation process. To investigate the evidence for denitrification, the hydrochemical characteristics and dual stable isotope composition of nitrate (15N and 18O) were measured over a 15-month period, 2015–2017, in 63 samples of field drainage in predominantly clay loam and sandy clay loam soils under mainly arable cultivation. Microbially-mediated denitrification in field drainage was indicated by the gradient of the linear regression of 15NNO3 and 18ONO3 compositions with a value of 0.58. Dual fractionation of the nitrate isotopes yielded enrichment factors for δ15NNO3 (−4.52‰) and δ18ONO3, (−4.51‰) within the reported ranges for denitrification in aquatic studies. Soil type influenced denitrification, with a positive relationship between percentage clay and δ15NNO3 and δ18ONO3 values. The same relationship was observed for denitrification rates calculated via a simple mass balance approach, which ranged from 11.0 to 26.3 kg N ha−1 and accounted for 30–73% of the leached soil nitrogen. Higher denitrification rates were recorded in drainage areas with a greater soil clay content (>20% by weight). Comparing calculated dentification rates for individual drain areas with median δ15NNO3 values of drain samples demonstrated that an isotopic enrichment of +1‰ is associated with a denitrification rate of 2.6 kg N ha−1. In conclusion, sustainable agricultural practices that maintain natural attenuation processes such as denitrification, for example by preserving and increasing the soil organic carbon content, are desirable to improve overall soil health to support ecosystem services that reduce nitrate pollution