Rapid quantification of biological nitrogen fixation using optical spectroscopy

Biological nitrogen fixation (BNF) provides a globally important input of nitrogen (N); its quantification is critical but technically challenging. Leaf reflectance spectroscopy offers a more rapid approach than traditional techniques to measure plant N concentration ([N]) and isotopes (δ15N). Here we present a novel method for rapidly and inexpensively quantifying BNF using optical spectroscopy. We measured plant [N], δ15N, and the amount of N derived from atmospheric fixation (Ndfa) following the standard traditional methodology using isotope ratio mass spectrometry (IRMS) from tissues grown under controlled conditions and taken from field experiments. Using the same tissues, we predicted the same three parameters using optical spectroscopy. By comparing the optical spectroscopy-derived results with traditional measurements (i.e. IRMS), the amount of Ndfa predicted by optical spectroscopy was highly comparable to IRMS-based quantification, with R2 being 0.90 (slope=0.90) and 0.94 (slope=1.02) (root mean square error for predicting legume δ15N was 0.38 and 0.43) for legumes grown in glasshouse and field, respectively. This novel application of optical spectroscopy facilitates BNF studies because it is rapid, scalable, low cost, and complementary to existing technologies. Moreover, the proposed method successfully captures the dynamic response of BNF to climate changes such as warming and drought.

Oral Cavity Cancer Tissues Differ in Isotopic Composition Depending on Location and Staging

The aim of this paper was to characterise the isotopic composition of oral squamous cell carcinoma (OSCC) specimens of different areas of the oral cavity. Secondly, we assessed whether there was a correlation between clinical stages of OSCC and isotopic abundance. The IRMS procedure was performed on 124 samples derived from 31 patients with OSCC of 15 N and 13 C to assess the isotopic composition. From each individual, four samples from the tumour, two from the margins, and two samples of healthy oral mucous membranes were derived. The two samples from the tumour and two samples from the margin were additionally subjected to histopathological assessment. Then, statistical analysis was conducted. Tumour infiltration tissues of the lower lip were characterised by higher mean δ13C values compared to samples derived from cancers of the other oral cavity regions (-23.82 ± 1.21 vs. -22.67 ± 1.35); (p = 0.04). The mean percentage of nitrogen content in tumour tissues was statistically higher in patients with the most advanced cancers (11.89 ± 0.03%) versus the group of patients with II and III stage cancers (11.12 ± 0.02%); (p = 0.04). In patients at stage IV, the mean δ13C value in the cancer samples equalled -22.69 ± 1.42 and was lower than that in patients at less severe clinical stages (p = 0.04). Lower lip cancer tissues differed in the isotopic abundance of carbon in comparison with tissues derived from the group of combined samples from other locations. Values of δ13C observed in specimens derived from lower lip cancers were similar to those observed in healthy oral mucous membranes. Cancer tissues obtained from patients in the last stage of OSCC had a different isotopic composition in comparison with those obtained from earlier stages. To confirm these observations, further research on larger groups of patients is needed.

Isotopic composition of serum zinc and copper in healthy children and children with autism spectrum disorder in North America

To better understand zinc and copper regulation and their involvement in various biochemical pathways as it relates to autism spectrum disorder (ASD), isotopic composition of serum zinc and copper were evaluated in both healthy children and children with ASD in North America. No significant difference in isotopic composition of serum zinc or copper with respect to healthy controls and ASD children were identified. However, the isotopic composition of serum copper in boys was found to be enriched in ⁶⁵Cu in comparison to previously published healthy adult copper isotopic composition. Furthermore, in both boys and girls, the average isotopic composition of serum zinc is heavier than previously published healthy adult isotopic zinc composition. There was also a negative association between total zinc concentrations in serum and the zinc isotopic composition of serum in boys. Finally, children with heavier isotopic composition of copper also showed a high degree of variability in their zinc isotopic composition. While numerous studies have measured the isotopic composition of serum zinc and copper in adults, this is one of the first studies which measured the isotopic composition of serum copper and zinc in children, specifically those diagnosed with ASD. The results of this study showed that age and gender specific normal ranges of isotopic composition must be established to effectively use isotopic composition analysis in studying various diseases including ASD.

Stable Isotopes Reveal Photoreduction of Particle-Bound Mercury Driven by Water-Soluble Organic Carbon during Severe Haze

Haze with high loading of particles may result in significant enrichment of particle-bound Hg (PBM), potentially impacting the atmospheric Hg transformation and transport. However, the dynamics of Hg transformation and the relative environmental effect during severe haze episodes remain unclear. Here, we report Hg isotopic compositions of atmospheric particles (PM_2.5, PM₁₀, and TSP) collected during a severe haze episode in Tianjin, China, to investigate the transformation and fate of Hg during haze events. All severe haze samples display significantly higher Δ¹⁹⁹Hg (up to 1.50‰) than global urban PBM, which cannot be explained by primary anthropogenic emissions. The high Δ¹⁹⁹Hg is likely caused by photoreduction of PBM promoted by water-soluble organic carbon (WSOC) during the particle accumulation period, as demonstrated by the positive correlations of Δ¹⁹⁹Hg with WSOC and relative humidity and confirmed by our laboratory-controlled photoreduction experiment. The results show that, on average, 21% of PBM are likely photoreduced and re-emitted back to the atmosphere as Hg(0), potentially requiring revision of atmospheric Hg budgeting and modeling. This study highlights the release of large portions of PBM back to the gas phase through photoreduction, which needs to be taken into account while evaluating the atmospheric Hg cycle and the relative ecological effects.

Raman Natural Gas Analyzer: Effects of Composition on Measurement Precision

Raman spectroscopy is a promising method for analyzing natural gas due to its high measurement speed and the potential to monitor all molecular components simultaneously. This paper discusses the features of measurements of samples whose composition varies over a wide range (0.005-100%). Analysis of the concentrations obtained during three weeks of experiments showed that their variation is within the error caused by spectral noise. This result confirms that Raman gas analyzers can operate without frequent calibrations, unlike gas chromatographs. It was found that a variation in the gas composition can change the widths of the spectral lines of methane. As a result, the measurement error of oxygen concentration can reach 200 ppm. It is also shown that neglecting the measurement of pentanes and n-hexane leads to an increase in the calculated concentrations of other alkanes and to errors in the density and heating value of natural gas.

Unraveling the Influence of Land-Use Change on δ13C, δ15N, and Soil Nutritional Status in Coniferous, Broadleaved, and Mixed Forests in Southern China: A Field Investigation

Natural isotopic abundance in soil and foliar can provide integrated information related to the long-term alterations of carbon (C) and nitrogen (N) cycles in forest ecosystems. We evaluated total carbon (TC), total nitrogen (TN), and isotopic natural abundance of C (δ¹³C) and N (δ¹⁵N) in soil and foliar of coniferous plantation (CPF), natural broadleaved forest (NBF), and mixed forest stands at three different soil depths (i.e., 0–10, 10–20, and 20–40 cm). This study also explored how soil available nutrients are affected by different forest types. Lutou forest research station, located in Hunan Province, central China, was used as the study area. Results demonstrated that the topsoil layer had higher TC and TN content in the mixed forest stand, resulting in a better quality of organic materials in the topsoil layer in the mixed forest than NBF and CPF. In general, soil TC, TN, and δ¹⁵N varied significantly in different soil depths and forest types. However, the forest type did not exhibit any significant effect on δ¹³C. Overall, soil δ¹³C was significantly enriched in CPF, and δ¹⁵N values were enriched in mixed forest. Foliar C content varied significantly among forest types, whereas foliar N content was not significantly different. No big differences were observed for foliar δ¹⁵N and δ¹³C across forest types. However, foliar δ¹³C and δ¹⁵N were positively related to soil δ¹³C and δ¹⁵N, respectively. Foliar N, soil and foliar C:N ratio, soil moisture content (SMC), and forest type were observed as the major influential factors affecting isotopic natural abundance, whereas soil pH was not significantly correlated. In addition, forest type change and soil depth increment had a significant effect on soil nutrient availability. In general, soil nutrient availability was higher in mixed forest. Our findings implied that forest type and soil depth alter TC, TN, and soil δ¹⁵N, whereas δ¹³C was only driven by soil depth. Moreover, plantations led to a decline in soil available nutrient content compared with NBF and mixed forest stands.

Extraction and Analysis of Available Boron Isotopes in Soil Using Multicollector Inductively Coupled Plasma Mass Spectrometry

As a result of the important roles of boron (B) in the growth of plants, the uptake of B by plants is dependent upon the existing form and content of available B in soil, which can bring about the local cycle of B isotope equilibrium. A method using water-heating extraction combined with three-step ion-exchange chromatography was developed for the extraction and isotopic analysis of available B in soil. The extraction efficiency and fractionation of B isotopic composition in the procedure were investigated. The results showed that, in the upper layers of soils, the change of δ¹¹B values was opposite that of the mass concentration and a similar variation between δ¹¹B and content occurred in the lower layers. The isotope of available B in soil can create a featured isotopic signature to further understand the geochemical details related to the soil properties and molecular mechanism of B uptake in plants.

Isotopic Heft on the B1 l Silent Mode in Ultra-Narrow Gallium Nitride Nanowires

Wurtzite semiconductor compounds have two silent modes, B_{1 l} and B_{1 h}. A silent mode is a vibrational mode that carries neither a dipole moment nor Raman polarizability. Thus, they are forbidden in both infrared reflectivity and Raman spectroscopy. Astonishingly, we detected the B_{1 l} mode in high-quality, ultra-narrow GaN nanowires using resonant Raman scattering, although the B_{1 h} was not observed, and there is no immediate explanation for this asymmetric finding. The Raman experiments were performed using several laser lines from 647 to 325 nm; the latter is a wavelength in which Raman becomes resonant. Actually, we observed the B_{1 l} mode only in resonance, indicating that the appearance of this mode is related to Fröhlich electron-phonon interactions; i.e., a dipole moment emerging in the B_{1 l} silent mode may not be present in the B_{1 h} mode. To shed light onto the physical origin of these observations, we performed density functional theory calculations of the lattice dynamics in GaN. We performed a careful analysis of the different physical mechanisms that allow the forbidden mode to appear to explain the physics underlying the nonzero dipole moment in the B_{1 l} mode, and the reason why this dipole moment is not present in the B_{1 h} mode.

[Isotope analysis of ammonium and nitrate: A review on measured methods and their application]

In the past several decades, a variety of methods have been developed for measuring the isotopic composition of ammonium (δ¹⁵N) and nitrate (δ¹⁵N and δ¹⁸O). This review summarized the advantages and disadvantages of these methods. Nowadays, the most popular method for measu-ring δ¹⁵N of ammonium is the combined hypobromite (BrO^-) and hydroxylamine (NH₂OH) me-thod, while for δ¹⁵N and δ¹⁸O of nitrate is the denitrifier method and the sodium azide (NaN₃) me-thod. These methods convert NH₄⁺ or NO₃^- into nitrous oxide (N₂O) and measure its isotopic compositions, with higher analytical precision because of the lower background concentration of atmospheric N₂O. Accordingly, these methods are suitable for the samples with lower N concentration, and normally require 10-60 nmol N. The development of new methods for measuring N isotopic composition has greatly stimulated the studies in nitrogen cycling worldwide.

Isotopic insights into methane production, oxidation, and emissions in Arctic polygon tundra

Arctic wetlands are currently net sources of atmospheric CH4 . Due to their complex biogeochemical controls and high spatial and temporal variability, current net CH4 emissions and gross CH4 processes have been difficult to quantify, and their predicted responses to climate change remain uncertain. We investigated CH4 production, oxidation, and surface emissions in Arctic polygon tundra, across a wet-to-dry permafrost degradation gradient from low-centered (intact) to flat- and high-centered (degraded) polygons. From 3 microtopographic positions (polygon centers, rims, and troughs) along the permafrost degradation gradient, we measured surface CH4 and CO2 fluxes, concentrations and stable isotope compositions of CH4 and DIC at three depths in the soil, and soil moisture and temperature. More degraded sites had lower CH4 emissions, a different primary methanogenic pathway, and greater CH4 oxidation than did intact permafrost sites, to a greater degree than soil moisture or temperature could explain. Surface CH4 flux decreased from 64 nmol m(-2)  s(-1) in intact polygons to 7 nmol m(-2)  s(-1) in degraded polygons, and stable isotope signatures of CH4 and DIC showed that acetate cleavage dominated CH4 production in low-centered polygons, while CO2 reduction was the primary pathway in degraded polygons. We see evidence that differences in water flow and vegetation between intact and degraded polygons contributed to these observations. In contrast to many previous studies, these findings document a mechanism whereby permafrost degradation can lead to local decreases in tundra CH4 emissions.

Linking soil, water, and honey composition to assess the geographical origin of argentinean honey by multielemental and isotopic analyses

The objective of this research was to investigate the development of a reliable fingerprint from elemental and isotopic signatures of Argentinean honey to assess its geographical provenance. Honey, soil, and water from three regions (Córdoba, Buenos Aires, and Entre Rı́os) were collected. The multielemental composition was determined by ICP-MS. δ(13)C was measured by isotopic ratio mass spectrometry, whereas the (87)Sr/(86)Sr ratio was determined using thermal ionization mass spectrometry. The data were analyzed by chemometrics looking for the association between the elements, stable isotopes, and honey samples from the three studied areas. Honey samples were differentiated by classification trees and discriminant analysis using a combination of eight key variables (Rb, K/Rb, B, U, (87)Sr/(86)Sr, Na, La, and Zn) presenting differences among the studied regions. The application of canonical correlation analysis and generalized procrustes analysis showed 91.5% consensus between soil, water, and honey samples, in addition to clear differences between studied areas. To the authors' knowledge, this is the first report demonstrating the correspondence between soil, water, and honey samples using different statistical methods, showing that elemental and isotopic honey compositions are related to soil and water characteristics of the site of origin.