Thomas A. Rahn

The biogeochemical controls of the δ15N and δ18O of N2O produced in landfill cover soils

We document an enrichment of both the δ18OAtm.O2 and δ15NAtm.N2 values of soil-derived N2O collected from landfill cover soils relative to tropospheric N2O. The isotopic values of N2O vary from −5.1‰ to +l9.4‰ and from +19.0‰ to +33.5‰ for δ 15NAtm.N2 and δ18OAtm.O2,, respectively. A tight linear correlation for δ18OAtm.O2 versus δ15NAtm.N2 is apparent, reflecting coupled microbial processes that produce N2O that may be isotopically enriched or depleted in relation to tropospheric N2O. Several explanations are provided to explain this correlation, including evidence for NH3 limitation during nitrification, which would be expected to diminish isotopic fractionation and consequently result in more enriched isotopic values of N2O. Desiccation effects on nitrification were also observed, which contribute to NH3 limitation and thus could influence the isotopic signature of N2O. Our results indicate that the N2O isotopic composition from soils may vary greatly depending on the season and soil moisture conditions and may at times be enriched in 15N and 18O relative to tropospheric N2O.

Understanding the Stable Isotope Composition of Biosphere‐Atmosphere CO2 Exchange

Stable isotopes of atmospheric carbon dioxide (CO2) contain a wealth of information regarding biosphere-atmosphere interactions. The carbon isotope ratio of CO2 (δ13C) reflects the terrestrial carbon cycle including processes of photosynthesis, respiration, and decomposition. The oxygen isotope ratio (δ18O) reflects terrestrial carbon and water coupling due to CO2-H2O oxygen exchange. Isotopic CO2 measurements, in combination with ecosystem-isotopic exchange models, allow for the quantification of patterns and mechanisms regulating terrestrial carbon and water cycles, as well as for hypothesis development, data interpretation, and forecasting. Isotopic measurements and models have evolved significantly over the past two decades, resulting in organizations that promote model-measurement networks, e.g., the U.S. National Science Foundations Biosphere-Atmosphere Stable Isotope Network, the European Stable Isotopes in Biosphere-Atmosphere Exchange Network, and the U.S. National Environmental Observatory Network.

Adaptation of continuous-flow cavity ring-down spectroscopy for batch analysis of δ13C of CO2 and comparison with isotope ratio mass spectrometry

Measurements of δ(13)C in CO(2) have traditionally relied on samples stored in sealed vessels and subsequently analyzed using magnetic sector isotope ratio mass spectrometry (IRMS), an accurate but expensive and high-maintenance analytical method. Recent developments in optical spectroscopy have yielded instruments that can measure δ(13)CO(2) in continuous streams of air with precision and accuracy approaching those of IRMS, but at a fraction of the cost. However, continuous sampling is unsuited for certain applications, creating a need for conversion of these instruments for batch operation. Here, we present a flask (syringe) adaptor that allows the collection and storage of small aliquots (20-30 mL air) for injection into the cavity ring-down spectroscopy (CRDS) instrument. We demonstrate that the adaptors precision is similar to that of traditional IRMS (standard deviation of 0.3‰ for 385 ppm CO(2) standard gas). In addition, the concentration precision (±0.3% of sample concentration) was higher for CRDS than for IRMS (±7% of sample concentration). Using the adaptor in conjunction with CRDS, we sampled soil chambers and found that soil-respired δ(13)C varied between two different locations in a piñon-juniper woodland. In a second experiment, we found no significant discrimination between the respiration of a small beetle (~5 mm) and its diet. Our work shows that the CRDS system is flexible enough to be used for the analysis of batch samples as well as for continuous sampling. This flexibility broadens the range of applications for which CRDS has the potential to replace magnetic sector IRMS.

Tree Mortality Decreases Water Availability and Ecosystem Resilience to Drought in Piñon‐Juniper Woodlands in the Southwestern U.S.

DOE Office of Science TES [SC DE-SC0008088]; NSF Ecosystems [NSF-DEB-1557176]; Pacific Northwest National Labs LDRD program; Los Alamos National Lab; College of Arts and Sciences at the University of New Mexico