About 10 years ago, reviews of the use of stable isotopes in animal ecology predicted explosive growth in this field and called for laboratory experiments to provide a mechanistic foundation to this growth. They identified four major areas of inquiry: (1) the dynamics of isotopic incorporation, (2) mixing models, (3) the problem of routing, and (4) trophic discrimination factors. Because these areas remain central to isotopic ecology, we use them as organising foci to review the experimental results that isotopic ecologists have collected in the intervening 10 years since the call for laboratory experiments. We also review the models that have been built to explain and organise experimental results in these areas.

Isotopic ecology ten years after a call for more laboratory experiments.

Isotopic ecology ten years after a call for more laboratory experiments. Biol Rev

Temperate forests of North America are thought to be significant sinks of atmospheric CO2. We developed a below-ground carbon (C) budget for well-drained soils in Harvard Forest Massachusetts, an ecosystem that is storing C. Measurements of carbon and radiocarbon ( 14 C) inventory were used to determine the turnover time and maximum rate of CO2 production from heterotrophic respiration of three fractions of soil organic matter (SOM): recognizable litter fragments (L), humified low density material (H), and high density or mineral-associated organic matter (M). Turnover times in all fractions increased with soil depth and were 2-5 years for recognizable leaf litter, 5-10 years for root litter, 40-100+ years for low density humified material and >100 years for carbon associated with minerals. These turnover times represent the time carbon resides in the plant + soil system, and may underestimate actual decomposition rates if carbon resides for several years in living root, plant or woody material. Soil respiration was partitioned into two components using 14 C: recent photosynthate which is metabolized by roots and microorganisms within a year of initial fixation (Recent- C), and C that is respired during microbial decomposition of SOM that resides in the soil for several years or longer (Reservoir-C). For the whole soil, we calculate that decomposition of Reservoir-C contributes approximately 41% of the total annual soil respiration. Of this 41%, recognizable leaf or root detritus accounts for 80% of the flux, and 20% is from the more humified fractions that dominate the soil carbon stocks. Measurements of CO 2 and 14 CO2 in the soil atmosphere and in total soil respiration were combined with surface CO2 fluxes and a soil gas diffusion model to determine the flux and isotopic signature of C produced as a function of soil depth. 63% of soil respiration takes place in the top 15 cm of the soil (O + A + Ap horizons). The average residence time of Reservoir-C in the plant + soil system is 81 years and the average age of carbon in total soil respiration (Recent-C + Reservoir-C) is 41 years. The O and A horizons have accumulated 4.4 kgC m -- 2 above the plow layer since abandon- ment by settlers in the late-1800's. C pools contributing the most to soil respiration have short enough turnover times that they are likely in steady state. However, most C is stored as humified organic matter within both the O and A horizons and has turnover times from 40 to

https://escholarship.org/content/qt0k7844v7/qt0k7844v7.pdf

Soil carbon cycling in a temperate forest: radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes

Physical constraints affecting bacterial habitats and activity in unsaturated porous media – a review

. Soil respiration – RS, the flux of CO2 from the soil to the atmosphere – is probably the least well constrained component of the terrestrial carbon cycle. Here we introduce the SRDB database, a near-universal compendium of published RS data, and make it available to the scientific community both as a traditional static archive and as a dynamic community database that may be updated over time by interested users. The database encompasses all published studies that report one of the following data measured in the field (not laboratory): annual RS, mean seasonal RS, a seasonal or annual partitioning of RS into its sources fluxes, RS temperature response (Q10), or RS at 10 °C. Its orientation is thus to seasonal and annual fluxes, not shorter-term or chamber-specific measurements. To date, data from 818 studies have been entered into the database, constituting 3379 records. The data span the measurement years 1961–2007 and are dominated by temperate, well-drained forests. We briefly examine some aspects of the SRDB data – its climate space coverage, mean annual RS fluxes and their correlation with other carbon fluxes, RS variability, temperature sensitivities, and the partitioning of RS source flux – and suggest some potential lines of research that could be explored using these data. The SRDB database is available online in a permanent archive as well as via a project-hosting repository; the latter source leverages open-source software technologies to encourage wider participation in the database's future development. Ultimately, we hope that the updating of, and corrections to, the SRDB will become a shared project, managed by the users of these data in the scientific community.

/pdf/a-global-database-of-soil-respiration-data-42g29ls8e4.pdf

A global database of soil respiration data

Newly available gas analyzers based on off-axis integrated cavity output spectroscopy (OA-ICOS) lasers have been advocated as an alternative to conventional isotope-ratio mass spectrometers (IRMS) for the stable isotopic analysis of water samples In the case of H2O, OA-ICOS is attractive because it has comparatively low capital and maintenance costs, the instrument is small and field laboratory portable, and provides simultaneous D/H and 16O/18O ratio measurements directly on H2O molecules with no conversion of H2O to H2, CO, or H2/CO2−water equilibration required Here we present a detailed assessment of the performance of a liquid-water isotope analyzer, including instrument precision, estimates of sample memory and sample mass effects, and instrumental drift We provide a recommended analysis procedure to achieve optimum results using OA-ICOS Our results show that, by using a systematic sample analysis and data normalization procedure routine, measurement accuracies of ± 08‰ for δD and ±01‰ δ18O ar

High-Precision Laser Spectroscopy D/H and 18O/16O Measurements of Microliter Natural Water Samples

A new H2O(liquid)−H2O(vapor) pore water equilibration and laser spectroscopy method provides a fast way to obtain accurate high resolution δD and δ18O profiles from single core samples from saturated and unsaturated geologic media. The precision and accuracy of the H2O(liquid−H2O(vapor) equilibration method was comparable to or better than conventional IRMS-based methods, and it can be conducted on geologic cores that contain volumetric water contents as low as 5%. Significant advantages of the H2O(liquid)−H2O(vapor) pore water equilibration method and laser isotopic analysis method include dual hydrogen- and oxygen-isotope assays on single small core samples, low consumable and instrumentation costs, and the potential for field-based hydrogeologic profiling. A single core is sufficient to obtain detailed vertical isotopic depth profiles in geologic, soil, and lacustrine pore water, dramatically reducing the cost of obtaining pore water by conventional wells or physical water extraction methods. In additi...

High Resolution Pore Water δ2H and δ18O Measurements by H2O(liquid)−H2O(vapor) Equilibration Laser Spectroscopy

Isotope hydrology of precipitation, surface and ground waters in the Okanagan Valley, British Columbia, Canada

Characterization of aqueous lead removal by phosphatic clay: equilibrium and kinetic studies.

Transport of the bacteria Klebsiella oxytoca and Burkholderia cepacia G4PR1 (G4PR1) was investigated in column experiments conducted under conditions that allowed us to quantify sorption under a range of ground water velocities. Column experiments (33 mm I.D. × 114 mm long columns) were conducted at four linear water velocities (0.5 to 14 cm.hr−1) through a medium to coarse grained silica sand. The peak C/Co concentrations for both bacteria were attenuated with respect to a conservative tracer (Cl−), and well-defined tailing was observed. Breakthroughs of both bacteria were influenced by the water velocity. In the case of G4PR1, the attenuation of the peak C/Co concentrations increased as the velocity decreased while the peak C/Co concentrations for K. oxytoca were similar at velocities between 3 and 13 cmhr−1 but decreased at the lowest velocity tested (0.6 cm-hr−1). The tailing reached constant C/Co values of between 2 × 10−3 and 5 × 10−3, and between 2 × 10−5 and 5 × 10−5 for K. oxytoca and G4PR1 after 2t0. A one-dimensional mathematical model for advective-dispersive transport that accounts for irreversible (kirr) and reversible (kf and kr) sorption was used to quantify the sorption process. Both irreversible and reversible sorption was required to obtain good fits to the measured K. oxytoca data. Results of this modeling suggested that kirr and kr are independent of velocity and an empirical relationship was developed relating kf to velocity. For G4PR1, the best fits were obtained using only reversible sorption. Results of the modeling suggested that kf was independent of velocity at all velocities tested and kr was independent of velocity at velocities between 3 and 13 cmhr−1. At the lowest velocity investigated (0.5 cmhr−1), the kf value decreased considerably. This study showed that sorption characteristics are bacteria specific, and are likely related to surface chemistry because G4PR1 is more hydrophobic than K. oxytoca. The study also showed that in order for bacterial transport experiments to be directly applicable to the subsurface, they should be conducted at velocities similar to those observed in the subsurface, or the relationship between the sorption parameter(s) and velocity should be known.

M. J. Hendry

Papers

High-Precision Laser Spectroscopy D/H and 18O/16O Measurements of Microliter Natural Water Samples

High Resolution Pore Water δ2H and δ18O Measurements by H2O(liquid)−H2O(vapor) Equilibration Laser Spectroscopy

Isotope hydrology of precipitation, surface and ground waters in the Okanagan Valley, British Columbia, Canada

Characterization of aqueous lead removal by phosphatic clay: equilibrium and kinetic studies.

Effects of Velocity on the Transport of Two Bacteria Through Saturated Sand