The Apparent Respiratory Quotient of Soils and Tree Stems and the Processes That Control It

The CO2/O2 fluxes ratio (apparent respiration quotient [ARQ]) measured in soils and plants contains valuable information about the respiratory-substrate stoichiometry and biotic and abiotic non-respiratory processes. We investigated ARQ variability by measurements in soil pore space air, and in headspace air from incubations of bulk-soil and tree stem tissues (both fresh and 24-hr stored tissues) in 10 measurement campaigns over 15 months in a Mediterranean oak forest. Mean (range) ARQ values were: soil air, 0.76 (0.60–0.92); bulk soil, 0.75 (0.53–0.90); fresh stem tissues, 0.39 (0.19–0.70); and stored stem tissues, 0.68 (0.42–1.08). The variability in tree stems was assumed to be controlled by CO2 re-fixation that lowered ARQ from 1.0, the value expected for carbohydrate respiration in plants. We estimate that the values of the stored tissues represent better stem metabolism since the fresh-tissue results contained a signal of wound-response O2 uptake that further lowered ARQ. The mean bulk-soil ARQ (0.75) was considerably lower than expected by soil organic matter (SOM) stoichiometry (0.95). This lower value might represent the stoichiometry of the SOM sub-pool that supports respiration, and/or oxidative depolymerization that increases O2 fluxes. Abiotic O2 uptake was demonstrated to reduce bulk-soil ARQ down to 0.37 and consume Fe2+, but estimated to have small effect under typical respiration rates. Soil-air ARQ was usually higher than bulk-soil ARQ and lower than root ARQ (which, when measured, ranged from 0.73 to 0.96), demonstrating the potential of ARQ to partition the autotrophic and heterotrophic sources of soil respiration. The limitations of this partitioning method are discussed

An integrative approach to understanding microbial diversity: from intracellular mechanisms to community structure

Trade-offs have been put forward as essential to the generation and maintenance of diversity. However, variation in trade-offs is often determined at the molecular level, outside the scope of conventional ecological inquiry. In this study, we propose that understanding the intracellular basis for trade-offs in microbial systems can aid in predicting and interpreting patterns of diversity. First, we show how laboratory experiments and mathematical models have unveiled the hidden intracellular mechanisms underlying trade-offs key to microbial diversity: (i) metabolic and regulatory trade-offs in bacteria and yeast; (ii) life-history trade-offs in bacterial viruses. Next, we examine recent studies of marine microbes that have taken steps toward reconciling the molecular and the ecological views of trade-offs, despite the challenges in doing so in natural settings. Finally, we suggest avenues for research where mathematical modelling, experiments and studies of natural microbial communities provide a unique opportunity to integrate studies of diversity across multiple scales

Tropospheric carbonyl sulfide mass balance based on direct measurements of sulfur isotopes

Significance Assessment for large-scale photosynthesis-climate feedbacks is needed. Carbonyl sulfide (COS) is an emerging proxy for the terrestrial photosynthesis. This proxy is limited by uncertainties related to the magnitudes of COS sources and sinks. Here, we demonstrate measurement-based assessments for the isotopic signal of: tropospheric COS, marine and anthropogenic COS emissions, and the isotopic fractionation of COS by plant uptake. All of these resulted in an isotopic mass balance for the COS budget which gives an important constraint for its sources.</jats:p

Tropospheric carbonyl sulfide mass-balance based on direct measurements of sulfur isotopes

Abstract Carbonyl sulfide (COS) is the major long-lived sulfur bearing gas in the atmosphere and a promising proxy for terrestrial gross primary production (GPP; CO2 uptake). However, large uncertainties in estimating the relative magnitude of the COS sources and sinks limit this approach. Isotopic measurements have been suggested as a novel tool to constrain COS sources, yet such measurements are currently scarce. Here we present, for the first time, a complete data-based tropospheric COS isotopic mass balance, which allows improved partition of the sources. We found an isotopic (δ34S±SE) value of 13.9±0.1‰ (versus V-CDT standard) for the troposphere, with an isotopic seasonal cycle driven by plant uptake. This seasonality agrees with a fractionation of -1.9±0.3‰ which we measured in plant-chamber experiments. Anthropogenic-influenced air samples indicated an anthropogenic COS isotopic signal of 8±1‰. Samples of seawater-equilibrated-air indicate that marine COS emissions have an isotopic signal of 13±0.4‰. Using our new data-based mass balance, we constrained the relative contribution of the two main tropospheric COS sources resulting in 26±11% for the anthropogenic source and 74±23% for the oceanic source. This constraint is important for a better understanding of the global COS budget and its improved use for GPP determination.</jats:p

Oxygen Isotope Signatures of Phosphate in Wildfire Ash

Atmospheric aerosol deposition is a significant source of phosphorus (P) in many terrestrial and marine ecosystems worldwide, influencing their biogeochemistry and primary production. Particles emitted from wildfires (hereafter, ash) are the second most important source of atmospheric P after airborne dust. In this study, we aim to identify the signature of ash oxygen isotopes in phosphate. This will enable the use of this signature for the separation of ash from other atmospheric P sources. We measured P concentrations and δ18OP in ash from natural and experimental fires and also from ash heated at different temperatures. The HCl and resin P concentrations (average ± SE) were 3.15 ± 0.35 and 1 ± 0.1 mg g–1, respectively. The HCl and resin δ18OP were 15.5 ± 0.4 and 14.7 ± 0.4‰ (average ± SE), respectively. Based on previous studies, we suggest possible isotope exchange reactions during the combustion process, between oxygen in phosphate and oxygen from other probable sources (i.e., the atmosphere, and CaCO3 and CaO formed in the ash). The unique isotopic signature in the ash, ranging from 11.5 to 19.4‰ in the HCl and resin P fractions, is different from that of other atmospheric P sources such as airborne tree pollen, which has δ18OP values between 19.2‰ and 29.6‰, and Saharan-dust samples collected in Israel, which have δ18OP values ranging from 20.7‰ to 22.6‰. Thus, the δ18OP can be used as a marker for identifying atmospheric P from wildfires and for estimating its importance to the global P cycle