Enhanced soil respiration in response to global warming may substantially increase atmospheric CO2 concentrations above the anthropogenic contribution, depending on the mechanisms underlying the temperature sensitivity of soil respiration. Here, we compared short-term and seasonal responses of soil respiration to a shifting thermal environment and variable substrate availability via laboratory incubations. To analyze the data from incubations, we implemented a novel process-based model of soil respiration in a hierarchical Bayesian framework. Our process model combined a Michaelis-Menten-type equation of substrate availability and microbial biomass with an Arrhenius-type nonlinear temperature response function. We tested the competing hypotheses that apparent thermal acclimation of soil respiration can be explained by depletion of labile substrates in warmed soils, or that physiological acclimation reduces respiration rates. We demonstrated that short-term apparent acclimation can be induced by substrate depletion, but that decreasing microbial biomass carbon (MBC) is also important, and lower MBC at warmer temperatures is likely due to decreased carbon-use efficiency (CUE). Observed seasonal acclimation of soil respiration was associated with higher CUE and lower basal respiration for summer- vs. winter-collected soils. Whether the observed short-term decrease in CUE or the seasonal acclimation of CUE with increased temperatures dominates the response to long-term warming will have important consequences for soil organic carbon storage.

Aim Plant root traits regulate belowground C inputs, soil nutrient and water uptake, and play critical roles in determining sustainable plant production and consequences for ecosystem C storage. However, the effects of elevated CO2 on root morphology and function have not been well quantified. We reveal general patterns of root trait responses to elevated CO2 from field manipulative experiments.Location North America, Europe, Oceania, Asia. MethodsThe meta-analysis approach was used to examine the effects of CO2 elevation on 17 variables associated with root morphology, biomass size and distribution, C and N concentrations and pools, turnover and fungal colonization from 110 published studies.Results Elevated CO2 increased root length (+26.0%) and diameter (+8.4%). Elevated CO2 also stimulated total root (+28.8%), fine root (+27.7%) and coarse root biomass (+25.3%), demonstrating strong responses of root morphology and biomass. Elevated CO2 increased the root:shoot ratio (+8.5%) and decreased the proportion of roots in the topsoil (-8.4%), suggesting that plants expand rooting systems. In addition, elevated CO2 decreased N concentration (-7.1%), but did not affect C concentration, and thus increased the C:N ratio (+7.8%). Root C (+29.3%) increased disproportionately relative to root N pools (+9.4%) under elevated CO2. Functional traits were also strongly affected by elevated CO2, which increased respiration (+58.9%), rhizodeposition (+37.9%) and fungal colonization (+3.3%). Main conclusionsThese results suggest that elevated CO2 promoted root morphological development, root system expansion and C input to soils, implying that the sensitive responses of root morphology and function to elevated CO2 would increase long-term belowground C sequestration.

Hemerythrins are oxygen-binding proteins found in the body fluids and tissues of certain invertebrates. Oxygen is bound at a nonheme iron centre consisting of two oxo-bridged iron atoms bound to a characteristic set of conserved histidine: aspartate and glutamate residues with the motifs H-HxxxE-HxxxH-HxxxxD. It has recently been demonstrated biochemically that two bacterial proteins bearing the same motifs do in fact possess similar iron centres and bind oxygen in the same way. The recent profusion of prokaryotic genomic sequence data has shown that proteins bearing hemerythrin motifs are present in a wide variety of bacteria, and a few archaea. Some of these are short proteins as in eukaryotes; others appear to consist of a hemerythrin domain fused to another domain, generally a putative signal transduction domain such as a methyl-accepting chemotaxis protein, a histidine kinase, or a GGDEF protein (cyclic di-GMP synthase). If, as initial evidence suggests, these are in fact hemerythrin-like oxygen-binding proteins, then their diversity in prokaryotes far exceeds that seen in eukaryotes. Here, a survey is presented of prokaryotic protein sequences bearing hemerythrin-like motifs, for which the designation 'bacteriohemerythrins' is proposed, and their functions are speculated.

Abstract. An aerosol time-of-flight mass spectrometer (ATOFMS) was deployed for the measurement of the size resolved chemical composition of single particles at a site in Cork Harbour, Ireland for three weeks in August 2008. The ATOFMS was co-located with a suite of semi-continuous instrumentation for the measurement of particle number, elemental carbon (EC), organic carbon (OC), sulfate and particulate matter smaller than 2.5 µm in diameter (PM 2.5 ). The temporality of the ambient ATOFMS particle classes was subsequently used in conjunction with the semi-continuous measurements to apportion PM 2.5 mass using positive matrix factorisation. The synergy of the single particle classification procedure and positive matrix factorisation allowed for the identification of six factors, corresponding to vehicular traffic, marine, long-range transport, various combustion, domestic solid fuel combustion and shipping traffic with estimated contributions to the measured PM 2.5 mass of 23%, 14%, 13%, 11%, 5% and 1.5% respectively. Shipping traffic was found to contribute 18% of the measured particle number (20-600 nm mobility diameter), and thus may have important implications for human health considering the size and composition of ship exhaust particles. The positive matrix factorisation procedure enabled a more refined interpretation of the single particle results by providing source contributions to PM 2.5 mass, while the single particle data enabled the identification of additional factors not possible with typical semi-continuous measurements, including local shipping traffic.

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