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The exchange of carbon between soil organic carbon (SOC) and the atmosphere affects the climate 1 , 2 and—because of the importance of organic matter to soil fertility—agricultural productivity 3 . The dynamics of topsoil carbon has been relatively well quantified 4 , but half of the soil carbon is located in deeper soil layers (below 30 centimetres) 5 – 7 , and many questions remain regarding the exchange of this deep carbon with the atmosphere 8 . This knowledge gap restricts soil carbon management policies and limits global carbon models 1 , 9 , 10 . Here we quantify the recent incorporation of atmosphere-derived carbon atoms into whole-soil profiles, through a meta-analysis of changes in stable carbon isotope signatures at 112 grassland, forest and cropland sites, across different climatic zones, from 1965 to 2015. We find, in agreement with previous work 5 , 6 , that soil at a depth of 30–100 centimetres beneath the surface (the subsoil) contains on average 47 per cent of the topmost metre’s SOC stocks. However, we show that this subsoil accounts for just 19 per cent of the SOC that has been recently incorporated (within the past 50 years) into the topmost metre. Globally, the median depth of recent carbon incorporation into mineral soil is 10 centimetres. Variations in the relative allocation of carbon to deep soil layers are better explained by the aridity index than by mean annual temperature. Land use for crops reduces the incorporation of carbon into the soil surface layer, but not into deeper layers. Our results suggest that SOC dynamics and its responses to climatic control or land use are strongly dependent on soil depth. We propose that using multilayer soil modules in global carbon models, tested with our data, could help to improve our understanding of soil–atmosphere carbon exchange. This study of whole-soil carbon dynamics finds that, of the atmospheric carbon that is incorporated into the topmost metre of soil over 50 years, just 19 per cent reaches the subsoil, in a manner that depends on land use and aridity.

Mass extinction at the Cretaceous–Paleogene (K-Pg) boundary coincides with the Chicxulub bolide impact and also falls within the broader time frame of Deccan trap emplacement. Critically, though, empirical evidence as to how either of these factors could have driven observed extinction patterns and carbon cycle perturbations is still lacking. Here, using boron isotopes in foraminifera, we document a geologically rapid surface-ocean pH drop following the Chicxulub impact, supporting impact-induced ocean acidification as amechanism for ecological collapse in the marine realm. Subsequently, surface water pH rebounded sharply with the extinction of marine calcifiers and the associated imbalance in the global carbon cycle. Our reconstructed water-column pH gradients, combined with Earth system modeling, indicate that a partial ∼50% reduction in global marine primary productivity is sufficient to explain observed marine carbon isotope patterns at the K-Pg, due to the underlying action of the solubility pump. While primary productivity recovered within a few tens of thousands of years, inefficiency in carbon export to the deep sea lasted much longer. This phased recovery scenario reconciles competing hypotheses previously put forward to explain the K-Pg carbon isotope records, and explains both spatially variable patterns of change in marine productivity across the event and a lack of extinction at the deep sea floor. In sum, we provide insights into the drivers of the last mass extinction, the recovery of marine carbon cycling in a postextinction world, and the way in which marine life imprints its isotopic signal onto the geological record.

Stable isotopes of carbon, nitrogen, and sulfur are used as ecological tracers for a variety of applications, such as studies of animal migrations, energy sources, and food web pathways. Yet uncertainty relating to the time period integrated by isotopic measurement of animal tissues can confound the interpretation of isotopic data. There have been a large number of experimental isotopic diet shift studies aimed at quantifying animal tissue isotopic turnover rate λ (%·day(-1), often expressed as isotopic half-life, ln(2)/λ, days). Yet no studies have evaluated or summarized the many individual half-life estimates in an effort to both seek broad-scale patterns and characterize the degree of variability. Here, we collect previously published half-life estimates, examine how half-life is related to body size, and test for tissue- and taxa-varying allometric relationships. Half-life generally increases with animal body mass, and is longer in muscle and blood compared to plasma and internal organs. Half-life was longest in ecotherms, followed by mammals, and finally birds. For ectotherms, different taxa-tissue combinations had similar allometric slopes that generally matched predictions of metabolic theory. Half-life for ectotherms can be approximated as: ln (half-life) = 0.22*ln (body mass) + group-specific intercept; n = 261, p<0.0001, r2 = 0.63. For endothermic groups, relationships with body mass were weak and model slopes and intercepts were heterogeneous. While isotopic half-life can be approximated using simple allometric relationships for some taxa and tissue types, there is also a high degree of unexplained variation in our models. Our study highlights several strong and general patterns, though accurate prediction of isotopic half-life from readily available variables such as animal body mass remains elusive.

The key components of crassulacean acid metabolism (CAM) – nocturnal fixation of atmospheric CO2 and its processing via Rubisco in the subsequent light period – are now reasonably well understood in terms of the biochemical reactions defining this water-saving mode of carbon assimilation. Phenotypically, however, the degree to which plants engage in the CAM cycle relative to regular C3 photosynthesis is highly variable. Depending upon species, ontogeny and environment, the contribution of nocturnal CO2 fixation to 24-h carbon gain can range continuously from close to 0% to 100%. Nevertheless, not all possible combinations of light and dark CO2 fixation appear equally common. Large-scale surveys of carbon-isotope ratios typically show a strongly bimodal frequency distribution, with relatively few intermediate values. Recent research has revealed that many species capable of low-level CAM activity are nested within the peak of C3-type isotope signatures. While questions remain concerning the adaptive significance of dark CO2 fixation in such species, plants with low-level CAM should prove valuable models for investigating the discrete changes in genetic architecture and gene expression that have enabled the evolutionary transition from C3 to CAM.

Higher atmospheric CO₂ concentrations (cₐ) can under certain conditions increase tree growth by enhancing photosynthesis, resulting in an increase of intrinsic water‐use efficiency (ᵢWUE) in trees. However, the magnitude of these effects and their interactions with changing climatic conditions are still poorly understood under xeric and mesic conditions. We combined radial growth analysis with intra‐ and interannual δ¹³C and δ¹⁸O measurements to investigate growth and physiological responses of Larix decidua, Picea abies, Pinus sylvestris, Pinus nigra and Pseudotsuga menziesii in relation to rising cₐ and changing climate at a xeric site in the dry inner Alps and at a mesic site in the Swiss lowlands. ᵢWUE increased significantly over the last 50 yr by 8–29% and varied depending on species, site water availability, and seasons. Regardless of species and increased ᵢWUE, radial growth has significantly declined under xeric conditions, whereas growth has not increased as expected under mesic conditions. Overall, drought‐induced stomatal closure has reduced transpiration at the cost of reduced carbon uptake and growth. Our results indicate that, even under mesic conditions, the temperature‐induced drought stress has overridden the potential CO₂ ‘fertilization’ on tree growth, hence challenging today's predictions of improved forest productivity of temperate forests.

The rise of atmospheric oxygen fundamentally changed the chemistry of surficial environments and the nature of Earth’s habitability 1 . Early atmospheric oxygenation occurred over a protracted period of extreme climatic instability marked by multiple global glaciations 2 , 3 , with the initial rise of oxygen concentration to above 10 −5 of the present atmospheric level constrained to about 2.43 billion years ago 4 , 5 . Subsequent fluctuations in atmospheric oxygen levels have, however, been reported to have occurred until about 2.32 billion years ago 4 , which represents the estimated timing of irreversible oxygenation of the atmosphere 6 , 7 . Here we report a high-resolution reconstruction of atmospheric and local oceanic redox conditions across the final two glaciations of the early Palaeoproterozoic era, as documented by marine sediments from the Transvaal Supergroup, South Africa. Using multiple sulfur isotope and iron–sulfur–carbon systematics, we demonstrate continued oscillations in atmospheric oxygen levels after about 2.32 billion years ago that are linked to major perturbations in ocean redox chemistry and climate. Oxygen levels thus fluctuated across the threshold of 10 −5 of the present atmospheric level for about 200 million years, with permanent atmospheric oxygenation finally arriving with the Lomagundi carbon isotope excursion at about 2.22 billion years ago, some 100 million years later than currently estimated. Sulfur isotope and iron–sulfur–carbon systematics on marine sediments indicate that permanent atmospheric oxygenation occurred around 2.22 billion years ago, about 100 million years later than currently estimated.

Revisions in isotopic source signatures reveal that global total fossil fuel methane emissions from industry plus natural geological seepage are much larger than thought. Global fossil fuel methane emissions revised Stefan Schwietzke et al . re-evaluate the global methane budget and the contribution of the fossil fuel industry to methane emissions on the basis of long-term global methane and methane carbon isotope records. They find that total fossil fuel methane emissions (fossil fuel industry plus natural geological methane seepage) are not increasing over time, but are 60–110 per cent greater than was previously thought. They also conclude that methane emissions from natural gas, oil and coal production and their usage are 20–60 per cent greater than inventories and that methane emissions from natural gas as a fraction of production have declined from about 8 per cent to 2 per cent over the past three decades. Methane has the second-largest global radiative forcing impact of anthropogenic greenhouse gases after carbon dioxide, but our understanding of the global atmospheric methane budget is incomplete. The global fossil fuel industry (production and usage of natural gas, oil and coal) is thought to contribute 15 to 22 per cent of methane emissions 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 to the total atmospheric methane budget 11 . However, questions remain regarding methane emission trends as a result of fossil fuel industrial activity and the contribution to total methane emissions of sources from the fossil fuel industry and from natural geological seepage 12 , 13 , which are often co-located. Here we re-evaluate the global methane budget and the contribution of the fossil fuel industry to methane emissions based on long-term global methane and methane carbon isotope records. We compile the largest isotopic methane source signature database so far, including fossil fuel, microbial and biomass-burning methane emission sources. We find that total fossil fuel methane emissions (fossil fuel industry plus natural geological seepage) are not increasing over time, but are 60 to 110 per cent greater than current estimates 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 owing to large revisions in isotope source signatures. We show that this is consistent with the observed global latitudinal methane gradient. After accounting for natural geological methane seepage 12 , 13 , we find that methane emissions from natural gas, oil and coal production and their usage are 20 to 60 per cent greater than inventories 1 , 2 . Our findings imply a greater potential for the fossil fuel industry to mitigate anthropogenic climate forcing, but we also find that methane emissions from natural gas as a fraction of production have declined from approximately 8 per cent to approximately 2 per cent over the past three decades.

Background/objectivesConsumption of whole vs. refined grain foods is recommended by nutrition or dietary guideline authorities of many countries, yet specific aspects of whole grains leading to health benefits are not well understood. Gastric emptying rate is an important consideration, as it is tied to nutrient delivery rate and influences glycemic response. Our objective was to explore two aspects of cooked rice related to gastric emptying, (1) whole grain brown vs. white rice and (2) potential effect of elevated levels of slowly digestible starch (SDS) and resistant starch (RS) from high-amylose rice.Subjects/methodsTen healthy adult participants were recruited for a crossover design study involving acute feeding and testing of 6 rice samples (50 g available carbohydrate). Gastric emptying rate was measured using a 13C-labeled octanoic acid breath test. A rice variety (Cocodrie) with high-amylose content was temperature-cycled to increase SDS and RS fractions.ResultsIn vitro starch digestibility results showed incremental increase in RS in Cocodrie after two temperature cycles. For low-amylose varieties, SDS was higher in the brown rice form. In human subjects, low-amylose and high-amylose brown rice delayed gastric emptying compared to white rices regardless of amylose content or temperature-cycling (p < 0.05).ConclusionsWhole grain brown rice had slower gastric emptying rate, which appears to be related to the physical presence of the bran layer. Extended gastric emptying of brown rice explains in part comparably low glycemic response observed for brown rice.

Stable carbon isotope ratios (δ13C) of terrestrial plants are employed across a diverse range of applications in environmental and plant sciences; however, the kind of information that is desired from the δ13C signal often differs. At the extremes, it ranges between purely environmental and purely biological. Here, we review environmental drivers of variation in carbon isotope discrimination (Δ) in terrestrial plants, and the biological processes that can either damp or amplify the response. For C3 plants, where Δ is primarily controlled by the ratio of intercellular to ambient CO2 concentrations (c i/c a), coordination between stomatal conductance and photo-synthesis and leaf area adjustment tends to constrain the potential environmentally driven range of Δ. For C4 plants, variation in bundle-sheath leakiness to CO2 can either damp or amplify the effects of c i/c a on Δ. For plants with crassulacean acid metabolism (CAM), Δ varies over a relatively large range as a function of the proportion of daytime to night-time CO2 fixation. This range can be substantially broadened by environmental effects on Δ when carbon uptake takes place primarily during the day. The effective use of Δ across its full range of applications will require a holistic view of the interplay between environmental control and physiological modulation of the environmental signal.