Respiratory Carbon Research Articles

Biological carbon pump estimate based on multidecadal hydrographic data

The transfer of photosynthetically produced organic carbon from surface to mesopelagic waters draws carbon dioxide from the atmosphere1. However, current observation-based estimates disagree on the strength of this biological carbon pump (BCP)2. Earth system models (ESMs) also exhibit a large spread of BCP estimates, indicating limited representations of the known carbon export pathways3. Here we use several decades of hydrographic observations to produce a top-down estimate of the strength of the BCP with an inverse biogeochemical model that implicitly accounts for all known export pathways. Our estimate of total organic carbon (TOC) export at 73.4 m (model euphotic zone depth) is 15.00 ± 1.12 Pg C year−1, with only two-thirds reaching 100 m depth owing to rapid remineralization of organic matter in the upper water column. Partitioned by sequestration time below the euphotic zone, τ, the globally integrated organic carbon production rate with τ > 3 months is 11.09 ± 1.02 Pg C year−1, dropping to 8.25 ± 0.30 Pg C year−1 for τ > 1 year, with 81% contributed by the non-advective-diffusive vertical flux owing to sinking particles and vertically migrating zooplankton. Nevertheless, export of organic carbon by mixing and other fluid transport of dissolved matter and suspended particles remains regionally important for meeting the respiratory carbon demand. Furthermore, the temperature dependence of the sequestration efficiency inferred from our inversion suggests that future global warming may intensify the recycling of organic matter in the upper ocean, potentially weakening the BCP.

Respiration and carbon use efficiency characteristics of soluble protein-derived carbon by soil microorganisms: A case study at afforested sites

Soluble protein plays a significant role in the release of carbon dioxide (CO2) in soils. However, our understanding of the respiration and C use efficiency (CUE) characteristics of soluble protein-derived C by soil microorganisms is limited. To address this issue, we sampled surface soils (0−10 cm) from seven tree monocultures and examined the temporal dynamics of turnover of 14C-labelled soluble protein-derived C by soil microorganisms. Two double first-order exponential kinetic decay models were applied to analyze the mineralization data (i.e., with and without abiotic protein-surface interactions). The model incorporating the immobilization of protein on the non-living solid phase exhibited the best fit to the experimental data (R2 > 99.6%). Our results suggest that 66.1−73.9% of the soluble protein-derived C was immobilized by the non-living solid phase in soils. After uptake by the soil microbial community, 8.0−13.8% of the C was rapidly respired as CO2, while 15.0−20.8% was used in anabolic processes, resulting in a CUE of 55.1–70.2%. However, there was little effect of forest type on protein turnover rate in the soil. The C:N ratio of soil microbial biomass (C/Nmic) was positively related to the CUE of protein and exhibited less variation within a forest type. Compared with soil microbial biomass C and N, C/Nmic could serve as a better indicator of the CUE of protein by soil microorganisms. This study sheds light on the respiration and CUE characteristics of soluble protein-derived C by soil microorganisms at afforested sites and enhances our understanding of the trade-off between the metabolism of protein-derived C by soil microorganisms and its immobilization by the non-living solid phase in soils.

Paleoproductivity and deep-sea oxygenation in Cosmonaut Sea since the last glacial maximum: impact on atmospheric CO2

The paleoproductivity in the Southern Ocean plays a crucial role in controlling the atmospheric CO2 concentration. Here, we present the sediment record of gravity core ANT37-C5/6-07, which was retrieved from the Cosmonaut Sea (CS), Indian Ocean sector of the Southern Ocean. We found that the change in the oxygen concentration in the CS bottom water is strongly correlated with the atmospheric CO2 fluctuations since the Last Glacial Maximum (LGM). Based on the change in the export production, we reconstructed the evolution history of the deep-water ventilation/upwelling in the study area. During the LGM, a large amount of respiratory carbon was stored in the deep Southern Ocean due to the effect of the low export productivity and restricted ventilation. The oxygen concentration was also low at this time. Despite the increase in paleoproductivity, the biological pump efficiency remained at a low level during the Last Deglaciation. Vast quantities of CO2 were released into the atmosphere through enhanced upwelling. The recovery of ventilation during this period facilitated the supply of oxygen-rich surface water to the deep ocean. Moreover, signals were identified during the transitions between the Heinrich Stage 1 (HS1), Antarctic Cold Reverse (ACR), and Younger Drays (YD) periods. During the Holocene, the productivity increased overall, and the oxygen in the bottom water was consumed but still remained at a high level. This may have been caused by the enhanced ventilation and/or the prevalence of East Cosmonaut Polynya (ECP) near Cape Ann.

Dryland nitrogen deposition induces microbiome‐driven increases in biocrust respiration and losses of soil carbon

AbstractBiocrusts are a dominant component in drylands worldwide and play critical roles in supporting soil microbial diversity and carbon (C) stocks. Nitrogen (N) fertilization associated with human activities threatens drylands, which are often considered N‐limited ecosystems. Here, we conducted a field experiment in two contrasting soil types (loess vs. sand) to investigate the impacts of low (30 kg N ha−1 year−1) and high (90 kg N ha−1 year−1) fertilization on moss‐biocrust dominated traits, soil nutrients, microbial taxonomic richness, soil C stocks and respiration rates (Rs). We showed that 5 months of N addition resulted in reductions in soil organic C content by 91% and increased both soil microbial richness and diversity. Our results further showed that relative to controls, low levels of N addition increased biocrust Rs by 52% through increased moss biomass and density (38% and 73%) and microbial taxonomic richness and diversity (18% and 23%), while no significant changes in biocrust Rs were observed after high levels of N addition. Considering multiple environmental factors simultaneously, we show that N fertilization indirectly promoted soil respiration and C losses via increases in microbial richness and diversity, which are critical drivers of soil function. Our work provides solid evidence that N deposition, even at low levels of N addition, can result in rapid losses of C in dryland soils. Our findings suggest that to maintain healthy dryland ecosystems and promote C, we must mitigate future land degradation and minimize anthropogenic N deposition.

Bacterial communities in residential wastewater treatment plants are physiologically adapted to high concentrations of quaternary ammonium compounds

AbstractBenzalkonium chloride (BAC) is a quaternary ammonium compound (QAC) widely used as the active ingredient of disinfectants. Its excessive discharge into wastewater is constant and in high concentrations, likely affecting the physiology of microbial communities. We compared the physiological community profile of activated sludge (AS) bacteria with and without prior in vitro exposure to a high concentration of BAC (10 mg L−1). We measured the community functional diversity, carbon substrate multifunctionality, and the median effective concentration that inhibits carbon respiration (EC50) using Biolog EcoPlates supplemented with a gradient of 0–50 mg L−1 of BAC. Surprisingly, we did not find significant differences in the physiological parameters among treatments. Certain abundant bacteria, including Pseudomonas, could explain the community's tolerance to high concentrations of BAC. We suggest that bacterial communities in wastewater treatment plants’ AS are “naturally” adapted to BAC due to frequent and high‐dose exposure. We highlight the need to understand better the effects of QACs in wastewater, their impact on the selection of tolerant groups, and the alteration in community metabolic profiles.

Variation of carbon source and sink along the environmental gradient from lakeside to highlands in Yuanchi swamp wetlands, Changbai Mountains, China.

Lacustrine wetlands have long-term carbon storage capacity and contribute significantly to regional carbon cycle, but it is unclear how its carbon sinks respond to climate change. We measured soil heterotrophic respiration carbon emissions (CO2 and CH4), vegetation carbon sequestration, and related environmental factors (temperature, water level, etc.) of five kinds of natural swamps (Phragmites marsh-L, Carex schmidtii marsh-C, Rhododendron capitatum swamp-D, Betula fruticose swamp-H, Larix olgensis swamp-LT)by using static chamber gas chromatography and relative growth equation methods, along the water environmental gradients from lakeside to highlands in Yuanchi of Changbai Mountains. We quantified the carbon source/sink function (CSS) and global warming potential (GWP) of various swamp types by estimating ecosystem net carbon balance, and revealed the variation patterns and formation mechanisms of CSS and GWP along the environmental gradients, aiming to explore the response of carbon source/sink of lakeside wetland in high altitude area to climate change. The results showed that marshes (L and C) were weak sources (-1.018 and -0.090 t C·hm-1·a-1) at the lower habitats of the water environment gradient, shrub swamps (D and H) were strong or weak sinks (1.956 and 0.239 t C·hm-1·a-1) at the middle habitats, forest swamp (LT) was strong source (-3.214 t C·hm-1·a-1) at the upper habitat. The spatial changes were promoted by water level and suppressed by soil temperature. For GWP, strong thermal radiation for marshes (from 44.682 to 59.282 t CO2·hm-1·a-1), cold radiation for shrub swamps (from -0.920 to -7.008 t CO2·hm-1·a-1), and weak thermal radiation for forest swamp (11.668 t CO2·hm-1·a-1), and their GWP was only promoted by soil temperature. Under current climate change background, marshes and forest swamp at both ends of the water environment gradient from lakeside to highlands played a positive feedback effect due to the increases of CH4 or CO2 emissions, while the middle shrub swamp still maintained a negative feedback effect in Yuanchi located the high-altitude area of the temperate Changbai Mountains.

Dark respiration explains nocturnal stomatal conductance in rice regardless of drought and nutrient stress.

The ecological mechanism underlying nocturnal stomatal conductance (gsn ) in C3 and C4 plants remains elusive. In this study, we proposed a 'coordinated leaf trait' hypothesis to explain gsn in rice plants. We conducted an open-field experiment by applying drought, nutrient stressand the combined drought-nutrient stress. We found that gsn was neither strongly reduced by drought nor consistently increased by nutrient stress. With the aforementioned multiple abiotic stressors considered as random effects, gsn exhibited a strong positive correlation with dark respiration (Rn ). Notably, gsn primed early morning (5:00-7:00) photosynthesis through faster stomatal response time. This photosynthesis priming effect diminished after mid-morning (9:00). Leaves were cooled by gsn -derived transpiration. However, our results clearly suggest that evaporative cooling did not reduce dark respiration cost. Our results indicate that gsn is more closely related to carbon respiration and assimilation than water and nutrient availability, and that dark respiration can explain considerable variation of gsn .

Variability and controls of the ocean acidification metrics pH and pCO2 in a large embayment of an Eastern Boundary Upwelling System (EBUS)

An assessment was undertaken of the spatial and temporal variability of pCO2 and pH in St Helena Bay, a highly productive, wide-open bay located in the Benguela, a major eastern boundary upwelling system (EBUS). Mechanisms controlling their patterns of variability and their linkage to the distribution of oxygen are explored. The mean surface pCO2 was 385 μatm, below the mean atmospheric concentration of 410 μatm, indicative of a system serving as a CO2 sink. The corresponding mean pH of bay surface waters was 8.3. The greatest variation from these means was driven by deep winter mixing causing a seasonal reversal of the air-sea pCO2 gradient. Mean surface pCO2 was also elevated above mean atmospheric levels in the nearshore (<15 m depth), a likely consequence of higher microbial respiration in the upper water column due to regular resuspension of detrital material. The nearshore of the bay is also sometimes subject to high biomass dinoflagellate blooms referred to as red tides, and their decay leads to exceptional pCO2 concentrations. Discharge from the Berg River and the associated degradation of terrestrially derived organic matter also contributes to higher pCO2 in the nearshore particularly during winter when rainfall is highest. Through the water column the important role of photosynthesis in determining the vertical distribution of pCO2 is evident in that pCO2 is shown to be a function of both Chl a and PAR. In St Helena Bay nearly 50% of water column biomass is located below the community compensation depth (i.e. where respiration carbon loss exceeds gross community production) leading to rapidly declining O2 and increasing pCO2 concentrations with depth. Resulting conditions of bottom water hypoxia/anoxia are therefore accompanied by more corrosive waters (pH declining to ∼7.4) and conditions of severe hypercapnia (pCO2 exceeding 2000 μatm) subjecting marine life to multiple stressors. St Helena Bay and similar bays in EBUS are likely to provide extrema in the deoxygenation and acidification of coastal waters.

ATP yield of plant respiration: potential, actual and unknown.

The ATP yield of plant respiration (ATP/hexose unit respired) quantitatively links active heterotrophic processes with substrate consumption. Despite its importance, plant respiratory ATP yield is uncertain. The aim here was to integrate current knowledge of cellular mechanisms with inferences required to fill knowledge gaps to generate a contemporary estimate of respiratory ATP yield and identify important unknowns. A numerical balance sheet model combining respiratory carbon metabolism and electron transport pathways with uses of the resulting transmembrane electrochemical proton gradient was created and parameterized for healthy, non-photosynthesizing plant cells catabolizing sucrose or starch to produce cytosolic ATP. Mechanistically, the number of c subunits in the mitochondrial ATP synthase Fo sector c-ring, which is unquantified in plants, affects ATP yield. A value of 10 was (justifiably) used in the model, in which case respiration of sucrose potentially yields about 27.5 ATP/hexose (0.5 ATP/hexose more from starch). Actual ATP yield often will be smaller than its potential due to bypasses of energy-conserving reactions in the respiratory chain, even in unstressed plants. Notably, all else being optimal, if 25 % of respiratory O2 uptake is via the alternative oxidase - a typically observed fraction - ATP yield falls 15 % below its potential. Plant respiratory ATP yield is smaller than often assumed (certainly less than older textbook values of 36-38 ATP/hexose) leading to underestimation of active-process substrate requirements. This hinders understanding of ecological/evolutionary trade-offs between competing active processes and assessments of crop growth gains possible through bioengineering of processes that consume ATP. Determining the plant mitochondrial ATP synthase c-ring size, the degree of any minimally required (useful) bypasses of energy-conserving reactions in the respiratory chain, and the magnitude of any 'leaks' in the inner mitochondrial membrane are key research needs.

Reviews and syntheses: Recent advances in microwave remote sensing in support of terrestrial carbon cycle science in Arctic–boreal regions

Abstract. Spaceborne microwave remote sensing (300 MHz–100 GHz) provides a valuable method for characterizing environmental changes, especially in Arctic–boreal regions (ABRs) where ground observations are generally spatially and temporally scarce. Although direct measurements of carbon fluxes are not feasible, spaceborne microwave radiometers and radar can monitor various important surface and near-surface variables that affect terrestrial carbon cycle processes such as respiratory carbon dioxide (CO2) fluxes; photosynthetic CO2 uptake; and processes related to net methane (CH4) exchange including CH4 production, transport and consumption. Examples of such controls include soil moisture and temperature, surface freeze–thaw cycles, vegetation water storage, snowpack properties and land cover. Microwave remote sensing also provides a means for independent aboveground biomass estimates that can be used to estimate aboveground carbon stocks. The microwave data record spans multiple decades going back to the 1970s with frequent (daily to weekly) global coverage independent of atmospheric conditions and solar illumination. Collectively, these advantages hold substantial untapped potential to monitor and better understand carbon cycle processes across ABRs. Given rapid climate warming across ABRs and the associated carbon cycle feedbacks to the global climate system, this review argues for the importance of rapid integration of microwave information into ABR terrestrial carbon cycle science.

Transcriptome analysis reveals insights into adaptive responses of two marine microalgae species to Nordic seasons

There is an increasing interest in algae-based biomass produced outdoors in natural and industrial settings for biotechnological applications. To predict the yield and biochemical composition of the biomass, it is important to understand how the transcriptome of species and strains of interest is affected by seasonal changes. Here we studied the effects of Nordic winter and summer on the transcriptome of two phytoplankton species, namely the diatom Skeletonema marinoi (Sm) and the eustigmatophyte Nannochloropsis granulata (Ng), recently identified as potentially important for biomass production on the west coast of Sweden. Cultures were grown in photobioreactors in simulated Nordic summer and winter, and the gene expression in two phases was quantified by Illumina RNA-sequencing. Five paired comparisons were made among the four conditions. Sm was overall more responsive to seasons since 70 % of the total transcriptome (14,783 genes) showed differential expression in at least one comparison as compared to 1.6 % (1403 genes) for Ng. For both species, we observed larger differences between the seasons than between the phases of the same season. In summer phase 1, Sm cells focused on photosynthesis and polysaccharide biosynthesis. Nitrate assimilation and recycling of intracellular nitrogen for protein biosynthesis were more active in summer phase 2 and throughout winter. Lipid catabolism was upregulated in winter relative to summer to supply carbon for respiration. Ng favored lipid accumulation in summer, while in winter activated different lipid remodeling pathways as compared to Sm. To cope with winter, Ng upregulated breakdown and transport of carbohydrates for energy production. Taken together, our transcriptome data reveal insights into adaptive seasonal responses of Sm and Ng important for biotechnological applications on the west coast of Sweden, but more work is required to decipher the molecular mechanisms behind these responses.

Exceptionally high respiration rates in the reactive surface layer of sediments underlying oxygen-deficient bottom waters

Organic carbon (OC) burial efficiency, which relates the OC burial rate to respiration in the seafloor, is a critical parameter in the reconstruction of past marine primary productivities. The current accepted theory is that sediments underlying oxygen-deficient (anoxic) bottom waters have low respiration rates and high OC burial efficiencies. By combining novel in situ measurements in anoxic basins with reaction-transport modelling, we demonstrate that sediments underlying anoxic bottom waters have much higher respiration rates than commonly assumed. A major proportion of the carbon respiration is concentrated in the top millimeter—the so-called ‘reactive surface layer’—which is likely a feature in approximately 15% of the coastal seafloor. When re-evaluating previously published data in light of our results, we conclude that the impact of bottom-water anoxia on OC burial efficiencies in marine sediments is small. Consequently, reconstructions of past marine primary productivity in a predominantly anoxic ocean based on OC burial rates might be underestimated by up to an order of magnitude.

Leaf physiological and morphological constraints of water-use efficiency in C3 plants.

The increasing evaporative demand due to climate change will significantly affect the balance of carbon assimilation and water losses of plants worldwide. The development of crop varieties with improved water-use efficiency (WUE) will be critical for adapting agricultural strategies under predicted future climates. This review aims to summarize the most important leaf morpho-physiological constraints of WUE in C3 plants and identify gaps in knowledge. From the carbon gain side of the WUE, the discussed parameters are mesophyll conductance, carboxylation efficiency and respiratory losses. The traits and parameters affecting the waterside of WUE balance discussed in this review are stomatal size and density, stomatal control and residual water losses (cuticular and bark conductance), nocturnal conductance and leaf hydraulic conductance. In addition, we discussed the impact of leaf anatomy and crown architecture on both the carbon gain and water loss components of WUE. There are multiple possible targets for future development in understanding sources of WUE variability in plants. We identified residual water losses and respiratory carbon losses as the greatest knowledge gaps of whole-plant WUE assessments. Moreover, the impact of trichomes, leaf hydraulic conductance and canopy structure on plants' WUE is still not well understood. The development of a multi-trait approach is urgently needed for a better understanding of WUE dynamics and optimization.

Decapping factor Dcp2 controls mRNA abundance and translation to adjust metabolism and filamentation to nutrient availability.

Degradation of most yeast mRNAs involves decapping by Dcp1/Dcp2. DEAD-box protein Dhh1 has been implicated as an activator of decapping, in coupling codon non-optimality to enhanced degradation, and as a translational repressor, but its functions in cells are incompletely understood. RNA-Seq analyses coupled with CAGE sequencing of all capped mRNAs revealed increased abundance of hundreds of mRNAs in dcp2Δ cells that appears to result directly from impaired decapping rather than elevated transcription. Interestingly, only a subset of mRNAs requires Dhh1 for targeting by Dcp2, and also generally requires the other decapping activators Pat1, Edc3 or Scd6; whereas most of the remaining transcripts utilize NMD factors for Dcp2-mediated turnover. Neither inefficient translation initiation nor stalled elongation appears to be a major driver of Dhh1-enhanced mRNA degradation. Surprisingly, ribosome profiling revealed that dcp2Δ confers widespread changes in relative translational efficiencies (TEs) that generally favor well-translated mRNAs. Because ribosome biogenesis is reduced while capped mRNA abundance is increased by dcp2&Delta, we propose that an increased ratio of mRNA to ribosomes increases competition among mRNAs for limiting ribosomes to favor efficiently translated mRNAs in dcp2Δ cells. Interestingly, genes involved in respiration or utilization of alternative carbon or nitrogen sources are up-regulated, and both mitochondrial function and cell filamentation are elevated in dcp2Δ cells, suggesting that decapping sculpts gene expression post-transcriptionally to fine-tune metabolic pathways and morphological transitions according to nutrient availability.

How old is too old? Implications of averaging 14C-Based estimates of ventilation age to assess the Pacific Ocean's role in sequestering CO2 in the past

Compilations of benthic foraminiferal 14C-based ventilation age estimates from the deep North Pacific have been used to argue the average age of bottom waters during the last glacial maximum was 3000 years, or about 1000 years older than modern. If true, glacial bottom waters would have accumulated additional respired carbon and contributed to the drop in atmospheric CO2. By the same reasoning, ventilation ages that were 1000 years older, would have lost dissolved oxygen and become anaerobic and thereby unable to support epibenthic fauna. The current study re-analyzed the Pacific 14C data and finds the glacial ventilation age average is biased by inclusion of data from low sediment accumulation rate cores that have been affected by bioturbation and contaminated with older specimens. Removal of data from cores with sedimentation rates less than 10 cm/ky reduces the old-bias and results in an average ventilation age that is close to the modern ocean.

Influence of mineral ions on the growth and fermentation performance of Dekkera bruxellensis GDB248.

The excess of minerals in the industrial substrates is detrimental for Saccharomyces cerevisiae ethanol fermentation performance. In this work, we sought to understand the effect of some of those minerals on the physiology of Dekkera bruxellensis. Three groups of minerals were classified on the basis of the aerobic growth profiles on glucose: neutrals (K+, Mg2+, P5+ and Zn2+), inducers (Mn2+ and Ca2+) and inhibitors (Al3+, Cu2+ and Fe2+). Cu2+ showed the highest mineral toxicity, and its effect was dependent of the level of medium aeration. On the other hand, copper stimulated respiration by increasing growth on respiratory carbon sources. Most growth inhibitors also hampered glucose fermentation, with changes in carbon distribution to metabolic routes dedicated to anabolic reactions and for alternative reduced co-factors oxidations to maintain cellular homeostasis. The negative effect of Cu2+ on yeast fermentation was partially alleviated by Mg2+ and Mn2+, similar to magnesium antagonism observed for S. cerevisiae. All these results might contribute to understand the action of these minerals in sugarcane substrates on the physiology of D. bruxellensis cells. Therefore, it represents one more step for the consolidation of the industrial use of this yeast in the production of fuel-ethanol as well as other biotechnological goods.

Fungal parasitism on diatoms alters formation and bio–physical properties of sinking aggregates

Phytoplankton forms the base of aquatic food webs and element cycling in diverse aquatic systems. The fate of phytoplankton-derived organic matter, however, often remains unresolved as it is controlled by complex, interlinked remineralization and sedimentation processes. We here investigate a rarely considered control mechanism on sinking organic matter fluxes: fungal parasites infecting phytoplankton. We demonstrate that bacterial colonization is promoted 3.5-fold on fungal-infected phytoplankton cells in comparison to non-infected cells in a cultured model pathosystem (diatom Synedra, fungal microparasite Zygophlyctis, and co-growing bacteria), and even ≥17-fold in field-sampled populations (Planktothrix, Synedra, and Fragilaria). Additional data obtained using the Synedra–Zygophlyctis model system reveals that fungal infections reduce the formation of aggregates. Moreover, carbon respiration is 2-fold higher and settling velocities are 11–48% lower for similar-sized fungal-infected vs. non-infected aggregates. Our data imply that parasites can effectively control the fate of phytoplankton-derived organic matter on a single-cell to single-aggregate scale, potentially enhancing remineralization and reducing sedimentation in freshwater and coastal systems.

Temperature sensitivity of soil organic carbon respiration along a forested elevation gradient in the Rwenzori Mountains, Uganda

Abstract. Tropical montane forests store high amounts of soil organic carbon (SOC). However, global warming may affect these stocks via enhanced soil respiration. Improved insight into the temperature response of SOC respiration can be obtained from in and ex situ warming studies. In situ warming via the translocation of intact soil mesocosms was carried out along an elevation gradient ranging between ca. 1250 m in the Kibale Forest to ca. 3000 m in the Rwenzori Mountains in Uganda. Samples from the same transect were also warmed ex situ. Ex situ results revealed that, following the elevation gradient, which represents a natural climate gradient, specific heterotrophic CO2 respiration decreased linearly by 1.01 ± 0.12 µgCh-1g-1 of SOC per 100 m of elevation increase. The coefficient of temperature sensitivity increased from 1.50 ± 0.13 in the lowest- to 2.68 ± 0.25 in the highest-elevation cluster, showing a linear increase of 0.09 ± 0.03 per 100 m of elevation increase. Additionally, respired CO2 was more depleted in 13C in the warmer lower elevations as compared to colder higher elevations, with a linear decrease of 0.23 ‰ ± 0.04 ‰ per 100 m of elevation increase. Furthermore, the microbial community structure indicated a weak trend along the elevation gradient, with higher elevations more dominated by fungi relative to bacteria. The results indicate an increased recalcitrance and decreased mineralisation of SOC, with elevation likely driven by decreasing soil temperature and pH. Subsequently, after 2 years of in situ warming (0.9 to 2.8 ∘C), specific heterotrophic SOC respiration tended to be lower for warmed soil compared to control soil. Furthermore, in warmed soils, δ13C values and SOC content tended to increase and decrease, respectively. Collectively, this points towards the increased mineralisation and depletion of readily available C during 2 years of warming. In conclusion, our results suggest that climate warming may trigger enhanced losses of SOC from tropical montane forests due to a combination of a higher temperature sensitivity of mineralisation and higher SOC content at higher elevations.

Changes in deep Pacific circulation and carbon storage during the Pliocene-Pleistocene transition

The global climate has been gradually cooling over the Cenozoic and is punctuated by the intensification of Northern Hemisphere Glaciation (NHG) from the latest Pliocene to earliest Pleistocene (∼3.1–2.5 millions of years ago, Ma). A decline of atmospheric CO2 is supposed as a prerequisite for the NHG, but the associated carbon-cycle processes remain elusive. Here we combine foraminiferal records of neodymium isotope and boron-calcium ratio, and simulations of an Earth system model, to investigate changes in the water-mass composition and carbonate-ion concentration of the deep Pacific Ocean during the NHG. Our proxy records have revealed a significant expansion of southern-sourced waters with increased respired carbon storage into the deep Pacific during the NHG. These changes may be explained by strengthened deep-water formation and biological-pump efficiency in the Southern Ocean due to Antarctic sea-ice growth, as suggested by our model experiments and evidence from the Sub-Antarctic region. These results provide key clues for quantifying the role of the dissolved inorganic carbon content of deep Pacific waters in modulating atmospheric CO2 during the NHG.

Sodium Lignosulfonate Effect on Physiological Traits of Cucumis sativus L. Seedlings

The application of pulp and paper mill sludge to agricultural soils is commonly considered as a strategy to improve soil properties, promote plant growth, and reduce the demand for costly chemical fertilization. The aim of this study was to evaluate if sodium lignosulfonate (sLS), one of the sludges of pulp production, may affect the biomass production, the respiration (R) and net CO2 assimilation rate (An) at the leaf level, and the content and accumulation of trace elements in the leaves of cucumbers grown under a sufficient nutrient supply or soil nutrient deficit. A pot culture experiment was conducted using sLS application rates of 0, 1.0, 2.5, 5.0, and 10 vol% to sandy loam soil. The decline in nutrient availability caused an increase in the R/An ratio and dramatically depressed biomass accumulation. The leaf Fe, Ni, Cr, Co, Al, and Pb contents were lower under low nutrient availability than under sufficient nutrient supply. Although sLS was not very effective in lessening the negative effect of nutrient deficiency on biomass accumulation, it reduced respiratory carbon losses and cell membrane permeability in the leaves of cucumbers grown under nutrient deficit. The reduction in the toxic level of leaf Mn in seedlings grown under sufficient nutrient availability and the toxic level of leaf Fe under a nutrient deficit might also be considered as a positive effect of the sLS application to sandy soil.