2,168 research outputs found

    Timescales of carbon turnover in soils with mixed crystalline mineralogies

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    Organic matter–mineral associations stabilize much of the carbon (C) stored globally in soils. Metastable short-range-order (SRO) minerals such as allophane and ferrihydrite provide one mechanism for long-term stabilization of organic matter in young soil. However, in soils with few SRO minerals and a predominance of crystalline aluminosilicate or Fe (and Al) oxyhydroxide, C turnover should be governed by chemisorption with those minerals. Here, we correlate mineral composition from soils containing small amounts of SRO minerals with mean turnover time (TT) of C estimated from radiocarbon (<sup>14</sup>C) in bulk soil, free light fraction and mineral-associated organic matter. We varied the mineral amount and composition by sampling ancient soils formed on different lithologies in arid to subhumid climates in Kruger National Park (KNP), South Africa. Mineral contents in bulk soils were assessed using chemical extractions to quantify Fe oxyhydroxides and SRO minerals. Because of our interest in the role of silicate clay mineralogy, particularly smectite (2 : 1) and kaolinite (1 : 1), we separately quantified the mineralogy of the clay-sized fraction using X-ray diffraction (XRD) and measured <sup>14</sup>C on the same fraction. <br><br> Density separation demonstrated that mineral associated C accounted for 40–70 % of bulk soil organic C in A and B1 horizons for granite, nephelinite and arid-zone gabbro soils, and &gt; 80 % in other soils. Organic matter strongly associated with the isolated clay-sized fraction represented only 9–47 % of the bulk soil C. The mean TT of C strongly associated with the clay-sized fraction increased with the amount of smectite (2 : 1 clays); in samples with &gt; 40 % smectite it averaged 1020 ± 460 years. The C not strongly associated with clay-sized minerals, including a combination of low-density C, the C associated with minerals of sizes between 2 µm and 2 cm (including Fe oxyhydroxides as coatings), and C removed from clay-sized material by 2 % hydrogen peroxide had TTs averaging 190 ± 190 years in surface horizons. Summed over the bulk soil profile, we found that smectite content correlated with the mean TT of bulk soil C across varied lithologies. The SRO mineral content in KNP soils was generally very low, except for the soils developed on gabbros under more humid climate that also had very high Fe and C contents with a surprisingly short, mean C TTs. In younger landscapes, SRO minerals are metastable and sequester C for long timescales. We hypothesize that in the KNP, SRO minerals represent a transient stage of mineral evolution and therefore lock up C for a shorter time. <br><br> Overall, we found crystalline Fe-oxyhydroxides (determined as the difference between Fe in dithionate citrate and oxalate extractions) to be the strongest predictor for soil C content, while the mean TT of soil C was best predicted from the amount of smectite, which was also related to more easily measured bulk properties such as cation exchange capacity or pH. Combined with previous research on C turnover times in 2 : 1 vs. 1 : 1 clays, our results hold promise for predicting C inventory and persistence based on intrinsic timescales of specific carbon–mineral interactions

    Living on borrowed time – Amazonian trees use decade‐old storage carbon to survive for months after complete stem girdling

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    Nonstructural carbon (NSC) reserves act as buffers to sustain tree activity during periods when carbon (C) assimilation does not meet C demand, but little is known about their age and accessibility; we designed a controlled girdling experiment in the Amazon to study tree survival on NSC reserves. We used bomb-radiocarbon (14C) to monitor the time elapsed between C fixation and release (‘age’ of substrates). We simultaneously monitored how the mobilization of reserve C affected δ13CO2. Six ungirdled control trees relied almost exclusively on recent assimilates throughout the 17 months of measurement. The Δ14C of CO2 emitted from the six girdled stems increased significantly over time after girdling, indicating substantial remobilization of storage NSC fixed up to 13–14 yr previously. This remobilization was not accompanied by a consistent change in observed δ13CO2. These trees have access to storage pools integrating C accumulated over more than a decade. Remobilization follows a very clear reverse chronological mobilization with younger reserve pools being mobilized first. The lack of a shift in the δ13CO2 might indicate a constant contribution of starch hydrolysis to the soluble sugar pool even outside pronounced stress periods (regular mixing). © 2018 The Authors. New Phytologist © 2018 New Phytologist Trus

    Human impacts on soil carbon dynamics of deep-rooted Amazonian forests and effect of land use change on the carbon cycle in Amazon soils

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    The main objective of these NASA-funded projects is to improve our understanding of land-use impacts on soil carbon dynamics in the Amazon Basin. Soil contains approximately one half of tropical forest carbon stocks, yet the fate of this carbon following forest impoverishment is poorly studied. Our mechanistics approach draws on numerous techniques for measuring soil carbon outputs, inputs, and turnover time in the soils of adjacent forest and pasture ecosystems at our research site in Paragominas, state of Para, Brazil. We are scaling up from this site-specific work by analyzing Basin-wide patterns in rooting depth and rainfall seasonality, the two factors that we believe should explain much of the variation in tropical soil carbons dynamics. In this report, we summarize ongoing measurements at our Paragominas study site, progress in employing new field data to understand soil C dynamics, and some surprising results from our regional, scale-up work

    Contribution of new photosynthetic assimilates to respiration by perennial grasses and shrubs: residence times and allocation patterns

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    Quantification of the fate of carbon (C) used by plant metabolism is necessary to improve predictions of terrestrial ecosystem respiration and its sources. Here, a dual isotope (C-13 and C-14) pulse-label was used to determine the allocation of new C to different respiratory pathways in the early and late growing seasons for two plant functional types, perennial grasses and shrubs, in the Owens Valley, CA, USA. Allocation differences between plant types exceeded seasonal allocation variation. Grasses respired 71 and 64% and shrubs respired 22 and 17% of the label below-ground in the early and late growing seasons, respectively. Across seasons and plant types, similar to 48-61% of the label recovered was respired in 24 h, similar to 68-84% in 6 d, and similar to 16-33% in 6-36 d after labeling. Three C pools were identified for plant metabolism: a fast pool with mean residence times (MRTs) of similar to 0.5 and similar to 1 d below- and above-ground, respectively; an intermediate pool with MRTs of 19.9 and 18.9 d; and a storage pool detected in new leaf early growing season respiration > 9 months after assimilation. Differences in allocation to fast vs intermediate C pools resulted in the mean age of C respired by shrubs being shorter (3.8-4.5 d) than that of the grasses (4.8-8.2 d)

    Models of soil organic matter decomposition: the SOILR package, version 1.0

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    Soil organic matter decomposition is a very important process within the Earth system because it controls the rates of mineralization of carbon and other biogeochemical elements, determining their flux to the atmosphere and the hydrosphere. SOILR is a modeling framework that contains a library of functions and tools for modeling soil organic matter decomposition under the R environment for computing. It implements a variety of model structures and tools to represent carbon storage and release from soil organic matter. In SOILR, organic matter decomposition is represented as a linear system of ordinary differential equations that generalizes the structure of most compartment-based decomposition models. A variety of functions is also available to represent environmental effects on decomposition rates. This document presents the conceptual basis for the functions implemented in the package. It is complementary to the help pages released with the software

    A special issue preface: radiocarbon in the Anthropocene

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    The Anthropocene is defined by marked acceleration in human-induced perturbations to the Earth system. Anthropogenic emissions of CO2 and other greenhouse gases to the atmosphere and attendant changes to the global carbon cycle are among the most profound and pervasive of these perturbations. Determining the magnitude, nature and pace of these carbon cycle changes is crucial for understanding the future climate that ecosystems and humanity will experience and need to respond to. This special issue illustrates the value of radiocarbon as a tool to shed important light on the nature, magnitude and pace of carbon cycle change. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'
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