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ABSTRACT The current soil carbon paradigm puts particulate organic carbon (POC) as one of the major components of soil organic carbon worldwide, highlighting its pivotal role in carbon mitigation. In this study, we compiled a global dataset of 3418 data points of POC concentration in soils and applied empirical modeling and machine learning algorithms to investigate the spatial variation in POC concentration and its controls. The global POC concentration in topsoil (0–30 cm) is estimated as 3.02 g C/kg dry soil, exhibiting a declining trend from polar regions to the equator. Boreal forests contain the highest POC concentration, averaging at 4.58 g C/kg dry soil, whereas savannas exhibit the lowest at 1.41 g C/kg dry soil. We developed a global map of soil POC density in soil profiles of 0‐30 cm and 0–100 cm with an empirical model. The global stock of POC is 158.15 Pg C for 0–30 cm and 222.75 Pg C for 0–100 cm soil profiles with a substantial spatial variation. Analysis with a machine learning algorithm concluded the predominate controls of edaphic factors (i.e., bulk density and soil C content) on POC concentration across biomes. However, the secondary controls vary among biomes, with solid climate controls in grassland, pasture, and shrubland, while strong vegetation controls in forests. The biome‐level estimates and maps of POC density provide a benchmark for modeling C fractions in soils; the various controls on POC suggest incorporating biological and physiochemical mechanisms in soil C models to assess and forecast the soil POC dynamics in response to global change.
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Particulate Organic Carbon Deconstructed: Molecular and Chemical Composition of Particulate Organic Carbon in the Ocean
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Abstract Biospheric particulate organic carbon (POCbio) burial and rock petrogenic particulate organic carbon (POCpetro) oxidation are opposing long‐term controls on the global carbon cycle, sequestering and releasing carbon, respectively. Here, we examine how watershed glacierization impacts the POC source by assessing the concentration and isotopic composition (δ13C and Δ14C) of POC exported from four watersheds with 0%–49% glacier coverage across a melt season in Southeast Alaska. We used two mixing models (age‐weight percent and dual carbon isotope) to calculate concentrations of POCbioand POCpetrowithin the bulk POC pool. The fraction POCpetrocontribution was highest in the heavily glacierized watershed (age‐weight percent: 0.39 ± 0.05; dual isotope: 0.42 (0.37–0.47)), demonstrating a glacial source of POCpetroto fjords. POCpetrowas mobilized via glacier melt and subglacial flow, while POCbiowas largely flushed from the non‐glacierized landscape by rain. Flow normalized POCbioconcentrations exceeded POCpetroconcentrations for all streams, but surprisingly were highest in the heavily glacierized watershed (mean: 0.70 mgL−1; range 0.16–1.41 mgL−1), suggesting that glacier rivers can contribute substantial POCbioto coastal waters. Further, the most heavily glacierized watershed had the highest sediment concentration (207 mgL−1; 7–708 mgL−1), and thus may facilitate long‐term POCbioprotection via sediment burial in glacier‐dominated fjords. Our results suggest that continuing glacial retreat will decrease POC concentrations and increase POCbio:POCpetroexported from currently glacierized watersheds. Glacier retreat may thus decrease carbon storage in marine sediments and provide a positive feedback mechanism to climate change that is sensitive to future changes in POCpetrooxidation.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3389/fmars.2020.00518
