G05_P01

What are soil microbes eating? Novel methods for determining the age of microbially utilized soil carbon

Finstad K¹,  Nuccio E¹, Grant K¹, Broek T², Pett-Ridge J¹, McFarlane K¹

¹Lawrence Livermore National Laboratory, Livermore, United States, ²Woods Hole Oceanographic Institution, Falmouth, 02543

Soils represent a large component of the global C cycle, storing more C than plants and the atmosphere combined. Microbial processing of organic matter in soil is a main driver of soil C cycling, yet we lack robust methods for accurately identifying the age of C respired by microbial communities, hindering our ability to predict how disturbance or climate change affects soil C persistence. The most common method for identifying the age of microbially respired C is through laboratory incubations where homogenized soils are incubated in sealed jars until sufficient CO₂ has accumulated for ¹⁴C measurement. However, comparison of lab incubation ¹⁴CO₂ to in situ field collections suggests that soil sampling and processing can cause incubated microbes to respire older C than they would under natural conditions. We therefore seek to develop a method to determine the age of microbially utilized C more accurately, such as the extraction and dating of microbial biomass. We find that in general, the Δ¹⁴C of CO₂ from lab incubations in surface soils is indistinguishable from the Δ¹⁴C of chloroform extracted microbial biomass, but the values diverge at depth, with the lab incubations often producing depleted values relative to the extracted biomass. These findings corroborate previous suspicions that laboratory incubations may bias the results and falsely suggest the consumption of older soil C than occurs under natural conditions. Future work is being conducted to refine this method and investigate the utility of ¹⁴C measurements on nucleic acids extracted from soil.

G05_P02

Divergence of compound class persistence in a California grassland soil

Grant K¹,  Repasch M^1,2, Finstad K¹, Broek T^1,3, McFarlane K¹

¹Lawrence Livermore National Laboratory, Livermore, United States, ²Institute of Arctic and Alpine Research, University of Colorado, Boulder, Boulder, United States, ³National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility, Woods Hole Oceanographic Institution, Woods Hole, United States

Soils store more carbon than the atmosphere and vegetation combined. Soil organic carbon (SOC) is composed of a complex mixture of plant and microbial derived organic compounds with distinct cycling timescales. The residence time of individual SOC components depends on a combination of factors, including compound reactivity, mineral association, and climate conditions, making it difficult to accurately quantify. However, radiocarbon analysis of specific compound classes can disentangle the mixture of SOC ages within a single sample. We modified methods to measure the Δ¹⁴C of distinct compound classes (lipids, amino acids, and carbohydrates) from bulk and physically fractionated grassland soils. Additionally, we measured the Δ¹⁴C of the water-extractable fraction (WEOC) and the residual acid-insoluble fraction. Samples were collected from a grassland meadow in Hopland, CA which receives 940 mm yr-1 of rainfall and is dominated by Avena barbata. We sampled a 1m soil pit at depth intervals (0-10, 10-20, 20-50, 50-100 m) to study changes in SOC persistence with depth. We used solid state ¹³C-NMR to measure the relative abundance of the target compound classes in soil. The Δ14C of bulk soil decreased from +28±6‰ at 0-10cm to -495±23‰ at the 50-100cm depth interval. The clay fraction (<63μm) had higher Δ¹⁴C values than both the bulk (<2mm) and sand (<2mm to >63μm) fractions. WEOC Δ¹⁴C values ranged from modern to about -45.6‰. Δ¹⁴C values of total extracted lipids ranged from 36±4‰ at the surface to -215±3‰ at depth. Quantifying the age distribution of distinct compound classes gives insight into SOC persistence.

G05_P03

Towards a comprehensive understanding of the drivers of the reservoir effect (dead carbon fraction) in stalagmites - a modelling approach

Lechleitner F¹,  Day C², Welte C³, Fohlmeister J⁴, Stoll H⁵

¹Department of Chemistry, Biochemistry and Pharmaceutical Sciences and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland, ²Department of Earth Sciences, University of Oxford, Oxford, UK, ³Department of Earth Sciences and Laboratory of Ion Beam Physics, ETH Zürich, Zürich, Switzerland, ⁴Federal Office for Radiation Protection, Berlin, Germany, ⁵Department of Earth Sciences, ETH Zürich, Zürich, Switzerland

A growing number of stalagmite ¹⁴C records have been generated over the past decades, from multiple climatic states, ecosystem types and lithologies. A number of processes have been identified that influence speleothem ¹⁴C activities, leading to deviations from the atmospheric value (the reservoir effect). However, it has so far been difficult to extract globally relevant relationships that describe the connection between environmental conditions and stalagmite ¹⁴C values.

Using numerical forward modelling and published datasets, we present a suite of sensitivity analyses that test the relative importance of different processes and carbon pools on the stalagmite reservoir effect. To evaluate model performance, we compare the model output of key chemical parameters in dripwaters relating to stalagmite ¹⁴C (δ¹³C, pH, Ca²+) from two independent geochemical models: PHREEQC-based CaveCalc (Owen et al., 2018) and a simpler calcite dissolution model (Fohlmeister et al., 2011).

Subsequenly, we test the sensitivity of the stalagmite reservoir effect to processes that depend on climate, ecosystem and lithological parameters above the cave. In particular we focus on the impacts of: i) a large pre-aged soil OM reservoir, ii) host rock dissolution under a suite of conditions from open to closed system, iii) pyrite oxidation. These processes have been suggested to play a significant role, particularly in stalagmites that have very high reservoir effect values (> 50%). Our results help to identify whether observed variations in stalagmite reservoir effect can be realistically explained by such processes, and contribute to a global understanding of the factors influencing the stalagmite reservoir effect.

G05_P04

Old-growth forest and adjacent prairie show contrasting soil carbon properties not linked to aboveground litter input and chemistry

McFarlane K¹,  Mambelli S², Porras R³, Wiedemeier D⁴, Schmidt M⁴, Dawson T², Torn M^2,3

¹Lawrence Livermore National Laboratory, Livermore, United States, ²University of California - Berkeley, Berkeley, United States, ³Lawrence Berkeley National Laboratory, Berkeley, United States, ⁴University of Zurich, Zurich, Switzerland

Old-growth coast redwood (Sequoia sempervirens) forests store more carbon in aboveground biomass per area than any ecosystem, in trees that are among the oldest, largest, and most productive plant species on earth.  Moreover, redwood litter contains high levels of aromatic compounds and is relatively resistant to decay. However, little is known about belowground carbon storage or turnover time in these forests.  We compared soil carbon storage, distribution, chemical composition, and age in an old-growth redwood forest and adjacent prairie with comparatively lower productivity and more decomposable litter. Contrary to what the relative litter quality would suggest, total soil carbon stocks to 110 cm depth were higher in prairie (350 Mg C ha-1) than in redwood (277 Mg C ha-1) even with the forest O-horizon included, although differences were limited to the top 50 cm. In addition, radiocarbon values reflected shorter turnover times for bulk soil and light density fractions in redwood than prairie throughout the sampled profile. Higher amounts of pyrogenic carbon and a higher degree of SOM stabilization, as indicated by light density fraction carbon molecular characterization with ¹³C-NMR spectroscopy, appear to be instrumental in explaining the larger soil carbon stocks and longer turnover times in prairie, while differences in fine-root carbon inputs likely contribute to comparatively shorter turnover times in redwood. We conclude that at these sites fire residues, root inputs, and soil properties influence soil carbon dynamics to a greater degree than the properties of aboveground litter.

G05_P05

Species-definite AMS ¹⁴C dating, ²¹⁰Pb and ¹³⁷Cs dating on a peat core from Jinchuan Mire, NE China

Misra S¹,  Kashyap S¹, Chou C¹, Chang T¹, Li H¹

¹National Taiwan University, Taipei, Taiwan

AMS ¹⁴C, ²¹⁰Pb and ¹³⁷Cs dating have been done on a 92-cm peat core from Jinchuan Mire in NE China, showing sedimentation rates ranging from 0.066 to 0.54 cm/year over the last 1050 years. Carex lehmanii (a plant of the genus Sedge in the Cyperaceae family) has been chosen for AMS ¹⁴C dating. A total of 110 AMS ¹⁴C dates on Carex from 85 horizons were yielded, in which 16 samples were treated by ABA treatment. The high resolution ¹⁴C dates show significantly variations throughout the core, indicating serious old carbon influence. The comparisons among the ¹⁴C, ²¹⁰Pb and ¹³⁷Cs dating results show mobilization problem of ²¹⁰Pb and ¹³⁷Cs. The detailed dating results exhibit that peat accumulation was 0.102, 0.54 (human impact) and 0.066 cm/yr during periods of 2018~1964, 1964~1950 and 1950~900 CE, respectively. Furthermore, the ABA treated Carex is generally older than the non-ABA treated Carex in the same depth, except samples below 85 cm depth, implying that old carbon influence between 85 and 92 cm was negligible. The decomposition of the peat plants in deeper layers which is related to plant species and climatic conditions and groundwater table which is related to climatic conditions and drilling sites may cause the variation of old carbon (in the dissolved CO₂ uptaken by Carex) influence on the Carex ¹⁴C ages. The Carex AMS ¹⁴C dates will help us not only to refine the previous age model of JCA but also to understand hydro-climatic information recorded by JCA core sediments.

G05_P06

Multi-Pool Monitoring of Organic and Inorganic Carbon at Milandre Cave, Switzerland – Implications for Future Paleoecosystem Proxies.

Rowan S¹,  Luetscher M², Szidat S¹, Laemmel T¹, Kost O³, Lechleitner F¹

¹University Of Bern, Bern, Switzerland, ²Swiss Institute for Speleology and Karst Studies, La Chaux-de-Fonds, Switzerland, ³ETH Zurich, Zurich, Switzerland

The organic matter (OM) fraction of speleothems, typically comprising 0.01-0.3% of the total carbon (Blyth et al., 2016), has the potential to offer information about past ecosystems. The provenances of speleothem OM are not well understood, though are speculated to be dominated by contributions from overlying vegetation and soil. Other potential sources include microbial activity within the karst system, cave fauna, and fossil carbon sourced from the carbonate (Blyth et al., 2016). The isotopic characterisation (𝛿¹³C and ¹⁴C) of stalagmite OM may give information about past ecological and climatic state of the surrounding region (Blyth et al., 2016).

Here we present the first results of a monitoring study of the organic and inorganic carbon fluxes in Milandre cave (Switzerland), whereby the main carbon source reservoirs will be monitored for two years. Our preliminary data includes 1) atmospheric, cave, soil, and well CO² 𝛿¹³C and ¹⁴C, and 2) cave drip water dissolved inorganic carbon (DIC) 𝛿¹³C, collected before significant degassing could take place. The cave gas samples are more depleted in ¹⁴C than soil and well gas samples. This suggests either an additional fossil reservoir of CO² contributing to the cave air or substantial influence from degassing of carbonate-derived CO₂ from drips in the cave. The DIC 𝛿¹³C is isotopically light, implying that the cave is an open system with a substantial contribution of biologically respired CO₂ feeding carbonate growth. This information will be used to constrain the source of speleothem OC and its suitability as a proxy.

G05_P07

Isotopic signatures of fine organic aerosol in the deciduous forest and photosynthetic isotopic discrimination: Insights from compound-specific radiocarbon analysis

Uchida M¹,  Kumata H, Kondo M^3,4, Chikaraishi Y⁵, Murayama S⁶, Mantoku K¹, Kobayashi T¹, Kawamura K⁵, Saigusa N¹, Koizumi H^3,8, Shibata Y⁹

¹Earth System Division, National Institute For Environmental Studies, Tsukuba, Japan, ²Faculty of Life Science, Tokyo University of Pharmacy and Life Science, , Japan, ³River Basin Research Center, Gifu University, , Japan, ⁴Now at Health and Environmental Risk Division, National Institute For Environmental Studies, Tsukuba, Japan, ⁵ Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan, ⁶National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan, ⁷River Basin Research Center, Gifu University, , Japan, ⁸Now at Waseda University, , Japan, ⁹Tokyo University of Science, , Japan

Organic aerosols including secondary organic aerosols have serious impacts on the Earth’s climate system directly by scattering and absorbing solar radiation and indirectly by acting as cloud condensation nuclei. Forests release large amounts of volatile organic compounds into the atmosphere. However, the sources and fate of organic compounds of biogenic origin in a forest ecosystem are not yet well understood. This knowledge is also of importance to well understanding accurate carbon isotopic discrimination (Δ) by photosynthesis at the ecosystem scale for modeling terrestrial uptake of carbon dioxide. To understand the origin and fate of biogenic organic compounds in forest aerosol, we measured stable carbon isotopic ratios (δ¹³C) and radiocarbon (¹⁴C) contents of n-fatty acids, n-alkanes, and total organic carbon in forest aerosols, soil, and plant material as well as atmospheric CO₂. Fine aerosol samples (PM₁₀) were collected at few-week intervals from August 2003 to November 2004 during the growing season at Takayama Experimental site (36˚80’N, 137˚26’E, 1420m a.s.l.) in a cool-temperate deciduous forest in Japan. Based on the results obtained, we discuss the sources and turnover time, and temporal variations of forest organic molecules as well as estimated plant wax-based photosynthetic isotopic discrimination(Δ) .

G05_P08

Peatland initiation and carbon accumulation history during the Holocene in Xinjiang, China

Zhao H^1,2,3,  Zhou W^1,2,4,5,6,7, Cheng P^1,2,5,6, Du H^1,2, Xian F^1,2,4, Shu P^1,2

¹The State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China, Xi'an, China, ²Shaanxi Key Laboratory of Accelerator Mass Spectrometry Technology and Application, Xi’an, 710061, China, Xi'an, China, ³Xi’an Institute for Innovative Earth Environment Research, Xi'an, China, ⁴CAS Center for Excellence in Quaternary Science and Global Change, , China, ⁵Xi’an Jiaotong University, Xi'an, China, ⁶Open Studio for Oceanic-Continental Climate and Environment Changes, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao, China, ⁷Beijing Normal University, interdisciplinary Research Center of Earth Sciences, Beijing, China

Peatland ecosystems are important terrestrial carbon reservoirs, understanding the responses of carbon dynamics to climate change will provide useful insights into projecting the fate of peatland carbon in the future. Most studies about the carbon dynamics of peatlands were focused on boreal and tropical peatlands now. However, there are rare data about carbon dynamics in the Xinjiang region, which is sensitive to climate change. Here, we studied the carbon accumulation histories during the Holocene by measuring the radiocarbon ages, total organic carbon(TOC), and bulk dry density(BDD) at Halashazi(ALT, 48°06′58.2′′N, 88°21′50.8′′E, 2456.7 msl) and Tielishahan peatlands(KNS, 48°48′87.4′′N, 86°54′60.0′′E, 1766.5 msl) in the Altay mountains. The result indicates a rapid carbon accumulation (averages are 98.4 and 107.2 g C/m²/yr in ALT and KNS profile, respectively) happened in the early Holocene(10.5-8.4ka), then carbon accumulation deposition flux decreased (59.9 and 60.4 g C/m²/yr) during the mid-Holocene(8.4-4.2ka), after that, it significantly elevated(58.1 and 69.9 g C/m²/yr) during the early late-Holocene(4.2-2.0ka), followed by a rapid decline(only recorded in KNS profile with the average of 71.9 g C/m²/yr) at the end of late Holocene(after 2.0ka). Systematic analysis with 51 basal ages of peatlands in Xinjiang also suggested the fast peat initiation during the early Holocene and late Holocene. The peatland initiation and accumulation histories in Xinjiang are linked with the temperature records, suggesting the possible causal connection between peatland dynamics and local temperature. Furthermore, the fast initiation and accumulation of peat during the late Holocene might have contributed to high atmospheric carbon dioxide concentrations.

G05_P09

Why do tree tissues have older radiocarbon ages than chronological ages?

Hilman B¹,  Solly E², Hagedorn F³, Kuhlman I¹, Herrera-Ramírez D¹, Trumbore S¹

¹Max-planck society, Jena, Germany, ²ETH Zurich, Zurich, Switzerland, ³Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland

Tree tissues and especially fine roots (≤ 2mm) often have older ¹⁴C ages than chronological ages (¹⁴C excess). The common explanation is that stored and old non-structural carbon (NSC) contributes to new tissue growth. Accordingly, the ¹⁴C excess increases with the proportion of stored vs. recently fixed NSC in the growth substrate. Here we suggest that rather than this proportion, what determines the ¹⁴C excess is the turnover of the NSC pool. We demonstrate this using measurements of needles, branches, and fine roots of two coniferous trees in a treeline ecotone in Stillberg, Switzerland. In such ecotones the non-structural carbohydrates (NSCarb) stocks increase with elevation, probably due to the fact that low temperatures suppress growth more strongly than photosynthesis. In addition, we did not expect large variations in the proportion of stored NSC in the growth substrate. We expected initially that increasing NSCarb stocks with elevation is a proxy for slower turnover rates and therefore greater ¹⁴C excess. However, we observed the opposite trend in the fine roots with turnover intensification and smaller excess ¹⁴C towards the treeline, while ¹⁴C excess in aboveground tissues did not vary with elevation. Based on current and previous results we conclude that the greater surplus of photo-assimilates in the treeline increases fluxes to both NSCarb and to belowground. The greater flux belowground speeds the turnover rate of the roots NSC and lowers its ¹⁴C excess.