Radiocarbon Signatures Research Articles

Effects of experimental drought on soil respiration and radiocarbon efflux from a temperate forest soil

AbstractSoil moisture affects microbial decay of SOM and rhizosphere respiration (RR) in temperate forest soils, but isolating the response of soil respiration (SR) to summer drought and subsequent wetting is difficult because moisture changes are often confounded with temperature variation. We distinguished between temperature and moisture effects by simulation of prolonged soil droughts in a mixed deciduous forest at the Harvard Forest, Massachusetts. Roofs constructed over triplicate 5 × 5 m2 plots excluded throughfall water during the summers of 2001 (168 mm) and 2002 (344 mm), while adjacent control plots received ambient throughfall and the same natural temperature regime. In 2003, throughfall was not excluded to assess the response of SR under natural weather conditions after two prolonged summer droughts. Throughfall exclusion significantly decreased mean SR rate by 53 mg C m−2 h−1 over 84 days in 2001, and by 68 mg C m−2 h−1 over 126 days in 2002, representing 10–30% of annual SR in this forest and 35–75% of annual net ecosystem exchange (NEE) of C. The differences in SR were best explained by differences in gravimetric water content in the Oi horizon (r2=0.69) and the Oe/Oa horizon (r2=0.60). Volumetric water content of the A horizon was not significantly affected by throughfall exclusion. The radiocarbon signature of soil CO2 efflux and of CO2 respired during incubations of O horizon, A horizon and living roots allowed partitioning of SR into contributions from young C substrate (including RR) and from decomposition of older SOM. RR (root respiration and microbial respiration of young substrates in the rhizosphere) made up 43–71% of the total C respired in the control plots and 41–80% in the exclusion plots, and tended to increase with drought. An exception to this trend was an interesting increase in CO2 efflux of radiocarbon‐rich substrates during a period of abundant growth of mushrooms.Our results suggest that prolonged summer droughts decrease primarily heterotrophic respiration in the O horizon, which could cause increases in the storage of soil organic carbon in this forest. However, the C stored during two summers of simulated drought was only partly released as increased respiration during the following summer of natural throughfall. We do not know if this soil C sink during drought is transient or long lasting. In any case, differential decomposition of the O horizon caused by interannual variation of precipitation probably contributes significantly to observed interannual variation of NEE in temperate forests.

Partitioning sources of soil‐respired CO2 and their seasonal variation using a unique radiocarbon tracer

AbstractSoil respiration is derived from heterotrophic (decomposition of soil organic matter) and autotrophic (root/rhizosphere respiration) sources, but there is considerable uncertainty about what factors control variations in their relative contributions in space and time. We took advantage of a unique whole‐ecosystem radiocarbon label in a temperate forest to partition soil respiration into three sources: (1) recently photosynthesized carbon (C), which dominates root and rhizosphere respiration; (2) leaf litter decomposition and (3) decomposition of root litter and soil organic matter >1–2 years old.Heterotrophic sources and specifically leaf litter decomposition were large contributors to total soil respiration during the growing season. Relative contributions from leaf litter decomposition ranged from a low of ∼1±3% of total soil respiration (6± 3 mg C m−2 h−1) when leaf litter was extremely dry, to a high of 42±16% (96± 38 mg C m−2 h−1). Total soil respiration fluxes varied with the strength of the leaf litter decomposition source, indicating that moisture‐dependent changes in litter decomposition drive variability in total soil respiration fluxes. In the surface mineral soil layer, decomposition of C fixed in the original labeling event (3–5 years earlier) dominated the isotopic signature of heterotrophic respiration.Root/rhizosphere respiration accounted for 16±10% to 64±22% of total soil respiration, with highest relative contributions coinciding with low overall soil respiration fluxes. In contrast to leaf litter decomposition, root respiration fluxes did not exhibit marked temporal variation ranging from 34±14 to 40±16 mg C m−2 h−1 at different times in the growing season with a single exception (88±35 mg C m−2 h−1). Radiocarbon signatures of root respired CO2 changed markedly between early and late spring (March vs. May), suggesting a switch from stored nonstructural carbohydrate sources to more recent photosynthetic products.

Dynamics of fine root carbon in Amazonian tropical ecosystems and the contribution of roots to soil respiration

AbstractRadiocarbon (14C) provides a measure of the mean age of carbon (C) in roots, or the time elapsed since the C making up root tissues was fixed from the atmosphere. Radiocarbon signatures of live and dead fine (<2 mm diameter) roots in two mature Amazon tropical forests are consistent with average ages of 4–11 years (ranging from <1 to >40 years). Measurements of 14C in the structural tissues of roots known to have grown during 2002 demonstrate that new roots are constructed from recent (<2‐year‐old) photosynthetic products. High Δ14C values in live roots most likely indicate the mean lifetime of the root rather than the isotopic signature of inherited C or C taken up from the soil.Estimates of the mean residence time of C in forest fine roots (inventory divided by loss rate) are substantially shorter (1–3 years) than the age of standing fine root C stocks obtained from radiocarbon (4–11 years). By assuming positively skewed distributions for root ages, we can effectively decouple the mean age of C in live fine roots (measured using 14C) from the rate of C flow through the live root pool, and resolve these apparently disparate estimates of root C dynamics. Explaining the 14C values in soil pore space CO2, in addition, requires that a portion of the decomposing roots be cycled through soil organic matter pools with decadal turnover time.

Radiocarbon – a low‐impact tool to study nutrient transport by soil fungi under field conditions

Here, we present a new in-situ method to study the uptake of amino acids by soil fungi. We injected 14C-labeled glycine into a marshland soil and measured the rate and the 14C signature of CO2 respired from sporocarps of Pholiota terrestris over 53.5 h and 2 m. We also determined the incorporation of glycine-C into sporocarp tissue. The 14C signature of the CO2 and tissue was quantified by accelerator mass spectrometry. After the label application, the rate of CO2 flux and its 14C signature from chambers with sporocarps were significantly higher than from chambers without sporocarps, and then declined with time. Postlabel, the 14C signature of the sporocarp tissue increased by 35 per thousand. We show that this approach can be used to study below-ground food webs on an hourly time-scale while minimizing the perturbation of competitive relationships among soil microorganisms and between plants and soil microorganisms. Additionally we show that care must be taken to avoid confounding effects of sporocarp senescence on rates and radiocarbon signatures of respired CO2.

Hydrocarbons in Lake Washington sediments. A 25-year retrospective in an urban lake.

Aliphatic and polycyclic aromatic hydrocarbon and stable and radiocarbon isotope distributions are compared for dated cores from the 1970s and 2000 for a 25-year retrospective in Lake Washington, Seattle, WA (USA). Contamination of Lake Washington sediments by petrogenic aliphatic hydrocarbons and pyrolytic polycyclic aromatic hydrocarbons via atmospheric deposition and stormwater runoff peaked between the 1950s and 1970s and has since decreased as stormwater inputs have been reduced. Radiocarbon signatures (delta14C, per 1000) of total organic carbon decrease (increased "age") in the depth interval of highest hydrocarbon concentration. Graphitic black carbon in the year 2000 core showed a historical profile similar to that of the PAH; however high background sediments deposited before the founding of Seattle indicates a considerable nonindustrial component derived from weathering in the watershed. Unlike hydrocarbon contamination, input of terrestrial organic matter (tracked by long-chain fatty alcohols) has increased throughout the late 20th century, documenting a shift in pollutant sources away from hydrocarbons and toward anthropogenic erosion of the region's soils.

Soil-productivity relationships and organic matter turnover in dry tropical forests of the Florida Keys

Soils and aboveground production in five types of upland forest in the Florida Keys were examined. Throughout the habitat gradient represented by these forest types, the soils were predominantly shallow and organic, forming in place directly on the limestone bedrock. However, the well-drained soils in the most productive broadleaved forests were deep enough to qualify as Histosols (Folists). Soils decreased in electrical conductivity and increased in nutrient content with increasing aboveground production. At 3–12 Mg ha−1 yr−1, production was within the range reported for dry tropical forests. Measured rates of decomposition were moderate or fast, and estimates of the organic C turnover of several soils based on their bomb radiocarbon signature were 100 years or less. In the face of these rapid turnover rates, we attribute the development of organic soils to the absence of mineral residues from weathering of the underlying limestone bedrock. Fast turnover of organic matter, and rapid and efficient cycling of nutrients are necessary to sustain the high production rates obtained on these shallow organic soils.

Bomb dating and age determination of large pelagic sharks

Despite their notoriety and role as apex predators, the longevity of large pelagic sharks such as the porbeagle (Lamna nasus) and shortfin mako (Isurus oxyrinchus) is unknown. Vertebral growth bands provide an accurate indicator of age in young porbeagle, but age validation has never been reported for any large shark species past the age of sexual maturity. Here, we report the first application of bomb radiocarbon as an age validation method for long-lived sharks based on date-specific incorporation of radiocarbon into vertebral growth bands. Our results indicate that porbeagle vertebrae recorded and preserved a bomb radiocarbon pulse in growth bands formed during the 1960s. Through comparison of radiocarbon assays in young, known-age porbeagle collected in the 1960s with the corresponding growth bands in old porbeagle collected later, we confirm the validity of porbeagle vertebral growth band counts as accurate annual age indicators to an age of at least 26 years. The radiocarbon signatures of porbeagle vertebral growth bands appear to be temporally and metabolically stable and derived mainly from the radiocarbon content of their prey. Preliminary radiocarbon assays of shortfin mako vertebrae suggest that current methods for determining shortfin mako age are incorrect.

Decomposition of peat from upland boreal forest: Temperature dependence and sources of respired carbon

The response of large stores of carbon in boreal forest soils to global warming is a major uncertainty in predicting the future carbon budget. We measured the temperature dependence of decomposition for upland boreal peat under black spruce forest with sphagnum and feather moss understory using incubation experiments. CO2 efflux rates clearly responded to temperature, which ranged from −10° to +8°C by ∼2°C increments. At temperatures below 0°C, significant decomposition was observed in feather moss peat but not in wetter sphagnum peat. Above 0°C, decomposition was exponentially related to temperature, corresponding to a Q(10) (the ratio of the rate of CO2 evolution at one temperature divided by that at a temperature 10°C cooler) of 4.4 for feather moss and 3.1 for sphagnum peat. The greatest change in CO2 evolution rate with temperature occurred between −2° and 0°C, which coincided with the phase transition of soil water. We saw no large change in the rate of CO2 evolution between incubation experiments separated by a 6 month storage period for feather moss peat. Stable C isotope measurements of evolved CO2 and the rate of change of CO2 evolution with time suggest different substrates are used to sustain heterotrophic respiration above and below freezing. Radiocarbon signatures of CO2 respired from both types of peat reflected significant contributions from C fixed in the last 35 years (“bomb” 14C) as well as C fixed prior to 1950. We observed no change in the Δ14C of respired CO2 with temperature. Isotopic signatures of peat components showed that a combination of substrates must contribute to the CO2 evolved in our incubations. Decomposition of fine roots (which made up less than 7% of the total peat C) accounted for ∼50% of respired CO2 in feather moss peat and for ∼30% of respired CO2 in sphagnum peat. Fine‐grained (<1 mm), more humified material that makes up 60–70% of the bulk peat organic carbon contributed significantly to heterotrophic respiration (∼30% in feather moss and ∼50% in sphagnum moss peat), despite slow decomposition rates. Increased temperatures caused enhanced decomposition from all pools without changing their relative contributions. Because the contribution of peat decomposition is a small portion of total soil respiration at the study site, increased respiration rates would be difficult to measure as increased fluxes in the field. Nonetheless, sustained warming could lead to significant loss of C from these peat layers.

Chemical and carbon isotopic evidence for the source and fate of dissolved organic matter in the northern Everglades

Surface waters in the Florida Everglades contain high levels ofdissolved organic carbon (DOC) compounds. δ13C values of DOCsamples collected from the northern Everglades indicate that less than about23%of the DOC was derived from sugarcane (the dominant agricultural crop in thearea), and the amount of DOC from sugarcane was greater during the dry period.Most of the DOC (> 50%) in the northern Everglades was in the low molecularweight ( modern” toabout 2400 years B.P., indicating that DOC was derived from both historic peatdeposits and modern vegetation. At each site, the HMW-DOC had older radiocarbonages than the LMW-DOC, and therefore contained a greater fraction of DOCderivedfrom the historic peat deposits. It appears that at least some of the old DOCcompounds from the historic peat deposits were decomposed during theirresidencein the surface water system in the northern Everglades, and the LMW-DOC wasmoremicrobially labile than the HMW-DOC. Our analysis suggests that accelerateddecomposition of organic matter in the historic peat deposits (due to land-usechange) could be a significant source of DOC and nutrients in the northernEverglades. Our data also suggest that the radiocarbon signature of DOC couldbeused as a sensitive indicator of the overall effectiveness of a wetlandrestoration project.

Evaluation of a protocol for the quantification of black carbon in sediments

Formation of highly condensed black carbon (BC) from vegetation fires and wood fuel combustion presumably transfers otherwise rapidly cycling carbon from the atmosphere‐biosphere cycle into a much slower cycling geological form. Recently reported BC fractions of total organic carbon (TOC) in surficial marine sediments span a wide range (2‐90%), leaving it presently unclear whether this variation reflects natural processes or is largely due to method differences. In order to elucidate the importance of BC to carbon burial the specificity of applied methods needs to be constrained. Here the operating range and applicability of a commonly used chemothermal oxidation (CTO) method is evaluated using putative BC standards, potentially interfering substances, and natural matrix standards. Test results confirm the applicability of the method to marine sediments. Integrity tests with model substrates suggest applicability to low‐carbon soils but only with a lower specificity to seawater particulate matter. The BC content of marine sediment samples in a set of studies employing the CTO method proved to be consistent with associated geochemical information. The radiocarbon content of the BC isolate in an environmental matrix standard was shown to be similar to the radiocarbon signature of pyrogenic polycyclic aromatic hydrocarbons (PAHs), here serving as molecular markers of combustion (fraction modern fM of BC was 0.065 ± 0.014 and of PAHs 0.056 ± 0.020), while being clearly distinct from the radiocarbon content of the bulk TOC (fM = 0.61 ± 0.08). Urgent questions such as the global accumulation rate of black carbon in soils and sediments may prove approachable with the chemothermal oxidation technique of BC quantification.

Radiocarbon in marine bacteria: Evidence for the ages of assimilated carbon

It is generally accepted that marine bacteria utilize labile, recently produced components of bulk dissolved organic matter. This interpretation is based largely on indirect measurements using model compounds and plankton‐derived organic matter. Here, we present an assessment of the relative proportions of modern and older dissolved organic carbon (DOC) utilized by marine bacteria. Bacterial nucleic acids were collected from both estuarine (Santa Rosa Sound, FL) and open‐ocean (eastern North Pacific) sites, and the natural radiocarbon signatures of the nucleic acid carbon in both systems were determined. Bacterial nucleic acids from Santa Rosa Sound were significantly enriched in radiocarbon with respect to the bulk DOC and were similar to the radiocarbon signature of atmospheric CO2 at the time of sampling, indicating that these bacteria exclusively assimilate a modern component of the estuarine bulk DOC. In contrast, bacterial nucleic acids from the oceanic site were enriched in 14C relative to the bulk DOC but depleted in 14C with respect to modern surface dissolved inorganic carbon (DIC) and suspended particulate organic carbon (POCsusp). This suggests that open‐ocean bacteria assimilate both modern and older components of DOC. The distinct radiocarbon signatures of the nucleic acids at these two sites (i.e., +120 ± 17‰ estuarine vs. −34 ± 24‰ oceanic) demonstrate that natural 14C abundance measurements of bacterial biomarkers are a powerful tool for investigations of carbon cycling through microbial communities in different aquatic systems.

Distributions of dissolved organic and inorganic carbon and radiocarbon in the eastern North Pacific continental margin

Temporal variations in the natural radiocarbon ( 14C) signatures of dissolved organic and inorganic carbon (DOC and DIC, respectively) in seawater have been studied previously (Druffel, E.R.M., Bauer, J.E., Williams, P.M., Griffin, S., Wolgast, D.M., 1996. Seasonal variability of radiocarbon in particulate organic carbon in the northeast Pacific. J. Geophys. Res. 101, 20 543–20 552; Bauer, J.E., Druffel, E.R.M., Williams, P.M., Wolgast, D.M., Griffin, S., 1998. Temporal variability in dissolved organic carbon and radiocarbon in the eastern North Pacific Ocean. J. Geophys. Res. 103, 2867–2882) at a long-term time-series station (Sta. M: 32°N, 123W) in the eastern North Pacific located at the eastern edge of the North Pacific abyssal plain. In June 1995 a transect was made from Sta. M inshore to approximately 500 m depth in order to evaluate the distributions of 14C in DOC and DIC from the abyssal plain to the upper continental slope. Concentrations and Δ 14C values of DOC in mixed layer waters (25 and 85 m) decreased toward the upper slope. In deeper waters, concentrations and Δ 14C values were in general similar at all three sites. Differences in DOC concentrations and Δ 14C-DOC between Sta. M and the rise and upper slope sites were explained in part by the mixing of DOC and Δ 14C along constant density ( σ t) surfaces. However, specific deviations from conservative behavior due to mixing were observed for Δ 14C-DOC at mesopelagic (∼700 m) and near-bottom (∼3600– 3900 m) depths of the continental rise. Comparable findings are reported for DIC, where σ t-normalized concentrations and Δ 14C values in Sta. M, rise and upper slope waters were similar, with the exception of slight increases in concentrations and Δ 14C values in near-bottom waters of the rise. These observations indicate that both DOC and DIC in continental rise and slope surface waters of the eastern North Pacific Ocean margin are comprised of a component of actively upwelled material derived from deeper offshore waters and a component of recent surface-derived material. Our data suggest that contributions of both 14C-enriched and 14C-depleted DOC and DIC to localized regions of the water column occur in waters of the continental rise and slope. The sources of material having anomalous Δ 14C values are not certain, but may include organic matter derived from off-shelf and slope transport, particulate organic carbon solubilization, and sediment porewaters. We also examine the relationships between Δ 14C-DOC and both Δ 14C-DIC and Δ 14C of suspended POC measured in a companion study as a means of evaluating similarities or differences in the mechanisms responsible for the distributions of each at all three sites.

Physical controls on dissolved inorganic radiocarbon variability in the California Current

We present depth profiles of Δ 14C and δ 13C of dissolved inorganic carbon (DIC) at Station M in the Eastern North Pacific. Several seasonal profiles are presented for the time period between 1991 and 1996. Comparison with GEOSECS data clearly shows changes in ocean radiocarbon profiles since 1973. The Δ 14C of DIC shows the most variability at depths of 450, 85, and 25 m, and the lowest variability at depths of 1600 and 2500 m. The largest variability in DIC Δ 14C occurs at 450 m, a depth marked by large fluctuations in the radiocarbon signatures of the source waters. The likely controls of DIC Δ 14C variability are physical changes in the circulation of the California Current System. A simple two-box model is used to show the importance of wind driven mixing at the surface. We discuss the likely effects of mesoscale eddies and ENSO on the DIC Δ 14C values at this site. We also show that remineralization of organic carbon (dissolved or particulate) is not responsible for the variability in the Δ 14C of DIC observed at Station M.

Gas Chromatographic Isolation of Individual Compounds from Complex Matrices for Radiocarbon Dating

This paper describes the application of a novel, practical approach for isolation of individual compounds from complex organic matrices for natural abundance radiocarbon measurement. This is achieved through the use of automated preparative capillary gas chromatography (PCGC) to separate and recover sufficient quantities of individual target compounds for (14)C analysis by accelerator mass spectrometry (AMS). We developed and tested this approach using a suite of samples (plant lipids, petroleums) whose ages spanned the (14)C time scale and which contained a variety of compound types (fatty acids, sterols, hydrocarbons). Comparison of individual compound and bulk radiocarbon signatures for the isotopically homogeneous samples studied revealed that Δ(14)C values generally agreed well (±10%). Background contamination was assessed at each stage of the isolation procedure, and incomplete solvent removal prior to combustion was the only significant source of additional carbon. Isotope fractionation was addressed through compound-specific stable carbon isotopic analyses. Fractionation of isotopes during isolation of individual compounds was minimal (<5‰ for δ(13)C), provided the entire peak was collected during PCGC. Trapping of partially coeluting peaks did cause errors, and these results highlight the importance of conducting stable carbon isotopic measurements of each trapped compound in concert with AMS for reliable radiocarbon measurements. The addition of carbon accompanying derivatization of functionalized compounds (e.g., fatty acids and sterols) prior to chromatographic separation represents a further source of potential error. This contribution can be removed using a simple isotopic mass balance approach. Based on these preliminary results, the PCGC-based approach holds promise for accurately determining (14)C ages on compounds specific to a given source within complex, heterogeneous samples.

Radiocarbon Signatures of Turnover Rates in Active Soil Organic Carbon Pools

AbstractIdentifying turnover rates in active soil organic carbon (SOC) pools is a critical step toward the understanding of soil nutrient cycling and global C cycles. The mean residence times (MRT) of the dominating active SOC pools in surface soils are usually less than a few decades. This range of MRT is difficult to determine because it is too short for radiocarbon dating and too long for most tracer techniques. I developed a procedure to construct radiocarbon signatures for identifying MRT in active SOC pools based on the residue bomb 14C in the atmosphere. The radiocarbon signature is specific to the year when a soil sample is taken. The sensitivity of the radiocarbon signatures is a function of MRT and the time when a soil sample is taken. This method is sensitive to MRT of active SOC pools in a range from 1 to 80 yr at the present time. Using those radiocarbon signatures, the MRT of active SOC pools in the Morrow Plots (Urbana, IL) and Sanborn Field (Columbia, MO) were estimated at 67 and 34 yr, respectively.

Concentrations and radiocarbon signatures of dissolved organic matter in the Pacific Ocean

We present evidence suggesting that only a portion of the dissolved organic carbon (DOC) in the seawater analyzed previously by Williams and Druffel (1987) was oxidized by the UV‐radiation method. High temperature catalytic (HTC) methods (Sugimura and Suzuki, 1988) used to reoxidize the central North Pacific gyre water samples reveal that the total DOC (DOCHTC) is about twice that of the UV‐oxidizable DOC (DOCuv). Indications from the original study suggest that this additional DOC contains higher concentrations of radiocarbon than in the DOCuv(Williams and Druffel, 1987). This evidence implies that DOC is older and thus more refractory with respect to biological utilization, yet is more chemically reactive with respect to photooxidation, than the fraction resistant to UV (DOCres).We report accelerator mass spectrometry (AMS) Δ14C measurements of humic, fulvic and hydrophilic acid fractions isolated from water collected at 180 m in the North Pacific (19°N, 158°W) using XAD macroreticular resins. Δ14C values of the humic material are less than those of DOCuv from a similar depth 1200 km further north (Williams and Druffel, 1987) indicating that these humic substances are part of the ‘old’ recycled DOCuv in the ocean.