Average Carbon Flux Research Articles

Influence of Alloying Elements on the Carburizing Behavior in Acetylene Atmosphere

Three steel types (AISI 1020, AISI 8620, AISI 4120) with similar carbon content and different Cr content were used as test specimens to closely examine the effect of alloying elements for carbon penetration and diffusion on the steel surface during vacuum carburizing. The carbon mass gain according to the carburizing time was measured using a microbalance, and the average carbon flux, which is an indicator of the carbon penetration rate, was calculated using the measured weight as a variable. The outermost surface of the carburized specimen was observed by scanning electron microscopy (SEM) and Raman spectroscopy (RS), and the reason for the change in carburization rate according to the steel type was identified in relation to the equilibrium carbon contents calculated from Thermo-Calc. The overall carbon distribution and distribution of alloy elements on the outermost surface were quantitatively analyzed using an electron probe microanalyzer (EPMA). On the surfaces of the AISI 1020 and AISI 4120 carburized specimens, graphite layers and grain boundary carbide were formed during the carburizing process, which hindered the carburization rate, while no abnormal layer was observed on the surface of the AISI 8620 carburized specimens, so the overall carburization results were excellent.

Graminoids vary in functional traits, carbon dioxide and methane fluxes in a restored peatland: Implications for modelling carbon storage

Abstract One metric of peatland restoration success is the re‐establishment of a carbon sink, yet considerable uncertainty remains around the time‐scale of carbon sink trajectories. Conditions post‐restoration may promote the establishment of vascular plants such as graminoids, often at greater density than would be found in undisturbed peatlands, with consequences for carbon storage. Although graminoid species are often considered as a single plant functional type (PFT) in land‐atmosphere models, our understanding of functional variation among graminoid species is limited, particularly in a restoration context. We used a traits‐based approach to evaluate graminoid functional variation and to assess whether different graminoid species should be considered a single PFT or multiple types. We tested hypotheses that greenhouse gas fluxes (CO2, CH4) would vary due to differences in plant traits among five graminoid species in a restored peatland in central Alberta, Canada. We further hypothesized that species would form two functionally distinct groupings based on taxonomy (grass, sedge). Differences in gas fluxes among species primarily reflected variation in leaf physiology related to photosynthetic efficiency and resource‐use, and secondarily by plant size. Multivariate analyses did not reveal distinct functional groupings based on taxonomy or environmental preferences. Rather, we identified functional groups defined by plant traits and carbon fluxes that are consistent with ecological strategies related to differences in growth rate, resource‐acquisition and leaf economics, representing plants with either a strategy to grow quickly and invest in resource capture or to prioritize structural investment and resource conservation. These functional groups displayed larger average carbon fluxes compared to graminoid PFTs currently used in modelling. Synthesis. Existing PFT designations in peatland models may be more appropriate for pristine or high‐latitude systems than those under restoration. Although replacing PFTs with plant traits remains a challenge in peatlands, traits related to leaf physiology and growth rate strategies offer a promising avenue for future applications.

Energy and carbon fluxes from an oil sands pit lake

Currently, post-mining landscape plans in the Athabasca Oil Sand Region include large watersheds terminating in pit lakes. In 2012, Base Mine Lake (BML), was constructed with the aim of demonstrating technologies associated with lake reclamation in the region. This paper examines the first 6.5 years of lake-atmosphere energy and carbon exchange. Energetically, BML behaved similar to other northern lakes, storing large quantities of heat in the spring and releasing it in the fall as sensible and latent heat fluxes. At various times a hydrocarbon sheen formed on the lake, which may have suppressed evaporation. However, simple linear relationships failed to statistically quantify the impacts and more comprehensive modelling of the variability may be required. At daily scales, variability in evaporation was well explained by the product of vapour pressure deficit and wind speed as well as the available energy (R2 = 0.74), while sensible heat was explained by the product of wind speed and the difference in air and surface temperature as well as available energy (R2 = 0.85). Spring CH4 fluxes were high, particularly around ice melt, with a maximum flux of 3.3 g m−2 day−1. Otherwise fluxes were low, except during irregular periods. The peak flux of these periods occurred following ~58 h of continuously falling pressure, relating cyclone activity to these large periods of methane emissions. Annually, CO2 and CH4 fluxes were initially high, with median fluxes of 231 mg CO2 m−2 h−1 and 23 mg CH4 m−2 h−1 in 2014. However, the median fluxes reduced quickly and over the least three years of the study (2017 through 2019) the median fluxes declined to 36 mg CO2 m−2 h−1 and 10 mg CH4 m−2 h−1. Overall, BML behaves similar to other boreal lake ecosystems with above average carbon fluxes compared to other constructed reservoirs.

Impact of physical and biological processes on temporal variations of the ocean carbon sink in the mid-latitude North Atlantic (2002–2016)

The ocean is currently a significant net sink for anthropogenically remobilised CO2, taking up around 24% of global emissions. Numerical models predict a diversity of responses of the ocean carbon sink to increased atmospheric concentrations in a warmer world. Here, we tested the hypothesis that increased atmospheric forcing is causing a change in the ocean carbon sink using a high frequency observational dataset derived from underway pCO2 (carbon dioxide partial pressure) instruments on ships of opportunity (SOO) and a fixed-point mooring between 2002 and 2016. We calculated an average carbon flux of 0.013 Pg yr−1 into the ocean at the Porcupine Abyssal Plain (PAP) site, consistent with past estimates. In spite of the increase in atmospheric pCO2, monthly average seawater pCO2 did not show a statistically significant increasing trend, but a higher annual variability, likely due to the decreasing buffer capacity of the system. The increasing ΔpCO2 led to an increasing trend in the estimated CO2 flux into the ocean of 0.19 ± 0.03 mmol m−2 day−1 per year across the entire 15 year time series, making the study area a stronger carbon sink. Seawater pCO2 variability is mostly influenced by temperature, alkalinity and dissolved inorganic carbon (DIC) changes, with 77% of the annual seawater pCO2 changes explained by these terms. DIC is in turn influenced by gas exchange and biological production. In an average year, the DIC drawdown by biological production, as determined from nitrate uptake, was higher than the DIC increase due to atmospheric CO2 dissolution into the surface ocean. This effect was enhanced in years with high nutrient input or shallow mixed layers. Using the rate of change of DIC and nitrate, we observed Redfieldian carbon consumption during the spring bloom at a C:N ratio of 6.2 ± 1.6. A comparison between SOO and PAP sustained observatory data revealed a strong agreement for pCO2 and DIC. This work demonstrates that the study area has continued to absorb atmospheric CO2 in recent years with this sink enhancing over time. Furthermore, the change in pCO2 per unit nitrate became larger as surface buffer capacity changed.

Regional-scale lateral carbon transport and CO<sub>2</sub> evasion in temperate stream catchments

Abstract. Inland waters play an important role in regional to global-scale carbon cycling by transporting, processing and emitting substantial amounts of carbon, which originate mainly from their catchments. In this study, we analyzed the relationship between terrestrial net primary production (NPP) and the rate at which carbon is exported from the catchments in a temperate stream network. The analysis included more than 200 catchment areas in southwest Germany, ranging in size from 0.8 to 889 km2 for which CO2 evasion from stream surfaces and downstream transport with stream discharge were estimated from water quality monitoring data, while NPP in the catchments was obtained from a global data set based on remote sensing. We found that on average 13.9 g C m−2 yr−1 (corresponding to 2.7 % of terrestrial NPP) are exported from the catchments by streams and rivers, in which both CO2 evasion and downstream transport contributed about equally to this flux. The average carbon fluxes in the catchments of the study area resembled global and large-scale zonal mean values in many respects, including NPP, stream evasion and the carbon export per catchment area in the fluvial network. A review of existing studies on aquatic–terrestrial coupling in the carbon cycle suggests that the carbon export per catchment area varies in a relatively narrow range, despite a broad range of different spatial scales and hydrological characteristics of the study regions.

Differences in carbon uptake and water use between a managed and an unmanaged beech forest in central Germany

Net atmospheric carbon dioxide exchange, total evapotranspiration and net primary production of two neighbouring beech (Fagus sylvatica L.) forests in central Germany differing in site management were measured using the eddy covariance technique and biometric methods. The unmanaged site was an old-growth forest with admixtures of ash (Fraxinus excelsior L.) and sycamore (Acer pseudoplatanus L.) trees whereas the managed site was a regularly thinned, even-aged (about 130years old), pure beech stand. Average carbon fluxes measured over seven years did not differ significantly between the two forests. Evapotranspiration was slightly higher (and consequently water use efficiency lower) at the unmanaged site. The maximum rates as well as the interannual variability of both net ecosystem exchange and net primary production were considerably larger at the managed, even-aged stand. The lowest annual carbon sequestration rates were observed in years with high fruit production in beech combined with cold and dry spells during leaf development which affected the carbon balance of the managed forest more than that of the unmanaged forest. In contrast, an extraordinarily dry period in late summer 2003 caused a stronger reduction in net carbon uptake in the old-growth forest which can probably be attributed to the contribution of ash to the ecosystem fluxes. Ash has a different phenology which made it more susceptible to the late summer drought. The relative importance of tree and structural (age, size) diversity, leaf area index and regenerative growth as well as the temporal frame and extent of single weather extremes for the forest–atmosphere exchange is discussed and it is concluded that site management and forest structure needs to be included in soil–vegetation–atmosphere-transfer models as it can often override effects of land use or plant functional type.

Characteristics of carbon flux in two biologically crusted soils in the Gurbantunggut Desert, Northwestern China

Abstract Different from soils in other ecosystems, soils in arid and semiarid areas are covered by biological soil crusts, which can both assimilate and release carbon via their physiological activities. Nevertheless, few studies have evaluated carbon flux in biologically crusted soils. In the current study, we investigated the daily carbon flux in moss- and cyanobacterial/lichen-crusted soils, as well as crust-removed soil (referred as bareland) in the Gurbantunggut Desert, Northwestern China from April to October 2010. We also investigated carbon flux in biologically crusted soils after 0, 2, 5, and 15 mm precipitation treatments. Carbon flux between crusted soil and bareland had no significant difference, with average values of 0.16, 0.2, and 0.12 μmol m− 2 s− 1 for moss-crusted soil, cyanobacterial/lichen crusted soil, and bareland, respectively. The corresponding Q10 values for the three soils were 2.9, 3.5, and 1.9, respectively. Precipitation significantly elicited carbon flux, reaching a maximum value of 2.4 μmol m− 2 s− 1. After precipitation, the net carbon flux in light was lower than that in dark respiration. No carbon influx was observed in light condition. The average carbon flux rate and total carbon production increased exponentially with the precipitation amount. This study indicates that biological soil crusts do not increase the respiration of soil, but increases the temperature dependence of soil carbon flux because of the higher microbial biomass than bareland.

Carbon emission and sequestration by agricultural land use: a model study for Europe

AbstractA model was developed to calculate carbon fluxes from agricultural soils. The model includes the effects of crop (species, yield and rotation), climate (temperature, rainfall and evapotranspiration) and soil (carbon content and water retention capacity) on the carbon budget of agricultural land. The changes in quality of crop residues and organic material as a result of changes in CO2 concentration and changed management were not considered in this model. The model was parameterized for several arable crops and grassland. Data from agricultural, meteorological, soil, and land use databases were input to the model, and the model was used to evaluate the effects of different carbon dioxide mitigation measures on soil organic carbon in agricultural areas in Europe. Average carbon fluxes under the business as usual scenario in the 2008–2012 commitment period were estimated at 0.52 tC ha−1 y−1 in grassland and −0.84 tC ha−1 y−1 in arable land. Conversion of arable land to grassland yielded a flux of 1.44 tC ha−1 y−1. Farm management related activities aiming at carbon sequestration ranged from 0.15 tC ha−1 y−1 for the incorporating of straw to 1.50 tC ha−1 y−1 for the application of farmyard manure. Reduced tillage yields a positive flux of 0.25 tC ha−1 y−1. The indirect effect associated with climate was an order of magnitude lower. A temperature rise of 1 °C resulted in a −0.05 tC ha−1 y−1 change whereas the rising CO2 concentrations gave a 0.01 tC ha−1 y−1 change. Estimates are rendered on a 0.5 × 0.5° grid for the commitment period 2008–2012. The study reveals considerable regional differences in the effectiveness of carbon dioxide abatement measures, resulting from the interaction between crop, soil and climate. Besides, there are substantial differences between the spatial patterns of carbon fluxes that result from different measures.