ALBERT

Description: We estimated CO2, CH4, CO and N2O emission fluxes over the British Isles and Western Europe using atmospheric radon observations and concentrations recorded at the Mace Head Atmospheric Research Station between 1996 and 2005. We classified hourly concentration data into either long-range European or regional sources from Ireland and UK, by using local wind speed data in conjunction with 222Rn and 212Pb threshold criteria. This leads to the selection of about 7% of the total data for both sectors. We then used continuous 222Rn measurements and assumptions on the surface emissions of 222Rn to deduce the unknown fluxes of CO2, CH4, CO and N2O. Our results have been compared to the UNFCCC, EMEP and EDGAR statistical inventories and to inversion results for CH4. For Western Europe, we found yearly mean fluxes of 4.1±1.5 106 kg CO2 km−2 yr−1 , 11.9±2.0 103 kg CH4 km−2 yr−1, 12.8±4.2 103 kg CO km−2 yr−1 and 520.2±129.2 kg N2O km−2 yr−1, respectively, for CO2, CH4, CO and N2O over the period 1996–2005. The method based upon 222Rn to infer emissions has many sources of systematic errors, in particular its poorly known and variable footprint, uncertainties in 222Rn soil fluxes and in atmospheric mixing of air masses with background air. However, these biases are likely to remain constant in the long-term, which makes the method quite efficient to detect trends in fluxes. Over the last ten years period, the decrease of the anthropogenic CH4, CO and N2O emissions in Europe estimated by inventories (respectively −30%, −35% and −23%) is confirmed by the Mace Head data within 2%. Therefore, the 222Rn method provides an independent way of verification of changes in national emissions derived from inventories. Using European-wide estimates of the CO/CO2 emission ratio, we also found that it is possible to separate the fossil fuel CO2 emissions contribution from the one of total CO2 fluxes. The fossil fuel CO2 emissions and their trends derived in that manner agree very well with inventories.

Description: The carbon (C) cycle in boreal regions is strongly influenced by fire, which converts biomass and detrital C mainly to gaseous forms (CO2 and smaller proportions of CO and CH4), and some 1–7% of mass to pyrogenic C (PyC). PyC is mainly produced as solid charred residues, including visually-defined charcoal, and a black carbon (BC) fraction chemically defined by its resistance to laboratory oxidation, plus much lower proportions of volatile soot and polycyclic aromatic hydrocarbons (PAHs). All PyC is characterized by fused aromatic rings, but varying in cluster sizes, and presence of other elements (N, O) and functional groups. There are several reasons for current interest in defining more precisely the role of PyC in the C cycle of boreal regions. First, PyC is resistant to decomposition, and therefore contributes to very stable C pools in soils and sediments. Second, it influences soil processes, mainly through its sorption properties and cation exchange capacity, and third, soot aerosols absorb solar radiation and may contribute to global warming. However, there are large gaps in the basic information needed to address these topics. While charcoal is commonly defined by visual criteria, analytical methods for BC are mainly based on various measures of oxidation resistance, or on yield of benzenepolycarboxylic acids. These methods are still being developed, and capture different fractions of the PyC "continuum". There are few quantitative reports of PyC production and stocks in boreal forests (essentially none for boreal peatlands), and results are difficult to compare due to varying experimental goals and methods, as well as inconsistent terminology. There are almost no direct field measurements of BC aerosol production from boreal wildfires, and little direct information on rates and mechanisms for PyC loss. Structural characterization of charred biomass and forest floor from wildfires generally indicates a low level of thermal alteration, with the bulk of the material having H/C ratios still 〉0.2, and small aromatic cluster sizes. For the more chemically-recalcitrant BC fraction, a variety of mainly circumstantial evidence suggests very slow decomposition, with turnover on a millennium timescale (5000–10 000 y), depending on environmental conditions and PyC properties, but the main limitation to PyC storage in soil is likely consumption by subsequent fires. Degraded, functionalized PyC is also incorporated into humified soil organic matter, and is transported to sediments in dissolved and particulate form. Boreal production is estimated as 7–17 Tg BC y−1 of solid residues and 2–2.5 Tg BC y−1 as aerosols. Primary research needs include basic field data on PyC production and stocks in boreal forests and peatlands, suitable to support C budget modeling, and development of standardized analytical methods and of improved approaches to assess the chemical recalcitrance of typical chars from boreal wildfires. To accomplish these goals effectively will require much greater emphasis on interdisciplinary cooperation.

Description: Black carbon (BC), from incomplete combustion of fuels and biomass, has been considered highly recalcitrant and a substantial sink for carbon dioxide. Recent studies have shown that BC can be degraded. We use soils sampled 100 years apart in a Russian steppe preserve to generate the first whole-profile estimate of BC stocks and turnover in the field. BC stocks (initially 2.5 kg m-2) decreased 25% with cessation of biomass burning. BC turnover in the soil was 293 y (best estimate; range 212–541 y), much faster than inert/passive carbon in soil models. Such results provide a new constraint on theories of soil carbon stabilization. Most importantly, BC cannot be assumed chemically recalcitrant in all soils; other explanations for very old soil carbon are needed.

Description: Retardation of soil organic carbon (SOC) decay after nitrogen addition to litter or soil has been suggested in several recent studies and has been attributed to a retardation in lignin decay. With our study we tested the long-term effect of mineral nitrogen fertilization on the decay of the SOC component lignin in arable soil. To achieve this, we tracked 13C-labeled lignin and SOC in an arable soil that is part of a 36-year field experiment with two mineral nitrogen fertilization levels. We could show that nitrogen fertilization neither retarded nor enhanced the decay of old SOC or lignin over a period of 36 years, proposing that decay of lignin was less sensitive to nitrogen fertilization than previously suggested. However, for fresh biomass there were indications that lignin decay might have been enhanced by nitrogen fertilization, whereas decay of SOC was unaffected. A retardation of SOC decay due to nitrogen addition, as found in other experiments, can therefore only be explained by effects on lignin decay, if lignin was actually measured.

Description: Black carbon (BC), from incomplete combustion of fuels and biomass, has been considered highly recalcitrant and a substantial sink for carbon dioxide. Recent studies have shown that BC can be degraded in soils. We use two soils with very low spatial variability sampled 100 years apart in a Russian steppe preserve to generate the first whole-profile estimate of BC stocks and turnover in the field. Quantities of fire residues in soil changed significantly over a century. Black carbon stock was 2.5 kg m−2, or about 7–10% of total organic C in 1900. With cessation of biomass burning, BC stocks decreased 25% over a century, which translates into a centennial soil BC turnover (293 years best estimate; range 182–541 years), much faster than so-called inert or passive carbon in ecosystem models. The turnover time presented here is for loss by all processes, namely decomposition, leaching, and erosion, although the latter two were probably insignificant in this case. Notably, at both time points, the peak BC stock was below 30 cm, a depth interval, which is not typically accounted for. Also, the quality of the fire residues changed with time, as indicated by the use benzene polycarboxylic acids (BPCA) as molecular markers. The proportions of less-condensed (and thus more easily degradable) BC structures decreased, whereas the highly condensed (and more recalcitrant) BC structures survived unchanged over the 100-year period. Our results show that BC cannot be assumed chemically recalcitrant in all soils, and other explanations for very old soil carbon are needed.

Description: Current climate change models predict significant changes in rainfall patterns across Europe. To explore the effect of drought on soil CO2 efflux (FSoil) and on the contribution of litter to FSoil we used rainout shelters to simulate a summer drought (May to July 2007) in an intensively managed grassland in Switzerland, and to reduce annual precipitation by around 30% similar to the hot and dry year 2003 in Central Europe. We added 13C-depleted as well as unlabelled grass/clover litter to quantify the litter-derived CO2 efflux (FLitter). Soil CO2 efflux and the 13C/12C isotope ratio (δ13C) of the respired CO2 after litter addition were measured during the growing season 2007. Drought significantly decreased FSoil in our litter addition experiment by 52% and FLitter by 74% during the drought period itself (May to July), indicating that drought had a stronger effect on the CO2 release from litter than on the belowground-derived CO2 efflux (FBG, i.e. soil organic matter (SOM) and root respiration). Despite large bursts in respired CO2 induced by the rewetting after prolonged drought, drought also reduced FSoil and FLitter during the entire 13C measurement period (April to October) by 32% and 33%, respectively. Overall our findings highlight i) the sensitivity of temperate grassland soils to changes in precipitation, a factor that needs to be considered in regional models predicting the impact of climate change, and ii) the need to quantify the response of the different components of soil CO2 efflux to fully understand climate change impacts on ecosystem carbon balance.

Description: Anthropogenic fires affected the temperate deciduous forests of Central Europe over millennia. Biomass burning releases carbon to the atmosphere and produces charcoal, which potentially contributes to the stable soil carbon pools and is an important archive of environmental history. The fate of charcoal in soils of temperate deciduous forests, i.e. the processes of charcoal incorporation and transportation, and the effects on soil organic matter are still not clear. In a long-term experimental burning site, we investigated the effects of slash-and-burn and determined soil organic carbon, charcoal carbon and nitrogen concentrations and the soil lightness of colour (L*) in the topmost soil material (0–1, 1–2.5 and 2.5–5 cm depths) before, immediately after the fire and one year after burning. The main results are that (i) only few charcoal particles from the forest floor were incorporated into the soil matrix by soil mixing animals. In 0–1 cm and during one year, the charcoal C concentrations increased only by 0.4 g kg−1 and the proportion of charcoal C to SOC concentrations increased from 2.8 to 3.4%; (ii) the SOC concentrations did not show any significant differences; (iii) soil lightness significantly decreased in the topmost soil layer and correlated with the concentrations of charcoal C (r=-0.87**) and SOC (r=−0.94**) in samples 0–5 cm. We concluded that the soil colour depends on the proportion of aromatic charcoal carbon in total organic matter and that Holocene burning could have influenced soil charcoal concentrations and soil colour.

Description: Retardation of soil organic carbon (SOC) decay after nitrogen addition to litter or soil has been suggested in several recent studies and has been attributed to a retardation in lignin decay. With our study we tested the long-term effect of mineral fertilization (N+P) on the decay of the SOC component lignin in arable soil. To achieve this, we tracked 13C-labeled lignin and SOC in an arable soil that is part of a 36-year field experiment (conversion from C3 to C4 crops) with two mineral fertilization levels. We could show that fertilization neither retarded nor enhanced the decay of old SOC or lignin over a period of 36 years, proposing that decay of lignin was less sensitive to fertilization than previously suggested. However, for new, C4-derived lignin there were indications that decay might have been enhanced by the fertilization treatment, whereas decay of new SOC was unaffected.

Description: The carbon (C) cycle in boreal regions is strongly influenced by fire, which converts biomass and detrital C mainly to gaseous forms (CO2 and smaller proportions of CO and CH4), and some 1–3% of mass to pyrogenic C (PyC). PyC is mainly produced as solid charred residues, including visually-defined charcoal, and a black carbon (BC) fraction chemically defined by its resistance to laboratory oxidation, plus much lower proportions of volatile soot and polycyclic aromatic hydrocarbons (PAHs). All PyC is characterized by fused aromatic rings, but varying in cluster sizes, and presence of other elements (N, O) and functional groups. The range of PyC structures is often described as a continuum from partially charred plant materials, to charcoal, soot and ultimately graphite which is formed by the combination of heat and pressure. There are several reasons for current interest in defining more precisely the role of PyC in the C cycle of boreal regions. First, PyC is largely resistant to decomposition, and therefore contributes to very stable C pools in soils and sediments. Second, it influences soil processes, mainly through its sorption properties and cation exchange capacity, and third, soot aerosols absorb solar radiation and may contribute to global warming. However, there are large gaps in the basic information needed to address these topics. While charcoal is commonly defined by visual criteria, analytical methods for BC are mainly based on various measures of oxidation resistance, or on yield of benzenepolycarboxylic acids. These methods are still being developed, and capture different fractions of the PyC structural continuum. There are few quantitative reports of PyC production and stocks in boreal forests (essentially none for boreal peatlands), and results are difficult to compare due to varying experimental goals and methods, as well as inconsistent terminology. There are almost no direct field measurements of BC aerosol production from boreal wildfires, and little direct information on rates and mechanisms for PyC loss. Structural characterization of charred biomass and forest floor from wildfires generally indicates a low level of thermal alteration, with the bulk of the material having H/C ratios still 〉0.2, and small aromatic cluster sizes. Especially for the more oxidation-resistant BC fraction, a variety of mainly circumstantial evidence suggests very slow decomposition, with turnover on a millennial timescale (in the order of 5–7 ky), also dependent on environmental conditions. However, there is also evidence that some PyC may be lost in only tens to hundreds of years due to a combination of lower thermal alteration and environmental protection. The potential for long-term PyC storage in soil may also be limited by its consumption by subsequent fires. Degraded, functionalized PyC is also incorporated into humified soil organic matter, and is transported eventually to marine sediments in dissolved and particulate form. Boreal production is estimated as 7–17 Tg BC y−1 of solid residues and 2–2.5 Tg BC y−1 as aerosols, compared to global estimates of 40–240 and 10–30 Tg BC y−1, respectively. Primary research needs include basic field data on PyC production and stocks in boreal forests and peatlands, suitable to support C budget modeling, and development of standardized analytical methods and of improved approaches to assess the chemical recalcitrance of typical chars from boreal wildfires. To accomplish these goals effectively will require much greater emphasis on interdisciplinary cooperation.

Description: Anthropogenic fires affected the temperate deciduous forests of Central Europe over millennia. Biomass burning releases carbon to the atmosphere and produces charcoal, which potentially contributes to the stable soil carbon pools and is an important archive of environmental history. The fate of charcoal in soils of temperate deciduous forests, i.e. the processes of charcoal incorporation and transportation and the effects on soil organic matter are still not clear. We investigated the effects of slash-and-burn at a long-term experimental burning site and determined soil organic carbon and charcoal carbon concentrations as well as the soil lightness of colour (L*) in the topmost soil material (0–1, 1–2.5 and 2.5–5 cm depths) before, immediately after the fire and one year later. The main results are that (i) only a few of the charcoal particles from the forest floor were incorporated into the soil matrix, presumably by soil mixing animals. In the 0–1 cm layer, during one year, the charcoal C concentration increased only by 0.4 g kg−1 and the proportion of charcoal C to SOC concentration increased from 2.8 to 3.4%; (ii) the SOC concentrations did not show any significant differences; (iii) soil lightness decreased significantly in the topmost soil layer and correlated well with the concentrations of charcoal C (r=−0.87**) and SOC (r=−0.94**) in the samples from the 0–5 cm layer. We concluded that Holocene biomass burning could have influenced soil charcoal concentrations and soil colour.