B22B-01 INVITED
Towards Soil and Sediment Inventories of Black Carbon
A body of literature on black carbon (BC) concentrations in soils and sediments is rapidly accumulating, but as of yet, there are no global or regional inventories of BC in either reservoir. Soil and sediment BC inventories are badly needed for a range of fields. For example, in oceanography a global sediment BC inventory is crucial in understanding the role of biomass burning in the development of stable marine carbon reservoirs, including dissolved organic carbon and sedimentary organic carbon. Again in the marine environment, BC likely strongly impacts the fate and transport of anthropogenic pollutants: regional inventories of BC in sediments will help develop better environmental remediation strategies. In terrestrial systems well-constrained natural BC soil inventories would help refine ecological, agricultural, and soil biogeochemical studies. BC is highly sorptive of nutrients including nitrogen and phosphorous. The presence of BC in ecosystems almost certainly alters N and P cycling; however, without soil BC inventories, we cannot know where BC has a significant impact. BC's nutrient sorptivity and water-holding capacity make it an important component of agricultural soils, and some researchers have proposed artificially increasing soil BC inventories to improve soil fertility. Natural soil BC concentrations in some regions are quite high, but without a baseline inventory, it is challenging to predict when agricultural amendment will significantly exceed natural conditions. And finally, because BC is one of the most stable fractions of organic carbon in soils, understanding its concentration and regional distribution will help us track the dynamics of soil organic matter response to changing environmental conditions. Developing effective regional and global BC inventories is challenging both because of data sparsity and methodological intercomparison issues. In this presentation I will describe a roadmap to generating these valuable inventories.
B22B-02 INVITED
Large variability of biochar stability and biochar properties
In general, charring or purposeful pyrolysis increases the stability of biomass. It is less clear, however, to what extent biochar properties influence its stability. Chemical and physical properties of biochars and biomass-derived black carbons (BC) vary greatly as a function of the type of biomass it was generated from and of the production temperature. We show that these properties greatly affect the stability of BC is a function of both these factors, with highly significant interactions. BC produced from corn stalks produced at 350°C decomposed much quicker when incubated at field capacity at 30°C for one year than those produced at 600°C. In contrast, there was hardly a difference noted between those two temperatures if oak was the precursor biomass. Such differences in labile carbon not only affect the proportion of stable carbon in BC, but also influence the quantification of long-term stability. Extrapolation from short-term decay to long-term stability may require prior knowledge about the decay rate of the labile fraction of BC. Some indications are provided for the short-term oxidation of BC.
B22B-03
Effects of Biomass Type and Combustion Conditions on the Bulk and Molecular Properties of Biochar-Derived Dissolved Organic Matter as Determined by Ultrahigh Resolution Mass Spectrometry
Biochar, the residual products of biomass combustion excluding vapor phase condensates, can be a major component of soil organic matter in regions prone to fires or where slash and burn or slash and char agriculture is practiced. Dissolution or leaching of biochar may be the primary mechanism for biochar turnover in soils as it is thought to be extremely refractory in solid form. Although researchers have made significant progress recently in understanding biochar's physical and chemical properties, we know of no comprehensive study of the relationship between these properties and the solubility or chemistry of dissolved organic matter (DOM) derived from biochar (or black carbon). Thus, we have thus begun a series of studies of the relationship between the chemical and morphological properties of biochars made from a variety of biomass types (woods and grasses) produced under a range of controlled conditions (temperature and atmosphere) and the quantity, quality and lability of leacheate derived from those biochars. After a series of time-course leaching experiments (3, 10 and 20 day), leacheate was quantified by total organic carbon analysis and incubated both abiotically and with microbe inoculates. The pre- and post- incubation leachates were then analyzed with the 9.4 T instrument at the National High Magnetic Field Laboratory in Tallahassee, FL, the same instrument that has been used in most of the pioneering mass spectrometry analyses of natural DOM. Traditional electrospray ionization (ESI) and the relatively new atmospheric pressure photoionization techniques were both employed to convert dissolved DOM molecules into gas-phase ions which were then identified by ultrahigh resolution FT-ICR MS. The unique chemical formulas of the 2,000 – 4,000 individual compounds identified were then characterized by van Krevelen analysis (elemental O/C vs H/C ratios), Kendrick mass defects, and double bond equivalencies. In this presentation we will use these molecular parameters to fingerprint biochar DOM as a function of biomass source, combustion conditions and degradative processes. These chemical fingerprints might later be used to trace biochar-derived DOM throughout the hydrosphere.
B22B-04
Char BC amendments for soil and sediment amelioration: BC quantification and field pilot trials
Background Activated char BC binds organic contaminants and possibly mercury so strongly that their bioaccumulation and transport to other environmental compartments are reduced. The advantages of black carbon amendment over many other remediation methods include i) it can be used as an in situ risk reduction method, ii) the price is low, and iii) it overcomes significant controversies associated with disposal of dredged and excavated materials. In this study BC amendment is used in pilot trials in the field for soil and sediment amelioration. Quantification of amended char BC Two methods for char BC quantification were tested: i) chemothermal oxidation (CTO) at a range of temperatures and ii) wet chemical oxidation with a potassium dichromate/sulfuric acid solution. The amount of BC amended to three soils was accurately determined by CTO at 375°C. For two sediments, much of the BC disappeared during combustion at 375°C, which could probably be explained by catalytic effects caused by sediment constituents such as metals, mineral oxides and salts. Attempts to avoid these effects through rinsing with acid before combustion did not result in higher char BC recoveries. CTO at lower temperatures (325-350°C) was a feasible alternative for one of the sediments. Wet oxidation with potassium dichromate/sulfuric acid proved to effectively function for BC quantification in sediments, since almost complete BC recovery (81-92 %) was observed for both sediments, while the amount of organic carbon remaining was low (5-16 %). Field pilots Earlier, we showed the effectiveness of BC amendment in the laboratory. In the laboratory it was shown that BC amendments (2 %) reduced freely dissolved porewater concentrations (factor of 10-50) and bioaccumulation (factor of 5). This presentation will describe 50 × 50 m pilot field trials in Norway (2007-2008): Trondheim Harbor (sediment) and Drammen (soil). The presentation will focus on physical monitoring (distribution of BC in the sediment by bioturbation), chemical monitoring (porewater concentrations, diffusive transport to water and air) and biological monitoring (biodiversity, bioaccumulation). The first results indicate that i) bioturbation mixes BC into the top 5 cm of the sediments, ii) 6 months after BC amendment, PAH leaching from soils is reduced by a factor of 2, iii) Hg leaching from sediments is reduced, iv) diffusive transport from sediment to water is reduced, and v) sediment biodiversity remains unaffected by the placement of a thin BC layer.
B22B-05 INVITED
Black carbon contribution to stabilised SOM in soil under slash and burn agriculture
Black carbon (BC) produced during slash and burn agriculture on tropical soils may enhance the soils organic matter content and hence their biological properties. However, once deposited on the soil surface, BC may be subject to erosion and/or microbial decomposition and thus not be preserved on site. Up to now, few studies have been carried out to assess the contribution of BC to the soils stable carbon pool on sites under slash and burn agriculture. The aim of the study was to assess the survival potential of BC in sloping tropical soils of clayey texture. The study was carried out in Northern Laos, where the soils are subjected to addition of black carbon produced by burning of agricultural crop residues. Our conceptual approach included the characterisation of (a) morphologically distinct BC forms and (b) chemical soil fractions. The samples were analysed for elemental content, chemical composition by 13C CPMAS NMR spectroscopy, carbon resistant to acid hydrolysis with HCl, carbon resistant to oxidation with acid dichromate solution and 14C activity. Our results indicated that BC produced by slash and burn agriculture was highly aromatic in nature. Its elemental composition as well as its susceptibility to be lost by chemical oxidation was dependent on its morphology. Acid hydrolysis did not lead to carbon loss from any BC form. We thus hypothesised that BC should be present in the hydrolysis resistant fraction isolated from soil. The charactersation of the chemical composition by 13C CPMAS NMR spectroscopy showed that the hydrolysis residue was composed of highly aromatic carbon. Considering the low lignin content of these soils and the good recovery of bulk soil aromatic carbon signal (80-100%) in the hydrolysis residue, we consider that this fraction may be suitable to assess BC contribution to clayey soils. We suggest that BC isolated as hydrolysis resistant C may represent up to 25% of the soils C as compared to 8% as isolated by acid dichromate oxidation. The 14C age of this fraction was older than that of the untreated soil and increased to more than 1000 years in B horizons. These observations suggest that BC may be preserved for long time, once incorporated into the mineral soil.
B22B-06
Organic Components and Elemental Carbon in Soils and Ambient Particles near Phoenix, AZ
In the desert southwest, fugitive dust emissions contribute significantly to ambient aerosol concentrations. Wind erosion from the arid land is a primary contributor to ambient particulate matter (PM) concentrations but, in regions including Central Arizona, desert lands have been converted for agriculture use and thus agriculture processes constitute another contributor. As the metropolitan Phoenix region expands into these agricultural lands, urban sources and construction also contributes to the ambient PM load. In an effort to identify and access relative contribution of these and other major PM sources in the region, a series of ambient PM samples and soil samples were collected near Higley, AZ, a suburb of Phoenix which has seen rapid urbanization onto agricultural lands between January and May 2008. The soil samples collected were resuspended and samples of resuspended dust were collected to represent particles smaller than 2.5 microns and 10 microns in aerodynamic diameter (PM2.5 and PM10 respectively). The size segregated soil and ambient PM samples were analyzed for bulk mass, elemental and organic carbon content, and a number of specific compounds including ions, metals, alkanes, organic acids, polycyclic aromatic hydrocarbons, and saccharides. The saccharide contribution to soil organic carbon has been studied to elucidate key factors in the soil carbon balance and markers have been developed for tracing fungal metabolites, plant growth and budding and organic matter decay. Using organic markers, the contribution of various sources to PM10 and PM2.5 levels have been determined by positive matrix factorization (PMF) of the ambient aerosol marker concentrations quantified from PM samples. Subsequently, samples of local soil from native and agricultural fields and local roadways wers size- segregated and analyzed in an effort to create a source profile for the dust in the area. A chemical mass balance model has been used to compare with the PMF results where sampled and resuspended agricultural soil, native soil and road dusts are used to characterize direct emissions of these sources to ambient fine and coarse particulate matter.
B22B-07
The production of black carbon during managed burning of UK peatlands: could managed burning of peatlands lead to enhanced carbon storage?
Peatlands are the UK's largest single terrestrial carbon store with carbon stored in UK peatlands than in forests of Britain and France combined. Unlike most northern peatlands in the peat soils of the UK are heavily managed for recreation and agriculture and due to their proximity to major centres of population are under more anthropogenic pressure than most peatlands. A typical management strategy on UK upland peats is the use of managed fire to restrict vegetation. Fires are used upon a 10-25 year rotation and are described as "cool" as they are designed to remove the crown of the vegetation without scorching the litter layer or the underlying soil. In this case the fire destroys primary productivity and limits litter production but produces char. Char is a low volume, highly refractory, high carbon content product while litter is a high volume, decomposable, lower carbon content product. Therefore, the question is if there are fire conditions under which the production of char causes more carbon to be stored in the peat than would have been stored if no fire management had been employed. This study combines field studies of recent managed burns and wildfires along with detailed vegetation studies from a long term monitoring site in order to assess litter, biomass and black carbon production. In the laboratory experimental burns were undertaken in order to assess the amount and controls upon char production and the carbon content of that char. Results of field and laboratory observations are used to model carbon accumulation under a series of fire management scenarios and the modelling shows that cools burns at long rotations could lead to higher carbon storage than if no fire had occurred, further in several cases more carbon accumulation occurred even if less depth of peat was generated.
B22B-08
Black Carbon and Sorption of PAHs in Natural Fire-Impacted Sediments From Oriole Lake (CA)
We investigated the occurrence of black carbon (BC) and polycyclic aromatic hydrocarbons (PAHs) in sediment samples from Oriole Lake (Kings Canyon and Sequoia National Park, California). Sequoia National Park provided a unique environment for this study, as its frequent natural fires are recorded in the giant Sequoias. From the tree record the fire record for the past ~ 500 years is known. Having this fire recorded provides a unique opportunity to reference the sediment layers to a known fire chronology. Ca. 4 m of sediment covering the last 5,000 years were taken in July 2007 at Oriole Lake. Cores were sliced into 1 cm increments, and analyzed for 210Pb and 137Cs for dating. Samples were extracted for PAH and analyzed for organic C and BC, according to a method by Gustafsson et al. (1997). Highest BC concentrations (> 0.5% d.w.) in the last century correlated with individual forest fire events in the region. Long-range transport of traffic emissions might have contributed to a gently rising baseline of sedimentary BC concentrations. PAH ratios (e.g., methylphenanthrenes versus phenanthrenes) indicated a prevalence of pyrogenic emissions. Sediments from the last 20 years were incubated with passive PE samplers to derive porewater concentrations. Results are used to discern whether the char-rich sediments display a stronger sorption of PAHs than can be explained by the combined OC and BC pools.