Browsing by Author "Gallagher, Morgan E."

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    Biochar-Induced Changes in Soil Hydraulic Conductivity and Dissolved Nutrient Fluxes Constrained by Laboratory Experiments
    (Public Library of Science, 2014) Barnes, Rebecca T.; Gallagher, Morgan E.; Masiello, Caroline A.; Liu, Zuolin; Dugan, Brandon
    The addition of charcoal (or biochar) to soil has significant carbon sequestration and agronomic potential, making it important to determine how this potentially large anthropogenic carbon influx will alter ecosystem functions. We used column experiments to quantify how hydrologic and nutrient-retention characteristics of three soil materials differed with biochar amendment. We compared three homogeneous soil materials (sand, organic-rich topsoil, and clay-rich Hapludert) to provide a basic understanding of biochar-soil-water interactions. On average, biochar amendment decreased saturated hydraulic conductivity (K) by 92% in sand and 67% in organic soil, but increased K by 328% in clay-rich soil. The change in K for sand was not predicted by the accompanying physical changes to the soil mixture; the sand-biochar mixture was less dense and more porous than sand without biochar. We propose two hydrologic pathways that are potential drivers for this behavior: one through the interstitial biochar-sand space and a second through pores within the biochar grains themselves. This second pathway adds to the porosity of the soil mixture; however, it likely does not add to the effective soil K due to its tortuosity and smaller pore size. Therefore, the addition of biochar can increase or decrease soil drainage, and suggests that any potential improvement of water delivery to plants is dependent on soil type, biochar amendment rate, and biochar properties. Changes in dissolved carbon (C) and nitrogen (N) fluxes also differed; with biochar increasing the C flux from organic-poor sand, decreasing it from organic-rich soils, and retaining small amounts of soil-derived N. The aromaticity of C lost from sand and clay increased, suggesting lost C was biochar-derived; though the loss accounts for only 0.05% of added biochar-C. Thus, the direction and magnitude of hydraulic, C, and N changes associated with biochar amendments are soil type (composition and particle size) dependent.
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    Forest soil carbon oxidation state and oxidative ratio responses to elevated CO2
    (Wiley, 2015) Hockaday, William C.; Gallagher, Morgan E.; Masiello, Caroline A.; Baldock, Jeffrey A.; Iversen, Colleen M.; Norby, Richard J.
    The oxidative ratio (OR) of the biosphere is the stoichiometric ratio (O2/CO2) of gas exchange by photosynthesis and respiration—a key parameter in budgeting calculations of the land and ocean carbon sinks. Carbon cycle-climate feedbacks could alter the OR of the biosphere by affecting the quantity and quality of organic matter in plant biomass and soil carbon pools. This study considers the effect of elevated atmospheric carbon dioxide concentrations ([CO2]) on the OR of a hardwood forest after nine growing seasons of Free-Air CO2 Enrichment. We measured changes in the carbon oxidation state (Cox) of biomass and soil carbon pools as a proxy for the ecosystem OR. The OR of net primary production, 1.039, was not affected by elevated [CO2]. However, the Cox of the soil carbon pool was 40% higher at elevated [CO2], and the estimated OR values for soil respiration increased from 1.006 at ambient [CO2] to 1.054 at elevated [CO2]. A biochemical inventory of the soil organic matter ascribed the increases in Cox and OR to faster turnover of reduced substrates, lignin and lipids, at elevated [CO2]. This implicates the heterotrophic soil community response to elevated [CO2] as a driver of disequilibrium in the ecosystem OR. The oxidation of soil carbon pool constitutes an unexpected terrestrial O2 sink. Carbon budgets constructed under the assumption of OR equilibrium would equate such a terrestrial O2 sink to CO2 uptake by the ocean. The potential for climate-driven disequilibriua in the cycling of O2 and CO2 warrants further investigation.
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    Impacts of biochar concentration and particle size on hydraulic conductivity and DOC leaching of biochar-sand mixtures
    (Elsevier, 2016) Liu, Zuolin; Dugan, Brandon; Masiello, Caroline A.; Barnes, Rebecca T.; Gallagher, Morgan E.; Gonnermann, Helge
    The amendment of soil with biochar can sequester carbon and alter hydrologic properties by changing physical and chemical characteristics of soil. To understand the effect of biochar amendment on soil hydrology, we measured the hydraulic conductivity (K) of biochar–sand mixtures as well as dissolved organic carbon (DOC) in leachate. Specifically, we assessed the effects of biochar concentration and particle size on K and amount of DOC in the soil leachate. To better understand how physical properties influenced K, we also measured the skeletal density of biochars and sand, and the bulk density, the water saturation, and the porosity of biochar–sand mixtures. Our model soil was sand (0.251–0.853 mm) with biochar rates from 2 to 10 wt% (g biochar/g total soil × 100%). As biochar (<0.853 mm) concentration increased from 0 to 10 wt%, K decreased by 72 ± 3%. When biochar particle size was equal to, greater than, and less than particle size of sand, we found that biochar in different particle sizes have different effects on K. For a 2 wt% biochar rate, K decreased by 72 ± 2% when biochar particles were finer than sand particles, and decreased by 15 ± 2% when biochar particles were coarser than sand particles. When biochar and sand particle size were comparable, we observed no significant effect on K. We propose that the decrease of K through the addition of fine biochar was because finer biochar particles filled spaces between sand particles, which increased tortuosity and reduced pore throat size of the mixture. The decrease of K associated with coarser biochar was caused by the bimodal particle size distribution, resulting in more compact packing and increased tortuosity. The loss of biochar C as DOC was related to both biochar rate and particle size. The cumulative DOC loss was 1350% higher from 10 wt% biochar compared to pure sand. This large increase reflected the very small DOC yield from pure sand. In addition, DOC in the leachate decreased as biochar particle size increased. For all treatments, the fraction of carbon lost as DOC ranged from 0.06 to 0.18 wt% of biochar. These experiments suggest that mixing sandy soils with biochar is likely to reduce infiltration rates, holding water near the surface longer with little loss of biochar-derived carbon to groundwater and streams.
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    Plant species, not climate, controls aboveground biomass O2:CO2 exchange ratios in deciduous and coniferous ecosystems
    (Wiley, 2017) Gallagher, Morgan E.; Liljestrand, Frasier L.; Hockaday, William C.; Masiello, Caroline A.
    The oxidative ratio (OR) is the O2:CO2 ratio associated with photosynthesis, respiration, and other ecosystem gas exchange processes and can be reported on the scale of an individual leaf, an ecosystem, up to the entire terrestrial biosphere. The OR of the terrestrial biosphere is used to partition anthropogenic CO2 between oceanic and terrestrial carbon sinks, and the ease of measurement of this property on smaller scales suggests its potential for other uses. However, controls on the natural variation of OR are not understood in either organic matter pools or fluxes, and this lack of basic information limits the use of the tracer. Here we assess the annual variability of the OR of photosynthesis over ~decade for two temperate forests, one coniferous and one deciduous, and show that the photosynthetic OR signature is strongly dominated by plant species. We determined the OR of this flux by measuring the OR of carbon pools that close on annual or shorter timescales (leaves and individual tree rings), via solid‐state 13C NMR spectroscopy and elemental analysis. Leaf litter OR is different between coniferous and deciduous forests, but tree bole OR is constant between species. There was no significant change in leaf litter OR with time, nor any correlations between leaf litter OR and temperature or precipitation. During this time growing season precipitation varied by 95% from the time period average, and growing season temperature by 22%, demonstrating that on the decadal scale photosynthetic OR is invariant over significant shifts in these climate parameters.
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    Short‐Term Changes in Physical and Chemical Properties of Soil Charcoal Support Enhanced Landscape Mobility
    (Wiley, 2017) Pyle, Lacey A.; Magee, Kate L.; Gallagher, Morgan E.; Hockaday, William C.; Masiello, Caroline A.
    Charcoal is a major component of the stable soil organic carbon reservoir, and the physical and chemical properties of charcoal can sometimes significantly alter bulk soil properties (e.g., by increasing soil water holding capacity). However, our understanding of the residence time of soil charcoal remains uncertain, with old measured soil charcoal ages in apparent conflict with relatively short modeled and measured residence times. These discrepancies may exist because the fate of charcoal on the landscape is a function not just of its resistance to biological decomposition but also its physical mobility. Mobility may be important in controlling charcoal landscape residence time and may artificially inflate estimates of its degradability, but few studies have examined charcoal vulnerability to physical redistribution. Charcoal landscape redistribution is likely higher than other organic carbon fractions owing to charcoal's low bulk density, typically less than 1.0ﾠg/cm3. Here we examine both the physical and chemical properties of soil and charcoal over a period of two years following a 2011 wildfire in Texas. We find little change in properties with time; however, we find evidence of enhanced mobility of charcoal relative to other forms of soil organic matter. These data add to a growing body of evidence that charcoal is preferentially eroded, offering another explanation for variations observed in its environmental residence times.