Thea Whitman

Characterization of biochars to evaluate recalcitrance and agronomic performance

Biochars (n=94) were found to have ash contents from 0.4% to 88.2%, volatile matter from 13.2% to 70.0%, and fixed carbon from 0% to 77.4% (w/w). Greater pyrolysis temperature for low-ash biochars increased fixed carbon, but decreased it for biochars with more than 20% ash. Nitrogen recovery varied depending on feedstock used to a greater extent (12-68%) than organic (25-45%) or total C (41-76%) at a pyrolysis temperature of 600 °C. Fixed carbon production ranged from no enrichment in poultry biochar to a 10-fold increase in corn biochar (at 600 °C). Prediction of biochar stability was improved by a combination of volatile matter and H:C ratios corrected for inorganic C. In contrast to stability, agronomic utility of biochars is not an absolute value, as it needs to meet local soil constraints. Woody feedstock demonstrated the greatest versatility with pH values ranging from 4 to 9.

Carbon Mineralizability Determines Interactive Effects on Mineralization of Pyrogenic Organic Matter and Soil Organic Carbon

Soil organic carbon (SOC) is a critical and active pool in the global C cycle, and the addition of pyrogenic organic matter (PyOM) has been shown to change SOC cycling, increasing or decreasing mineralization rates (often referred to as priming). We adjusted the amount of easily mineralizable C in the soil, through 1-day and 6-month preincubations, and in PyOM made from maple wood at 350 °C, through extraction. We investigated the impact of these adjustments on C mineralization interactions, excluding pH and nutrient effects and minimizing physical effects. We found short-term increases (+20-30%) in SOC mineralization with PyOM additions in the soil preincubated for 6 months. Over the longer term, both the 6-month and 1-day preincubated soils experienced net ∼10% decreases in SOC mineralization with PyOM additions. Additionally, the duration of preincubation affected interactions, indicating that there may be no optimal preincubation time for SOC mineralization studies. We show conclusively that mineralizability of SOC in relation to PyOM-C is an important determinant of the effect of PyOM additions on SOC mineralization.

Dynamics of microbial community composition and soil organic carbon mineralization in soil following addition of pyrogenic and fresh organic matter

Pyrogenic organic matter (PyOM) additions to soils can have large impacts on soil organic carbon (SOC) cycling. As the soil microbial community drives SOC fluxes, understanding how PyOM additions affect soil microbes is essential to understanding how PyOM affects SOC. We studied SOC dynamics and surveyed soil bacterial communities after OM additions in a field experiment. We produced and mixed in either 350 °C corn stover PyOM or an equivalent initial amount of dried corn stover to a Typic Fragiudept soil. Stover increased SOC-derived and total CO2 fluxes (up to 6x), and caused rapid and persistent changes in bacterial community composition over 82 days. In contrast, PyOM only temporarily increased total soil CO2 fluxes (up to 2x) and caused fewer changes in bacterial community composition. Of the operational taxonomic units (OTUs) that increased in response to PyOM additions, 70% also responded to stover additions. These OTUs likely thrive on easily mineralizable carbon (C) that is found both in stover and, to a lesser extent, in PyOM. In contrast, we also identified unique PyOM responders, which may respond to substrates such as polyaromatic C. In particular, members of Gemmatimonadetes tended to increase in relative abundance in response to PyOM but not to fresh organic matter. We identify taxa to target for future investigations of the mechanistic underpinnings of ecological phenomena associated with PyOM additions to soil.

Biochar projects for mitigating climate change: an investigation of critical methodology issues for carbon accounting.

Biochar is a potential tool in our fight against climate change, driven by its high carbon stability and supported by its roles in bioenergy and soil fertility. We consider methodology aspects of biochar systems used for carbon management and investigate the criteria for establishing additionality, baselines, permanence, leakage, system drivers, measurement, verification, economics and development for successful stand-alone projects and carbon offsets. We find that explicitly designing a biochar system around ‘true wastes’ as feedstocks combined with safe system drivers could minimize unwanted land-use impacts and leakage. Applying baselines of biomass decomposition rather than total soil carbon is effective and supports a longer crediting period than is currently standard. With biochar production introduced into bioenergy systems, under a renewable biomass scenario, the change in emissions increases with higher fuel use, instead of decreasing. Biochars may have mean residence times of over 1000 years, but can be accounted for more effectively using a recalcitrant and labile fraction.

Biochar systems for smallholders in developing countries : leveraging current knowledge and exploring future potential for climate-smart agriculture

Biochar is the carbon-rich organic matter that remains after heating biomass under the minimization of oxygen during a process called pyrolysis. There are a number of reasons why biochar systems may be particularly relevant in developing-country contexts. This report offers a review of what is known about opportunities and risks of biochar systems. Its aim is to provide a state-of-the-art overview of current knowledge regarding biochar science. In that sense the report also offers a reconciling view on different scientific opinions about biochar providing an overall account that shows the various perspectives of its science and application. This includes soil and agricultural impacts of biochar, climate change impacts, social impacts, and competing uses of biomass. The report aims to contextualize the current scientific knowledge in order to put it at use to address the development climate change nexus, including social and environmental sustainability. The report is organized as follows: chapter one offers some introductory comments and notes the increasing interest in biochar both from a scientific and practitioners point of view; chapter two gives further background on biochar, describing its characteristics and outlining the way in which biochar systems function. Chapter three considers the opportunities and risks of biochar systems. Based on the results of the surveys undertaken, chapter four presents a typology of biochar systems emerging in practice, particularly in the developing world. Life-cycle assessments of the net climate change impact and the net economic profitability of three biochar systems with data collected from relatively advanced biochar projects were conducted and are presented in chapter five. Chapter six investigates various aspects of technology adoption, including barriers to implementing promising systems, focusing on economics, carbon market access, and sociocultural barriers. Finally, the status of knowledge regarding biochar systems is interpreted in chapter seven to determine potential implications for future involvement in biochar research, policy, and project formulation.

A dual-isotope approach to allow conclusive partitioning between three sources

Stable isotopes have proved to be a transformative tool; their application to distinguish between two sources in a mixture has been a cornerstone of biogeochemical research. However, quantitatively partitioning systems using two stable isotopes (for example, 13C and 12C) has been largely limited to only two sources, and systems of interest often have more than two components, with interactive effects. Here we introduce a dual-isotope approach to allow conclusive partitioning between three sources, using only two stable isotopes. We demonstrate this approach by partitioning soil CO2 emissions derived from microbial mineralization of soil organic carbon (SOC), added pyrogenic organic matter (PyOM) and root respiration. We find that SOC mineralization in the presence of roots is 23% higher (P<0.05) when PyOM is also present. Being able to discern three sources with two isotopes will be of great value not only in biogeochemical research, but may also expand hitherto untapped methodologies in diverse fields.

Spatiotemporal dynamics of the bacterial microbiota on lacustrine Cladophora glomerata (Chlorophyta)

The branched periphytic green alga Cladophora glomerata, often abundant in nearshore waters of lakes and rivers worldwide, plays important ecosystem roles, some mediated by epibiotic microbiota that benefit from host‐provided surface, organic C, and O2. Previous microscopy and high‐throughput sequencing studies have indicated surprising epibiont taxonomic and functional diversity, but have not included adequate consideration of sample replication or the potential for spatial and temporal variation. Here, we report the results of 16S rRNA amplicon‐based phylum‐to‐genus taxonomic analysis of Cladophora‐associated bacterial epibiota sampled in replicate from three microsites and at six times during the open‐water season of 2014, from the same lake locale (Picnic Point, Lake Mendota, Dane Co., WI, USA) explored by high‐throughput sequencing studies in two previous years. Statistical methods were used to test null hypotheses that the bacterial community: (i) is homogeneous across microsites tested, and (ii) does not change over the course of a growth season or among successive years. Results indicated a dynamic microbial community that is more strongly influenced by sampling day during the growth season than by microsite variation. A surprising diversity of bacterial genera known to be associated with the key function of methane‐oxidation (methanotrophy), including relatively high‐abundance of Crenothrix, Methylomonas, Methylovulum, and Methylocaldum–showed intraseasonal and interannual variability possibly related to temperature differences, and microsite preferences possibly related to variation in methane abundance. By contrast, a core assemblage of bacterial genera seems to persist over a growth season and from year to year, possibly transmitted by a persistent attached host resting stage.

Microbial community assembly differs by mineral type in the rhizosphere

Inputs of root carbon (C) fuel growth of nearby soil microorganisms. If these microbes associate with soil minerals, then mineral-microbiome complexes near roots could be a gateway towards stabilization of soil carbon and may influence the quantity and quality of persistent SOM. To investigate the interactions between roots, soil minerals, and microbes, we incubated three types of minerals (ferrihydrite, kaolinite, quartz) and a native soil mineral fraction near roots of a common Californian annual grass, Avena barbata, growing in its resident soil. We followed microbial colonization of these minerals for 2.5 months – the plant’s lifespan. Bacteria and fungi that colonized mineral surfaces during this experiment differed across mineral types and differed from those in the background soil, implying microbial colonization was the result of processes in addition to passive movement with water to mineral surfaces. Null model analysis revealed that dispersal limitation was a dominant factor structuring mineral-associated microbial communities for all mineral types. Once bacteria arrived at a mineral surface, capacity for rapid growth appeared important, as ribosomal copy number was significantly correlated with relative enrichment on minerals. Glomeromycota (a phylum associated with arbuscular mycorrhizal fungi) appeared to preferentially associate with ferrihydrite surfaces. The mechanisms enabling colonization of soil minerals may be foundational to the overall soil microbiome composition and partially responsible for the persistence of C entering soil via plant roots.

Spatiotemporal Dynamics of the Bacterial Microbiota and Methanotrophic Bacteria on Lotic Cladophora glomerata (Chlorophyta)

The periphytic green alga Cladophora glomerata is found growing abundantly in dense mats in lakes and rivers worldwide, often co-occurring in eutrophic lakes with near-shore waters saturated in methane. This alga hosts a diverse microbial community, but the spatiotemporal dynamics of the alga’s bacterial microbiota over a growth season have not been characterized. In this study, replicate samples of Cladophora were collected in 2014 from multiple locales in Lake Mendota at multiple times during the summer growth season to test the hypothesis that the bacterial community changes over time and is geographically heterogeneous. Genetic sequencing of epibiontic bacteria using the 16S rDNA biomarker showed significant differences in community structure and composition over time and space, suggesting a dynamic microbial community that is strongly influenced by sampling time and weakly by sampling site. Of particular importance are high diversity and relative abundance of likely methane-oxidizing (methanotrophic) bacteria, especially Crenothrix, Methylomonas, and Methylocaldum, which showed distinctive site preferences. Different patterns were observed in many aerobic heterotrophic bacteria, such as Meiothermus, Leadbetterella, and Flectobacillus and non-oxygenic phototrophic bacteria such as Rhodobacter. Comparison to results of a similar 2011 study from the same site revealed a core bacterial assemblage that persists between years and over a growth season, but also opportunistic bacterial genera and ecological guilds whose populations increase, decrease, or peak over different timeframes. Evidence for a highly dynamic microbial community growing on Cladophora glomerata warrants further study to determine the most influential factors and how these factors influence freshwater macroalgae or related submerged photosynthetic organisms in environmental, industrial, or biotechnological systems.