Permanence of Soil Applied Biochar

Biochar, derived from organic materials through pyrolysis, consists of both Stable and Less-Stable carbon types, with its production process, carbon stability, and implications in policy-making and construction highlighting its potential for carbon sequestration, material durability, and environmental sustainability.

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Biochar and Its Carbon Content

What is Biochar?

Biochar is a specialized form of charcoal produced through the thermal decomposition of organic materials like plant waste, wood, and agricultural residues in a controlled environment with limited oxygen, a process called pyrolysis. This results in a porous, black, carbon-rich substance that, when added to soil, significantly enhances its fertility and health. This unique structure allows for better water retention in sandy soils and improved aeration in compacted or clayey soils. Additionally, biochar has a remarkable ability to capture and store carbon. By nature, plants absorb carbon dioxide from the atmosphere during photosynthesis, and upon their decay, this carbon can potentially return to the atmosphere. Converting these plants into biochar captures a substantial portion of this carbon, storing it in a stable form and thereby aiding in the reduction of CO2 emissions. This function makes biochar an attractive strategy to combat climate change. Beyond soil amendment, biochar has applications in water filtration due to its pollutant adsorption properties. Some researchers are even investigating its potential to reduce methane emissions in livestock farming when incorporated into feed. Another significant advantage of biochar is its durability; unlike other organic materials that might decay rapidly in soil, biochar can remain effective for centuries or even longer. Overall, biochar offers a sustainable and eco-friendly solution to multiple agricultural and environmental challenges, presenting an opportunity for both enhanced soil health and a proactive approach to climate change.

How is Biochar Made?

Biochar is produced through a specialized process called pyrolysis, which involves the thermal decomposition of organic materials in an oxygen-limited environment. During pyrolysis, biomass—like wood, agricultural residues, or other plant-based materials—is heated to high temperatures, typically ranging between 300°C to 700°C. As the material heats up, it begins to break down, releasing volatile compounds and gases. What remains is a black, carbon-rich substance: biochar. The absence or limitation of oxygen during this process is crucial; it prevents the complete combustion of the biomass, ensuring that the end product retains a high carbon content. This very process distinguishes biochar from other charred products like charcoal. By converting biomass into biochar, not only are we producing a material that can benefit soil health and crop productivity, but we're also creating a means to capture and store carbon, making it a promising solution in the realm of climate change mitigation.

Why is Biochar Important?

Biochar's importance transcends its foundational role as a soil enhancer. Delving deeper, its unique properties hold significant promise in addressing several pressing environmental concerns. At its core, biochar is a carbon sink. Plants, during their lifecycle, naturally pull carbon dioxide, a principal greenhouse gas, from our atmosphere. Typically, when these plants decay or are combusted, much of this absorbed carbon is released back into the environment. However, converting these plants into biochar through pyrolysis effectively 'traps' a notable portion of this carbon, keeping it sequestered for extended periods. This not only reduces CO2 emissions but also aids in actively mitigating the advancing effects of climate change. Its porous structure, while being instrumental in promoting soil health, also finds value in water filtration systems, capturing pollutants and ensuring cleaner water sources. In an era marked by increasing environmental challenges, biochar emerges not just as a simple soil additive, but as a versatile tool that addresses the intertwined issues of soil degradation, water pollution, and climate change. Researchers' growing interest in biochar's potential, especially its role in diminishing methane emissions from livestock—a significant contributor to global warming—further underscores its relevance and potential in contemporary ecological and agricultural paradigms. Including biochar in our environmental strategies provides a multidimensional solution that intertwines agriculture with sustainability, heralding a path towards a more balanced coexistence with our planet.

Two Types of Carbon in Biochar

Stable Carbon (PAC)

Stable Carbon, also referred to as Pyrogenic Anorganic Carbon (PAC), is one of the primary types of carbon found in biochar. PAC is characterized by its long-term persistence in the soil. Think of it as the marathon runner of carbon, enduring for centuries or even millennia. Its resilience stems from its molecular structure which resists breakdown and decomposition. This stable nature of PAC makes biochar a powerful tool in carbon sequestration, capturing atmospheric CO2 and storing it in the ground, thereby playing a pivotal role in the fight against climate change.

Less-Stable Carbon (SPC)

On the other side of the spectrum is Less-Stable Carbon or Short-lived Pyrogenic Carbon (SPC). SPC can be likened to a sprinter, present for a brief, intense period. Unlike PAC, SPC tends to break down and decompose over shorter time scales, ranging from a few years to decades. Despite its relatively fleeting presence, SPC has its unique significance. As it decomposes, it gradually releases nutrients, benefiting plant growth and supporting microbial life in the soil. While not as lasting as PAC in terms of carbon storage, SPC contributes to soil fertility and health, showcasing how both types of carbon in biochar offer distinct advantages to our environment.

Why Does this Matter for Policy?

The differentiation between Stable Carbon (PAC) and Less-Stable Carbon (SPC) in biochar is paramount for policy making, primarily due to the implications each type has for carbon sequestration and climate change mitigation.

Stable Carbon, or PAC, represents a long-term carbon storage solution. When biochar with a high content of PAC is applied to soils, it can remain in that state for hundreds to thousands of years, effectively sequestering carbon and preventing it from returning to the atmosphere as carbon dioxide. Policies that support the production and use of biochar rich in PAC could significantly advance a country's or region's climate goals, allowing for a tangible method to capture and store atmospheric carbon in soil, thus directly addressing rising atmospheric CO2 levels.

On the other hand, Less-Stable Carbon (SPC) has a shorter lifespan in the soil, degrading and potentially returning to the atmosphere over a span of years to decades. While SPC still offers benefits for soil health and short-term carbon storage, its quicker turnover rate means that it doesn't provide the same long-term carbon sequestration benefits as PAC. As a result, policy makers need to be cautious. If the focus is purely on immediate carbon offsets without understanding the longevity of the sequestration, it could lead to an overestimation of the climate benefits of certain biochar products.

Therefore, understanding and differentiating between PAC and SPC becomes crucial for informed policy making. To craft effective and long-lasting climate policies, officials need to consider the proportion of these two types of carbon in biochar products and their respective roles in carbon sequestration. Only then can true, sustainable benefits be reaped both for the environment and for the broader goals of combating climate change.

Why Does this Matter for Construction?

The differentiation between Stable and Less-Stable carbon in biochar has profound implications for the realm of construction. For one, the durability and lifespan of construction materials are inextricably linked to the stability of the carbon within them. Materials infused with biochar possessing a greater proportion of Stable Carbon (PAC) tend to showcase heightened resistance to degradation, promising enhanced durability and possibly a longer material lifespan. On the flip side, those enriched with Less-Stable Carbon (SPC) might have a shortened longevity, potentially compromising structural reliability. The push for a greener construction footprint brings into focus the role of PAC-rich biochar. It presents a commendable opportunity to sequester carbon, acting as a counterbalance to the emissions synonymous with construction activities. However, an SPC-rich biochar might not extend the same carbon offset advantages due to its less enduring nature. The carbon type can also sway the physical and chemical properties of the construction materials, with PAC possibly offering superior insulation characteristics, while SPC might influence the material's moisture retention capacities. It's also worth noting that as biochar with a higher SPC concentration deteriorates, it may release certain compounds, raising potential indoor air quality and health concerns. Lastly, from an economic vantage point, the carbon stability in biochar could dictate its market price and demand. Those offering long-lasting benefits, owing to a higher PAC content, might be more sought-after, thereby influencing construction costs and decisions. In essence, the stability spectrum of carbon in biochar plays a pivotal role in shaping its applicability and value in construction scenarios.

What Do We Need to be Careful About?

When delving into the realm of biochar, there are vital considerations that warrant attention. While biochar showcases significant promise in terms of carbon sequestration and diverse applications, it's essential to approach its production and use responsibly. The feedstock source, the conditions under which it's produced, and its intended application play a paramount role in its overall environmental impact. Furthermore, over-reliance on biochar could divert focus from more holistic and sustainable environmental practices. As with any emerging solution, it's crucial to strike a balance, ensuring that the pursuit of biochar benefits does not inadvertently lead to environmental degradation or overlook other equally vital environmental strategies.

Biochar that was produced at pyrolysis temperatures above 550°C and presenting a molar H:C ratio below 0.4 is highly persistent when applied to soil. 75% of such biochar carbon consists of stable polycyclic aromatic carbon (PAC) and will persist after soil application for more than 1000 years independent of the soil type and climate. The remaining 25% of the biochar carbon may be considered as semi-persistent carbon (SPC) presenting a mean residence time in soil of 50 to 100 years depending on soil type and climate. Soil applied biochar contains thus two distinct carbon pools with different degrees of permanence and therefore has two different carbon sink values. The climate service obtained from the stable fraction of biochar (75% of the C-content) can be considered of equal permanence as geological storage (e.g., DACCS, BECCS, Enhanced Weathering).

Biochar is a heterogenous carbonaceous material (EBC, 2012) that consists of two distinct carbon pools with different degrees of persistence when applied to soil:

The persistent aromatic carbon (PAC) pool, which consists of larger clusters of aromatic carbon rings (see figure 1), generally with more than seven aromatic rings, is not susceptible to degradation. The PAC pool has a mean residence time (MRT) in soil largely exceeding 1000 ye-
ars (Bowring et al., 2022; Howell et al., 2022), independent of common environmental factors such as soil humidity, temperature, freeze-thaw-cycles, and biological activity or agricultural practices like tillage. Cluster sizes of aromatic
carbons (how the carbon ring structures cling together) may be more important for persistence than the sheer number of aromatic rings in a molecule (Mao et al., 2012; McBeath & Smernik, 2009; Nguyen et al., 2010).

Schematic representation of different molecular forms of carbon in biochar. Aromatic and Heteroaromatic Carbon has closed-ring molecules, lower H/Corg and O/Corg ratio and increased persistence. Aliphatic Carbon has open-chain molecules, high H/Corg and O/Corg ratio and is prone to decat. Polycyclic Aromatic Carbon has fused closed-ring molecules, very low H/Corg and O/Corg ration and >7 fused rings are highly stable.
Figure. 1: Schematic representation of different molecular forms of carbon in biochar.

The semi-persistent carbon (SPC) pool, which contains aliphatic, small aromatic, and heteroaromatic carbon species, is more degradable in soil (Rombolà et al., 2016). Some compounds of the semi-persistent carbon pool can degrade within the first year after soil application; others will persist for decades and even centuries depending on the chemistry of the aliphatic and small aromatic compounds and their physical placement within the porous structure of the biochar. On average, the SPC fraction of
biochar has an MRT in the order of at least 50 to 100 years, depending on the biochar composition (i.e., distribution of aliphatic carbon, small aromatics, heteroaromatics), the soil type, and the climate (Bowring et al., 2022; Hilscher & Knicker, 2011; Lehmann et al., 2015; Pisani et al., 2014; Schmidt et al., 2011; Singh et al., 2012). With an MRT of 49 years for soil organic carbon (Schmidt et al., 2011), considering the evidence that pyrogenic carbon persists longer in soil than soil organic carbon (S. Lutfalla et al., 2017; Schmidt et al., 2011) and the calculated MRT of 91 years for 49% of soil applied pyrogenic carbon in six field trials with none optimized biochars, the application of an MRT of 50 years for the semi-persistent carbon pool of biochar with an H:C < 0.4 is conservative and presents a considerable margin.

In the environment, each of the carbonaceous compounds separated in those two respective carbon pools shows distinct degradation dynamics that can be described by an individual degradation curve. If biochar is incubated for one or two, or even eight years, as done in controlled incubation laboratory studies (Kuzyakov et al., 2014; Lehmann et al., 2015) and the resulting degradation data are then mathematically extrapolated into the far future, the prediction of the degradation dynamic is erroneous because it assumes that (1) the biochar consists only of semi-persistent carbon pools (aliphatics and small clusters of aromatic and heteroaromatic rings) and (2) the exponential biological degradation model is valid, despite degradation mechanisms being rather physico-chemical than biological. Over a time scale of several thousands of years, persistent aromatic carbon moieties may eventually also be degraded (Bowring et al., 2022), but this fact is barely contained in degradation data measured only during the first decade after biochar was incubated in soil.

All biochar incubation studies observed that the rate of degradation slows down exponentially with time and that the experimental data can be fitted mathematically with bi- or trimodal decay functions (Lehmann et al., 2015; Wang et al., 2016; Zimmerman & Gao, 2013). However, such mathematical fitting may be misleading as it cannot account for qualitative transitions (e.g., removal of physical protection of compounds inside the biochar structure or) occurring decades and centuries after the latest measured datapoints. As shown by Lutfalla et al. (2019), the small number of existing data sets presenting decadal degradation data of carbon in soil cannot be fitted by such bi- or trimodal decay functions. Therefore, projecting the degradation behavior of the semi-persistent carbon pool onto the degradation curve of the entire biochar is not adequate and biases our understanding of long-term carbon dynamics.

The percentage of PAC in a given biochar depends mainly on the pyrolysis conditions (i.e., temperature, residence time, heating rate, particle size, carrier gas, pressure) but also on the feedstock characteristics (i.e., lignin and ash content of biomass) (McDonald-Wharry, 2021). The PAC content can be quantified by hydrogen pyrolysis HyPy) (Ascough et al., 2009; Rombolà et al., 2016) or by Raman spectroscopy (McDonald-Wharry, 2021; McDonald-Wharry et al., 2013). HyPy analysis is reliable and proven but too complex to be utilized in commercial labs and thus too expensive to be used in routine analysis as for example in the EBC certification process. Raman spectroscopy as well as the newer Mid-Infrared or Rock-Eval methods are more cost-efficient analytical method and are currently under methodological evaluation for the EBC. Other parameters of biochar, such as production conditions (e.g., temperature) and elemental composition (i.e. using molar H:C and O:C ratio), which are frequently used as proxies for the degree of aromatization rely on poorly constrained databases with low analytical quality. The dataset used e.g., by Woolf et al. (2021) and the IPCC (IPCC, 2019) to calculate biochar persistence based on highest treatment temperature (HTT) and H:C proxies, contained only one biochar series where HTT was actually measured (Budai et al., 2014) all other temperatures were estimates. Pyrolysis temperature, i.e., the actual temperature inside the biomass particle during conversion, cannot be measured accurately in most industrial pyrolyzers and does not capture the effects that heating rate, residence time, particle size, and pressure have on the formation of PAC (Santín et al., 2017). Moreover, when using literature data on H:C to parameterize mathematical functions on biochar persistence more care needs to be taken to only use correctly analyzed carbon and hydrogen contents of materials that are really biochar. The IPCC (2019) and Woolf (2021) included e.g. many materials with H:C ratios above 0.7 which are clearly not biochars (EBC, 2012; IBI, 2015) and considered implausible data resulting from insufficient described analyses (e.g., how was the biochar dried before measuring the H-content). While the molar H:C ratio can be measured with sufficient precision, some fractions of biochars with low H:C are not necessarily PAC and may be considered semi-persistent only (Howell et al., 2022). The H:C ratio is thus a proxy with limited significance for the quantification of biochar persistence.

Since PAC is not yet analyzed for every EBC-certified biochar, average biochar data from literature should be used with caution. Conservative security margins must be used to estimate the persistent biochar content and thus the portion of biochar carbon that will endure as C-sink for more than 1000 years. Based on the degradation experiments published so far and considering that the calculated decay functions express only the degradation dynamic of the semi-persistent biochar carbon pool, the carbon calculated as remaining after 100 years is regarded as the minimum PAC fraction of biochars with an H:C ratio < 0.4.

Fig. 2: Sequestration curve of a 1000 tons carbon sink made from soil applied biochar with an H:C ratio below 0.4. The persistent aromatic carbon (PAC) pool presents 750 t carbon that will remain over more than 1000 years in the terrestrial system.The semi-persistent carbon (SPC) pool has a minimum MRT of 50 years and was modeled on bi-modal exponential decay function. The complete SPC-decay occurs over 350 years. Thus, the total carbon sink decreases to 87.5% after 50 years and reaches the stable PAC plateau of 75.0% of total pyrogenic carbon after 350 years. The decay function isTotal PyC(x)=a*e^(-kf*x) + b*e^(-ks*x) + Pwith a = 45.423, kf = 0.513, b = 212.007, ks = 0.009448, P = 742.5 nd x = year after soil application. The decay curve of thesemi-persistent carbon pool is an approximation covering multiple discrete (physical) degradation events rather than a continually harmonious decomposition.

Applying the conventionally assumed average degradation rate of 0.3% per year, the 74% of carbon remaining after 100 years (Camps-Arbestain et al., 2015; H.-P. Schmidt et al., 2020) can be considered PAC. This corresponds well to the experimental data presented by Howell et al. (2022), finding 75% stable polycyclic aromatic carbon for various engineered biochars with H:C ratios below 0.4 using the HyPy quantification method. Biochars with H:C ratios above 0.4 are likely to have a distinct labile carbon pool of incomplete pyrolyzed biomass or condensates that may become subject to more rapid biological degradation. Dedicated research for those materials is needed to assess the persistence of pyrolyzed biomass presenting high H:C ratios above 0.4 (Pulcher et al., 2022).

To the best of the current scientific knowledge, it is safe to assume that biochar with an H:C ratio below 0.4 can be best described by a 2-pool-model presenting (1) A persistent aromatic carbon (PAC) pool of 75% with an MRT of >1000 (1400-14400 years, see annex), competitive with geological carbon storage and suitable for CO2-emission compensation and (2) A semi-persistent carbon (SPC) pool with an MRT of at least 50 years offering an additional, valuable climate cooling service, yet of a different quality than persistent aromatic carbon (PAC).

Annex 1

Mean residence times of natural pyrogenic carbon

Calculations of global inputs and deposits of naturally produced pyrogenic carbons (PyC) can attest to how robust and conservative the assumption of these average persistence rates over 100 years is. Forest, bush and steppe fires are examples of incomplete combustion, which transform part of the biomass into chars, i.e., PyC. According to recent surveys of natural fires, 5-15% of the biomass carbon involved in the fire is converted to PyC (Santín etal., 2016). Natural PyCs are similar in structure and material properties to industrially produced biochar. However, it can be assumed that the stability of high HTT industrial biochar, and thus the mean residence time (MRT), is even higher than that of natural PyC (Howell et al., 2022; Santín et al., 2017) due to more controlled and homogeneous high-temperature conditions.

Mainly through forest and steppe fires, about 0.114-0.383 Gt (Giga tons) of pyrogenic carbon (PyC) are generated each year (Santín et al., 2016). Globally, the total mass of PyC in soils is 71-212 Gt, in nearshore sediments 400-1200 Gt, and in further ocean sediments 80-240 Gt (Bird et al., 2015; Santín et al., 2016), resulting in a global PyC pool of 550-1,650 Gt (excluding PyC in water bodies and groundwater sediments). Based on the dimension of the global PyC pool and the annual input of PyC of 0.114 - 0.383 Gt given above, the average MRT of natural PyC can be calculated as MRT = GlobalPyC-pool/annualPyCinput.

The MRT range of natural PyC could thus be calculated as (550 Gt / 0.383 Gt a-1 to 1,650 Gt / 0.114 Gt a-1 =) 1,440 to 14,500 years. This time frame is confirmed by Bowring et al. (2022) who determined a minimum MRT of 2,760 years using the same data basis but without including
sedimentary PyC. Using the extrapolation of Reisser et al. (2016), according to which the PyC content of soil organic carbon (SOC) is 14%, and the global content of SOC is 1,500 to 3,000 Gt (Scharlemann et al., 2014) the global PyC content in soils would be about 210 - 420 Gt (Leifeld et al., 2018). From the annual PyC input of 0.114 - 0.383 Gt, the MRT for PyC in soils would be (210 Gt / 0.382 Gt a-1 to 420 Gt / 0.114 Gt a-1) 550 to 3,700 years. Since the MRT of PyC in sediments is significantly higher than in soils, the difference between the two calculations is plausible. Note, however, that most of the PyC in nearshore sediments is originally derived from PyC leached from soils (Coppola & Druffel, 2016), so that much longer MRTs than the calculated 550 to 3,700 years would result for soil-PyC, except that the pyrogenic carbon would no longer be found in soils but as deposits in sediments (Coppola et al., 2014).


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This presentation was partially funded by the National Fish and Wildlife Foundation Innovative Nutrient and Sediment Reduction Grant Program.

Disclaimer: The views and conclusions contained in this video are those of the authors and should not be interpreted as representing the opinions or policies of the I.S. Government or the National Fish and Wildlife Foundation and its funding sources. Mention of trade names or commercial products does not constitute their endorsement by the U.S. Government, or the National Fish and Wildlife Foundations or its funding sources.

Statement: This material is based on work supported by the U.S. Environmental Protection Agency (Assistance Agreement No. CB96358201) and the National Fish and Wildlife Foundation’s Chesapeake Bay Stewardship Fund, which promotes community-based efforts to develop conservation strategies to protect and restore the diverse natural resources of the Chesapeake Bay.