Circular Economy Benefit Indicators for Using Biochar in Regenerative Agriculture

doi:10.21203/rs.3.rs-2956227/v1

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Circular Economy Benefit Indicators for Using Biochar in Regenerative Agriculture

https://doi.org/10.21203/rs.3.rs-2956227/v1

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In spite of decades of political commitments, research, and applications of innovations for reducing climate change's causes and catastrophic impacts, the global CO₂ emissions have continued to rise. Achieving more stringent reduction targets requires rapid activities towards carbon neutrality in many ways to accelerate the transition to equitable, sustainable, livable, post-fossil carbon societies. Conventional agriculture, is responsible for 14% of global annual greenhouse gas (GHG) emissions and is causing an additional 17% through land use changes, mostly in developing countries. This is due to GHG emissions from its food production activities that include the farming activities, food transportation to food processing facilities, distribution, marketing, food usage, and food waste. The negative impacts of current agricultural practices underscore the urgency to transition to holistic, regenerative agricultural practices where negative carbon emission technologies such as the usage of biochar and other approaches are used to dramatically decrease the GHG emissions as well as to make the transition to circular economies via food chain, carbon neutrality. It is unfortunate that at this time, carbon sequestration progress and implementation of circular economy principle is well documented and to be practically achievable, the progress needs to be quantitatively measured. The authors of this paper, proposed a system of indicators for measuring progress toward carbon neutrality in agriculture based on circular economy principles. To accelerate the needed transition to regenerative agriculture, the extensive production and usage of biochar is essential.

biochar

carbon neutrality

climate change

circular economy

indicators

regenerative agriculture

Sustaining carbon balance is essential for humans and other living organisms on our planet. But the carbon cycle balance has been disrupted by various human activities, primarily through conventional farming activities, deforestation and burning of fossil carbon as fuels. Conventional agriculture, food production and distribution significantly contribute to greenhouse gas emissions. Agriculture is responsible for 14% of the annual greenhouse gas (GHG) emissions and induces an additional 17% of emissions through land use changes, mostly in developing countries. Enhanced irrigation, polluting agricultural chemicals, and pollution caused by food production, transportation, processing, usage and wastage account for almost one-third of all the greenhouse gases emitted by humans (Bolesnikov and Dinda 2022). There are many challenges that we must address, which are contributing to climate change. Among them is the current net annual increase of the planet's population of 81,000,000, the global population is projected to reach 9.7 billion by 2050 (UN 2022). These and sea level rise due to global warming will result in widespread food insecurity and increased demand for a wide array of natural resources and raw materials (Venkata Mohan et al. 2016). Therefore, due to the increasing human population and exponential increase in demand for food, it is of utmost importance to discover and implement means of net regenerative agriculture to help to capture more carbon in the soil to help to slow down the global warming while helping to enhance and ensure the fertility of the soil to effectively and efficiently produce nutritious food for the human and other animal needs. This process must be designed and monitored to help to bring the carbon cycle back into balance. A crucial element of this equation is that global human population stabilization must also be achieved. However, that is not the primary focus for this article, although it is totally essential to be globally achieved in the coming few decades.)

Despite decades of political efforts, research and innovations, on the causes and catastrophic impacts of climate change, global CO₂ emissions have continued to rise. They are 60% higher today than in 1990 (Stoddard et al. 2021). Our present emissions trajectory has left us on course for an average temperature rise of more than 3 ^oC above preindustrial levels by 2100, indicating a significant gap between current emissions and those needed to limit global warming to 1.5–2^oC, as set forth by the 2015 Paris Agreement (Wei et al. 2018). Specifically, to achieve the 2 ^oC target will require implementation of negative emission approaches to achieve 87% of carbon emissions reductions. To achieve, the 1.5 ^oC target necessitates practically all emission trajectories to achieve the negative emissions to achieve the net-zero emissions goal by the middle of this century (IPCC 2022).

Recent scientific research indicates that the world economy would become 10% smaller if the 2050 net-zero emissions and the Paris Agreement targets on climate change are unmet. Under the current trajectory, the global GDP would decrease 11–14% by mid-century compared to a world without climate change (Swiss Re 2021). According to the Paris Agreement, nations must set targets for reducing carbon emissions and other greenhouse gasses by 2050. Extensive attention has recently been focused on agriculture and its role(s) in transforming sustainable societies worldwide (Dumortier et al. 2020).

Measures reaching carbon neutrality

To keep the planet from warming more than 1.5°C above pre-industrial levels, most countries, including the EU, the U.S. and other countries, have set goals to reach net zero emissions by 2050. Net zero means that all greenhouse gas emissions are counterbalanced by an equal amount of eliminated emissions. Achieving this will require rapid activities towards carbon neutrality. Carbon neutrality is understood as maintaining a balance between emitting carbon and absorbing carbon from the atmosphere in various carbon sinks. Since CO₂ that’s already in the atmosphere will affect the climate for hundreds to thousands of years, the IPCC maintains that carbon dioxide removal (CDR) technologies will be critical to capture 100 to 1000 gigatons of CO₂ this century. To mitigate climate change, various carbon dioxide removal (CDR) technologies for enhancing C capture from the atmosphere through industrial means and C sequestration in terrestrial and marine ecosystems are being investigated and in many places are being implemented. These include bioenergy with C capture and storage (Hanssen et al. 2020), enhanced rock weathering by spreading crushed minerals, which are naturally capable of adsorbing CO₂ on land or in the ocean (Beerling 2017); afforestation and reforestation (Forster et al. 2021); soil C sequestration via biochar, compost, direct biowaste incorporation, and conservation tillage, among others (Murphy 2020); ocean fertilisation through the application of iron ore/and other nutrients for promoting the growth of photosynthetic plankton (Emerson 2019); coastal wetlands restoration; and direct air capture using chemicals to remove CO₂ directly from the atmosphere (Beuttler et al. 2019).

Agriculture and strategies for achieving carbon neutrality

In the scientific world, a consensus is evolving that we need a drawdown of substantial amounts (at least 1000 Gt) of CO₂ from the atmosphere to help the earth’s climate system to stabilize at a safer temperature (Amonette et al. 2021). Evidence from several thousand years ago, points to ‘agriculture’ as the first instance of human-caused increases in GHGs. Agriculture and associated land-use changes continue to be major sources of biogenic GHGs.

The greenhouse gas emissions from agriculture are expected to rise. It has been estimated that nitrous oxide (N₂O) emissions from the agricultural sector will increase 35–60% by 2030 which are attributed to fertilizer use and manure production and usage (FAO 2003). Methane (CH₄) emissions increase directly with increasing livestock numbers and they are expected to rise 60% between 1990 and 2030 (Panchasara et al. 2021). The agricultural sector depends on natural resources and consumes approximately 70% of global freshwater while occupying 40% of the global land area (Kaniaska 2016).

Although soils contribute significantly (37% mainly as N₂O and CH₄) to agricultural emissions, improved soil management can substantially reduce these emissions and can help to sequester some of the CO₂ removed from the atmosphere by plants as carbon in the soil organic matter. In addition to decreasing GHG emissions and sequestering carbon, wise soil management that increases organic matter and tightens the soil nitrogen (N) cycle can yield powerful synergies, such as enhanced fertility and productivity, increased soil biodiversity, reduced erosion, runoff, and water pollution, to buffer crop and pasture systems against the impacts of climate change (Paustian et al. 2016). Mitigating nitrous oxide (N₂O) emissions from agriculture through improved management practices of fertilizer and livestock waste are critical to achieving carbon neutrality goals (Aggarwal et. al. 2018). While improved waste management and animal feed have the potential to mitigate N₂O emissions, according to Galik et al. (2017), improved nitrogen fertilizer usage practices hold the best potential for immediate mitigation. Methane emissions from enteric fermentation and livestock manure, comprise a significant share of GHG emissions from the agricultural sector. The USDA has estimated that non-CO₂ emissions from agriculture can be reduced by at least 25% from current levels by 2050 through expansion of existing mitigation practices, widespread deployment of recent technological innovations, and expansion of outreach and technical assistance efforts (The White House 2016).

All of these better practices have roles to play in present and future policies and investments in resilient infrastructure. These practices align with possible scenarios of carbon emission policies and infrastructure adaptation. Increased carbon sequestration can provide a new revenue stream for farmers through voluntary carbon credits that will help restore the balance between atmospheric carbon and soil organic carbon (Zhao et al. 2019). Such restoration will have downstream effects of healthier humans, animals and the environment, as stated by the ‘One Health’ principle of WHO. Regenerative approaches in agriculture can help to bridge the gap between human and environmental health. According to UN forecasts, “if we meet key goals in food and agriculture, cities, energy, and materials, along with health and well-being, we have the potential to open up market opportunities worth up to $12 trillion a year in less than 15 years” (Elkington 2017). Improved agricultural and land use practices can reduce/sequester 204.2 and 273.9 gigatons of CO₂ equivalent emissions by 2050 (Hawken 2020).

The 6th wave of innovation (initiated by such climate change mitigation strategies as carbon farming, and carbon neutrality) can be the transition to holistic agricultural practices through a system’s thinking approach. In sustainability science, co-creation principles have been advocated as a working principle to achieve local consensus for action within the circular bioeconomy (Itten et al. 2021).

All of these goals are valuable for focusing on the proposed agricultural strategies and practices, such as sustainable agriculture, carbon recycling and carbon farming, nature-based negative emission technologies and regenerative agriculture/farming (Fig. 1).

Several of the 17 UN Sustainable Development Goals (SDGs) are particularly relevant to sustainable agriculture, including SDG 2 ‘Zero hunger’, SDG 3 ‘Good health and well-being’; SDG 12 ‘Responsible production and consumption’, SDG 13 ‘Climate action’ and SDG 15 ‘Life on land’.

According to the ‘Farm to Fork’ and Biodiversity Strategies, there is a wide-range of ambitious targets to achieve to ensure that the EU food system is on a transformative path towards greater sustainability. In this strategy, biochar is among 25 farming practices, suggested in the ‘Farm to Fork’ strategy for carbon capture and storage and it is the one with the highest positive impact on carbon capture and storage (about 30–40%) (Liu et al. 2016). Those with the greatest relevance to agricultural production are: to contribute to a reduction of at least 55% in net GHG emissions by 2030; reduction, by 50%, of the use and risk of chemical pesticides, and reduction in use of more hazardous pesticides by 50% by 2030; reduction of nutrient losses by at least 50% while ensuring that there is no deterioration in soil fertility. This will reduce the use of fertilizers by at least 20% by 2030; reduction by 50% of usage of antimicrobials for farmed animals and in aquaculture by 2030; reaching 25% of agricultural land under organic farming by 2030; a minimum of 10% area under high-diversity landscape features (EASAC 2022). As part of agricultural transition for achieving the goals, pyrolysis of biomass for producing biochar can and should be an important facet.

In some countries, we find that negative emission technologies such as usage of biochar can be part of the solution as it has the potential to offset between 13 and 40% of the GHG emissions and already have other co-benefits within the Norwegian agricultural sector (Tisserant et al. 2021).

Pyrolysis of biomass for biochar production

In this context, biochar use in agriculture, has direct benefits in relation to modern agricultural practices. Pyrolysis is widely employed to produce biochar since it preserves one-third of the feedstocks as persistent biochar products while also generating bio-oils and non-condensable gases (Soni and Karmee 2020). Biochar production from widely distributed waste(s) has socio-economic and environmental significance in achieving C neutrality. The possibility of producing biochar with multiple functions in a sustainable way can help to position the biochar industry as a viable contributor in creating a more sustainable and prosperous future for all people and for the environment (Chen et al. 2019).

In sustainable agriculture, biochar use can help to sustain a balance between higher food production and biosphere conservation, and it improves water conservation, reduction in need for fertilizers and pesticides, and it helps to foster increased biodiversity of agroecosystem, while increasing economic stability.

Biochar usage, together with other practices such as afforestation and reforestation, agroforestry, protecting soils and enhancing soil organic carbon, conversion of cropland to fallow/permanent grassland, and restoration of peatlands and wetlands (Bolesnikov and Dinda 2022) demonstrated the efficiency in increasing soil carbon sequestration. In this way, biochar plays a direct role in the carbon cycling strategy which is effective in strengthening the man-made carbon cycle which synergistically integrates with the natural ones and makes their benefits available at a higher rate to natural processes with a higher selectivity towards a single product or goal.

Carbon farming is an agricultural approach designed to utilize the capacity of agricultural soils and forests to absorb surplus carbon from the atmosphere, and to store it in the soil (converted to plant material and/or to soil organic matter), where the carbon enhances the health and productivity of the soils. By incorporating biochar into the soil, which helps to increase the amount of carbon stored in the soil organic matter, together with natural addition of carbon from dead plant materials. Integration of biochar into the soil, helps to reduce carbon losses from decomposition processes. Burning fossil fuels emits carbon dioxide into the atmosphere faster than photosynthesis and other practices can remove it. The damage caused by the imbalance is disruption of the carbon cycle rather than by the carbon itself (Bolesnikov and Dinda 2022).

Negative emission technologies (NET) are obligatory for reaching the 2015 Paris Accord climate change goals (Lu et al. 2019). Nature-based NETs on land rely on biomass C sequestration through interventions, such as reforestation and afforestation, sustainable forest management, soil C sequestration from increased inputs to soils, and biochar additions (Goll et al. 2021).

Using organic fertilizers and crop residues in agricultural soils enhances C sequestration. New technologies need to be developed and implemented to improve the C sequestration efficiency, e.g., by repeated changes of redox conditions similar to rice paddies (Liang et al. 2017) and by promoting microbial diversity (Siedt et al. 2021) and an abundance of soil organic carbon powering the “microbial C pump” and improving the storage of microbial mass in soils (Kastner and Miltner 2018). This ‘system’s’ approach may require additional fertilizing measures when leaving crop residues in (poor) agricultural soils. Biochar amendments can be a practical approach to increase SOC stocks due to the stable (on a millennium time scale) nature of the C contained in the biochar incorporated into agricultural soils (Yin et al. 2021).

Biochar is included in the list of nature-based NETs, which are of two types: 1) bioenergy with carbon capture and storage (BECCS) and 2) production and integration of biochar into agricultural soils (Amonette et al. 2021). As a near-term alternative to BECCS, some biomass intermediate pyrolysis poly-generation (BIPP) systems have been developed for the production of large quantities of biochar for agricultural applications (Yang et al 2021).

The term “poly-generation” means that the system has multiple outputs in its technology. The BIPP technology represents intermediate pyrolysis, which operates with a residence time of 30 min at a 600^oC (Zhang et al. 2018), fast pyrolysis yields relatively more biochar (33–37% increased yield). The pyrolysis reactor has a heat recovery system that decomposes 80% of the biofuels to generate electric power. The remaining 20% of the biofuels can be used for household purposes. Compared to slow pyrolysis, intermediate pyrolysis poly-generation provides more heat and provides the opportunity for electricity generation through the generated pyrolysis gas and bio-oil in a continuous production system. An advantage is the high stability biochar which is 60–80% of the carbon will be stable for more than 100 years when integrated into the soils (Masek et al. 2013) in a continuous production system. However, the feedstock and the pyrolysis conditions determine the preliminary stability of the biochar, whereas soil properties, climate conditions and farming practices defines the ultimate recalcitrance of biochar. Last but not least the BIPP system has good feedstock flexibility, enabling the use of many different biomass sources with the possibility to make many different types of biochar products (Chen et al. 2016). Regarding climate change mitigation, Lehmann et al. (2021) concluded that widespread agricultural applications of biochar could reduce global emissions of GHGs by 3.4 to 6.3 Pg CO₂e/year and the feedstock of the biochar would be plant residues.

Yang et al. (2021) concluded that the BIPP char producing systems have triple, near-term benefits: a. addressing climate change, b. providing energy and c. producing biochar for improving soil quality. The wide-scale deployment of BIPP systems, could significantly reduce air pollution, by decreasing in emissions of SO₂, NO_x, BC, and primary PM_2.5. In addition, deployment of BIPP systems could have more pronounced regional effects in different provinces. The calculations suggest that China alone, can achieve around 13–31% of the global removal goals for BECCS by focusing strongly between now and 2050 on the deployment of BIPP systems including biochar production and usage in agricultural soils. These estimations are based on the detailed, but conservative analyses, by only using agricultural, forest residues, and energy plants (Yang et al. 2021).

By supporting and sustaining soil microbial processes and enhancing plant health, biochar brings many benefits of regenerative agriculture (regenerative farming). Regenerative agriculture is defined as a system of farming principles that are designed to maintain agricultural productivity, increase biodiversity and in particular, to restore and maintain soil biodiversity and to enhance ecosystem services, including carbon capture and storage (Oberč and Arroyo Schnell (2020)). “Regenerative agriculture,” based on recycling farm waste and using composted materials from other sources, combined with improved land use practices that enhance soil carbon sequestration, can potentially store 3.5 to 11 billion tons of carbon dioxide emissions per year” in agricultural soils (Mann 2021). Regenerative practices such as no-tilling, no pesticide spraying, cover cropping, crop rotation, and managed grazing practices can help to further increase carbon storage in agricultural soils.

In contrast to related agricultural approaches, regenerative agriculture is not viewed as defined ‘a prior,’ by a given set of rules and practices, instead the goals that should be achieved are set and the practices and technologies which contribute to achieving these goals will be developed and adopted over time (EASAC 2022). The main components of regenerative agriculture include: soil-health restoration, increased health of animals and humans eating the better quality plants produced on soils that have been treated with biochar, enhanced capture and carbon-storage, and reversal of biodiversity losses.

The progress in achieving the carbon neutrality targets needs to be measured. This could be supported by the usage of an indicator framework. One objective of the authors of this article is to outline the direct carbon neutrality indicators, which can be applied to agriculture in working to achieve circular economy goals. The carbon neutrality indicators were described and examples provided based on the expanding literature pertaining to the potential usage of biochar in agricultural production processes.

The ‘Linear Economy,’ is comprised of the traditional ‘take-make-use-dispose’ model of production and consumption, needs to be reworked for agricultural production to keep pace with the world’s projected population growth and the resultant increased demand for food and other materials. Burning waste crop residues may be a cost-effective management option in a linear model that overlooks adverse environmental effects and losses of biomass nutrients. Given the anticipated increasing demands for global food production and for substantial GHG mitigation, it’s unlikely that farmers and society will be able to ignore these costs and benefits for much longer. The European Commission’s, Circular Economy Strategy and “Closing the Loop” Action Plan (European Commission 2015) noted the high value of bio-based resources and biochar specifically, that can help the EU to implement a circular bioeconomy (Kourmentza et al. 2018).

A “circular bioeconomy” is defined as the overlap of the circular economy and bioeconomy (Carus and Dammer 2018), an innovative research-based approach to optimize the sustainable management and utilization of bio-based resources (Rajesh Banu et al. 2020). Carus and Dammer (2018) suggested that the European Union’s 2012 bioeconomy and 2015 circular economy were focused upon biologically originated products, biomass, and food wastes.

The Institute for European Environmental Policy (2018) stated that the implementation of a circular bioeconomy was created to fulfil the United Nation’s Sustainable Development Goals (SDGs) and commitments to both sustainable consumption and reduced GHG emissions. Recently the European Commission started the ‘Green Deal’ program according to the principles of CE. Net regenerative agriculture, reforestation and afforestation with increased species diversity can contribute to the needed stabilisation or even to reversal of climate change.

De Pascale et al. (2021) reviewed various CE indicator methods and classified them according to spatial levels (micro, meso and macro) and to application levels. However, the CE indicators for the manufacturing sector cannot be directly applied to the agricultural sector, e.g. there is a need to differentiate among technical and biological resource materials, the perishable nature of products, the close link with natural ecosystems and the strong seasonality of bio-production, by using slowing, closing and regenerative features.

Velasco-Munoz et al. (2021) developed the CE indicator framework to support sustainable, restorative, and regenerative agriculture. With the objective to focus upon carbon neutrality, they developed a new set of indicators for carbon neutrality in agriculture. The indicators in this set are related to materials and processes which directly affect carbon neutrality. The set was formulated for usage within agricultural boundaries and were designed to address the need to observe the progress of the carbon neutrality of individual farms. The indicators were described along with the benefits of biochar for progressing in carbon neutrality as highlighted in the following section. The indicators were divided into three groups of resource loops as suggested by Velasco-Munoz et al. (2021), namely slowing, closing and regenerative. The slowing (S) indicators present the operations that help to save and optimise the use of resources. The closing (C) indicators indicated the processes aimed at reusing agricultural materials, but for other than the original applications and contributing to the resource cascading approach. The regenerative (R) indicators are focused on preserving and enhancing natural resources.

Each group of indicators to carbon neutrality for individual farms are presented in Figs. 2–4.

Slowing (S) resource loop indicators

S1. Resource export index

It demonstrates the extent to which local household nutrient production exceeds both individual household demand plus the demand from green areas (De Kraker et al. 2009). In the case of biochar usage in agriculture, it will provide markets for biomass that will/can lead to positive changes in land usage (Yrjala et al. 2022).

S2. Carbon balance

It is calculated by summing direct and indirect CO₂ emissions and subtracting avoided CO₂ emissions (Fernandez-Mena et al. 2020). Greenhouse gas (GHG) emissions include agricultural activities (e.g. livestock enteric fermentation and emissions related to organic fertilizer material’s storage and application), material transportation (truck and boat emissions per kg of material transported and per km for local and global materials flows) (ADEME 2017). Additionally, the GHG emissions avoided by biogas production and substitution for fossil energy usage are estimated (European Environmental Agency 2015). For estimating soil C sequestration, separate calculations of C from organic fertilisers, from crop residues and from grasslands grazing, should be considered.

Buffer zones (used to prevent excess nutrient runoff into water systems) have been seen as potential land areas for energy biomass production in the Netherlands (Meeusen et al. 2000) and in Denmark (Christen and Dalgaard 2013). Vassura et al. (2017) studied oat (Avena sativa) production impact on GHG and use of the field buffer zone biomass for biochar production. It was observed that producing one metric ton of oat flakes from cradle-to-gate generates 700 kg of CO₂eq emissions. Buffer zones in willow biomass and biochar production may enable the full neutralization of the global warming potential of the oat production within the agricultural system.

Biochar production from the side flows could mitigate 350 kgCO₂eq/t of the oat production. Willow buffer zones could be used for biomass production. This would enable additional biochar production and increase the potential to sequester a maximum of 390 kgCO₂eq/t oats, which could theoretically, lead to carbon-neutral oat production.

Ankuda and Baltrėnaitė (2021) studied Tora’ willow (Salix sp. Wall) as energy plant and mugwort (Artemisia dubia Wall.) for biochar production and amendment for improvement of sandy soil quality. They concluded that mugwort biomass was better than willow biochar suited for incorporation into non-fertile sandy soil with the objective of stopping its degradation and improving its properties.

Erenstein (2011) reported that biochar integration into the soil is an effective crop residue management option to eliminate the detrimental effects of crop residue burning. It also contributes to improved soil health and fertility and leads to sustainable agriculture with substantial carbon sequestration; all of which are of great significance. Moreover, better calorific values and cleaner fuel-sourced by-products from biochar production (thermochemical) technologies than fossil fuels, can play significant roles in crop residue management. They were recently promoted, over other technologies by (Lehmann 2007).

Rice is an essential crop that requires improved crop residue management practices due to surplus amounts of rice straw that is not readily useful for the rice-farm’s livestock and due to shorter cropping gaps. In addition, crop residue incorporation as a blend with other lignocellulosic biomasses, such as weeds, can be used for sustainable management through thermochemical decomposition processes. Its further incorporation as a blend with other inorganic materials like sewage sludge and fly ash for carbon capture and enhanced soil productivity, should be researched in the context of seeking to achieve sustainable agriculture (Singh et al. 2015).

S3. Overall greenhouse gas balance

This is calculated as the CO₂ equivalents emitted per unit product (Casson-Moreno et al. 2020). Lehmann et al. (2021) concluded that widespread agricultural applications of biochar could reduce global emissions of GHGs by 3.4 to 6.3 Pg CO₂e/year (0.92 to 1.72 PgC), with half of that value stemming from increased soil carbon sequestration. These estimates are likely to be overly optimistic, because available plant residues on croplands contain about 1.5 Pg C (Wolf et al. 2015), not all of which can be gathered, and captured in pyrolysis, and converted to biochar. For instance, only about 50% of the carbon in plant residues is retained in biochar; the remainder is emitted to the atmosphere as CO₂ during pyrolysis (Gaunt and Lehmann 2008). Schlesinger and Amundson (2018) suggested that a more realistic estimate is 0.18 Pg C/year of net carbon sequestration using biochar on croplands globally.

Apart from CO₂, agriculture is a significant source of methane (CH₄) and nitrous oxide (N₂O) emissions. As the global warming potential of N₂O and CH₄ are 298 and 235 times greater, respectively, than the equivalent mass of CO₂ in the atmosphere (Forster et al. 2007), the reduction of all three of them will significantly contribute to the overall balance of GHGs and help to slow down or reverse global warming.

Diversification of biochar use in agriculture can help to achieve even higher reductions of GHG emissions. For instance, biochar use as biofiltration loading in biofilters can reduce ammonia (NH₄⁺) concentration by up to 87% and thus contribute to reduced releases of nitrogen compounds (Baltrėnaitė et al. 2016; Baltrėnas and Baltrėnaitė 2020). Biochar use as an additive to the sewage sludge in the farm-scale waste anaerobic digesters showed a decreased effect on CO₂ and H₂S emissions and increased CH₄ yield (Baltrėnas and Baltrėnaitė 2020).

S4. Partial nitrogen balance

The nitrogen balance was evaluated by the difference in farmer-managed N inputs and N outputs (Tadesse et al. 2019). Most GHGs emissions are caused by nitrogen fertilizer production and soil N₂O emissions from nitrogen fertilizer usage. Nitrogen fertilizers are produced by the natural gas steam reforming and the Haber-Bosch process, which consume large amounts of fossil natural gas (Uusitalo and Leino 2019).

There is documented evidence about N₂O emission reductions by some types of biochar amended on soil, e. g. Yanai et al. (2007) based upon municipal biowaste-based biochar. They documented 73% reduction in N₂O emissions. Similarly, Rondon et al. (2006) found a positive effect on N₂O reduction by using mango tree wood biochar. This confirms reduced release of N compounds in agriculture with a positive effect on climate change due to agricultural usage of biochar.

S5. Indicator of circular economy efficiency of the biofertilizer

This indicator was designed to document the percentage of bio-fertilizer produced relative to the amount of raw material used, which indicates the production rate of the biofertilizer during the process. Biochar-based fertilizers (BF), as a new type of environmental protection fertilizer, have received attention in the fields of agriculture and environmental protection (Agarwal et al. 2022). The BFs are made by mixing biochar with organic or inorganic fertilizers which can be used as a slow-release nutrient carrier (Gao et al. 2022). The BF not only confers the advantage of soil improvement but also continuously supplies nutrients in combination with the fertilizer requirements of crops, thereby reducing nutrient losses and achieving the dual functions of slow release and carbon fixation (Guo et al. 2021). As a slow-release fertilizer, BF can maintain high nutrient levels in the soil for a long time, which is attributed to the fact that biochar retains nitrogen, phosphorus, potassium, and other nutrients (Rasse et al. 2022).

The results showed that the net income following BF treatments increased by 4.30–12.76% in 2020 and 26.13–51.35% in 2021, similar to the results of previous studies (Zhang et al. 2022). Furthermore, the N, P, and K accumulation following BF application increased by 19.77–58.01%, 0.70–25.05%, and 16.11–54.93% in 2020, and by 43.94–133.78%, 18.42–70.50%, and 35.54–101.65% in 2021, respectively, compared to the TFP (traditional fertilizers treatment) treatment group (Zhang et al. 2022).

S6. Avoided carbon emissions for bioenergy systems

This indicator was designed to help to quantify the reduction in fossil energy usage by substitution of renewable energy, measured in tonnes of CO₂ equivalent (Zabaniotou 2018). Gasification of coal and biomass as a net carbon-negative power source was proposed for environmentally-friendly electricity generation. A recent meta-analysis quantified the impacts of biochar application on GHG emissions and crop yields by employing yield-scaled GHG intensity (GHGI) (Liu et al. 2019). Their results documented that biochar significantly reduced yield-scaled GHGI by 29%. Experimental conditions and properties of the soils influenced the reductions in yield-scaled GHGI. They documented that in dry lands, the reduction was much more evident with biochar use compared to its effectivity in wet paddy soils.

S7. Land use and land-use changes related to bioenergy feedstock production

This indicator to help to quantify the total land area for bioenergy feedstock production compared to the total national area, including agricultural land, and forest land (Zabaniotou 2018). Biochar provides bioenergy and biochemical products from pyrolysis gases and tars (e.g. heat, power, vehicle fuel, lubricants) (Yrjala et al. 2022). Therefore, the production of biochar from farm waste biomass (including e.g. biomass from buffer zones) would contribute to the total land area used for bioenergy, and thereby, they can be used to improve and increase carbon capture and increase agricultural productivity.

S8. Water quality

This indicator pertains to the amount of pollutants and nutrients entering waterways (Zoboli et al. 2016). Biochar improves soil nutrient fluxes that can decrease eutrophication. This is supported by several benefits that biochar has on soil. Biochar has high nutrient retention and thus the timing of nutrient applications becomes less critical with regard to nutrient losses. The physical characteristics of biochar (e.g. pore structure) may change the pore-size distribution of the soil and alter percolation patterns, the residence time of soil water and the leaching flow pathways. Biochar can reduce the leaching losses in regions of high rainfall. Finally, biochar contains high CEC and its application to soil favours increase of negative charge (Major et al. 2009). Biochar also provides soil contamination remediation and, thereby, reduces emissions to water (Yrjala et al. 2022).

S9. Contaminant footprint

The contaminant footprint indicator pertains to the contaminant density in the environment, the amount of which is influenced by the increased amount of extracted resources from the Earth.. The environmental self-purification capacity starts to decline as far as the contamination footprint in the biosphere increases (Baltrėnaitė et al. 2018). In the boundaries of agriculture, it encompasses the contaminants (such as potentially toxic elements, including Zn, Ni, Cu, Cr) entering the agricultural system with fertilizers and amendments (e.g. sewage sludge, slurry). The quality of the amendments, including biochar, and the risk they may pose to the environment must be carefully monitored and limited. The aerogenic contaminants may enter the agricultural system and be deposited on the soil via aerogenic transport from stationary and defused pollution sources. Apart from the direct impacts on the reduction of the natural capacity, they cause the formation of secondary/tertiary contaminants. For example, fertilisers containing a reactive form of metals promote the formation of environmentally persistent free radicals (Baltrėnaitė-Gedienė et al. 2022), which produce reactive oxygen species (ROS) and can result in rhizosphere and plant toxicity (Zhang et al. 2019).

Closing (C) resource loops’ indicators

C1. Circularity indicator of component i

This is the indicator that pertains to the amount of component i that extends its lifetime in the upstream processes relative to the component present in the waste (Cobo et al. 2018). Biochar addition to soil highly extends the SOC’s lifetime which is a highly positive indication of C circularity. The priming effect on rates of stabilization or turnover of the persistent soil carbon supports carbon retention within agroecosystems (Woolf and Lehmann 2012).

C2. Waste output index

For this indicator, the nutrient system boundaries are expanded, considering the amounts of nutrients available or the total inputs of nutrients that can be used in nearby agricultural lands as well as those that go to households and urban greens are kept within the system; they can be considered as having been recycled, whereas nutrients going to the landfills or lost during treatment are regarded as exported wastes (De Kraker et al. 2019). Biochar production from waste material (e.g. crop waste and biomass from buffer zones) provides markets for biomass that will/can lead to positive changes in land-usage (Yrjala et al. 2022).

C3. Fertiliser nutrient balance (Nitrogen balance)

This indicator is focussed upon the fertilization inputs and crop outputs (Fernandez-Mena et al. 2020). The nitrogen balance component is defined by three indicators, such as fertilization inputs and crop outputs (potential local flows in terms of fertilization, bioenergy and animal feeding), use of N from chemical fertilizers, recycled sewage sludge (% of N recycled), and nitrogen leaching (N losses and N use) (Schils et al. 2018; IPCC 2013). Biochar improves soil nutrient fluxes and can have a substantial impact on efficient nitrogen balance e.g. according to Aller et al. (2018).

C4. Renewable energy production

This indicator focusses upon the system’s capacity to produce renewable energy using anaerobic digestors on the farms (Fernandez-Mena et al. 2020). This indicator of renewable energy production includes the amounts (m³) of produced biogas and the amounts of electricity (kWh) generated. Since biochar production provides bioenergy and biochemical products from pyrolysis gases and tars (e.g. heat, power, vehicle fuel, lubricants) (Yrjala et al. 2022), there are many strategies for using the end-product, biochar, to facilitate zero-waste and the development of circular economies (Hu et al. 2021). Matrapazi and Zabaniotou (2020) observed that the utilization of spent coffee grounds leads to the formation of three valuable products, which can be sold or used in situ or off-site, thereby helping to close energy and material loops which include:

biogas with a low calorific value of 14 MJ/Nm3, is clearly lower than that of natural gas (38 MJ/Nm3), but is still a good fuel to be used in situ for the needs of the plant, thereby helping to close the energy loops;

biooil which is of low quality and needs to be upgraded before sale, although, its properties make it a rather satisfactory fuel that can be fed smoothly into existing combustion systems, in situ, thereby helping to close energy loops;

biochar, which can be used as a soil amendment or as a fertilizer for the city's gardening needs, thereby helping to close that material loop.

Anaerobic digestion is an important technology for organic waste treatment and bioenergy recovery, thereby it can play a vital role in developing the world's circular economy (Fagerström et al. 2018) by mitigating emissions of GHG, extracting energy from the organic compounds via functional microorganisms, avoiding the release of hazardous odours and substances into the environment, and recycling nutrients in the form of organic fertilisers.

Supplementation by materials such as biochar and activated carbon is a promising strategy to enhance anaerobic digestion (Dang et al. 2016), as this strategy does not involve modification of the infrastructures. It has been demonstrated that biochar, derived from gasification or pyrolysis processes, can be added to anaerobic digesters to improve anaerobic digestion stability and to enhance methane production. It has been found that biochar plays a crucial role in enhanced anaerobic digestion via several mechanisms, including:

promoting direct, interspecies electron transfer in anaerobic digestion among different microbial species due to biochar's good conductivity (Liu et al. 2012)

enhancing microbial growth through biochar's immobilization effects (Fidel et al. 2017),

adsorbing inhibitive compounds like heavy metals, ammonia and volatile fatty acids in the anaerobic bioreactors (Sunyoto et al. 2016), and

increasing bioreactor buffering capability by the biochar's relatively high alkalinity (Fidel et al. 2017).

In addition, the economic feasibility of the biochar addition strategy with a dosage of 15 g/L in AD processes for enhancing methane yield has been documented (Zhang et al. 2020b). Biochar use as an additive to the sewage sludge (10% v/v) in the farm-scale waste anaerobic digesters showed increased CH₄ yield by 10% (Baltrėnas and Baltrėnaitė 2020). Furthermore, some authors have reported that adding biochar can improve the bio-availability of the micro and macronutrients of the digestate that can be used in agricultural fertigation (Zhang et al. 2020a).

C5. Nutrient (Nitrogen) recycling index

This indicator focusses upon the ratio of the recycled nitrogen to the total nitrogen used in the agricultural system (Rufino et al. 2009). Using biochar in agriculture, facilitates NO₂ removal from the soils (Uusitalo and Leino 2019). It improves soil nutrient fluxes that may decrease eutrophication (Yrjala et al. 2022).

C6. Nutrient (Nitrogen) use efficiency

This indictor focusses upon nitrogen (N) use efficiency as the ratio between the harvested N outputs and the managed N inputs, presented in percentages (Tadesse et al. 2019). The N use efficiency and the N recycling enhancing measures can include livestock feed quality improvements, exchange of crop residues and manure between crop and livestock compartments within and among farms, and provision of alternative and low-cost energy sources for satisfying the fuel demands of the farm families. Using biochar as a soil amendment can reduce the nitrogen fertilizer requirements of crops and pastures, particularly in the case of manure-based biochar, which has a greater N content than wood-based biochar. Moreover, biochar particles react with soil minerals and organic matter to generate charged surfaces that attract nitrate and ammonium ions and thus reduce their losses through leaching and volatilization (Singh et al. 2010). Reducing N fertilizer requirements contributes to GHG mitigation benefits because manufacturing N fertilizers is a particularly GHG-intensive process, typically releasing 2–10 g CO₂e (CO₂ equivalents) per g fertilizer-N due to high natural gas consumption (Williams et al. 2006).

Regenerating (R) resource loops’ indicators

R1. Consumption of fossil-P-fertilizers

This indicator focusses upon the total fossil-P-fertiliser consumption (Zoboli et al. 2016). As discussed by Zoboli et al. (2016), there are several ways to reduce the P demand and consumption in agriculture. It can be achieved by a balanced and healthy diet, increased usage efficiency in crop production, performing animal feeding in accordance to growth phases, lowering phytate-P content, adding phytase enzymes and using more digestible feedstuffs (Schoumans et al. 2014). As biochar reduces the agricultural sector’s usage of pesticides and other agrochemicals (Yrjala et al. 2022) it can also help to reduce the consumption of fossil-P-fertilizers.

R2. Effective cation-exchange capacity (CEC eff.)

This indicator focusses upon the CEC eff. That pertains to the total amount of exchangeable cations that the soil can adsorb on its surfaces (Mosquera-Losada et al. 2019). The CEC eff. is strongly associated with the available surface area of the soil and thereby, it effectively reflects the reactive soil surfaces to which SOC may be adsorbed. It may also represent a direct measure of SOC sorption to soil surfaces at the existing pH conditions (Solly et al. 2020). The CEC eff. is an important indicator of soil-fertility that was documented to be positively affected by high doses (5–10 Mg/ha) of bio-waste-based fertilizers (containing urban sewage sludge, chicken manure, wood ash, animal by-products, quicklime, wood kindling) (Mosquera-Losada et al. 2019). Biochar also increases soil CEC and thus it can be considered as an effective material for increasing the CEC eff.

R3. Species richness

This indicator focusses upon the species richness of soil fertilized with bio-waste-based fertilizers. The species richness was measured by the total number of species (i.e. annual alpha-biodiversity) and the annual abundance diagrams. The high doses of the bio-waste-based fertilisers improved the soil fertility and therefore, increased pasture production and modified its botanical composition, which was more sensitive to the variation of soil fertility than the production. These results indicated that the botanical composition could be considered to be a good bio-indicator of changes in soil fertility when bio-waste-based fertilisers are used in agriculture. The soil and pasture results associated with the bio-waste amendments were similar to those obtained when the conventional practices of the area were used. Therefore, the partial or total substitution of mineral fertilisers by bio-waste-based fertilisers is a valuable step in adopting the circular economy concepts in agriculture (Mosquera-Losada et al. 2019). Additionally, the agricultural, circular economy is improved by using biochar which improves the soil’s nutrient status and improves its porosity, pH, ion exchange capacity and root growth positively as well as it improves the soil’s microbial processes (Yrjala et al. 2022).

R4. Soil quality

This indicator is used to measure the percentage of land on which the soil quality—especially in terms of organic carbon—is maintained or improved relative to the total land on which bioenergy feedstocks are cultivated or harvested (Zabaniotou 2018). The biochar improves soil nutrient status and improves porosity, pH, ion exchange capacity and root growth, positively, and as well as it improves soil microbial processes. Biochar provides soil contamination remediation and, thereby, improves the land for the production of safe food products. Biochar reduces the agricultural sector’s usage of pesticides and other agrochemicals (Yrjala et al. 2022). Many studies have documented that biochar can reduce the mineralization rate of organic matter (Cross and Sohi 2011). More importantly, when determining the long-term impacts on native soil carbon, it is not the priming effect of labile soil carbon but the effects of priming on rates of stabilization or turnover of the persistent soil carbon (Woolf and Lehmann 2012), which has a direct, positive impact on GHG emission reduction.

R5. Biological diversity in the landscape

This indicator is used to measure biodiversity (biological diversity), as nationally recognised areas of high biodiversity value have, relative to the total land on which bioenergy feedstocks are cultivated (Zabaniotou 2018). Soil biological diversity, which is structurally the primary level of diversity development in the ecosystem, is positively affected by the biochar amendment. Such physicochemical properties of biochar as pH, electrical conductivity (EC), volatile organic compounds, surface structure and chemical element composition have been shown to affect the soil microbial community structure, diversity and abundance. This is manifested by a more favourable microbial habitat and substrate for microbial metabolic activities, promoting ecologically important functions such as enzyme activities, element cycling, decontamination of soil contaminants and plant productivity (Palansooriya et al. 2019).

Work to achieve Carbon neutrality includes an array of processes designed to help to achieve the global climate change goal of net zero emissions by 2050. Agricultural processes have been estimated to contribute about 30% to the global carbon dioxide emissions. Among the carbon dioxide removal technologies, which are crucial in achieving carbon neutrality, biochar has been documented to be effective and economical for the agricultural sector. Agricultural progress toward carbon neutrality should be monitored. The extensive set of indicators that are being used, have documented biochar’s effectivity in making the needed changes. The framework for twenty indicators for monitoring the carbon neutrality progress in the individual farm was presented. The twenty indicators were divided as follows: a. Nine addressed ‘slowing resource loops.’ b. Six addressed, ‘closing resource loops.’ c. Five addressed regenerative resource loops. The increasing breadth and depth of the multiple benefits of increased agricultural usage of biochar in carbon neutrality activities within the farm were presented. Much has been learned, but much more is needed to more effectively accelerate the transition to equitable, sustainable, livable societies in the context of climate changes, COVID-19 like pandemics and the current net annual increase in the human population of about 81,000,000. Let’s co-work locally, regionally and globally to achieve the needed changes before it is too late!

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Competing Interests

The authors have no relevant financial or non-financial interests to disclose.

Author Contributions

All authors contributed to the study's conception and design. Edita Baltrėnaitė-Gedienė: concept, design of the work, analysis, artworks, manuscript drafting, revision. Donald Huisingh: design of the work, revision. Kim Yrjälä: revision, interpretation of data. All authors read and approved the final manuscript.

Ethics approval and consent to participate

This is not applicable.

Consent for publication

This is not applicable.

Availability of data and materials

This is not applicable.

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Circular Economy Benefit Indicators for Using Biochar in Regenerative Agriculture

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Abstract

Figures

Introduction

Measures reaching carbon neutrality

Agriculture and strategies for achieving carbon neutrality

Pyrolysis of biomass for biochar production

Methods

Results and discussion

Slowing (S) resource loop indicators

S1. Resource export index

S2. Carbon balance

S3. Overall greenhouse gas balance

S4. Partial nitrogen balance

S5. Indicator of circular economy efficiency of the biofertilizer

S6. Avoided carbon emissions for bioenergy systems

S7. Land use and land-use changes related to bioenergy feedstock production

S8. Water quality

S9. Contaminant footprint

Closing (C) resource loops’ indicators

C2. Waste output index

C3. Fertiliser nutrient balance (Nitrogen balance)

C4. Renewable energy production

C5. Nutrient (Nitrogen) recycling index

C6. Nutrient (Nitrogen) use efficiency

Regenerating (R) resource loops’ indicators

R1. Consumption of fossil-P-fertilizers

R2. Effective cation-exchange capacity (CEC eff.)

R3. Species richness

R4. Soil quality

R5. Biological diversity in the landscape

Conclusion

Declarations

References

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