Biochar application to agricultural soils
Why and how biochar is valuable for agricultural production
Biochar, short for “bio-charcoal” or “bio-coal,” is a charcoal of plant origin made by burning organic matter from agricultural and forestry waste (also called biomass) in a controlled process called pyrolysis [1]. It appears as small black fragments and has several uses, such as soil amendment for agriculture or wastewater treatment. It is presented as an excellent carbon capture agent.
Human-caused climate change and unsustainable agriculture have led to episodes of droughts, fertilizer leaching, and food insecurity worldwide [2]. Faced with current and future challenges, biochar could be a determining factor in developing a sustainable agricultural future while contributing to the mitigation of climate change impacts [3]. Biochar has been recognized to positively affect soil quality and crop yields while keeping soil health intact [4].
Different biochar production techniques
The principle of biochar production is based on thermochemical conversion. This process includes several methods, such as pyrolysis (the most used), hydrothermal carbonization, gasification, and torrefaction. Pyrolysis is the process of thermal decomposition of organic matter in an oxygen-free environment at a temperature between 250 and 900 °C [5].
The technique chosen for production must be appropriate depending on the type of biomass and the process conditions such as heating rate, temperature, residence time, etc. These conditions are crucial as they can affect the physical and chemical states of the resulting biochar [5].
Factors affecting biochar properties
The reaction conditions during the pyrolysis process are mainly responsible for the properties of biochar. Factors such as raw materials, temperature, particle size, heating rate, etc., primarily influence the properties of biochar [6].
- The raw material is characterized by two types: (i) woody biomass and (ii) non-woody biomass. Woody biomass mainly includes tree residues and forest residues. It is characterized by low moisture content, little debris, less void, high density, and calorific value. Non-woody biomass includes animal, industrial and agricultural solid waste [5].
- The carbonization temperature influences the physicochemical properties and structure of biochar, for example, elemental composition, pore structure, surface area, and functional groups [5].
- Residence time refers to the duration of combustion.
Biochar application in agriculture
About 6,000 years ago, the Amerindians living in the Amazon rainforest discovered that the use of charcoal could transform their poor and infertile soils called “oxisols,” equivalent to ferritic soils, into fertile soils called “terra preta,” which means black soil. These people created a type of agriculture called “slash and burn agriculture,” which consisted of cutting trees from the forest and those resulting from the clearing of fields and carbonizing this biomass rather than burning it completely. The charcoal was then incorporated into the soil with other natural fertilizers, such as manure. This is why these soils have been called anthrosols.
Terra preta is found in hectare pockets in the Amazon rainforest (Brazil), generally consisting of oxisols. Terra preta is a very fertile soil in the long term; it contains 70 (seventy) times more carbon, has a high cation exchange capacity (CEC), is also rich in phosphorus, calcium, and magnesium, and abounds in a diversity of soil microorganisms. Terra preta has remained very productive for crops, which today justifies the marketing of terra preta as a substrate in agriculture and horticulture [8].
Phases of biochar interaction with soil and plant
The interactions between biochar, soil, and plants in the context of the annual crop cycle can be considered according to three phases:
- Phase 1: Short-term (1 to 3 weeks) reactions of biochar in the soil and effects on seed germination and seedlings.
- Phase 2: Medium-term (1 to 6 months) with the creation of reactive surfaces on biochar, effects on plant growth and yield, from sowing to harvest.
- Phase 3: Long-term (>6 months) interactions as biochar “ages” in the soil and its effects on subsequent crop cycles.
Biochar is generally applied at planting time or 1 to 3 weeks before planting [9].
Phase 1: Short-term reactions (1- 3 weeks)
- Chemical Effects
After application to soil, water entering the biochar pores dissolves soluble organic and mineral compounds on the biochar’s outer and inner surfaces. These solutes increase dissolved organic carbon (DOC), cations, and anions in the soil solution, which increases electrical conductivity and pH and decreases redox potential Eh.
The magnitude of changes in soil solution composition depends on the specific composition of biochar and soil. The release of DOC and nutrient ions from biochar is rapid during the first week and much slower over the following weeks [9].
Biochar also has an important effect on nutrient retention. The most direct explanation for the effect of biochar on nutrient retention is that it acts as an adsorbent. Adsorption is a phenomenon that occurs at the interface between a liquid phase and a solid phase, forming bonds between the surface of the solid phase and the molecules present in the liquid phase. Organic matter generally has a specific adsorption capacity, and the carbonization of matter typically increases the adsorption capacity. Most studies suggest that biochar adsorbs more cations than anions, and several experiments have shown the fixation of ammonium from an aqueous solution [10, 11].
- Physical Effects
Biochars generally increase the soil water retention capacity, especially in coarse-textured soils, decrease bulk density, and increase porosity, with more significant effects observed at rates exceeding 40 T.ha-1 [12].
Biochar can also impact water infiltration into soils, for example, by moderating the reduction in infiltration rate during high-intensity rainfall in soils prone to surface sealing, as observed at a 2% mass fraction by Abrol et al. (2016) [13]. The reduction in sealing leads to a decrease in runoff and erosion rates [9].
Phase 2: Medium-Term Reactions (1 to 6 Months)
The effects of biochar in later periods differ from the first stage, which is dominated by the dissolution of biochar compounds. In phase 2, plant roots intercept and interact with the biochar. Root hairs penetrate the pores of the biochar, roots wrap around the biochar, and very small biochar particles can attach to the root surface.
Phase 3: Long-Term Reactions
Several studies have examined the longer-term interactions as biochar “ages” in the soil by studying the effects on general soil properties and plant growth when biochar has been applied in previous crop cycles or by examining biochar particles extracted from the soil. Degradation by cultivation, exposure to wetting-drying and freezing-thawing cycles, and ingestion by soil fauna can lead to further fragmentation of biochar particles and oxidation of biochar surfaces exposed to micro-aggregate detachment [14].
The aging of biochar through interactions with soil minerals and microbes generally leads to functionalized surfaces consisting of organo-mineral macro aggregates, which can protect biochar and newly added organic matter, thus stabilizing newly added carbon for long periods in the soil. Residual effects of a single biochar application on pH have been observed, as well as some secondary yield benefits [9].
Biochar for composting
One of the most promising uses of biochar is as an additive in organic waste composting. Several studies have well-documented the benefits of biochar as an additive in the composting process [15][16].
Adding biochar to compost batches provides a suitable habitat for microorganisms and improves the environmental conditions for microbial growth (e.g., moisture, aeration, and nutrient availability).
Increased microbial activity and improved aeration in the composting batch with biochar lead to accelerated degradation of organic matter [16].
Potential limitations of biochar application on crop yields
While most studies have shown positive effects of biochar application on crop yield, negative impacts on plant productivity have also been reported.
The first cause of the negative effect of biochar amendment is the excessive content of potentially toxic substances in biochar. Indeed, biochar can contain toxic compounds such as Polycyclic aromatic hydrocarbons (PAHs), heavy metals, and salts that can harm plant germination and growth as well as soil microbial activity. The levels of toxic compounds in biochars depend on the feedstock type and the pyrolysis conditions [17].
Secondly, a negative alteration of soil properties can occur after adding biochar. This can be explained by the following reasons: (i) a decrease in nitrogen availability due to microbial immobilization of nitrogen in soil amended with high C/N ratio biochars; (ii) an excessive increase in the pH of neutral and alkaline soils beyond the optimum for soil fertility (i.e. over-liming) leading to the immobilization of key nutrients, such as P, Mn, Fe and B; (iii) an increase in the mobility of toxic metals or metalloids; (iv) a negative priming effect in the soil which interferes with the mineralization process, reducing nutrient availability [17].
What are the key takeaways regarding the use of biochar?
Using biochar has shown good potential and beneficial contributions to increase sustainability and promote the circular economy in the agricultural sector. However, biochar also represents a new technology whose use needs to be more strongly supported by a broader base of studies revealing the inherent variability of the conditions in which this technology can be applied.
As a soil amendment, biochar has improved soil physical properties and fertility and increased crop yields, particularly in soils with low pH, coarse texture, and/or low nutrient availability. Biochar in the composting process improves the degradation of organic matter and the agronomic value of final composts and decreases N losses and GHG emissions, but some studies have not found a synergistic effect of combining biochar and compost on plant growth. Indeed, despite its multiple uses, biochar is not a standardized material, so its properties and effects can vary from matrix to matrix. An appropriate combination of the type of biochar, the purpose of its use, and the optimal application rate needs to be explored. Selecting the proper biomass and optimizing pyrolysis conditions are critical factors in creating a biochar with high agronomic value for its use in agriculture.
References
[1] Comité scientifique français de la désertification, « Le biochar est-il un puits à carbone si efficace qu’on le dit ? », Futura Planète, 2009. https://www.futura-sciences.com/planete/actualites/developpement-durable-biochar-il-puits-carbone-si-efficace-quon-dit-19347/ (consulté le 12 septembre 2022).
[2] B. Glaser et J. J. Birk, « State of the scientific knowledge on properties and genesis of Anthropogenic Dark Earths in Central Amazonia (terra preta de Índio) », Geochimica et Cosmochimica Acta, vol. 82, p. 39‑51, avr. 2012, doi: 10.1016/j.gca.2010.11.029.
[3] V. Alling et al., « The role of biochar in retaining nutrients in amended tropical soils », Journal of Plant Nutrition and Soil Science, vol. 177, no 5, p. 671‑680, 2014, doi: 10.1002/jpln.201400109.
[4] E. W. Bruun, P. Ambus, H. Egsgaard, et H. Hauggaard-Nielsen, « Effects of slow and fast pyrolysis biochar on soil C and N turnover dynamics », Soil Biology and Biochemistry, vol. 46, p. 73‑79, mars 2012, doi: 10.1016/j.soilbio.2011.11.019.
[5] P. Yaashikaa, P. S. Kumar, S. Varjani, et A. Saravanan, « A critical review on the biochar production techniques, characterization, stability and applications for circular bioeconomy | Elsevier Enhanced Reader », 2020. https://reader.elsevier.com/reader/sd/pii/S2215017X20300023?token=233B58BAA75A5921C4DA2A60C44723D3792DD3AD97B565864D699AD28DB77AA95F853734FB0E94AEA40D6E8EA4A0C950&originRegion=eu-west-1&originCreation=20220829165000 (consulté le 29 août 2022).
[6] S. M. Shaheen et al., « Wood-based biochar for the removal of potentially toxic elements in water and wastewater: a critical review », International Materials Reviews, vol. 64, no 4, p. 216‑247, mai 2019, doi: 10.1080/09506608.2018.1473096.
[7] M. Tripathi, J. N. Sahu, et P. Ganesan, « Effect of process parameters on production of biochar from biomass waste through pyrolysis: A review », Renewable and Sustainable Energy Reviews, vol. 55, p. 467‑481, mars 2016, doi: 10.1016/j.rser.2015.10.122.
[8] L. G. Onana, « Le Biochar : un charbon biologique adapté aux sols tropicaux (…) – IED afrique | Innovations Environnement Développement », IED INNOVATION ENVIRONNEMENT DEVELOPPEMENT AFRIQUE. https://www.iedafrique.org/Le-Biochar-un-charbon-biologique.html (consulté le 12 septembre 2022).
[9] S. Joseph et al., « How biochar works, and when it doesn’t: A review of mechanisms controlling soil and plant responses to biochar », GCB Bioenergy, vol. 13, no 11, p. 1731‑1764, 2021, doi: 10.1111/gcbb.12885.
[10] C. Hollister, J. Bisogni, et J. Lehmann, « Ammonium, Nitrate, and Phosphate Sorption to and Solute Leaching from Biochars Prepared from Corn Stover (Zea mays L.) and Oak Wood (Quercus spp.) », Journal of environmental quality, vol. 42, p. 137‑144, mai 2013, doi: 10.2134/jeq2012.0033.
[11] Y. Yao, B. Gao, M. Zhang, M. Inyang, et A. R. Zimmerman, « Effect of biochar amendment on sorption and leaching of nitrate, ammonium, and phosphate in a sandy soil », Chemosphere, vol. 89, no 11, p. 1467‑1471, nov. 2012, doi: 10.1016/j.chemosphere.2012.06.002.
[12] P. R. Quin et al., « Oil mallee biochar improves soil structural properties—A study with x-ray micro-CT », Agriculture, Ecosystems & Environment, vol. 191, p. 142‑149, juin 2014, doi: 10.1016/j.agee.2014.03.022.
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Further reading
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