When Biochar Works and When it Does Not
Biochar Update 5
Below is a simple explanation of the last 20 years of biochar research based on this review paper (which includes many of the top biochar researchers) .
Different Inputs for Biochar + Comparison
Biochar has a ton of feedstock — A wide range of biochar types produced from feedstocks including woody residues, crop straw, animal manures, sewage sludge, and food wastes.
A review of 5400 studies (Ippolito et al., 2020) found that wood-based feedstocks generally produced biochar with the highest surface area.
Straw-based feedstocks gave the highest cation exchange capacity (CEC). High CEC means you have a higher negative charge, which allows water along with its nutrients to have a strong bond with soil (specifically clay and organic matter). This helps increase plant yield.
Manure feedstocks produced biochars with the highest N (nitrogen) and P (phosphorus) content. The highest content does not mean that plants use it efficiently
Temperature Comparison
Normal Temperature is about 350°C to over 750°C
HTTP (or temperatures) above 500°C produced biochars that were more persistent in soil, with higher ash contents and pHs.
Biochar pH ranges from 4.6 to 9.3. Above 500°C would probably create a pH of 8+ which we don’t since it negatively affects nutrient availability look below.
Benefits of Biochar
Reviews and meta-analyses show that biochar generally lowers soil acidity This means it increases pH — so optimal for soil that has higher acidity 6.5pH and below.
Biochar increases buffering capacity. Buffer capacity quantifies the resistance of a solution to changes in pH after the addition of OH- or H+. Making pH less likely to change after biochar is added/already changed the pH
Increases dissolved and total organic C. Higher soil organic carbon promotes soil structure or tilth meaning there is greater physical stability. This improves soil aeration (oxygen in the soil) and water drainage and retention and reduces the risk of erosion and nutrient leaching.
Increases cation exchange capacity (CEC). Cation, atom, or group of atoms that bears a positive electric charge. CEC measures are the soil’s ability to hold cations by electrical attraction. The five most abundant exchangeable cations in the soil are calcium (Ca++ ), magnesium (Mg++), potassium (K+), sodium (Na+) and aluminum (Al+++) — which are helpful for plant growth. The higher your charge increases the number of nutrients held by soil, increasing available ions for plants.
Increased Available nutrients and water retention. Nutrients are found in water in ions form and biochar holds water 5 to 7x its weight so it keeps more water instead of it evaporating from the soil.
Increased Aggregate stability. The smaller pieces of sand, silt, and clay have been mixed with OM to form larger structures called aggregates. Soil aggregate stability refers to the soil’s ability to not break down in the event of stresses (ex. wind and water erosion).
Reduces bulk density. Bulk density is an indicator of soil compaction. Soil compaction occurs when soil particles are pressed together, reducing pore space between them. When you reduce pore space, there is less area for plants/roots to grow — impacting yield. High bulk density is an indicator of soil compaction.
Increase microbial activity. As plants rely on soil microorganisms to mineralize (break down) organic nutrients for growth and development. Mineralize means to break down compounds. The seems to be similar to water breaking down nutrients and compounds into Ions.
They increase nitrogen uptake, increase plant growth, and support faster initial growth of plant roots without any synthetic fertilizers needed (Ingham 1985).
They found a drastic 50% reduction in the need for NPK fertilizers in plots that were inoculated with the microbes (Thilagar 2016). — Interesting
Because it naturally does fertilizers do you don’t need them as much if at all anymore — biochar enables that.
Accelerate nutrient cycling. A nutrient cycle is a repeated pathway of a particular nutrient or element from the environment through one or more organisms and back to the environment. Examples include the carbon cycle, the nitrogen cycle, and the phosphorus cycle. The faster the cycles happen the faster it gets to plants. The faster it gets to plants the more photosynthesis cycles that can happen (because the nutrients are available) the faster a plant can create its food, leading to faster growth.
Reduce leaching. Leaching in agriculture is the removal of a solute (nutrients) from a porous solid (soil) using a liquid solvent (water). That solvent is excess water coming through rainfall or irrigation. Reducing leaching means reducing the number of available nutrients disappearing. The more nutrients the more plant growth.
Immediately after heavy rain, much of the air (ideal air is 25% of soil) is forced out and replaced with water. Gravity, evaporation, and plants will reduce the level of water, which is then replaced with air. Losing out on available soil nutrients that the plant needs.
Assumption: because of cohesion when the water level evens out it will be below where it needs to be to connect to roots to provide nutrients (due to gravity) and the water that will evaporate will move the water upwards away from the root. Reducing the available nutrients for the plant.
Reduce volatilization of nitrogen. Volatile means the compound/nutrient evaporates at room temperatures, which also happen to be around the range of ideal temperature for plant growth. Volatilization is the loss of N through the conversion of ammonium to ammonia gas. Nitrate and nitrite are both anions with a negative charge and are quickly leached through the soil. Ammonium is a cation, and its positive charge helps it stick to negatively charged clay and OM, slowing down the leaching process (compared with anions).
Increases rate seed germination. Germination rate = seeds grown/seeds planted. Higher germination = more plants in the process of growth. Increasing potential for yield. Seeds normally take 1–3 weeks to germinate, germination is usually the growth of a plant contained within a seed; it results in the formation of the seedling.
Increase flowering. Flowering refers to plant reproduction
Increase resistance to disease — Needle Mover. Generally, it’s estimated that various pests (insects, weeds, nematodes, animals, diseases) each year cause crop yield losses of 20–40%. More precisely, some data maintains that crop diseases cause average yield losses of 42% for the most important food crops.
Acclimation to abiotic stressors. Acclimatization here simply means the adaptation of plantlets to a new environment. Abiotic stresses include as low or high temperature, deficient or excessive water, high salinity, heavy metals, and ultraviolet radiation, are hostile to plant growth and development, leading to great crop yield penalties worldwide.
Many studies report that biochar increases plant productivity, with an average yield increase of 10%–42% (Table 1), although negative effects have also been recorded (Jeffery et al., 2017; Macdonald et al., 2014; Ye et al., 2020).
Average yield increase comes from everything that improves plant nutrition and the mitigation of the negative things.
Studies reporting positive responses have commonly used biochar application rates of 5–20 Mg ha−1 (Table 1); however, applications of biochar–fertilizer mixes at low rates (<1 Mg ha−1 biochar) have also increased yields,
5–20Mg per hectare is what they used in the studies is super small scale. This metanalysis shows for wide-scale biochar they see anything from 2tons per hectare to 60 tons per hectare of biochar.
Stage 1: Short-term (1–3 weeks) reactions of biochar
Chemical effects
After application to soil, water entering biochar pores dissolves soluble organic and mineral compounds on biochar outer and inner surfaces.
This is how biochar reduces leaching by having it solve on the surfaces of biochar providing a negative electric charge to attract ions so they don’t get leached.
The soluble compounds mentioned above when dissolved increase dissolved organic carbon (DOC), cations, and anions in the soil solution (Silber et al., 2010), which increases the electrical conductivity and pH.
High CEC means you have a higher negative charge, which allows water along with its nutrients to have a strong bond with soil (specifically clay and organic matter). This helps increase plant yield. An increase in pH is okay as long as it does not make soil pH above 7.5.
The release of DOC (dissolved organic carbon) and nutrient ions from biochar (Kim et al., 2013) is rapid over the first week and much slower over the following weeks (Mukherjee & Zimmerman, 2013).
This suggests that biochar has a strong start capacity, but then plateaus in the future. CEC (cations exchange capacity) is low from 50 to 200 mmol kg−1. Normal soil samples are 13.5 cmol/kg-1. CEC increases over time as more functional groups form on biochar surfaces.
Low-temperature biochar (HTT — highest treatment temperature < 450°C) and biochar produced in facilities with incomplete separation of pyrolysis vapors generally have higher contents of water-soluble organic compounds. The more soluble they are in the water the more available nutrients for the plant.
Low-temperature biochar can be hydrophobic initially due to the accumulation of aliphatic compounds in pores and on the surface; such compounds are usually lost during pyrolysis at higher temperatures. Hydrophobicity can inhibit water uptake by biochar particles (Gray et al., 2014), but this effect dissipates over time. Most biochars are usually alkaline — meaning they have a pH of over 7. (HTT >450°C) have relatively lower levels of water-soluble compounds. What is good for a plant is 5.5 to 7.0 pH.
Biochar is a reductant — meaning it gives out electrons to other elements or compounds. Biochar (especially those made at >400°C) can have a high content of free radicals, which can lead to the formation of reactive oxygen species (Pignatello et al., 2017; Ruan et al., 2019; Yu & Kuzyakov, 2021) and strongly accelerate oxidation reactions. This acceleration leads to oxidation not only of biochar itself but also of SOM and plant residues (Du et al., 2020) and is especially intensive in soils with fluctuating water levels (Merino et al., 2020) or with high content of iron (oxyhydr)oxides (Merino et al., 2020; Yu & Kuzyakov, 2021).
Physical Changes
Some studies (e.g., Uslu et al., 2020) that reported negative effects of biochar on germination at very high rates (120 Mg ha−1) applied biochar directly to seeds in a petri dish. Hypothesis: Studies that have negative outcomes for biochar rely on petri dish data when it only makes sense to do it on the field where you have microbes, minerals, organic compounds interact with biochar.
Germination rates were not affected by the addition of BCF at <700 kg ha−1 in pot or field trials, while seedling growth was the same or greater than with NPK fertilizer alone.
At high application rates, biochar with high levels of soluble salts could inhibit germination and seedling growth through osmotic stress. Kochanek et al. (2016) showed that biochars containing karrikins, a class of water-soluble organic molecules associated with plant response to fire, can accelerate germination and early growth of plants
Main takeaway: applying biochar a few weeks in advance of sowing supports seedling growth through the development of a beneficial rhizosphere microbiome.
Physical and chemical reactions in the soil
Stage 2: Medium-term (1–6 months) creation of reactive surfaces on biochar, effects on plant growth and yield from seedling to harvest
The surface area and porosity increase. porosity is the gap between solid particles, which contains water and air.
There is also a fine layer of organic matter with a high concentration of C–O, and C–N functional groups forms around the external and some of the internal pore surfaces of the biochar and BCF. What absorbs cations — positively charged ions.
Biochar pores may become filled with organic matter and minerals, protecting organic matter from microbial decomposition (Pignatello et al., 2017) and reducing the availability of nutrients.
Microagglomerates that form on internal and external biochar surfaces, consisting of nanoparticulate minerals bound with organic molecules, have a significant concentration of –C–O, –C=O, –COOH, or –NH functional groups (Joseph et al., 2010). Recent research indicates that many of the reactions described above related to biochar occur on or in the micro agglomerates. Gases such as NH3, N2O, and CH4 produced through biotic and abiotic reactions of fertilizers in soils and/or through chemical reactions on the surfaces of the biochar can diffuse into the nanopores (<50 nm), where they can react with oxidants and reductants, especially if the pores contain water, which reduces N loss and GHG emissions (Section 4.3; Chiu & Huang, 2020; Quin et al., 2015).
Meta-analyses have shown that biochar increases microbial biomass and activities (Pokharel et al., 2020), particularly in high-N soils (Zhang et al., 2018) and with biochars produced at low temperature from nutrient-rich feedstocks. Biochars, particularly those made at low temperature from crop residues, cause shifts in microbial community composition, increasing the ratios of fungi to bacteria, and gram-positive to gram-negative bacteria (Zhang et al., 2018).
A good resource for understanding the value of Fungi vs Bacteria = Fungi are generally much more efficient at assimilating and storing nutrients than bacteria. One reason for this higher carbon storage by fungi lies in the chemical composition of their cell walls. They are composed of polymers of chitin and melanin, making them very resistant to degradation. Bacterial membranes, in comparison, are phospholipids, which are energy-rich. They degrade easily and quickly and function as a food source for a wide range of microorganisms.
The meta-analysis by Pokharel et al. (2020) identified that biochar increased microbial biomass C and the activities of the enzymes urease by 23%. Urease, an enzyme that catalyzes the hydrolysis of urea, forming ammonia and carbon dioxide. Ammonia (nitrogen ion-helps plants) and CO2 which is required to start the photosynthesis process.
Increases alkaline phosphate activity by 25%. Alkaline phosphatases in plants play a major role in the supply and metabolism of inorganic phosphate for the maintenance of cellular metabolism (Tabaldi et al., 2007; Mishra and Dubey, 2008). One of the most important nutrients to living organisms is Inorganic phosphate (Pi). It is required in ATP formation, kinase/phosphatase signaling and in the synthesis of lipids, carbohydrates, and nucleic acids — I think ATP is the one I know about. I should probably learn about the other things inorganic P creates.
Increases dehydrogenase activity by 20%. Dehydrogenases are enzymes that catalyze reduction reactions through the transfer of hydrogen ions (protons) from the substrate to an acceptor or co-enzyme — this is in the step at the start of the 2nd step of photosynthesis. This increase in enzyme activities, as well as the shift in microbial community diversity and activity (Jaiswal, Elad, et al., 2018), are directly dependent on (i) pH increase after biochar addition, as soil acidity is the main factor regulating the microbial composition
Fungi and bacteria inhabit the larger nutrient-rich pores of biochar (>2 µm) where they mine the nutrients in the biochar and those that have been absorbed from fertilizers. The adsorption of root exudates, microbial metabolites, and microbial neuromas increases SOM levels and thus increases soil organic carbon. In low P soils, arbuscular mycorrhizal fungi (AMF) invade the pores of biochar, especially biochars with high P content on the pore surface, which can increase plant P uptake (Gujre et al., 2020; Solaiman et al., 2019; Vanek & Lehmann, 2015). Blackwell et al. (2015) found that a phosphorus-enhanced BCF increased root colonization to 75% compared with 20% 10 | JOSEPH et al. in mineral fertilizer and unfertilized control and increased P uptake efficiency
Nutrient Responses
Much of the N within the biochar C matrix (e.g., heterocyclic-N) is unavailable to plants (Clough et al., 2013; Torres Rojas et al., 2020), whereas most K in biochar is present in soluble forms, released in the short term after application to soil (Silber et al., 2010), and is readily available to plants. Meta-analyses have found that biochar application commonly increases P availability, particularly when applied to acidic or neutral soils, and for biochar produced from low C: N feedstocks (e.g., manure, crop residues), and produced at low temperatures. Lower temperatures again — manure and crop residues would be a good place to start.
However, P availability can be low in Ca-rich and K-poor feedstocks such as sewage sludge (Buss et al., 2018, 2020; Torres-Rojas et al., 2020; Wang et al., 2019) because pyrolysis can convert plant-available organic P into inorganic P that is less available in the short term (Buss et al., 2020; Rose et al., 2019).
For example, a meta-analysis found that biochar reduces N leaching on average by 26%, though it can increase ammonia volatilization at biochar application rates 40 Mg ha−1 and with biochar pH > 9 (Haider et al., 2020; Liu, Zhang, et al., 2018).
Biochar accelerates the mineralization of organic matter and nutrient cycling, and AMF root colonization, which can increase N and P uptake by plants, as discussed above (Solaiman et al., 2019) and can also improve root growth under water stress (Mickan et al., 2016). Particles on the surface of biochars consisting of carbon-coated minerals are particularly effective in reducing the bioavailability of heavy metals (Kumar & Prasad, 2018). Essential and non-essential heavy metals generally produce common toxic effects on plants, such as low biomass accumulation, chlorosis, inhibition of growth and photosynthesis, altered water balance and nutrient assimilation, and senescence, which ultimately cause plant death.
Effect on disease
Generally, no impact is found at low rates (<2 Mg ha−1), positive impacts are seen at moderate rates (2–20 Mg ha−1), and negative impacts at relatively high rates (>50 Mg ha−1). Rates that are beneficial for plant growth in non-diseased systems can result in disease promotion in pathogen-infected systems (Jaiswal et al., 2015).
Chenopodium quinoa and maize both grew significantly better in biochar treatments, which was attributed to improved plant traits (lower proline content and less negative osmotic potential) rather than to increased root zone water content (Ahmed et al., 2018; Kammann et al., 2011).
Tests with heat stress and biochar in Arabidopsis indicated early micro stresses primed the plants to cope better with subsequent acute heat stress