NATIONAL POND SERVICE – EDUCATIONAL RESOURCES
Biochar Usage and Benefits for Aquatic Habitats
Benefits of Biochar in Ponds and Lakes
Biochar is a charcoal-like substance that’s made by burning organic material from agricultural and forestry wastes (also called biomass) in a controlled process called pyrolysis. Biochar is black, highly porous, lightweight, fine-grained and has a large surface area. Although it looks a lot like common charcoal, biochar is produced using a specific process to reduce contamination and safely store carbon. During pyrolysis organic materials, such as wood chips, leaf litter or dead plants, are burned in a container with very little oxygen.
Biochar is hygroscopic. Thus it is a desirable soil material in many locations due to its ability to attract and retain water. This is possible because of its porous structure and high specific surface area. As a result, nutrients such as phosphate, and agrochemicals are retained for the plants benefit. Plants are therefore healthier, and less fertilizer leaches into surface or groundwater. Biochar benefits for soil may include but are not limited to
- enhancing soil structure and soil aggregation;
- increasing water retention;
- decreasing acidity;
- reducing nitrous oxide emissions;
- improving porosity;
- regulating nitrogen leaching;
- improving electrical conductivity; and
- improving microbial properties.
Chemical-physical Properties of Biochar
The properties of biochar vary depending on the feedstock and production temperature, as discussed above. Consequently there is considerable variability in the chemical and physical properties of different biochars. The table below summarizes data from our literature review. Some conclusions from the literature are summarized below.
- Biochar has a large surface area.
- Cation exchange capacity (CEC) decreases as pyrolysis temperature increases. This is due to the loss of volatile organic content and associated functional groups as temperature increases. As CEC decreases, the ability of biochar to retain negatively charged chemicals, such as phosphate, decreases.
- Non-wood vegetative feedstocks have a greater CEC than wood feedstocks. This is due to a greater percentage of aliphatic compounds and associated functional groups. Non-wood feedstocks primarily consist of grasses.
- Sludges and manure-based biochars have high nutrient content and are thus not satisfactory for managing stormwater
Effect of Biochar on Retention and Fate of Phosphorus
Biochar is not likely to provide significant phosphorus retention in bioretention practices unless impregnated with cations (e.g. magnesium) during production at relatively low temperatures (e.g. less than 600oC.)
Biochar may have several properties for managing stormwater, such as increased water and pollutant retention, improving soil physical properties, and attenuating bacteria and pathogens. Biochar has been examined as a potential amendment to engineered media in bioretention or other stormwater control practices. With respect to phosphorus, information from the literature is mixed. Below are summaries from several studies.
- Mohanty et al. (2018) observed that biochar does not absorb phosphate efficiently. Phosphorus retention can be enhanced by impregnating biochar with cations such as magnesium and zinc.
- [https://stormwater.pca.state.mn.us/index.php?title=Biochar_and_applications_of_biochar_in_stormwater_management#References Reddy et al. (2014) found that biochar reduced influent phosphate concentrations by 47% in column experiments. Influent concentrations were 0.57 and 0.82 mg/L for unwashed and washed biochar, respectively. These concentrations are on the high end of concentrations found in urban stormwater.
- Yaoa et al. (2011) observed retention in biochar-(sugar beet source)amended soils that were fertilized. Adsorption was dominated by magnesium oxides and maximum adsorption occurred at pH values less than 4.
- Zhaoa et al. (2013) studied different feedstocks and observed high phosphorus concentrations in animal-based feedstocks and wastewater sludge (0.065 – 0.44%) compared to other feedstocks (0.007 – 0.07%)
- Iqbal et al. (2015) examined leaching of phosphorus from compost (80% yard and 20% food waste) and co-composted biochar (100% fir-forest slash). They found biochar amendments did not significantly reduce the leaching of phosphorus compared to the compost only treatment. Phosphorus leached from biochar, but because phosphorus concentrations in biochar are low, this leaching contributed little total phosphorus. Leached phosphorus was primarily in the form of orthphosphate.
- Han et al. (2018) found that addition of biochar to soil led to increased desorption of phosphorus during winter freeze-thaw cycles.
- Soinne et al. (2014) observed no effect of biochar on phosphorus retention in a sandy and two clay soils.
Effect of Biochar on Retention and Fate of Other Pollutants
- Nitrogen. Biochar effects on nitrogen retention depend on the properties of the biochar and stormwater runoff. Biochars produced at relatively low temperatures (less than 600oC) provide some retention of organic nitrogen and ammonium nitrogen in stormwater runoff. Mechanisms for nitrogen retention include adsorption of ammounium and nitrogen immobilization. Leaching of nitrogen may decrease due to increased water holding capacity (Iqbal et al., 2015; Gai et al., 2014; Zheng et al., 2013; Ding et al., 2010).
- Metals. Biochar enhance retention of metals in stormwater runoff. (Reddy et al., 2014; Domingues et al., 2017; Iqbal et al., 2015)
- Organics. Biochar significantly retains polynuclear aromatic hydrocrabons in stormwater runoff (Reddy et al., 2014; Domingues et al., 2017; Ulrich et al., 2017; Iqbal et al., 2015)
- Bacteria and viruses. Biochar effects on bacteria and virus retention are a function of the particle size of the biochar. Fine-grained biochars enhance removal of bacteria in stormwater runoff through straining of microorganisms (Reddy et al., 2014; Sasidharan et al., 2016; Yang et al., 2019).
- Dissolved organic carbon. Biochar shows limited retention of dissolved carbon in stormwater runoff (Iqbal et al., 2015).
- Greenhouse gas emissions. Biochar effectively sequesters carbon and reduces loss of greenhouse gases when incorporated into soil or media, particularly soil with high organic matter content (Zhaoa et al., 2013; Mohanty et al., 2018; 37. Agyarko-Mintah et al., 2017).
Effect of Biochar on Soil Physical and Hydraulic Properties
Because of a large surface area and internal porosity, biochar can affect soil physical properties (Mohanty et al., 2018; Herrera Environmental Consultants, 2015; Iqbal et al., 2015; Imhoff, 2019; Jien and Wang, 2013). These effects are most pronounced in soils with low organic matter concentration.
- Porosity and surface area. Biochar significantly increases the porosity of most soils.
- Water holding capcity. Biochar significantly increases the water holding capacity of soil.
- Hydraulic conductivity. Biochar increases the hydraulic conductivity of fine- and medium-grained soils and may decrease the hydraulic conductivity of coarse-grained soils.
- Structure. Biochar enhances aggregation in soils, thus enhancing soil structure and potentially increasing soil macroporosity.
Phosphorus is a macronutrient essential for the growth of plants and other biological organisms. This element is one of the fundamental building blocks that constitute nucleic acids (DNA and RNA), complex carbohydrates and phospholipids. In most cases of freshwater bodies, the limiting nutrient in regards to algal growth is likely to be phosphorus (Manahan, 2009). The common forms of phosphorus present in aqueous solutions are orthophosphate, polyphosphate and organic phosphate (Tchobanoglous et al., 2003). Generally, wastewater contains orthophosphate and small amounts of organic phosphate (Grubb, 2000). Industrial wastewaters from some industries might contain phosphate levels greater than 10 mg/L (Akay et al., 1998).
The most significant difference of the phosphorus cycle compared to other element cycles is that no gaseous compounds exist. Therefore, it is only found in soil and aquatic environments.
Since phosphorus is not readily available from the atmosphere, it is deemed the limiting nutrient. Overall, inorganic phosphorus is discharged into water bodies from numerous natural and human sources. When plants and animals die, decomposition of the biomass by bacterial activities converts organic phosphorus to inorganic phosphorus, which is then released back to the environment.
The major steps of the phosphorus cycle in aquatic environments are summarized below (Bitton, 2010).
- Mineralization: Organic phosphorus compounds are mineralized to orthophosphate by microorganisms such as bacteria (e.g., Bacillus Subtilis), and fungi (e.g., Penicillium). The enzymes accountable for the decomposition of phosphorus compounds are phosphatases.
- Assimilation: Microorganisms assimilate phosphorus into their cells.
- Precipitation of Phosphorus: In the aquatic environment, the solubility of orthophosphate is affected by the pH and the presence of other minerals, Al3+, Ca2+, Fe3+, and
- 4 Mg2+. Precipitation leads to formations of insoluble compounds, such as Fe3(PO4)2.8H2O and AlPO4.2H2O.
- Solubilisation of Insoluble Phosphorus: Microorganisms’ metabolic activity contributes to the solubilisation of phosphorus compounds. The process involves enzymes, production of organic and inorganic acids, production of CO2, and production of H2S.
Sources of Phosphorus
Since phosphorus is usually the limiting nutrient in lakes and rivers, in order to reverse or slow down the eutrophication process, the inputs of phosphorus to the water bodies must be abridged. This can be accomplished by identifying the sources of phosphorus and potential mitigation methods for their reduction.
The natural source of phosphorus to lakes is from the weathering of rock and from decomposition of organic matter (Pery and Vanderklein, 1996). However, it is extremely difficult to regulate the natural inputs of phosphorus. As in the case of many lakes, the major sources of phosphorus are anthropogenic. These nutrient sources are categorized into non-point sources and point sources (Smith, 2003).
Integrating Biochar into the soil structure can enhance not only the nutrient contents but also the water and nutrient retention. Its high surface area combined with improved soil structure boost water-holding capacity of soil. As shown in a study conducted by Tryon (1948), soil water retention capacity increased by about 18% upon addition of 45% (by volume) biochar to a sandy soil. Laird et al., (2010) also reported a reduction of nutrient leaching from Mid-western agricultural soil due to biochar application. Lehmann et al. (2003) observed that amendment of Biochar considerably reduced the leaching of N. This helps to mitigate eutrophication indirectly since the amounts of nutrients effused into water bodies are reduced.
Soil’s Best Friend
Because of Biochar’s physical and chemical nature, it has a unique ability for attracting and holding moisture, nutrients, and agrochemicals, even retaining difficult to hold nutrients like nitrogen and phosphorous. Nitrogen tends to run-off regular soils, upsetting ecosystem balance in streams and riparian areas.
Biochar also holds gasses; recent research has proven biochar-enriched soils reduce carbon dioxide (CO2) and nitrous oxide (N2O) emissions by 50-80%. N2O is a significant greenhouse gas, 310 times more potent than CO2.
Biochar’s immense surface area and complex pore structure (a single gram can have a surface area of over 1000 square yards) provides a secure habitat for micro-organisms and fungi. Certain fungi form a symbiotic relationship with plant root fibers and this allows for greater nutrient uptake by plants. There is speculation that this fungi may play a part in terra preta’s ability to regenerate itself.
Persistency in Soil— It is undisputed that Biochar is more persistent than any form of organic matter commonly applied to soil. Because of Biochar’s long-term persistence in soil (more than 2,500 years and counting), all the associated benefits of nutrient retention, water retention and overall soil fertility are longer lasting than with common fertilizers alone. Biochar, comparatively inert, doesn’t break down like other organic soil amendments and resists chemical and microbial degradation, especially when buried.