Biochar as a habitat for soil microorganisms
The pore space of pyrolyzed biomass increases during charring by several thousand folds and is related to charring temperature and feedstock materials.
Figure 1. Biochars derived from corncobs (Zea mays L.) produced at incremental pyrolysis temperatures (350–650°C in 100°C intervals) were characterized using elemental analysis, specific surface area, scanning electron microscopy, x-ray diffractometry, and Fourier-transform infrared spectra to estimate the relationship between the physical and chemical properties of biochars and the treatment temperature. Generally, carbonization, aromatization, and specific surface area increased with the elevation of temperatures.
Estimates of the resulting surface area of different biochars range from 10 to several hundred square meters per gram (m2 - g–1), which provides a significantly increased surface area for microbial colonization .
Root-biochar-bacteria interaction: 3D projection
Source: Property of Robert Cirino. Bioimaging Center, Delaware Biotechnology Institute, University of Delaware. Using a Zeiss 5LIVE microscope, a 3D projection of a lettuce root (blue), fungal hyphae(blue), and bacteria (green) colonization is visualized. Dyes: Syto 13, Calcofluor white.
The porous structure of biochar, its high internal surface area and its ability to adsorb soluble organic matter, gases and inorganic nutrients are likely to provide a highly suitable habitat for microbes to colonize, grow and reproduce, particularly for bacteria, actinomycetes and arbuscular mycorrhizal fungi. Some members of these groups may preferentially colonize biochar surfaces depending upon the physical and chemical characteristics of different biochars.(Figure 2)
Figure 2. The porous structure of biochar invites microbial colonization. Source: (left photo) S. Joseph; (right photo) Yamamoto, in Lehmann and Joseph (2009).
Depending upon the size of a given pore, different microbes will or will not have access to internal spaces. Several authors have suggested that the biochar pores may act as a refuge site or microhabitat for colonizing microbes, where they are protected from being grazed upon by their natural predators (Saito and Muramoto, 2002; Warnock et al., 2007) or where microbes that are less competitive in the soil environment can become established (Ogawa, 1994).
The above graphic shows the basic process of how Biochar becomes a safe habitat for micro-organisms and storage of nutrients and moisture, the symbiotic relationship between plants, fungus and micro-organisms, how plants feed this micro-life carbohydrates in return for moisture and nutrients.
1. Biochar applied around a trees root zone or drip line.
2. Plant roots (white) linking up with common stringy soil fungus (pink). The plant carbohydrates produced from photosynthesis are shared with the fungus in return for nutrients and moisture.
3. Fungus linking up with Biochar full of moisture, nutrients and micro-organisms
4. Micro-organisms embedded into the porous structure of Biochar share nutrients with fungus in return for plant produced carbohydrates.
The pore size variation observed across biochar particles from different feedstocks and pyrolysis conditions is such that the microflora could, indeed, colonize and be protected from grazing, especially in the smaller pores (see Table 1).
Table 1. Pore diameters in wood and bamboo biochar compared to the ranges in the diameter of various soil microorganisms
The high porosity of biochar may also allow it to retain more moisture. Pietikäinen et al. (2000) reported that two biochars, one prepared from humus and one from wood, had a higher water-holding capacity (WHC) (2.9mL g–1 dry matter) than activated carbon (1.5mL g–1 dry matter) or pumice (1.0mL g–1 dry matter). An increase in the WHC of biochar may result in an overall increase in the WHC of the soils to which it is added.
For biochars with a high mineral-ash content, the porosity will continue to increase as the ash is leached out over time; thus, the capacity of the biochar to retain water, provide surfaces for microbes to colonize, and for various elements and compounds to become adsorbed is also likely to increase over time. Smaller pores will attract and retain capillary soil water much longer than larger pores (larger than 10μm to 20μm) in both the biochar and the soil.
Water is the universal biological solvent and its presence in biochar pores increases the ‘habitability’ of biochar substantially.
Very little is known currently about the functionality of microbial extracellular enzymes interacting with biochar of different compositions. This is an important area for future research.