Post-processing of biochars to enhance plant growth responses: a review and meta-analysis

Abstract

A number of processes for post-production treatment of “raw” biochars, including leaching, aeration, grinding or sieving to reduce particle size, and chemical or steam activation, have been suggested as means to enhance biochar effectiveness in agriculture, forestry, and environmental restoration. Here, I review studies on post-production processing methods and their effects on biochar physio-chemical properties and present a meta-analysis of plant growth and yield responses to post-processed vs. “raw” biochars. Data from 23 studies provide a total of 112 comparisons of responses to processed vs. unprocessed biochars, and 103 comparisons allowing assessment of effects relative to biochar particle size; additional 8 published studies involving 32 comparisons provide data on effects of biochar leachates. Overall, post-processed biochars resulted in significantly increased average plant growth responses 14% above those observed with unprocessed biochar. This overall effect was driven by plant growth responses to reduced biochar particle size, and heating/aeration treatments. The assessment of biochar effects by particle size indicates a peak at a particle size of 0.5–1.0 mm. Biochar leachate treatments showed very high heterogeneity among studies and no average growth benefit. I conclude that physiochemical post-processing of biochar offers substantial additional agronomic benefits compared to the use of unprocessed biochar. Further research on post-production treatments effects will be important for biochar utilization to maximize benefits to carbon sequestration and system productivity in agriculture, forestry, and environmental restoration.

Biochar granulation reduces substrate erosion on green roofs

“…The use of granulated or pelletized biochars (Thomas 2021) or chemical dust suppressants (Jin et al 2019;Wang et al 2021) has been suggested as a means to mitigate biochar and substrate erosion. Granulated biochars (formed by a granulation process with a binding agent) are expected to be resistant to wind and water erosion because they have higher bulk density and lower total porosity compared with unprocessed biochars (Liao et al 2022a).…”

“…This effect may also help alleviate biochar and substrate total erosion by reducing surface wind speed (Hong et al 2020) and intercepting rainwater that could result in splash erosion (Durán Zuazo and Rodríguez Pleguezuelo 2008). Nevertheless, plant responses may vary with biochar post-processing (Thomas 2021) and the effects may also be species-specific (Liao and Thomas 2019).…”

Optimizing Biochar Particle Size for Plant Growth and Mitigation of Soil Salinization

“…Our results suggesting a clear optimum biochar size are consistent with recent meta-analyses. Thomas [38] pooled data from 23 studies (involving 112 comparisons) and concluded that a biochar particle size of 0.5-1.0 mm generally resulted in an optimal plant growth response regardless of soil type or texture. Edeh et al [63] suggest a size <2 mm is best for sandy soil because this size category best improves soil hydrological properties.…”

“…Small biochar particles also can show increased ash content and pH [39]. Although very few studies have examined plant growth responses across a wide range of biochar particle sizes, a recent meta-analysis presents evidence for an optimal biochar particle size of 0.5-1.0 mm [38]. Prior studies on biochar particle size effects have focused on agricultural soils; in urban ecosystems, compost-amended soils are common, and are likely to show distinct responses to both salt exposure and biochar amendments.…”

An investigation of drum granulation of post‐pyrolysis washed biochar

“…The optimum size will likely depend on the soil particle size and the plant species. [3,12] Dust generation from biochar is a crucial health issue, whether during the initial spreading onto soils or when remaining on the surface of soils. [13][14][15][16] Biochar is a fine powder with particle sizes below 600 μm and frequently with a high proportion of particles below 10 microns [15,17] ; some biochar particles would therefore be easily airborne during processing and handling, spreading, and also once in contact with the soil.…”