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.
Highlights
Pyrolyzed organic matter, which derived from waste material streams from forestry and agriculture, has received considerable recent attention for a wide range of applications
In the case of searches related to post-treatment effects, search terms included descriptions for the set of terms in potential use to describe biochar, in conjunction with a set of terms for plant growth (“plant” or agricult*” or “crop” or “tree” or “seedling” plus “growth”, “yield”, “biomass”, “performance”), and terms used to describe individual post-processing methods or the resulting products (“sieving”, “grinding”, “sieved”, “ground”, “nano”, “pelletization”, “pellet”, “granulation”, “granule”, “leaching”, “leached”, “washing”, “washed”, “ageing”, “aging”, “aged”, “weathering”, “weathered”, “heat-treatment”, “activation”, “acid-treatment”, “acidification”, “acidulation”, “base-treatment”, “basification”)
For the data considered here, the pooled response ratio statistic for unmodified biochars relative to no-biochar controls (R = 0.241 ± 0.056; p < 0.001) corresponds to a 27.3% increase in plant biomass or yield; for modified biochars relative to controls (R = 0.366 ± 0.072; p < 0.001), the proportional increase is 44.2%. In each of these analyses, the Q-test for heterogeneity among comparisons is significant (p < 0.001). This variation is clearly attributable to post-processing type, which was found to be significant as a moderator of effects (p = 0.005); effects of other moderators examined were not significant
Summary
Which derived from waste material streams from forestry and agriculture, has received considerable recent attention for a wide range of applications. The net carbon balance of such a system depends on a number of factors, with the principle being: (1) baseline emissions of feedstock materials if not used in biochar production; (2) the efficiency of bioenergy produced and its distribution; (3) the degree to which biochar can act as a substitute for carbon-emitting technologies; (4) the carbon footprint of the feedstock supply chain and processing; (5) the effects of biochar on net carbon uptake in managed ecosystems This fifth factor is often critical to achieve long-term carbon-negative impacts, since photosynthesis remains one of the only processes that can be harnessed on a large scale to directly take up atmospheric CO2. The potential for a positive feedback loop—in which production and application of biochar yields additional capacity to produce biochar—is contingent on biochar applications having positive effects on net productivity and carbon balance of managed ecosystems where it is deployed as a soil amendment
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