Biochar: the Future of Sustainable Agriculture?

Fig 1. Burning Charcoal (Source)

Using biochar as a form of soil amendment technique have been discussed a lot in scientific literature and popular press in recent years. It has been almost been regarded as a silver bullet to solve all problems in agricultural activity. So for today's post, I would like to give a brief overview of biochar regarding it potential implications when it is added to the soil. 

Biochar and its Historical Perspective

Biochar is charcoal added into soils as a form of fertilizer. It can be made from any form of biomass via pyrolysis (heating in the absence of oxygen). The history behind the idea of using charcoal as soil amendment traces back to the discovery of terra preta, an anthropogenic, dark-colored soil created by the indigenous people of the Amazon 2500 years ago. It was said to be created by continuously adding charcoal derived from cooking and/or fresh biomass waste. Terra preta was very versatile and extremely high in organic content. Therefore, people are trying to regenerate similar solid by adding modernly produced biochar. 

Claimed Benefits

The list of potential benefits of the use of biochar is almost endless: 

• Improve fertilizer efficiency and reduce run-off due to improved nutrient retention through cation absorption. 
• Increase yields due to the more available nutrients (Liang et al. 2006). 
• Increase water quality by continuous retention of pollutants such as heavy metals, pesticides, herbicides, etc. 
• Increase soil pH (Lehmann et al., 2003; Van Zwieten et al., 2010a). 
• Increase soil microbial biomass (Liang et al., 2010; Jin, 2010) . 
• Have high climate mitigation potentials through the reduction of greenhouse gas emission (Lehmann et al., 2006; Laird, 2008; Sohi et al., 2010). Biochar's high recalcitrant nature slows the rate of photosynthetically fixed carbon return to the atmosphere. It reduces nitrous oxide (Cayuela et al., 2013) and methane emission (Jeffery et al., 2016). 
• Source of renewable bio-energy, which can improve agricultural productivity. 

Fig 2. Chart showing biochar's benefits (Source: International Biochar Initiative)

Drawbacks

Concerning the potential benefits stated above, some drawbacks were found as well:

• Spokas et al. (2011) found that biochar has no effect whatsoever on crop yield, in some cases, it may actually lead to a reduction in yield due to its water and nutrient sorption ability. 
• The sorption of pesticide and herbicide could reduce their efficacy and could in turn become a source of contaminant. 
• In high pH soils, increasing the pH is not desirable for certain type of crops sensitive to pH. 
• The reduction in methane production is not universal and could even increase methane emission (Wang et al. 2017a). 
• Fine ash of biochar may pose potential risk for respiratory diseases.  

Some Thoughts

Most of the trials and experiments on biochar were conducted in laboratory conditions and most of them are carried out to focus on the benefits. While little was carried out to find any possible shortcomings of using biochar. Moreover, most trials were conducted for a fairly short term of around 2 years. The long-term effect of biochar on our soil, plants and atmosphere is still an open question. Hence, more research is required to determine a more comprehensive understanding before proceeding with the promotion of biochar as a soil improving amendment. 

Currently, my personal opinion is mainly skeptical about its climate mitigation potentials as biochar require burning lots of biomass. Large quantities of biomass would have to be harvested and burned to have any measurable impact on the global atmosphere. Not the mention burning itself would produce lots of pollutants. However, I am not against the small-scale use of waste charcoal by individual farmers to improve agricultural productivity, particularly in low-fertility and degraded soils.