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Evaluating the Safety of Biochar use in Agriculture

“..Biochars produced by different methods will have different characteristics including persistent free radical properties, and thus their risks must be evaluated individually”

Biochar is a form of charcoal used for agricultural purposes. It is a carbonised biomass produced from sustainable sources by heating organic waste matter, such as crop residue or manure in low-oxygen conditions.

Biochar is typically added to soil as a sustainable and environmentally friendly means of providing long-term improvements in soil quality and crop yield1,2. In addition, biochar improves water quality by preventing nutrients from leaching out of the soil into groundwater3. This in turn results in more nutrients and agrochemicals being retained in the soil and available to support the growth of crops.

Furthermore, the carbon in biochar resists degradation and so biochar can hold carbon in the soil for hundreds to thousands of years3. It is therefore also a useful tool in the battle against global climate change, since the oil and gas by-products of biochar production can replace fossil fuel use and carbon is stably sequestered in the soil. When the biochar is buried in the ground as a soil enhancer the system can become “carbon negative.”

Realisation of the potential of biochars to both enhance and protect the environment whilst increasing agricultural yields has resulted in them attracting much attention and being the focus of numerous research projects. Although the potential risks associated with using biochars as soil enhancers have been studied, unexplained negative impacts on plant germination and growth have been reported after the addition of biochar to the soil4.

It has been suggested that this may be because potential risks were assessed solely through the analysis of heavy metals, polyaromatic hydrocarbons, and tars. Since free radicals are generated during biomass charring and can occur in relatively persistent forms, it is possible that these radicals are still present in the biochar and inhibit plant growth. In order to develop protocols to minimise the negative effects of biochar, it was important to determine whether such hypotheses are correct.

The generation and properties of free radicals has recently been investigated in three types of biochar: corn stalks, rice and wheat straws5. Each type of biochar was produced several times using different temperatures. The levels of free radicals in each of the biochars, lignin and cellulose controls were monitored by electron paramagnetic resonance (EPR) spectrometry using a Bruker A300−6/1, Xband EPR spectrometer.

By using EPR spectroscopy, free radicals with relatively long lifetimes (they were still present after a month) were detected in the biochars5. These induced reactive oxygen species on contact with water, which have significant potential to negatively impact plant growth. The inhibition of root growth varied according to the temperature used to produce the biochar, with lower temperatures being associated with lower risk5.

These findings highlight the need for further investigations into the potential risks associated with biochars, bearing in mind that biochars produced by different methods may have different characteristics, including generation of persistent free radicals, and thus carry different levels of risks.

Contact Bruker for more information about EPR.

References

  1. Oh T, et al. Effect of aqueous extract of biochar on germination and seedling growth of lettuce (Lactuca sativa L.). J. Fac. Agric., Kyushu Univ. 2012;57(1):55−60.
  2. Alburquerque JA; et al. Enhanced wheat yield by biochar addition under different mineral fertilization levels. Agron. Sustain. Dev. 2013;33(3):475−484.
  3. Ippolito JA, et al. Environmental benefits of biochar. J. Environ. Qual. 2012;41(4):967−972.
  4. Solaiman ZM, et al. Biochars influence seed germination and early growth of seedlings. Plant Soil 2012;353(1−2):273−
  5. Liao S, et al. Detecting free radicals in biochars and determining their ability to inhibit the germination and growth of corn, wheat and rice seedlings. Environ Sci Technol. 2014;48(15):8581‑8587.