environmentalresearchweb: Why did you decide to pursue this approach?

Simon Jeffery (SJ): The term biochar is a bit of a misnomer as it suggests that there is a single type of material called "biochar". Actually there exists a wide range of different types of chars, with different chemical and physical properties. When such a range of materials are applied to complex systems, such as the soil, under a wide range of climatic conditions, a wide range of responses is all but inevitable. The approach that we used – meta-analysis – allowed us to bring together a wide range of experiments, undertaken with different soil/char/climatic variables, and to identify key trends and patterns that can point towards mechanisms and so increase our understanding of the topic.

environmentalresearchweb: What's significant about your results?

SJ: There are numerous claims that biochar will increase crop yields. Our results show that yes, under certain conditions biochar does have the potential to increase crop yields. However, our study is the first to show that when applied to soils that are not depleted in nutrients and that already have a pH that is favourable to plant growth, and so are already producing near their yield limit, biochar is likely to have no significant effect on yields and in some cases may actually lead to reduced yields. [Editor's note: In the tropics the average increase in yield following biochar application was 25% but in temperate latitudes it was zero, the study found.]

environmentalresearchweb: How does your research build on previous work?

SJ: This work contains an order of magnitude more studies than previous such research, with a far more even global distribution. Previous work had a strong bias to studies in tropical soils and reported significantly positive findings. Our work builds on that by allowing analysis of temperate soils, which are usually functioning nearer to their yield maximum, and showing that significant positive effects are usually only seen in more poor or weathered soils.

environmentalresearchweb: What are the implications of your findings? How would you like to see policies change as a result?

SJ: The implications are that we cannot expect wide-scale application of biochar to soils, applied with no consideration of the local soil or environment, to lead to crop-yield increases. Specific matching of biochar properties with soil/climate requirements is required to maximize the potential yield benefits of biochar in any place, and in some places no type of biochar addition is likely to have a yield effect. In such places, if biochar is to be applied, other potential ecosystem services that biochar can interact with, such as improved water-holding capacity and reduction of greenhouse-gas emissions, should be considered.

environmentalresearchweb: What are the next steps for your research?

SJ: The weakness of meta-analysis is that it is statistical in nature. While it can point towards potential mechanisms, we need empirical science to test whether those are indeed the key mechanisms. We are taking our research forward by undertaking experimental work to test hypotheses thrown up by the results of the meta-analysis. The aim is to ensure that we have a robust understanding as to how biochar works, or does not work, under a wide range of realistic application scenarios, so that we can effectively guide policy as it is being developed to maximize the potential benefits and to minimize any trade-offs between benefits following biochar application to soil.

• Jeffery and colleagues reported their meta-analysis in an Environmental Research Review.

Related links

Related stories