Quantifying biochar content in a field soil with varying organic matter content using a two-temperature loss on ignition method.

Abstract While the use of biochar as a soil amendment for agronomic and environmental management is gaining popularity, quantification of biochar in soil is still challenging. The objective of this work was to develop a fast, simple and inexpensive method to quantify biochar content in field soil with varying organic matter content – the two-temperature loss on ignition (LOI) method. In this approach, biochar mass fraction in a biochar-amended soil is computed by measuring the dry mass of biochar/soil mixture after heating sequentially at two temperatures: low temperature (LT), and high temperature (HT). This method requires the LOI profile for pure soil and pure biochar that are representative of soil and biochar in the field. Although the soil LOI profile may vary due to spatial variation in soil organic matter (SOM) content, the method only requires that the relative soil LOI at LT with respect to LOI at HT is uniform because of similarity in SOM chemical composition. In this method, LT and HT are selected such that the maximum difference in LOI exists at these temperatures between pure soil and biochar. The method was tested by quantifying the biochar content in roadway filter strips with and without a wood biochar pyrolyzed at high temperature (550 °C). The estimates of biochar content from the method matched independent measurements for soils with low (−0.23 ± 0.09 CI%, CI = 95% confidence interval, versus actual 0%) and high (3.9 ± 0.3 CI% versus actual 4.0 ± 1.1 CI%) biochar mass fraction. The method is applicable when SOM content is low to moderate (e.g. <15%) and mostly composed of labile organic compounds, and when biochars are pyrolyzed at moderate to high temperatures (i.e. >400 °C) and composed of relatively low ash content (e.g. <30%). Graphical abstract Unlabelled Image Highlights • Two-temperature loss on ignition (LOI) method developed to quantify biochar in soil. • Spatial variation of soil organic matter (SOM) content is accounted for in the method. • Method applied to biochar-amended roadway soil and error was <3%. • Method is applicable for soils with low to moderate SOM content. • Method is applicable for biochars pyrolyzed at high temperatures with low ash content. [ABSTRACT FROM AUTHOR]