Comparison of different methods for estimating microbial biomass in biochar-amended soils

Biochar use as a soil amendment can improve soil functions, enhances microbial activity, and increases crop production. However, due to its high sorptive capacity, it may interfere with traditional methods for determining soil microbial biomass, specifically chloroform fumigation-incubation (FI) and fumigation-extraction (FE). This study aimed to assess the impact of biochar on microbial biomass determination using traditional methods and a new CO2 high pressurization (CO2HP) technique. Five treatments were set up: unamended soil (control), and soil amended with two types of biochar, produced at 440 ◦C (B440) or 880 ◦C (B880), at two application rates (20 or 40 t ha-1). Following cell lysis by fumigation or CO2HP, released microbial C (ΔCmic) was estimated by determining microbial respiration over a 10-d incubation (FI and CO2HP-I methods, respectively) or by extracting soluble organic C (FE, and CO2HP-E, respectively), while released microbial N (ΔNmic) was estimated by determining extractable total N exclusively by FE and CO2HP-E methods. Without biochar, ΔCmic estimates were similar across FE, FI, and CO2HP-E methods. Contrarily, CO2HP-I method greatly overestimated ΔCmic compared to the other three methods, particularly at the higher biochar rate, suggesting that the adsorption of CO2 within biochar pores during CO2HP treatment and subsequent slow release during incubation could have produced artifacts. The presence of B880 resulted in a decrease in ΔCmic values, which might have been caused by an acclimation of microbial biomass to new habitat. Contrarily, the addition of B440, increased ΔCmic when determined by the FE method, compared to FI and CO2HP-E methods. This suggested an overestimation of extractable C after fumigation, possibly due to adsorption of CHCl3 by the B440 biochar, rich in functional groups, that might have bound CHCl3. We concluded that biochar interfered with the determination of ΔCmicand ΔNmic as a function of both type and amount added.