The Link Between Soil Chemical and Microbial Properties in Heavy Metal Contaminated Soils As Remediated with Biochar.

Heavy metal(loid)s contamination negatively affect soil microorganisms. Some organic amendment such as biochar can improve soil carbon and nutrient content, increase soil pH and specific porous area, and also play a role on reducing metal toxicity by sorption process. Microbial activity and biomass are important assessments of soil quality and revitalization.

In this research, soil was homogenize mixed with cadmium (25 and 50 mg kg⁻¹) and lead (2500 and 5000 mg kg⁻¹) both separately and combined. Then we added macadamia nutshell biochar into spiked soils at 5% (wt/wt) as a remediation amendment. After 49 days incubation at 25°C, both soil chemical (e.g., pH and metal potential availability) and biological properties (e.g., metal bioavailability and microbial characteristics) were detected. We use microbial carbon use efficiency (CUE) as microbial indicator for monitoring soil functional recovery. Microbial CUE was calculated as follows:

CUE = ΔMBC/ (ΔMBC+ ƩCO₂-C) (1)

where, ΔMBC is the change in microbial biomass carbon, measured by fumigation-extraction method; ƩCO₂-C is the CO₂ accumulation from microbial respiration determined using titration of NaOH with 0.03 M HCL.

Results showed that heavy metal toxicity significantly (p < 0.001) reduced microbial activity as well as microbial CUE. This demonstrated the detrimental metal toxicity on soil microorganisms. Scanning electron microscope (SEM) images and fourier transform infrared spectroscopy (FTIR) analysis showed the high porous structure of biochar particles. Biochar had significant (p < 0.001) positive effects on reducing soil potential Cd and Pb concentration. In addition, microbial CUE increased by 38%, 49% and 39% at the end of incubation. It was significantly (p < 0.001) different from metal contaminated samples due to biochar application. Biochar is benefit to soil microorganisms and microbial CUE as well as improves microbial living environment.