The soil microbiota is an important mediator of soil organic C (SOC) dynamics, and it is sensitive to environmental stress such as heavy metals. Biochars have been found to be effective in reducing the bioavailability of heavy metals in soil, thereby coming metal toxicity to soil biota. In this research, we applied biochar to heavy metal contaminated soil and investigated the use of a simple microbial C use efficiency (CUE) as an indicator in relation with soil functional recovery.

 

Soil was spiked with Cd (25 and 50 mg kg^-1) and Pb (2500 and 5000 mg kg^-1) both separately and combined. A set of spiked and uncontaminated soil samples was treated with biochar (5% wt/wt) and all the samples were incubated at 25°C for 7 days. 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.

After 7 days incubation, the accumulative microbial respiration (MR) CO₂ was 109.88±9.57, 36.62±1.58, 44.84±3.26 and 52.08±3.52 (µg CO­₂-C g^-1 soil) in control, Cd (single), Pb (single) and Cd+Pb (mixed) contaminated soils in the absence of biochar treatment, while 96.23±6.29, 123.29±10.25, 89.86±5.80 and 98.23±6.99 (µg CO₂­-C g^-1 soil) with biochar amendment. This indicates the detrimental effects of Cd and Pb on microbial activity. Results showed the difference in CUE between control and metal contaminated soil (CUE decreased 0.18, 0.27 and 0.31 in Cd, Pb and Cd+Pb soils, respectively) is less  in biochar added treatments (CUE decreased 0.15, 0.21 and 0.36 in Cd, Pb and Cd+Pb soils, respectively). This demonstrated that a biochar was effective in remediation of heavy metal contaminated soil thereby improving microbial CUE.