Effect of corn stover-modified biochar on some biological properties of a Cd-contaminated calcareous soil

In the past few decades, accumulation of heavy metals in soils has increased as a result of human activities. Microorganisms are one of the important parts of soil, that their activity can be affected by the high levels of heavy metals such as Cadmium (Cd). Soil biological parameters can be used for evaluating the quality of contaminated soils. Biochar as a soil amendment can control bioavailability of heavy metals in contaminated soils and reduce their transfer risk to the food chain. The objective of this study was to evaluate the effect of corn stover biochar and modified- corn stover biochar on cadmium (Cd) availability and microbiological properties of a Cd-contaminated soil.

A soil sample was collected from the surface layer (0-20 cm) and spiked with different concentrations of Cd (Cd(NO3)2). The contaminated soil was placed in periodic wetting-drying (WD) cycles for about five months in room temperature. Then, corn stover biochar (BC) and modified corn stover biochar with FeCl2 (Fe-BC) were prepared at 500°C and mixed with the Cd-contaminated soil at 2% (w/w). The soil samples were incubated for 90 days. At the end of incubation period, Cd availability and some of microbiological properties of soil including soil microbial respiration (BR), substrate induced respiration (SIR), microbial biomass carbon (MBC) and dehydrogenase activity were measured and metabolic quotient (qCO2) were determined. This study was carried out as a factorial experiment based on a randomized complete design with two factors including Cd contamination (0, 15 and 30 mg Cd kg-1 soil) and biochar type (control (CK), corn stover biochar (BC) and modified-corn stover biochar (Fe-BC)) in three replications under labratory condition.

The results indicated that application of corn biochar and modified-corn biochar led to increase of soil pH (0.19 and 0.21 unit respectively), cation exchange capacity (6.7 and 10.9%), organic carbon (157 and 200%) and decrease of concentration of available Cd (21.8-41.5%) in the soil compared to the control. In general, soil Cd contamination led to a significant reduction in the soil microbiological properties. Application of corn stover biochar and modified-corn stover biochar significantly increased microbial respiration (24.7-58.7%), substrate-induced respiration (16.8-42.5%), microbial biomass carbon (24.5-58.6%) and dehydrogenase activity (28.8-108%). The effects of Fe-BC treatment on changes in Cd availability and soil biological properties were greater than BC treatment.

In general, results of this study indicated that FeCl2 modified-corn stover biochar can be suitable organic amendment for Cd stabilization and may improve soil microbial activities in calcareous Cd-contaminated soils.