Reduction in CO2 emission from normal and saline soils amended with coal fly ash

Fly ash can reduce CO2 emission from soils via biochemical (i.e., inhibition of microbial activity) and physicochemical (i.e., carbonation) mechanisms. This study investigated the effects of fly ash amendment on biochemical and physicochemical reduction in CO2 emission from normal and saline soils. The physicochemical mechanisms of reduction in CO2 emission by fly ash were estimated in a batch experiment with carbonate solution as a CO2 source by the scanning electron microscope (SEM) and inductively coupled plasma analyses. Biochemical mechanisms of reduction in CO2 emission by fly ash were investigated in a 3-day laboratory incubation experiment with normal and saline soils in the absence and presence of fly ash. Finally, the effects of fly ash amendment at a variety rate from 2 to 15 % (w/w) on CO2 emission from normal and saline soils in the presence of additional organic carbon source (glucose) were investigated through a 15-day laboratory incubation study. In the batch experiment with carbonate solution, both the SEM image of fly ash and changes in soluble Ca and Mg concentrations during reaction with carbonate suggested that the formation of CaCO3 and MgCO3 via carbonation was the principal physicochemical mechanism of carbonate removal by fly ash. In the 3-day incubation study conducted to examine biochemical mechanisms of reduction in CO2 emission by fly ash, microbial respiration of saline soil was inhibited (P < 0.05) by fly ash due to high pH, salinity, and boron concentration of fly ash; meanwhile, for normal soil, there was no inhibitory effect of fly ash on microbial respiration. In the 15-day incubation with glucose, fly ash application at a variety rates from 2 to 15 % (w/w) reduced CO2 emission by 3.6 to 21.4 % for normal and by 19.8 to 30.3 % for saline soil compared to the control without fly ash. For saline soil, the reduction in CO2 emission was attributed primarily to inhibition of microbial respiration by fly ash; however, for normal soil in which suppression of microbial respiration by fly ash was not apparent, carbonation was believed to play an important role in reduction of CO2 emission. Therefore, fly ash may be helpful in reducing CO2 emission from normal soils via carbonation. For saline soil, however, fly ash needs to be carefully considered as a soil amendment to reduce CO2 emission as it can inhibit soil microbial activities and thus degrade soil quality.