Influence of Biochar Addition on the Denitrification Process and N2O Emission in Cd-Contaminated Soil

Abstract

The aim of this study was to investigate the effect of biochar addition on the denitrification process and N2O emission in Cd-contaminated soil. Four different biochars, i.e., dairy manure and rice straw pyrolyzed at 350 and 550 °C, respectively, were added into a Cd-contaminated soil and incubation experiments were conducted for 8 weeks. Results showed that Cd had an inhibitory effect on denitrifying reductase enzymes and reduced the abundance of functional genes. On the contrary, amendment with the biochars increased denitrifying enzyme activity and gene abundance, and thus, enhanced the denitrification process. Labile carbon (C) in the biochar-amended soil, which was calculated based on the two-pool exponential model, was the key factor to facilitate this process. As a less important factor, elevated soil pH by biochar addition also increased denitrifying activity as well as the nosZ abundance. Decrease of Cd bioavailability by the biochar addition was beneficial to the denitrification process. Addition of the biochars with higher amount of NO3 −-N, especially the rice straw-derived biochars, increased cumulative N2O emission by more than ten times relative to the Cd-contaminated soil. With the great amount of labile C and NO3 −-N, the treatment of biochars prepared at 350 °C released the larger amount of CO2 and N2O than other treatments. The biochar addition could totally release the heavy metal stress and restore the Cd-contaminated soil in terms of bacterial community.