Influence of Biochar Amendment on Soil Denitrifying Microorganisms in Double Rice Cropping System

Abstract

At present, it is not explicit how biochar regulates the microbial process of denitrification in paddy fields. Therefore, a field experiment was carried out in a double rice cropping system with three wheat straw biochar treatments:no biochar treatment (CK), added 24 t·hm-2 biochar (LC), and added 48 t·hm-2 biochar (HC). Real time PCR (qPCR) and terminal-restricted fragment length polymorphism (T-RFLP) technology were adopted to analyze the abundances and microbial community structures of denitrification functional genes (narG, nirK and nosZ) in the fallow season and rice season. Due to its alkalinity, biochar amendment increased soil pH by 0.2-0.8. Biochar amendment also increased soil NH4+-N and NO3--N contents by 21.1%-32.5% and 63.0%-176.0% in the fallow season due to the presence of soluble N. Nevertheless, it reduced NH4+-N content by 48.8%-60.1% in the rice season due to the adsorption of biochar. The amendment increased soil MBN content in the fallow season, which may be a result of the large surface of biochar supplying nutrients and a suitable survival environment for the microorganisms. In the fallow season, compared to CK treatment, the increased soil NH4+-N and NO3--N with biochar amendment promoted the conversion of NH4+-N to NO3--N, and thus increased the abundances of narG and nosZ (P<0.05). The higher soil pH with biochar addition increased the abundances of nosZ and altered the community structures of narG and nosZ in the fallow season. Biochar amendment altered the denitrification process, but it did not change N2O emissions in the fallow season, which might reduce NO3--N leaching losses. In the rice season, biochar amendment increased nosZ abundance (P<0.05). HC increased the nirK gene abundance, which contributed to increased N2O emission in the rice season (P<0.05). Biochar converted the community structures of nirK and nosZ by decreasing the NH4+-N content in the rice season. The changes of the narG community structure with HC treatment contributed to the increased N2O emission. In conclusion, biochar amendment can influence the microbes involved in soil denitrification by changing the soil properties, and thus impact the N2O emissions and NO3--N leaching.