Assessment of bacterial community composition, methanotrophic and nitrogen-cycling bacteria in three soils with different biochar application rates

The increased use of biochar as a soil amendment to alleviate the impact of agricultural practices on climate change has been a motivation for many studies to determine the effects of biochar on soil properties, particularly the abundance and activities of soil microbes and related biological processes. This study investigates the impact of different application rates of wood-derived biochar on community structure, nitrogen-cycling and methanotrophic bacteria in three soil types. Biochar was added at 0, 2.5, 5 and 10% w/w to black clay loam (BCL, Vertosol), red loam (RL, Dermosol) and brown sandy loam (BSL, Kurosol) soils. Soil chemical analysis and 16S rRNA gene amplicon sequencing using the IIlumina Mi-Seq platform were conducted on initial samples and after 10-month incubation. The results indicated that the addition of biochar loading levels to the different soils had a significant impact on NH4 and NO3, total C and N, pH, electrical conductivity (EC) and soil moisture content. These changes were reflected in significant differences in the bacterial diversity between biochar treatments in the BSL and RL soils, while the BCL soil was more resilient to change. Complete ammonia-oxidising (Nitrospira) and nitrite-oxidising bacteria (NOB) were more abundant than standard ammonia-oxidising bacteria (AOB) in all soils. Increased biochar loading raised the abundance of nitrifying bacteria in BCL soil while Nitrospira became more abundant in BSL soil. Biochar addition affected the abundance of certain N-2 fixer groups in a soil-dependent manner. Strong positive correlations were observed in Rhizobium (r = 0.99) and Azospirillum abundance (r = 0.70) with increased biochar loading rates in BCL. Greater biochar loading also significantly increased the relative abundance of methanotrophs, especially in BCL soil. The impact of biochar on community structure and nitrogen-cycling bacteria depended on soil types and biochar rates which correlated to the differences in soil properties. Overall, the abundance of nitrogen-cycling bacterial groups seemed to be most affected by the changes in soil conditions, including aeration, C/N ratio, nutrients and pH in relation to biochar application in different soils. These changes show that short-term biochar loading influences community structure and leads to increases in populations of methanotrophic and nitrifying bacteria.