Biochar addition affected the dynamics of ammonia oxidizers and nitrification in microcosms of a coastal alkaline soil

Biochar amendments have frequently been reported to alter microbial communities and biogeochemical processes in soils. However, the impact of biochar application on bacterial (AOB) and archaeal ammonia oxidizers (AOA) remains poorly understood. In this study, we investigated the responses of AOB and AOA to the application of biochar derived from cotton stalk at rates of 5, 10, and 20 % by weight to a coastal alkaline soil during a 12-week incubation. The results showed that the amoA gene of AOB consistently outnumbered that of AOA, whereas only the AOA amoA gene copy number was significantly correlated with the potential ammonia oxidation (PAO) rate (P < 0.01). The significant decrease of PAO rates in biochar treatments occurred after incubation for 4-6 weeks, which were distinctly longer than that in the control (2 weeks). The PAO rates were significantly different among treatments during the first 4 weeks of incubation (P < 0.05), with the highest usually in the 10 % treatment. Biochar application significantly increased the abundance of both nitrifiers in the 4 weeks of incubation (P < 0.05). Biochar amendment also decreased AOA diversity, but increased AOB diversity, which resulted in different community structures of both nitrifiers (P < 0.01), as shown by the differences between the 5 % biochar and the control treatments. We conclude that biochar application generally enhanced the abundance and altered the composition of ammonia oxidizers; the rate of biochar application also affected the rate and dynamics of nitrification, and the risk for increasing the alkalinity and N leaching of the studied soil was lower with a lower application rate.