Biochar and organic fertilizer amendments stimulate the dynamics of ammonia oxidizers community in a slightly alkaline soil

Soil treated with only biochar or organic fertilizer has been extensively documented to influence the soil nitrification process due to its impact on soil characteristics. However, the dynamic response of ammonia oxidizers to the combined application of biochar and organic fertilizer in slightly alkaline soil is yet to be studied. Therefore, this study intended to evaluate the abundance and composition of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in incubated soil by using quantitative real-time PCR (qPCR) and terminal restriction fragment length polymorphism (T-RFLP) analysis of amoA genes. The incubation lasted 106 days and involved 4 treatments: the control (CK); 2% (w/w) organic fertilizer (O); 2% (w/w) biochar (B); and a combination of 2% (w/w) organic fertilizer and biochar (OB). Our results showed that AOB and AOA abundance increased significantly and changed over time in the biochar-treated soil at late stages of the incubation. AOA abundance increased nearly 19 times more than AOB abundance in the biochar treatment as compared to the control, indicating that biochar is more crucial to promote AOA abundance than AOB abundance. Moreover, T-RFLP analysis showed that the interaction between biochar and organic fertilizer significantly altered AOA community composition over time due to the effect of biochar and organic fertilizer amendments on soluble organic nitrogen (SON). The soil treated with organic fertilizer had the specific community compositions at the end of the incubation, suggesting that organic fertilizer changed the AOB community composition by increasing available phosphorus (AP), available potassium (AK), and total nitrogen (TN). Our findings, therefore, revealed that biochar can improve AOA abundance and composition by enhancing SON, while organic fertilizer can greatly increase AOB by regulating AP, AK, and TN.