Combined application of biochar and N fertilizer shifted nitrification rate and amoA gene abundance of ammonia-oxidizing microorganisms in salt-affected anthropogenic-alluvial soil

Biochar has been globally recognized as a novel and eco-friendly soil ameliorant to mitigate hazardous effects of salinization on soil health, and mediate carbon and nitrogen (N) nutrient cycling in agricultural ecosystems. However, knowledge pertaining to the regulatory mechanism of biochar application on nitrification in the salt-affected soil is limited. In this study, field plot experiment for three consecutive years and an aerobic incubation experiment with three biochar application rates (7.5, 15, and 30 ton ha−1) and N fertilizer (225 kg N ha−1 yr−1) were conducted. We investigated the effect of combined application of biochar and N fertilizer on soil chemical and microbial properties, nitrification kinetic parameters, and the abundance of ammonia monooxygenase (subunit A, amoA) gene of ammonia-oxidizing bacteria (AOB) and archaea (AOA). Our results showed that biochar addition increased soil pH, cation exchange capacity, available potassium, and microbial biomass carbon, and decreased the numbers of amoA-AOB gene in the salt-affected soil applied with N fertilizer. The nitrification process and fitted kinetic parameters were negatively responsive to the combined application of biochar and N fertilizer, which reduced the nitrification potential and potential nitrification rate, and prolonged the duration of nitrification. The nitrification rate and abundance of amoA-AOB and amoA-AOA genes decreased under biochar and N fertilizer addition, and biochar played the predominant role in the inhibiting effect, which aggravated with the increase of biochar rate. The AOB dominated the autotrophic nitrification with the nitrification rate having higher positive correlation with abundance of amoA-AOB gene than with that of amoA-AOA gene. The abundance of both amoA-AOB and amoA-AOA genes showed significantly negative correlation with nitrate nitrogen (NO3−-N) concentration. In conclusion, biochar and N fertilizer addition inhibited the autotrophic nitrification rate mainly by greatly reducing the abundance of amoA-AOB gene, which was adaptive to the changing soil microhabitat traits induced by biochar application and N fertilization.