Organic fertilizer and Bacillus amyloliquefaciens promote soil N availability via changing different mineralization-immobilization turnover rates in acidic soils

Biofertilizer and organic fertilizer (OF) are potential alternative strategies for agriculture sustainability. However, their effects on soil gross nitrogen (N) transformation and the underlying mechanisms remain largely unknown. Therefore, in this study, using Bacillus amyloliquefaciens (BA) as a biofertilizer agent, a N-15 tracer was employed to quantify soil gross N transformation rates in acidic soils under tea cultivation with four treatments: no fertilizer application (F0, control), only BA application (BA treatment), only OF application (OF treatment), and BA plus OF applications (BAOF treatment). Compared with F0 treatment, BA and OF treatments increased soil nitrate (NO3-) and total inorganic N concentrations. The gross rates of total organic N mineralization (M-Ntotal) and total NH4+ immobilization were significantly decreased by BA and BAOF treatments but increased by OF treatment. Soil microbial biomass N concentration; 4-beta-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), and urease activities; and ureC abundance were decreased by BA and BAOF treatments, whereas NAG and LAP activities and ureC abundance were increased by OF treatment; these modifications were probably responsible for the opposite effect on MNtotal. Meanwhile, BA, OF, and BAOF treatments significantly increased the gross autotrophic nitrification (O-NH4) rate by 105.2%, 270.5%, and 129.6%, respectively, which was attributed to increased soil pH and ammonia-oxidizing archaea abundance. BAOF treatment led to a higher gross NO3- immobilization (I-NO3) rate than F0 treatment. The net NO3- production, which indicates N loss risk, increased with BA and OF treatments and decreased with BAOF treatment. Thus, BA and OF applications increased soil N availability and N loss risk associated with enhanced O-NH4, whereas BAOF treatment reduced N loss risk by enhancing I-NO3. This study provides insights into the mechanisms underlying the effects of OF and BA applications on N availability and N loss risk in acidic soils, especially emphasizes the effects of OF plus BA applications on soil N availability and N loss, which would be great benefit for agricultural production and environment.