Biochar and neem seed cake co-amendment effects on soil nitrogen cycling and NH3 volatilization in contrasting soils

In a 28-day incubation study, ball milling technologies were applied to enhance the sorptive and functional properties of pristine biochar. The effect of ball-milled (BM) biochar, neem seed cake, and their co-amendment was evaluated on nitrification, ammonia (NH3) volatilization, and the abundance of nitrifying microbial communities in three contrasting tropical soils of different pH (acidic, neutral, and alkaline). The amendments were applied at 2% dry w/w to soils fertilized with ammonium chloride (NH4Cl). Results showed that in neutral and alkaline soils, the co-amendment led to a 40% and 64% increase in NH3 volatilization, respectively, compared to control due to significant ammonium (NH4+) retention and temporary nitrification inhibition. Conversely, in acidic soil, BM biochar and neem seed cake amplified nitrification by 23% and 62%, respectively, compared to sole amendments, while neem seed cake increased NH3 volatilization by 56% compared to BM biochar + neem seed cake due to NH4+ retention, altered soil pH, and changes in nitrifying microbial community. The abundance of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and complete ammonia oxidizers (Comammox) was altered by changes in soil pH and N availability modulated by BM biochar and neem seed cake. Correlation analysis revealed significant relationships between soil organic matter (SOM), NH4+, and NO3- on the abundance of nitrifying microorganisms. The study affirms the efficacy of BM biochar-neem seed cake co-amendment on nitrification inhibition but indicates potential N losses by NH3 volatilization depending on soil type, highlighting the need for soil type-specific management strategies to optimize N retention while minimizing environmental impacts.