Smaller Nano-Silica Particles Promote Ammonium Dominance and Mitigate N2O Emissions in Flooded Tropical Paddy Soil

Silicon (Si)-based amendments can regulate soil nitrogen (N) cycling and reduce gaseous N losses, but the effect of nano-Si particle size on mineral N dynamics and nitrous oxide (N2O) emissions under flooded tropical paddy soil conditions remains unclear. This study aimed to elucidate how nano-Si particle size regulates mineral N dynamics and N2O emissions, with particular emphasis on whether smaller particles promote ammonium N (NH4+-N) dominance and more effectively mitigate N2O emissions than larger particles. A 30-day flooded incubation experiment was conducted using tropical paddy soil amended with 15 or 50 nm nano-Si particles, each applied at 67 mg kg−1 dry soil, alongside an unamended control. Mineral N dynamics, extracellular enzyme activities, N-cycling functional genes, and N2O emissions were evaluated. Compared with the control, 15 nm and 50 nm nano-Si significantly increased NH4+-N concentration by 16.4% and 5.25% while reducing nitrate N (NO3−-N) concentration by 40.8% and 23.0%, respectively. The NO3−-N/NH4+-N ratio decreased significantly by 49.8% and 22.1%, indicating a particle-size-dependent shift toward NH4+ dominance. The 15 nm treatment significantly enhanced β-N-acetylglucosaminidase and leucine aminopeptidase activities, supporting organic N turnover and NH4+-N accumulation. It also significantly reduced the abundance of the nitrite reductase gene nirS, involved in denitrification, whereas the N2O reductase gene nosZ, responsible for N2O reduction, showed no consistent treatment-specific response. Consistently, cumulative N2O emissions decreased significantly by 33.6% and 18.2% under 15 nm and 50 nm treatments, respectively. These results indicate that both nano-Si treatments significantly shifted mineral N dynamics toward NH4+-N dominance and reduced cumulative N2O emissions compared with the control. These responses were consistently stronger under the 15 nm treatment than under the 50 nm treatment, demonstrating that smaller nano-Si particles more effectively limit NO3−-N accumulation, promote NH4+-N retention, and mitigate N2O emissions in flooded tropical paddy soil. Nevertheless, direct measurements of N-transformation rates, comparisons with conventional Si sources, and field-scale validation are required before practical application.