Differential bacterial ammonia oxidation patterns in soil particles from two contrasting forests: The importance of interfacial interactions

Soil inorganic nitrogen (N) deficiency limits forest productivity in subalpine regions. The traditional view holds that subalpine coniferous forests are characterized by lower soil inorganic N availability, and such differences are greatly affected by the diversity and abundance of ammonia-oxidizing bacteria. It is known that interfacial interactions between bacteria and solid particles in agricultural and aquatic ecosystems can alter bacterial metabolic activity and biogeochemical processes. However, whether bacteria-soil interfacial interactions induce different ammonia oxidation processes in distinct soil particles remain unknown. Herein, we collected soil particles of different sizes from two contrasting forest sites (a spruce-dominated natural forest and spruce plantation) and examined the effect of soil-Nitrosomonas europaea (a model ammonia-oxidizing bacteria) interfacial interactions on the oxidation of ammonium (NH4+-N) using batch adsorption and semipermeable membrane experiments. The metabolic activity of bacteria was also examined using an isothermal microcalorimetric method. Upon adsorption on soil particles, the ammonia oxidation rate and bacterial metabolism activity were slower than those of free bacteria. The inhibition effect ceased when the bacteria were physically isolated from the soil particles by a membrane, indicating the importance of interfacial interactions in the ammonia oxidation process. Additionally, clay particles were more effective than silts and sands in binding cells and greatly reduced the ammonia oxidation reaction. Across the two forests, natural forest soils exhibited a higher bacterial adsorption capacity but lower bacterial metabolic activity, resulting in a fewer loss of NH4+-N. Our results suggested that stronger bacteria-soil interfacial interactions (especially for clay fractions) were responsible for maintaining high soil inorganic N availability in forests, highlighting the need to incorporate interfacial interactions into biogeochemical models of N cycling in forest ecosystems.