Response of microorganisms to a 5-year large-scale nitrogen loading in immature volcanic ash soil in an oak-dominated forest

Increasing atmospheric nitrogen (N) deposition in terrestrial ecosystems influences aboveground and belowground carbon (C) storage. In belowground systems, N fertilization in field experiments has often been reported to suppress soil microbial biomass and mineralization; however, the effects on soil microorganisms are not always consistent. Here, we investigated N load response of microorganisms after 2 and 5 years in organic layers and mineral soils in a temperate forest dominated by Quercus crispula using large-scale field N fertilization (9 ha, 100 kg N ha⁻¹ year⁻¹ for 5 years, urea) on immature volcanic ash soil (with a high buffering capacity). In the organic layers, N loading increased the total C concentration and KCl-extractable organic C content. Additionally, the amount of extractable organic C in the soil did not change after one month of laboratory incubation, possibly due to the low microbial use. These results likely indicate the accumulation of recalcitrant C (possibly due to decreased oxidase activity). Although the fungal-to-bacterial composition ratio did not change, the bacterial biomass increased by 18% and 26% in the second and fifth years, respectively, in the N-fertilized plots. Furthermore, the abundance of ammonia-oxidizing bacterial AmoA increased, which was correlated with potential nitrification. These changes may result from plant litter N content and litter quantity and subsequently change in soil environments, especially with increased soil N availability. In the mineral soils, N loading changed soil environments to a lesser extent than the organic layers; however, the fungal biomass decreased by 42% and 44% in the second and fifth years, respectively. This could be linked to a decrease in resource investment into symbionts (ectomycorrhizal fungi derived from oak roots) by underground plants. These findings suggest that in an oak-dominated forest, the influence of N loading between the organic layers and mineral soils on microorganisms varies, enhancing our understanding of belowground C dynamics.