Conversion from natural coniferous forests to cedar plantations increase soil nitrogen cycling through changing microbial community structures

Converting natural forests to coniferous plantations in temperate regions often impact soil nitrogen (N) cycling. However, belowground microbial communities responsible for these changes in N cycling are not well understood, particularly in forest ecosystems dominated by coniferous trees. Additionally, the influence of different soil acidity levels on these microbial community changes and their effect on N cycling have not been thoroughly investigated. In order to gain a deeper understanding of these impacts, we conducted a study to examine the effects of the conversion from natural forests dominated by coniferous trees to pure cedar plantations on soil N cycling, as well as the root-associated and soil microbial communities using high-throughput amplicon sequencing technique in two areas with different acidity levels. Our results showed that the conversion increased inorganic N content (i.e., the sum of ammonium and nitrate N), accompanied by a decrease in the relative abundances of ectomycorrhizal fungi and an increase in saprotrophic fungi within class Leotiomycetes regardless of acidity. Additionally, the conversion increased nitrate N content only in the less acidic area (pH about 5.5) but not in the more acidic area (pH below 5). Relative abundances of copiotrophic bacteria, including Burkholderia sp. and Bradyrhizobiaceae, which are enriched in roots, were significantly lower in cedar plantations than in natural forests. These copiotrophic bacteria were significantly lower in the less acidic area than in the more acidic area. The relative abundances of these copiotrophic bacteria were negatively correlated with ammonia-oxidizing archaeal abundances, leading to higher nitrate N content in cedar plantations in the less acidic area. Our findings have practical implications, suggesting that retaining natural forests dominated by ectomycorrhizal-associated coniferous trees and introducing these types of trees into pure cedar plantations may be an effective strategy for reducing nitrate production, particularly in less acidic areas.