Relationship between forest mycorrhizal type and nitrogen-cycling functions of soil microbial communities: Implications from the rhizosphere scale to the ecosystem scale

Chikae Tatsumi, Graduate School of Agriculture, Kyoto University, Kyoto, Japan, Takeshi Taniguchi and Norikazu Yamanaka, Arid Land Research Center, Tottori University, Tottori, Japan, Sheng Du, Institute of Soil and Water Conservation, Chinese Academy of Science, Shaanxi, China, Ryunosuke Tateno, Field Science Education and Research Center, Kyoto University, Kyoto, Japan

Background/Question/Methods
The mycorrhizal type of the dominant tree species has been recognized as a potentially significant factor controlling soil C and N cycling. Ectomycorrhizal (ECM) fungi are thought to compete with free-living soil microbes through their access to organic N, which slows down the decomposition of soil organic matter, although arbuscular mycorrhizal (AM) fungi do not cause this competition. However, there is no clarity regarding what form of N the ECM fungi and free-living microbes compete for, or which microbial groups compete with ECM fungi for N. To better understand this competition, we collected soils from ECM and AM forests in the drylands of East Asia and measured the soil extractable N content. Additionally, we analyzed the fungal and prokaryotic community functions using amplicon sequencing followed by functional analysis with FUNGuild and PICRUSt pipelines and quantified the ammonia-oxidizing amoA gene abundance. Furthermore, to distinguish the effects of soil physicochemical properties from those of ECM, we also conducted multiple field experiments, including reciprocal soil transplantation and artificial rainfall experiments in the ECM and AM forests, and analyzed the soil microbial and N properties as described above.
Results/Conclusions
The ECM forests had consistently lower ammonia-oxidizer abundance and nitrate N content than the AM forests. The estimated saprotrophic fungal and organic N-decomposing prokaryotic abundances were not lower in both the forests. The extractable organic N (EON) and ammonium N contents were also not lower in the ECM forest. The artificial rainfall experiment results supported the finding that the ECM forests always had low ammonia-oxidizer abundance and nitrate N content, regardless of the rainfall event. Reciprocal transplant experiments showed that the ammonia-oxidizer abundance and the nitrate N content tended to be lower when placed in the ECM than in the AM forest, regardless of soil origin. Thus, the presence of ECM fungi consistently suppressed the population of ammonia-oxidizers rather than that of saprotrophic fungi and prokaryotes, resulting in low nitrate N content but not low EON or ammonium N content in the ECM forest. These results imply that ECM fungi compete with ammonia oxidizers. Moreover, the low nitrate N content in the ECM forest may increase plant dependence on ECM fungi, which would increase plant C investment in soils, resulting in higher soil C content in the ECM than in AM forests. Our findings imply that the difference in microbial N use at the rhizosphere scale controls C and N cycling at the ecosystem scale.