Mycorrhizal fungi are thought to have a large impact on soil carbon storage by controlling soil organic matter (SOM) decomposition via the modification of soil nitrogen (N) availability for free-living microbes. The N competition hypothesis suggests that ectomycorrhizal (ECM) fungi access organic N and successfully compete for N with free-living microbes; thereby, they slow the decomposition caused by free-living microbes, although arbuscular mycorrhizal (AM) fungi do not take part in this competition. However, despite the importance of soil N and free-living microbial dynamics in this “N competition hypothesis”, few studies have focused on soil N cycling and the microbial community. Therefore, this study examined the mechanism of the mycorrhizal effects on soil N cycling mediated by soil fungal and prokaryotic community.
Since the community is strongly affected by soil physicochemistry, we used an aridity gradient with large differences in soil moisture and pH to separate the mycorrhizal and physicochemical effects. Soil samples were collected from AM-symbiotic black locust forests under three different aridity levels, and from ECM-symbiotic oak forests under two aridity levels ranging from 450 to 600 mm mean annual rainfall. The soil extractable N dynamics, and abundance, composition and function of the soil microbial communities, were measured.
Results/Conclusions
The soil nitrate N content was higher in the AM forests, and the soil-dissolved organic N (DON) content was higher in the ECM forests. The fungal communities of the AM and ECM forests differed completely, i.e., ECM fungi dominated in the ECM forests and saprotrophic fungi dominated in the AM forests. Ammonia-oxidizing prokaryotes were more abundant in the AM forests. All of these differences occurred regardless of larger changes in the soil physicochemical properties along the aridity gradient.
In the ECM forests, the growth and activity of free-living saprotrophic fungi and ammonia-oxidizing prokaryotes, which are responsible for SOM degradation and nitrification, respectively, were suppressed, probably due to the N competition with ECM fungi, resulting in higher DON and lower nitrate N contents than in the AM forests. ECM fungi should absorb larger amounts of DON and ammonium N. Since the aridity level caused large changes in the soil physicochemical properties, but had smaller effects on soil extractable N and microbial dynamics, the differences in N and microbes between the two forests were not caused by the soil physicochemical properties. This study supports the hypothesis of N competition between ECM fungi and free-living saprotrophic fungi and ammonia-oxidizing prokaryotes.