Arbuscular mycorrhizal fungi promote soil ammonia but not nitrate assimilation in the Mu Us Desert

Although arbuscular mycorrhizal fungi (AMF) have been established to assimilate nitrogen (N) and transport this to host plants, the potential role of AMF in soil N processes, particularly the assimilation of inorganic N, has not been comprehensively investigated. Compared with other terrestrial ecosystems, deserts are typically nitrogen-poor environments. Numerous species of xerophytic shrubs growing in deserts, such as Artemisia ordosica, form efficient symbiotic relationship with AMF, and play an important role in ecological restoration in China. However, whether arbuscular mycorrhizal symbioses are advantageous with respect to the competition for N resources and inhibition of N assimilation by other soil microbes in nitrogen-poor soils has yet to be established. To gain further insights in this regard, we established a culture system of the xerophyte A. ordosica and AMF, and investigated the effects of AMF on soil nitrate reductase (NR) and nitrite reductase (NiR) activities, the abundance of genes involved in N processes, and soil microbial composition based on controlled experiments designed to elucidate the roles of AMF in soil inorganic N assimilation. We accordingly found that AMF enhanced the assimilation of soil NH₄⁺-N and NO₂⁻-N by promoting soil NiR activity, upregulating the abundance of gltB and gltD genes, and influencing the soil microbial community. However, AMF were found to inhibit soil NR activity and showed no promotive effect with respect the assimilation of NO₃⁻-N in the soil. Although compared with natural conditions, our experiments simplified the structure of the soil microbial community and may have amplified AMF function, on the basis of our findings, we identified a new N-utilization mechanism of AMF and A. ordosica symbionts. Specifically, we found that the fungal partners promote soil NH₄⁺-N and NO₂⁻-N assimilation, but not that of NO₃⁻-N, while also promoting the growth of A. ordosica. This can reduce the likelihood of soil NO₃⁻-N being consumed by other soil microbes. The findings of this study will contribute to enhancing our understanding of the survival of desert plants and AMF in nitrogen-deficient environments.