Plant-microbial interactions for nitrogen absorption converge between arbuscular- and ectomycorrhizal-dominated ecosystems at high nitrogen availability

Nitrogen (N) availability is one of the critical limiting factors regulating plant growth, microbial activity, and the interactions between plants and soil microorganisms. The competition between plants and microorganisms, represented by the ratio of microbial N immobilization and plant N uptake (measured as Nim: PNU ratio), generally reflects the degree of N limitation in a terrestrial ecosystem. However, the key factors driving the pattern of Nim: PNU ratio across global ecosystems remain little understood. Here, using a global data set of 1022 observations from 184 studies, we examined the relative importance of mycorrhizal associations, N availability, climate, plant, and soil properties on the Nim: PNU ratio. Our results show that mycorrhizal fungi type (arbuscular mycorrhizal (AM) vs. ectomycorrhizal (EM) fungi) in combination with soil N availability explain the variation in the Nim: PNU across terrestrial ecosystems. In AM fungi-associated ecosystems, the relation between Nim and PNU displayed a weaker negative correlation (r =-0.06, p < 0.001), whereas there was a stronger positive correlation (r = 0.25, p < 0.001) in EM fungi-associated ecosystems. Those results indicated that the AM-associated plants display a weak interaction with soil microorganisms for N absorption, while EM-associated plants cooperate with soil microorganisms. Further, we found that the Nim: PNU ratio for both AM- and EM-associated ecosystems gradually converge at a stable value (14.7 vs. 13.5, p > 0.05) with greater N availability. Our study thus highlights that plant-microbial interaction for N absorption both equalize and stabilize at increased N supply, and both these mechanisms primarily depend on the mycorrhizal association of plants in terrestrial ecosystems.