Symbiotic fungal inoculation promotes the growth of Pinus tabuliformis seedlings in relation to the applied nitrogen form

Background: Nitrogen (N) deposition has become a major driving factor affecting the balance of terrestrial ecosystems, changing the soil environment, element balance and species coexistence relationships, driving changes in biodiversity and ecosystem structure and function. Human-induced nitrogen input leads to a high NH₄⁺/ NO₃^- ratio in soil. However, relatively few studies have investigated the effects of different nitrogen sources on forest plant-microbial symbionts. In this study, the effects of four nitrogen sources, N free, NH₄Cl, L-glutamic acid, and Na(NO₃)₂ (N-, NH₄⁺-N, Org-N, and NO₃^--N) on four fungal species, Suillus granulatus (Sg), Pisolithus tinctorius (Pt), Pleotrichocladium opacum (Po), and Pseudopyrenochaeta sp. (Ps), which were isolated from the roots of Pinus tabulaeformis, were studied in vitro. The effects of inoculation with the four fungi on the growth performance, nutrient uptake and nitrogen metabolism-related enzymes of Pinus tabuliformis under different nitrogen source conditions were subsequently studied.

Results: The biomass and N concentration of the Sg and Po strains were the highest under the NO₃^--N treatment, while the biomass and N concentration of the Pt and Ps strains were significantly greater under the NH₄⁺-N and NO₃^--N treatments than under the Org-N and N- treatments. All four fungi could effectively colonize the roots of P. tabuliformis and formed a symbiotic relationship with it. Under all nitrogen conditions, the inoculation of the four fungi had positive effects on the growth, root development and nutrient concentration of the P. tabuliformis seedlings. Under the Org-N and NO₃^--N treatments, the nitrate reductase (NR) activity of the inoculated plants was significantly greater than that of the noninoculated control (CK) plants. Under all nitrogen conditions, the glutamine synthetase (GS) activity of the inoculated plants was significantly greater than that of the CK plants.

Conclusions: The four fungi can establish good symbiotic relationships with P. tabuliformis seedlings and promote their growth and development under different nitrogen source treatments.