Potential role of common mycorrhizal networks in improving plant growth and soil physicochemical properties under varying nitrogen levels in a grassland ecosystem

Majority of plants are linked below-ground through common mycorrhizal networks (CMNs). The formation of CMNs between the roots of conspecific and heterospecific plant individuals influences the distribution of limiting nutrients, e.g., nitrogen (N) among the interconnected plants. In various ecosystems, the availability of N is being increased substantially; however, the effect of elevated N on the existence and functionality of CMNs in semi-arid regions of Inner Mongolia remains largely unexplored. Therefore, to verify the development of CMNs and their functionality under increasing N concentration, we incorporated different levels of N fertilizer, i.e., N0 = 0, N1 = 7.06, N2 = 14.12, and N3 = 21.18 mg kg−1, and established Leymus chinensis (L. chinensis) as a donor and the recipient plant in a specially designed portioned root-box. For the establishment of CMNs among the plants, compartments were separated via 37 μm mesh. The 15N isotope tracer technique and microscopic analysis evidenced the presence of CMNs because the extraradical hyphae originated from the donor side and crossed 37 μm mesh of the recipient plant compartment. The 15N recovery was highest at N1 in the donor (18.38%) and recipient (7.89%) plants, while the average transfer rate of 15N to recipient plant was 21–65% with a maximum transfer rate at N2 treatment. The establishment of CMNs significantly improved the donor and recipient plants chlorophyll contents and plant biomass at low N levels. Moreover, the AM fungal extraradical hyphae also released a special kind of protein called glomalin-related soil proteins (GRSPs) that improved the soil structure in terms of mean weight diameter (MWD), thereby improving the soil organic carbon (SOC). Interestingly, the mycorrhizal network had a more profound impact on its neighboring plants than the host plant. In short, the present investigation emphasizes the presence of CMNs in the typical steppe of Inner Mongolia with vital roles in nutrient transfer and improvement of plant growth and soil properties.