Labile Carbon Input Mitigates the Negative Legacy Effects of Nitrogen Addition on Arbuscular Mycorrhizal Symbiosis in a Temperate Grassland

Nitrogen (N) deposition and carbon (C) addition significantly influence the dynamics of plant-microbe interactions, particularly altering the symbiotic relationship between plants and arbuscular mycorrhizal fungi (AMF). However, the effects and underlying mechanisms of labile C input on the relationship between AMF and various plant species in a nitrogen-enriched environment remain a knowledge gap. A seven-year field experiment was conducted to examine how six levels of N and three levels of labile C addition impact AMF colonization in four key plant species: <i>Leymus chinensis</i> (Trin. ex Bunge) Tzvelev, <i>Stipa baicalensis</i> Roshev., <i>Thermopsis lanceolata</i> R. Br. and <i>Potentilla bifurca</i> Linn. Our results showed that N and C additions exert significantly different effects on the relationship between AMF and various plant species. Labile C addition mitigated historical N negative effects, particularly for <i>S. baicalensis</i>, enhancing AMF infection and promoting nutrient exchange under high-N and low-C conditions. The relationship between AMF and both <i>L. chinensis</i> and <i>T. lanceolata</i> changed to weak mutualism under low-N and high-C conditions, with significant decreases in vesicular and arbuscular abundance. Plant root stoichiometry plays a critical role in modulating AMF symbiosis, particularly under high-N and -C conditions, as reflected in the increased AMF infection observed in <i>T. lanceolata</i> and <i>P. bifurca</i>. Our findings emphasize the species-specific and nutrient-dependent AMF symbiosis, revealing that targeted C input can mitigate the legacy effects of N enrichment. Effective nutrient management is of crucial importance for ecological restoration efforts in temperate grasslands affected by long-term N enrichment.