Root Symbionts Contributions to Productivity of Grass–Legume Mixtures Under Organic Versus Synthetic Nitrogen Fertilization

This study assessed the effects of nitrogen (N) fertilization source on forage yield, nutritional status, and the activity/abundance of autochthonous root symbionts in fescue–lotus mixtures. Specifically, the contribution of arbuscular mycorrhizal fungi (AMF) and N-fixing rhizobia (NFR) was assessed under manure (M) and urea (U) fertilization. The aim was to determine whether autochthonous root symbiont populations enhance pasture productivity and nutritional status, mainly under organic fertilization. A nine-month pot experiment was conducted growing fescue–lotus mixtures under two soil conditions: A-soil, with autochthonous root symbiont populations; and R-soil, in which root symbiont populations (AMF and NFR) were reduced via formaldehyde application + UV treatment. Both soil conditions received either no fertilizer (Control) or equal N doses (120 kg N ha^− 1 y^− 1) supplied as cattle dry manure or urea. Fertilization with M resulted in root symbionts activity/abundance similar to the Control. Fertilization with U reduced root mycorrhizal colonization and nodulation. Plant growth was significantly higher in A-soil than in R-soil, with the legume component showing positive response. Under R-soil conditions, U fertilization resulted in the highest pasture growth, favoring grasses at the expense of legumes. Overall, M appears to be a more suitable fertilizer than U, as it better preserves root symbionts and may also contribute to increased productivity in the mixture. These findings suggest that M may be less disruptive to the studied root symbionts (AMF and NFR) than U under these conditions. These findings could have implications for the development of management strategies designed to preserve potentially beneficial root symbionts in pasture-based livestock systems.