Saprotrophic-ectomycorrhizal fungal interactions affect poplar performance

Mycorrhizal and saprotrophic fungi are key players in plant nutrition in forest ecosystems, affecting nutrient availability and plant nutrient acquisition, but the impact of their interaction on plant performance remains largely understudied. Their interaction is particularly important under nutrient-limited conditions (e.g., nitrogen limitation) as they may compete for resources or engage in facilitative interactions that ultimately affect plant nutrient uptake and growth. Here, we used a simplified, plant-centric experimental design to investigate the effects of fungal interactions on plant performance. Poplar ( Populus × canescens ) plantlets were grown under nutrient-poor conditions for 23 weeks with a single nutrient source: a mixture of 15 N-labelled poplar (labile) and beech (recalcitrant) leaf litter. Plants were inoculated with Pholiota squarrosa (saprotrophic), Laccaria bicolor (ectomycorrhizal), both, or neither. We analysed growth, nitrogen uptake, and photosynthetic performance. Ectomycorrhizal-inoculated plants showed greater growth, root development, and nitrogen accumulation than non-inoculated controls or those inoculated with saprotrophic fungi alone. Photosynthetic performance, particularly at 16 weeks, was also enhanced. In contrast, saprotrophic fungi increased nitrogen concentration in roots but did not improve plant biomass. Plant biomass and root architecture did not differ between EMF-only and dual-inoculated plants, suggesting that the addition of saprotrophic fungi did not further enhance or impair these traits. However, for nitrogen-related traits, dual-inoculated plants showed intermediate values between EMF-only and STF-only treatments. Despite these trends, statistical analysis did not detect a significant interaction between fungal guilds. These findings indicate that ectomycorrhizal fungi play a stronger role in promoting plant performance under nitrogen-limited conditions, likely through enhanced nutrient uptake and photosynthetic efficiency. Saprotrophic fungi alone did not promote plant growth under the experimental conditions, nor did their presence alter the benefits conferred by ectomycorrhizal fungi. • Root symbionts, ectomycorrhizal fungi, enhance plant growth in poor substrates • Saprotrophic fungi offer limited benefits for plant growth • Their combination gives intermediate effects without enhancing ectomycorrhizal gains • 15 N-labelled leaf litter traced nutrient uptake in controlled, nutrient-poor setups