Complementarity of Fine Roots and Ectomycorrhizal Fungi in Nitrogen Acquisition Along a Gradient of Intraspecific Competition Intensity

Plant roots can detect and react to the presence of competitors' roots. Intraspecific competition usually constrains root proliferation to minimize the overlap of root systems between competitors, especially in resource-impoverished environments. However, it remains largely unclear whether and how this decline in root nutrient-scavenging capacity can be complemented by other nutrient-acquisition strategies. Here, we leveraged 25 of 41-year-old Pinus Sylvestris var. mongolica monocultures with stand densities ranging from 350 to 1500 trees per hectare, reflecting a gradient of intraspecific competition intensity. In these stands, we measured variables of ecosystem nitrogen (N) status, needle N resorption efficiency, and nine root morphological, physiological and mycorrhizal traits. Results showed that increasing stand densities decreased soil N transformation rates and carbon-:N-acquisition enzyme ratios, indicating an increasing degree of N deficiency. High-density stands had lower root length density than low-density stands, implying intense intraspecific competition causing root segregation. Conversely, stand density was positively correlated with the relative abundance of ectomycorrhizal fungi (EMF) with genetic potential to produce class II peroxidases. Collectively, these findings highlight that the decline of plant-available soil N may account for root segregation under intraspecific competition and suggest the complementarity of fine roots and EMF in nutrient acquisition at the intraspecific level.