Contrasting responses of new pioneer and fibrous roots exposed to nitrogen deposition: a field study using three woody species

Aims Within root systems, framework pioneer roots mainly function in soil exploration and resource transport, while fibrous roots and their associated fungi extend the foraging soil space for resource absorption. However, the plasticity of pioneer and fibrous roots in response to elevated nutrient availability has received relatively little attention. Methods For three hardwood species, newly formed root branches were harvested in order to compare pioneer and fibrous roots for variation in morphology, mycorrhizal colonization and anatomy, as well as their trait shifts in response to fertilization. Results Compared with the fibrous roots, pioneer roots were generally larger in diameter, stele and cortex size, root tissue density, and hydraulic conductivity, but lower in specific root length (SRL) and mycorrhizal colonization rate (MCR). Fertilization generally decreased SRL and MCR in fibrous roots, and increased conduit diameters in pioneer roots. Additionally, fertilization enhanced hydraulic conductivity for all three species: 162, 45 and 263% in pioneer roots, and 46, 13 and 172% in fibrous roots for Juglans mandshurica Maxim., Fraxinus mandshurica Rupr. and Phellodendron amurense Rupr., respectively. Conclusions Pioneer and fibrous roots responded differently to fertilization. The resulting enhanced resource transportation capacity may serve to mitigate the toxic effect in fibrous roots and avoid pioneer roots becoming a bottleneck for absorbed water and nutrient in an expanding root system, suggesting tightly associated traits and functional equilibrium within an individual root and even the root branches. The observed trait shifts and coordination between pioneer and fibrous roots would facilitate resource acquisition in changing soil conditions.