Contrasting nutrient-mediated responses between surface and deep fine root biomass to N addition in poplar plantations on the east coast of China

Dynamics of fine root biomass (FRB) is fundamental to understanding carbon allocation and storage in forests. However, our understanding of FRB responses and its vertical distribution with respect to atmospheric nitrogen (N) deposition is limited. In this study, we applied five N addition rates (0, 5, 10, 15, and 30 g m⁻² yr⁻¹) to explore effects of N addition on FRB and its distribution patterns along soil profile (0–100 cm) in poplar plantations on the east coast of China. We found that N addition significantly decreased total FRB and altered its vertical distribution. Specifically, N addition decreased FRB in the surface (0–20 cm) and increased it in the deep (20–100 cm) soil layers. Structural equation models (SEM) showed that soil available nutrients dominantly regulated FRB and its vertical distribution in poplar plantations. Decreased FRB and increased root distribution to deep soil layers indicate that trees can not only regulate carbon allocation between above- and below- ground tissues, but also optimize nutrient uptake by redistributing their roots vertically under different nutrient environments.