Using isotopic tracer to understand nitrogen use efficiency and root functions across root orders of poplar

Abstract

As a keystone species for afforestation and bioeconomic applications, poplar plays vital roles in paper production, bioenergy systems, and ecological remediation. While nitrogen critically regulates poplar's rapid growth, the species' preferential utilization mechanisms for different N fertilizers remain poorly characterized. This study employed an integrated approach combining 15 N isotopic tracing, root order anatomical analysis, and nitrogen transformation gene profiling to investigate N allocation patterns and root functional specialization in poplar. Results demonstrated significantly higher ¹⁵N uptake rates (6.26 %-120.79 %) in roots, trunks, branches, and leaves under urea treatment compared to compound fertilizer. Root absorption capacity showed an inverse correlation with root order, indicating functional differentiation across branching hierarchies. Residual fertilizer was primarily retained within the 0–40 cm soil horizon, suggesting limited vertical leaching. Molecular analysis revealed divergent microbial mediation-both fertilizers enhanced soil N mineralization but through distinct biochemical pathways: urea primarily stimulated assimilatory nitrate reduction (upregulating nasB, nasA, nasD and nirA genes), whereas compound fertilizer activated dissimilatory pathways (nirB gene upregulation). These findings elucidate the physiological basis of N utilization preferences in poplar, demonstrating root system modularity where lower-order roots specialize in nutrient acquisition while higher orders focus on transport. This work provides mechanistic insights into optimizing fertilization strategies for poplar plantations while advancing our understanding of functional-structural relationships in woody root systems.