Internal storage dominates external supply: The primacy of nitrogen-storage traits in driving root nitrogen reallocation

Moreover, leaf N resorption—an aboveground conservation mechanism typically induced under low soil N availability—positively covaried with RNR, jointly contributing to plant nutrient conservation. Soil processes, including net N mineralization and microbial community composition (e.g., fungal-to-bacterial ratios), negatively and positively associated with RNR, respectively, by modulating external N supply. The overwhelming dominance of storage traits over supply factors (91.2% vs. 8.5%) reveals a ‘hierarchical control’ of RNR.

In this hierarchy, the plant’s genetic and structural allocation to storage organs (e.g., biomass) sets the ‘capacity ceiling’, while soil N availability merely fine-tunes the ‘realized flux’ within that predefined capacity. Within the multidimensional root economics space, RNR aligned positively with root diameter (RD) and negatively with root N concentration, while remaining orthogonal to root tissue density (RTD).

The orthogonality between RNR and RTD suggests that physiological remobilization is distinct from structural conservation. While RTD represents the metabolic cost of building root tissues (the 'structural' axis), RNR reflects the flexibility of the 'labile' N-pool. This implies that 'thick' roots (high RD) can be highly active N-reserves regardless of whether they are physically dense or porous, highlighting a decoupling of structural tissue life-span from internal nutrient turnover speed.

This integrative synthesis provides direct evidence supporting the integration of RNR into the root economics space and highlights the primacy of N-storage traits in shaping plant nutrient conservation strategies, with environmental factors playing supportive roles.