Absorptive Roots Prioritize Chemical over Morphological Investment Under Litter Addition in a Qinling Pine-Oak Mixed Forest.

The root economics spectrum predicts coordinated trait shifts to a heterogeneous soil environment, yet how roots strategically respond to litter-driven nutrient patches is not fully understood. We conducted a litter addition experiment (CK: 0, Low: 30, Medium: 60, High: 120 g) in a Qinling mixed forest, quantifying root responses of Pinus tabuliformis (Pt) and Quercus aliena var. acuteserrata (Qa). Soil inorganic nitrogen (NH4+-N and NO3--N) increased significantly only under high litter inputs (by 138% and 130%, respectively; p < 0.001), indicating a threshold effect. Root carbon and nitrogen concentration generally increased under the Medium and High litter addition treatments compared to the Low treatments (p < 0.05), while morphological traits remained conservative (p > 0.05). Species identity showed no significant effect in the multivariate root trait syndrome (redundance analysis, p = 0.716), though species-specific responses were observed in the root carbon concentration (Pt: p < 0.05; Qa: n.s.). These results demonstrate a hierarchical foraging strategy where physiological plasticity dominates over morphological change, challenging the root economics spectrum and providing a multidimensional framework for predicting root function in heterogeneous environments.