Research on the Ecological Stoichiometry Characteristics of Root Systems and Soil Carbon, Nitrogen, and Phosphorus in the Broadleaf Korean Pine Forest of Northeast China

Ecological stoichiometry is a key indicator for assessing plant nutrient use and biogeochemical cycling in ecosystems. Here, we investigated natural broadleaved–Korean pine (Pinus koraiensis) forests in Northeast China, measuring C, N, and P concentrations in Korean pine roots (coarse and fine) and in soils at 0–20 cm and 20–40 cm to characterize root–soil stoichiometric patterns. The results show that: (1) fine roots (479.3 ± 21.6 g kg⁻¹) had lower C than coarse roots (515.9 ± 13.8 g kg⁻¹), whereas their N (10.71 ± 3.14 g kg⁻¹) and P (1.06 ± 0.33 g kg⁻¹) were 2.3 × and 1.9 × those of coarse roots (N: 4.61 ± 0.96 g kg⁻¹; P: 0.57 ± 0.14 g kg⁻¹). With decreasing latitude, root C:N generally declined; C:P exhibited a high–low–high pattern along latitude, while N:P increased progressively. The coarse- and fine-root C:N:P ratios were 905:8:1 and 452:10:1, respectively. (2) Average soil C, N, and P concentrations at 0–20 cm and 20–40 cm were 80.45 ± 15.97 g kg⁻¹, 5.59 ± 0.83 g kg⁻¹, 0.83 ± 0.25 g kg⁻¹ and 44.28 ± 17.93 g kg⁻¹, 3.24 ± 0.89 g kg⁻¹, 0.67 ± 0.26 g kg⁻¹, respectively. Soil C:P and N:P showed highly consistent spatial variation (y = 0.0526x + 1.7352, R² = 0.7984; y = 0.0534x + 1.498, R² = 0.8348). (3) Climate, soil, and biotic factors explained 89.3% and 94.8% of the variation in fine- and coarse-root nutrient traits, respectively. Mean annual temperature, mean January temperature, and mean annual precipitation significantly influenced root N and N:P; soil P significantly affected fine-root C, N, P and the ratios C:N and N:P; stand density significantly affected root C, N, and C:N. Integrating root–soil stoichiometry indicates that these broadleaved–Korean pine forests are overall N-limited in Northeast China. These findings provide guidance for the sustainable management of regional forests.