Changes in Surface Soil Organic Carbon Fractions and Their Pool Management Indices Along an Altitudinal Gradient in Karst Mountains in Relation to the Expansion Degrees of Chimonobambusa utilis

Soil organic carbon fractions and pool management indices are critical for the ecosystem function of bamboo forests; however, their response to varying degrees of expansion of <i>Chimonobambusa utilis</i> (EDCU) and altitudinal gradients remains poorly understood in high-altitude karst regions. In this study, 225 samples (three replicate soil samples, each with five duplicate samples) were collected from 45 typical soil sites in the Jinfo high-altitude karst mountains, China. This study investigated the effects of three EDCUs (low, moderate, and high expansion) and five altitudinal gradients (1300-1500 m, 1500-1700 m, 1700-900 m, 1900-2100 m, and 2100-2300 m) on root elemental composition, soil properties, soil organic fractions, and pool management indices. The results revealed that root total C, N, R_C:P, and R_N:P decreased with increasing altitude, whereas root total C, N, P, and R_C:N also increased significantly with increasing EDCU. Compared with those at low and moderate EDCU, the POC:SOC (34.12%), HFOC (32.73 g kg^-1), and HFOC:SOC (37.07%) ratios were highest at high EDCU along the altitudinal gradient of 1700-1900 m. Meanwhile, the L (2.38), LI (2.01), and CMI (174.55) ratios reached their highest values at moderate expansion degrees of <i>Chimonobambusa utilis</i> within the altitudinal gradient of 1900-2100 m. Moreover, redundancy discriminant analysis (RDA) and structural equation modeling (SEM) revealed that the soil carbon pool management index was significantly positively associated with soil properties through direct pathways and negatively correlated with root elemental composition through indirect pathways. In general, the quality of the carbon pool in <i>Chimonobambusa utilis</i> is optimal within the moderate expansion degrees of <i>Chimonobambusa utilis</i> within the altitudinal gradient of 1900-2100 m. The findings of this study establish a theoretical basis for the expansion of <i>Chimonobambusa utilis</i> in high-altitude karst regions and provide scientific evidence to support the increase in the carbon sequestration capacity of bamboo forest ecosystems in these mountainous areas.