# 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

## TL;DR

This study explores how the expansion of a bamboo species and altitude affect soil carbon in karst mountains, offering insights for sustainable forest management.

## Contribution

The study reveals how bamboo expansion and altitude influence soil carbon fractions and management indices in high-altitude karst regions.

## Key findings

- Soil carbon pool quality is optimal during moderate bamboo expansion at 1900–2100 m altitude.
- Higher bamboo expansion increases certain soil organic carbon fractions like POC:SOC and HFOC:SOC.
- Carbon and phosphorus limitations vary with bamboo expansion and altitude, affecting soil properties.

## Abstract

This study investigated the effects of the degrees of expansion of Chimonobambusa utilis (EDCU) and altitudinal gradients on root elemental composition, soil properties, soil organic fractions, and pool management indices. The results demonstrated in this study showed the effects of expansion degrees of Chimonobambusa utilis and altitudal gradients on root elemental composition, soil properties, soil organic carbon fractions, and carbon pool management indices. The results showed that the carbon pool management index was positively correlated with soil properties and carbon fractions but negatively correlated with root elemental composition. Overall, reducing limitations on root-mediated organic carbon inputs along altitudinal gradients may enhance the adaptability of Chimonobambusa utilis to nutrient-poor environments. Moreover, soil carbon pool quality reached optimal levels during moderate expansion phases within the 1900–2100 m altitudinal gradient. These findings provide a theoretical foundation for understanding the carbon sequestration potential of Chimonobambusa utilis expansion in high-altitude karst regions. Soil enzyme activity and stoichiometric ratios are indirectly influenced by soil bulk density and root characteristics. Overall, both carbon (C) and phosphorus (P) limitations were observed at the lowest MRB level (<20%), whereas higher MRB levels exhibited P limitation exclusively, with no significant C limitation. These findings provide a scientific basis for promoting green and sustainable management practices in bamboo-invaded broad-leaved forest ecosystems.

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 Chimonobambusa utilis (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, RC:P, and RN:P decreased with increasing altitude, whereas root total C, N, P, and RC: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 Chimonobambusa utilis 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 Chimonobambusa utilis is optimal within the moderate expansion degrees of Chimonobambusa utilis within the altitudinal gradient of 1900–2100 m. The findings of this study establish a theoretical basis for the expansion of Chimonobambusa utilis 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.