Soil-specific changes of soil organic carbon and its fractions in a chronosequence of Chinese fir plantations in southern subtropical China

Abstract

Understanding soil-specific organic carbon dynamics is crucial for optimizing carbon sequestration strategies in subtropical plantations. However, the variations in soil organic carbon (SOC) and its fractions in a chronosequence of plantation forest soils across different soil types remain poorly understood. In this study, we examined SOC, particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) in topsoil (0–20 cm) and subsoil (20–100 cm) along a chronosequence of Chinese fir (Cunninghamia lanceolata [Lamb.] Hook) plantations in red and yellow soils of southern subtropical China. Our results revealed a fundamental divergence in carbon stabilization pathways: red soils presented geochemical preservation mediated by precipitation-driven nutrient shifts, while yellow soils showed dominant physical protection. In red soils, SOC, MAOC contents and the MAOC/SOC ratio significantly increased with stand age, driven primarily by geochemical preservation via non-crystalline iron-aluminum oxides (variance explained, VE = 15.8% and 30.2% in the topsoil and subsoil, respectively). Conversely, yellow soils showed a structural-driven pathway, while topsoil SOC and MAOC contents decreased with stand age, whereas subsoil SOC, POC and MAOC contents increased. Additionally, the POC/SOC ratio in both layers exhibited a rising tendency as stand age increased. This preferential accumulation of labile carbon in yellow soils was facilitated by structural improvements, specifically, alleviated compaction (decreased bulk density; VE = 26.6% and 16.2% in the topsoil and subsoil, respectively), enhanced aggregate stability (mean weight diameter; VE = 13.0%), and improved hydrological conditions (soil water content; VE = 38.1%). Our study suggests that forest carbon management must be soil-dependent, focusing on geochemical stabilization in red soils and physical protection in yellow soils to effectively stabilize SOC.

Keywords

Chinese fir plantations / Soil type / Stand age / Soil organic carbon / Carbon fraction

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