Variation of soil carbon accumulation across a topographic gradient in a humid subtropical mountain forest

Abstract

Topography is tightly coupled with soil organic carbon (SOC) cycling in sloping landscapes. However, we know little about the spatial variation of SOC accumulation and persistence along a topographic gradient and the controlling processes. Here, we assessed the spatial variation of SOC and its composition along a topographic gradient in a humid mountain forest. The variables associated with environmental factors, topographical traits, carbon input from plants, and soil physico-chemical properties were analyzed to assess their contributions. Results showed that both SOC and mineral-bound organic carbon (MOC) contents were comparable among the topographic positions (ridge, middle slope, lower slope and valley). However, particulate organic carbon (POC) content decreased significantly from ridge to valley. Our measured environmental variables explained 67%, 74% and 77% of the variations in SOC contents for 0–10 cm, 10–20 cm and 20–40 cm soils, respectively. Soil physico-chemical properties (including pH and soil reactive Fe/Al oxides) were the main driver on SOC and MOC variations. In contrast, the variation in POC was more explained by topographical traits and carbon input. We also observed significantly lower SOC stability for ridge soil than valley soil. The significant topographic patterns for SOC fraction and SOC stability suggested that the soil carbon cycling processes were dependent on landscape positions. Future carbon budget and carbon dynamic researches in the humid sloping landscapes should take into account the topographic effects, especially for the free and light carbon fractions.