Soil is the largest carbon pool in terrestrial ecosystems. In areas with complex terrain, the topographic position can modify the hydrologic conditions and other biophysical variables within ecosystems, which then lead to the spatial pattern of soil organic carbon (SOC) cycling. Understanding SOC dynamics along topographic gradients are therefore important to the accurate estimation of carbon budget in mountain ecosystem.   

In order to identify the topographic effects on soil carbon cycling, the Global Change Biology Group led by Prof. LIU Feng of Wuhan Botanical Garden investigated the spatial variation of SOC content and its composition along topographic transects (ridge, middle slope, lower slope and valley) in a humid mountain forest and assessed the driving factors of the variations.   

Plant distribution showed a clear spatial pattern in this humid subtropical forest. But, SOC and mineral-bound organic carbon (MOC) were comparable among the topographic positions. In contrast, particular organic carbon (POC) content and SOC stability varied significantly from ridge to valley. The lack of topographic patterns for SOC and MOC contents was probably caused by the similar soil properties (including pH and soil minerals) along the toposequence. The highest POC content in the ridge was mainly associated with the high carbon input from vegetation and the lowest transport with surface runoff.  

These results further emphasized topography could regulate the processes of soil carbon accumulation. Future carbon budget and dynamics studies in the humid forest landscapes should take into account the topographic effects, especially on the free and light carbon fractions.   

This work was supported by the National Natural Science Foundation of China. Relevant research results have been published in Biogeochemistry entitled “Variation of soil carbon accumulation across a topographic gradient in a humid subtropical mountain forest”. 

Variation partitioning results of SOC, POC, and MOC in 0-10 cm, 10-20 cm, and 20-40 cm depths against the variables of topography, carbon input, and soil properties. Values <0 were not shown. Significant values (p < 0.05) after 999 permutations are indicated with asterisk. Re meant the unexplained variation (Image by WBG)