Soil Organic Carbon Deficits from Converting Primary Forests to Plantations and Secondary Forests in Subtropical China

Soil organic carbon (SOC) plays a critical role in supporting key ecosystem functions and services to humanity. Despite the widespread conversion of primary forests to plantations and secondary forests, how it affects SOC content and distribution of SOC remains uncertain. We evaluated SOC content, its components in two fractions, particulate organic carbon (a mean residence time shorter than decades) and, mineral-associated organic carbon (a mean residence time of multiple decades to centuries) and the fine root characteristics (biomass and necromass) across Chinese fir plantations and secondary forests during stand development (5–41 years old) in subtropical China, employing primary forests as a reference. We found that SOC content differed significantly between forest types and soil depths, but not with stand age within plantations or secondary forests. In topsoil (0-10cm), SOC was on average 32.8 mg g -1 in primary forests, 27.8 mg g -1 in secondary forests and 22.3 mg g -1 soil in plantations, i.e., primary forests had 47% higher and secondary forests had 25% higher SOC contents than Chinese fir plantations. SOC content in the subsoil (40-60 cm) was approximately 6 mg C g -1 and did not change significantly with forest types or stand ages. The higher topsoil SOC in primary forests and secondary forests resulted primarily from particulate organic carbon preservation caused by higher fine root biomass. The fraction of mineral-associated organic carbon to SOC increased with soil depth and was higher in plantations than that of secondary and primary forests in topsoil, which was associated with a higher amount of fine root necromass that helps form mineral-associated organic carbon. Despite 40-year stand development in plantations and secondary forests, SOC content nor its fractions changed with stand age. Our results reveal persistent SOC deficits from converting primary forests to plantations and secondary forests and highlight the urgent needs to conserve remaining primary forests and increase forest ages by minimizing forest disturbances.