ABSTRACT Afforestation has been well documented to enhance soil organic carbon (SOC) pools. However, the capacity of soils to mitigate global warming depends critically on SOC persistence, which is fundamentally governed by the accumulation of non‐labile SOC fractions following afforestation. Here, we employed physical fractionation of organic carbon via size/density separation. SOC and its two functionally distinct fractions, particulate organic carbon (POC) and mineral‐associated organic carbon (MAOC), were quantified in corn–soybean rotation fields and pine plantation forests located in two geological backgrounds: clasolite and limestone areas in southwest Guangxi Province, China. The plantation forests were established in former corn–soybean fields 30–40 years ago. Bulk SOC and MAOC contents in croplands were 74.8% and 86.3% higher, respectively, in limestone areas than in clasolite areas, whereas SOC and its fractions in forests showed no significant differences between the two lithological types. In limestone areas, SOC, POC and MAOC did not differ significantly between croplands and forests, whereas in clasolite areas, these fractions increased by 41.3%, 139.1% and 37.7%, respectively, after afforestation. The proportions of MAOC in croplands constituted 95.7% and 94.0% of bulk SOC in limestone and clasolite areas, respectively, but were reduced by 4.9% and 4.5% following afforestation. These results suggest that afforestation reduces SOC stability by decreasing the proportion of MAOC. Incorporating the effects of afforestation on SOC stability into Earth system models is essential for improving predictions of climate‐carbon cycle feedback, particularly given the potential prevalence of our observed pattern.
Zheng et al. (Thu,) studied this question.