Vertical distributions of organic carbon fractions under paddy and forest soils derived from black shales: Implications for potential of long-term carbon storage

Abstract Black shales are characterized by a high content of organic carbon (C). Few studies have focused on the influence of land use on soil organic C (SOC) fractions from soils derived from black shale (black shale soils). The objective of this study was to elucidate the influence of land use on SOC fractions in black shale soils combining chemical determination and stable C isotope analysis techniques. Herein, we determined labile organic C (LOC), semi-labile organic C (Semi-LOC), and recalcitrant organic C (ROC) fractions in various depths of soils in paddy fields (0–70 cm) and forests (0–120 cm) from black shale distribution region in Hunan province, China, and then investigated δ13C values of these soils. Results showed that the contents of LOC, Semi-LOC, and ROC in paddy soils (1.63–7.35 g kg−1, 0.35–1.21 g kg−1, and 3.75–14.8 g kg−1, respectively) and forest soils (0.73–4.94 g kg−1, 0.12–0.89 g kg−1, and 1.44–8.96 g kg–1, respectively) are significantly decreased with increasing depth. The contribution made by LOC to SOC in paddy soils was significantly lower than that in forest soils, while the contribution made by ROC to SOC was significantly higher in paddy soils than that in forest soils. In these two land uses, the δ13C values were higher in SOC compared to the ROC fraction, while the δ13C values were close in the ROC fraction below 20 cm soil depth. Our study indicated that i) new C is mainly limited to the surface soil layer (0–10 cm) in forests, while it can be leached along the soil profiles in paddy fields; ii) the estimated ROC pool is ~900 Pg within the 0–100 cm soil layer in terrestrial ecosystems, which should better represent the ability of soil C sequestration.