Organic Carbon Storage and 14C Apparent Age of Upland and Riparian Soils in a Montane Subtropical Moist Forest of Southwestern China

Upland and riparian soils usually differ in soil texture and moisture conditions, thus, likely varying in carbon storage and turnover time. However, few studies have differentiated their functions on the storage of soil organic carbon (SOC) in sub-tropical broad-leaved evergreen forests. In this study, we aim to uncover the SOC storage and ¹⁴C apparent age, in the upland and riparian soils of a primary evergreen broad-leaved montane subtropical moist forest in the Ailao Mountains of southwestern China. We sampled the upland and riparian soils along four soil profiles down to the parent material at regular intervals from two local representative watersheds, and determined SOC concentrations, δ¹³C values and ¹⁴C apparent ages. We found that SOC concentration decreased exponentially and ¹⁴C apparent age increased linearly with soil depth in the four soil profiles. Although, soil depth was deeper in the upland soil profiles than the riparian soil profiles, the weighted mean SOC concentration was significantly greater in the riparian soil (25.7 ± 3.9 g/kg) than the upland soil (19.7 ± 2.3 g/kg), but has an equal total SOC content per unit of ground area around 21 kg/m² in the two different type soils. SOC δ¹³C values varied between -23.7 (±0.8)‰ and -33.2 (±0.2)‰ in the two upland soil profiles and between -25.5 (±0.4)‰ and -36.8 (±0.4)‰ along the two riparian soil profiles, with greater variation in the riparian soil profiles than the upland soil profiles. The slope of increase in SOC ¹⁴C apparent age along soil depth in the riparian soil profiles was greater than in the upland soil profiles. The oldest apparent age of SOC ¹⁴C was 23,260 (±230) years BP (before present, i.e., 1950) in the riparian soil profiles and 19,045 (±150) years BP in the upland soil profiles. Our data suggest that the decomposition of SOC is slower in the riparian soil than in the upland soil, and the increased SOC loss in the upland soil from deforestation may partially be compensated by the deposition of the eroded upland SOC in the riparian area, as an under-appreciated carbon sink.