Quantification of Soil Organic Carbon Storage and Turnover in Two Mangrove Forests Using Dual Carbon Isotopic Measurements

Abstract

Mangrove ecosystems are highly productive and play an important role in tropical and global coastal carbon (C) budgets. However, soil organic carbon (SOC) storage and turnover in mangrove forests are still poorly understood. Based on C and C isotopic measurements of soil cores from two natural mangrove forests in Southern China, SOC density was 674.41 Mg ha⁻¹ in one forest (site 1, a Aegiceras corniculatum-dominated high tidal flat) and 372.83 Mg ha⁻¹ in the other forest (site 2, a Bruguiera gymnorrhiza + Kandelia candel-dominated middle tidal flat). SOC storage in the upper 100 cm in mangrove forests in China was estimated to be 13.65–24.68 Tg. SOC δ¹³X values in the two mangrove forests ranged from − 29.36 to − 25.90 ‰. SOC δ¹³C was enriched with depth at 20–70 cm at site 1 (which is similar to the trend in most terrestrial ecosystems) but not at site 2, probably because the latter but not the former forest experienced frequent tidal flushing of the surface soil. SOC δ¹³C enrichment with depth at site 1 was not due to depletion of δ¹³C of atmospheric CO₂ by fossil fuel emissions or to the difference between root and leaf ¹³C, but possibly resulted from preferential microbial decomposition; this inference was supported by the Rayleigh distillation model, which also indicated that C was mainly from the parent A. corniculatum forest. C and stable C isotopic measurements indicated that tidal fluctuation greatly impacted SOC deposition in these mangrove forests; the high tidal flat (site 1) and the middle tidal flat (site 2) showed terrestrial and oceanic deposition characteristics, respectively. ¹⁴C from the testing of thermonuclear weapons had penetrated to 50–60 cm in the two forests. SOC turnover time varied with depth. The surface SOC turnover time at both sites was about 0.5 year, because most surface SOC consisted of easily decomposed litter. SOC turnover time at 20–60 cm at site 1 and at 25–50 cm at site 2 was 4.44–26.04 year. Abundant roots apparently accounted for the short SOC turnover times at these middle soil layers.