Source and stability of soil organic carbon jointly regulate soil carbon pool, but source alteration is more effective in mangrove ecosystem following Spartina alterniflora invasion

Spartina alterniflora invasion seriously affects blue carbon function in mangrove ecosystems. However, the mechanisms by which S. alterniflora invasion affects the sources and stability of soil organic carbon (SOC) in mangrove ecosystems have not been fully elucidated. In this study, stable isotopes and nuclear magnetic resonance spectroscopy techniques were used to investigate the impacts of S. alterniflora invasion on SOC sources, decomposition characteristics, and SOC pool stability in the Zhangjiang Estuary, Fujian Province, China. The results showed that SOC content in the mangrove was 8.25–51.46 g kg−1, which was 21.3 % and 93.4 % higher than those in the S. alterniflora and tidal flat, respectively. Additionally, the SOC storage at depths of 0–30 cm was 26.0 % and 96.9 % higher in mangrove than in S. alterniflora and tidal flats, respectively. The values of soil δ13C and δ15N were lower, whereas the C/N ratio was higher in mangrove than in S. alterniflora. A Bayesian stable isotope mixing model revealed that mangrove plant tissues were the highest contributor to SOC (40.8 ± 18.7 %) in mangrove topsoil because of litter effects. However, in the S. alterniflora and tidal flat ecosystems, SOC was mainly derived from allochthonous sources with particulate organic matter (POM) proportions of 72.1 ± 3.3 % and 56.4 ± 5.3 %, respectively. Alkyl C (16.3–41.4 %) was the dominant functional group in all soils, followed by O-alkyl C and aromatic C. Alkyl C, A/O-A, and hydrophobicity index (HI) were significantly lower in S. alterniflora soil than in mangrove and tidal flat soils. Therefore, the SOC sources and stability, including fractions and chemical composition, were substantially altered in mangrove ecosystems after S. alterniflora invasion, leading to decreases in SOC pools. Nevertheless, source alteration was more effective in mangrove wetland invaded by S. alterniflora. These findings may provide new perspectives regarding blue carbon sources, decomposition, stabilization, and sequestration in mangrove wetlands invaded by S. alterniflora.