Impacts of Age and Expansion Direction of Invasive Spartina alterniflora on Soil Organic Carbon Dynamics in Coastal Salt Marshes Along Eastern China

Invasion of Spartina alterniflora has been reported to modify carbon (C) cycling processes and pools of the native salt marsh ecosystems. However, it remains unclear how the invasion age and expansion direction of S. alterniflora impact the soil organic carbon (SOC) dynamics. In this study, by considering the continuous invasion of S. alterniflora into mudflat (seaward) and native species Suaeda salsa (landward) salt marshes over a period of 20 years chronosequence, we quantify their impacts on SOC dynamics along the coastal salt marshes of Yancheng National Nature Reserve of China, based on the extensive field sampling and remote sensing data. The results showed that (1) S. alterniflora invasion increased SOC concentration and density, which averaged 8.39 g C kg−1 and 0.0085 g C cm−3 seaward and 6.26 g C kg−1 and 0.0065 g C cm−3 landward, respectively; (2) soil organic carbon showed a significantly positive relationship with total nitrogen (TN), salinity, and water content and a negative relationship with pH and bulk density for S. alterniflora landward expansion. For seaward expansion, it showed a similar relationship, but there are higher correlations with soil properties except an insignificant relationship with pH; and (3) soil organic carbon and carbon accumulation rate are significantly higher in seaward than in landward. However, for landward expansion, soil organic carbon concentration keeps increasing linearly with invasion time, while for seaward expansion, it increased to a maximum in 11 years of invasion and then decreased. Our results showed that it is very important to assess the effects of invasion on SOC dynamics in salt marsh ecosystems by considering both the expansion direction and invasion age.