Spartina alterniflora invasion alters soil bulk density in coastal wetlands of China

Abstract Although empirical studies show that invasive Spartina alterniflora Loisel. ( S. alterniflora ) has reduced soil bulk density (BD), the mechanisms that underpin this response are still unclear. To explore this pattern, we used a space‐for‐time substitution approach and collected soil samples from the eastern coast of China, where S. alterniflora ecosystems were established a short period of time after initial invasion. We used structural equation modelling to test the hypothesis that S. alterniflora invasion has an indirect influence on BD through the interacting effects of soil and plant variables, including plant productivity, soil organic carbon (SOC), total nitrogen (TN), pH, salinity, sand, and clay content. These variables, which are associated with S. alterniflora invasion, were assumed to represent multiple physicochemical and biological processes. The model showed that these processes explain 82% of the BD variation in response to S. alterniflora invasion. We observed that SOC had the most direct influence on BD, followed by salinity and soil sand content. We also found that S. alterniflora invasion had an important direct influence on the measured variables. Although plant productivity was not directly related to BD, it was indirectly associated with the impacts of plant productivity on salinity. Our results highlight the occurrence of BD variability within an interacting system of soil and plants wherein S. alterniflora invasion leads to a decrease in BD. This work also highlights that identifying the mechanisms driving BD variability in response to S. alterniflora invasion would help to improve our understanding of soil development in S. alterniflora marshes.