Changes in sediment nutrients following Spartina alterniflora invasion in a subtropical estuarine wetland, China

Invasion of the exotic cordgrass, Spartina alternifiora, has become a global crisis and its spread is leading to significant alterations in the functions of ecosystems. However, alteration in nutrient levels caused by invasion of non-native plant species is poorly understood, and the effects of invasive plants on sediment nutrients found in previous studies are contradictory. In this study, we hypothesized that an exotic plant species invasion will change the chemical characteristics of an ecosystem over time. To test this hypothesis, the spatial distribution of S. alternifiora in 2006, 2010 and 2014 was analyzed by aerial images, Google Earth satellite images and field investigations. Furthermore, according to the space time reciprocal principle, a primary field survey with three locations (S1, S2 and S4) was conducted in 2014, and sediment nutrient levels, including total carbon (TC), total nitrogen (TN) and total phosphorus (TP) were quantified to investigate the invasive effects of S. alternifiora. We also investigated six locations (S1 S6) in 2016 in the same manner to compare the changes in sediment nutrients between the two years. The area infested with S. alternifiora increased from 10.8 ha in 2006, to 26.2 ha in 2010, and 37.5 ha in 2014, with a 35.6% annual expansion rate between 2006 and 2010, and a 10.8% annual expansion rate between 2010 and 2014, respectively. Considering the two field survey results, we found that S. alternifiora invasion increased sediment nutrient levels in the early-term (i.e., S1-S4 period, approximately 10 years), and then significantly decreased TC, TN and TP content to levels even lower than bare beach. Analyses of stoichiometry characteristics of the sediment nutrients also verified these two apparent stages. The results showed that early-term invasion of S. alternifiora and subsequent invasion alter sediment nutrients differently, and these results may explain the seemingly contradictory increases and decreases in nutrient levels reported in previous studies. Our results provide important information that furthers our understanding of the impact of S. alterniflora invasion.