Tidal salt marshes are one of the most effective natural habitats for sequestering carbon. Yet, we know little about how this important ecosystem service is impacted by nutrient pollution and invasion by non-native plants. In salt marshes, 80% of the carbon is stored in soil and below-ground decomposing organic material. Changes in soil carbon develop over decades. Therefore, to understand the potential impact of invasion and nutrient pollution, it is necessary to assess both changes to-date in carbon standing stock as well as changes in the underlying processes – especially below-ground decomposition. We conducted two field experiments in San Francisco Bay, CA, USA in a salt marsh invaded by Lepidium latifolium (perennial pepperweed). Both experiments were carried out in paired native and invaded plots, half of which were treated monthly with nitrogen and phosphorus fertilizer. First, we measured carbon standing stock. Above-ground biomass and litter were collected in ½m2plots. Below-ground biomass was collected at 3 depth ranges (0-10cm, 10-20cm, and 20-30cm). Soil cores were taken in 2cm increments to a depth of 30cm. All samples were analyzed for C and N content. Second, we conducted a 2-year experiment to track rates of below-ground decomposition using temporally replicated litter bags.
Our carbon storage experiment shows nutrient addition increases both above- and below-ground biomass, with corresponding increases in those carbon pools. In contrast, invasion by L. latifolium decreases both above- and below-ground biomass. We found differences in soil carbon content between native and invaded plots and this relationship varies with depth. In the decomposition experiment, we found invasive L. latifolium’s roots decompose dramatically faster than those of the dominant native plant, Sarcocornia pacifica (pickleweed). Additionally, roots in invaded plots decomposed faster than roots in native plots. Nitrogen and phosphorus addition notably increased rates of decomposition of both native and invasive root tissue. Additionally, deep roots (30cm) decomposed more slowly than shallow roots (10cm). Overall, invasion by L. latifolium increased rates of decomposition and decreased biomass, thereby decreasing carbon storage in salt marshes. Nutrient addition, on the other hand, increased rates of decomposition but also increased biomass. Therefore, determining the overall impact of nutrient addition on salt marsh carbon storage will require carbon balance modeling. These results show elevated nutrients and invasion by non-native plants have important implications for blue carbon in salt marsh ecosystems, and need to be a future focus for both wetland management and development of blue carbon policy.