# Beyond Invasion: How Phragmites australis Modifies Soil Architecture and Carbon Storage in Long Island Sound Salt Marshes

## TL;DR

Invasive Phragmites australis changes soil structure in salt marshes but does not consistently improve carbon storage compared to native plants.

## Contribution

The study reveals that Phragmites australis creates uneven carbon storage, challenging the belief that it enhances carbon sequestration.

## Key findings

- Phragmites australis soils had higher and more variable bulk density but no consistent carbon storage advantage.
- Phragmites australis caused heterogeneous carbon hotspots, unlike the uniform distribution in native stands.
- Higher soil density from Phragmites may improve marsh resilience but reduces reliable carbon sequestration.

## Abstract

This study investigated soil total organic carbon stocks and physical properties under invasive Phragmites australis and native Sporobolus alterniflorus (formerly Spartina alterniflora) in southern Connecticut tidal marshes. The findings challenge the assumption that P. australis invasion necessarily increases carbon sequestration. While P. australis soils consistently exhibited significantly higher and more variable bulk density and dry weight, this did not result in a consistent carbon storage advantage. Notably, P. australis created highly heterogeneous localized concentrations of carbon compared to the more consistent distribution in native stands. We conclude that while P. australis provides a denser soil structure that may increase resilience against sea-level rise, its complex impact on carbon dynamics may reduce the reliability of these marshes as consistent blue carbon sinks. Management strategies must weigh the benefit of geomorphic stability against the loss of uniform carbon sequestration and native biodiversity.

Coastal salt marshes are essential for climate change mitigation due to their high carbon storage capacity, which is influenced by soil type, hydrology, and floristic composition. Over the past century, invasive Phragmites australis has displaced native Sporobolus alterniflorus (formerly Spartina alterniflora) across salt marshes on the Long Island Sound, and it is widely hypothesized that its larger biomass and rapid growth enhance soil carbon sequestration. This study tested that hypothesis by comparing total organic carbon stocks and physical soil properties in two southern Connecticut marshes over multiple seasons. Our results show that mean soil bulk density was significantly higher under P. australis than S. alterniflorus at both locations. However, this did not translate to superior carbon storage. Analysis showed a significant seasonal effect but no significant overall difference in median TOC between species, indicating that P. australis is competitive in total mass only due to its higher soil density. Notably, Levene’s test for homogeneity of variance was significant (p = 0.039), revealing that P. australis creates highly heterogeneous “hot spots” of carbon storage compared to the relatively uniform distribution found in native stands. These findings suggest that while P. australis invasion results in a more physically dense and potentially resilient marsh platform—relevant for surviving sea-level rise and filtering nutrient runoff—it may simultaneously compromise the stability and uniformity of regional carbon sinks. Management strategies should consider these tradeoffs when prioritizing the protection of native S. alterniflorus for consistent carbon sequestration.