Differential effects of Phragmites australis versus Spartina alterniflora biochar on salt marsh soil improvement and carbon stabilization

Biochar is an effective strategy for improving physicochemical properties of coastal salt marsh soil and enhancing carbon stabilization. In this study, Spartina alterniflora ( S . alterniflora ) and Phragmites australis ( P . australis ) were used as biochar materials. Four pyrolysis temperatures (350 °C, 450 °C, 550 °C, and 650 °C) and three addition amounts (1%, 2%, and 3%) were applied to explore the effects of the two biochars on the physicochemical properties and carbon components of salt marsh soil. The results showed that both S. alterniflora biochar (SBC) and P . australis biochar (PBC) significantly increased soil pH, cation exchange capacity, and nutrients (SBC: 5.3%–188.6%; PBC: 1.4%–200.9%). The addition of biochar at 3% significantly increased soil total nitrogen (TN), Nitrate nitrogen (NO 3 − ), and available phosphorus (AP) contents, thereby effectively enhancing soil nutrient supply capacity. The addition of SBC and PBC altered the composition of soil organic carbon (SOC), significantly increasing the proportion of mineral-associated organic carbon and thereby promoting SOC stability, particularly under high addition amounts and pyrolysis temperatures. These findings suggest that both SBC and PBC hold promise for improving soil carbon stabilization in coastal salt marsh soils. This study provides a valuable approach for saline–alkali soil remediation in coastal wetlands and offers a potential pathway for the resource utilization of S. alterniflora and P . australis .