Distinct mowing effects on organic carbon storage and dynamics in Spartina alterniflora and Phragmites australis sediments

Mowing is widely employed in coastal wetland management to control invasive plants and resource utilization. Although its effects on greenhouse gas emissions are well studied, impact on sediment organic carbon (OC) dynamics remains incomplete. This study investigated a temperate estuarine wetland by analyzing sediments from Spartina alterniflora, Phragmites australis, and mudflats. Sediment total organic carbon (TOC), total nitrogen (TN), labile OC (water extractable OC, WEOC), relatively stable OC (Fe-bound OC, OC-Fe), inorganic nutrients, and lignin phenols were measured to assess mowing effects across vegetation types. Mowing induced distinct impacts on vegetated sediments. In S. alterniflora sediments, frequent mowing (three times in one year with biomass removal) weakened plant activities, led to reductions in clay content and total Fe, decreased but more recalcitrant WEOC pool, fluctuating nutrient levels, and declines in OC-Fe fractions, indicating ongoing OC consumption without fresh inputs. Conversely, P. australis sediments exhibited increased TOC and TN post-mowing, associated with root mortality and litter input, reflected by shifted δ¹³C and δ¹⁵N, elevated C/N ratios, enhanced microbial signals in the dissolved organic matter, and further confirmed by lignin phenols. Notably, OC-Fe_PP (co-precipitated OC-Fe) increased after mowing in P. australis, likely due to the association of plant-derived lignin- and phenolic-rich OC with Fe (hydr)oxides. Overall, mowing reduced the relative contribution of OC-Fe in both species but promoted short-term OC accumulation in P. australis sediments. These findings highlight vegetation-dependent mowing effects on sediment OC pools and underscore the need for careful coastal wetland management to balance blue carbon preservation with ecological sustainability.