Contribution of belowground plant components to salt marsh soil volume

Abstract

Accumulation of belowground plant material (live roots, rhizomes, and dead material) is critical to vertical accretion of tidal salt marsh soils and their ability to maintain elevation with respect to sea-level rise. Many studies utilize a single, global relationship between soil organic carbon and belowground volume, which does not include the pore space created by belowground growth, thus does not appropriately reflect the contribution of vegetation to soil accretion. We develop factors that relate live belowground mass to the volume of live roots and rhizomes for four species in a St. Lawrence estuary salt marsh at la Pocatière, Quebec. Spartina alterniflora, Spartina patens, and invasive Phragmites australis were identified in the aboveground growth, while only tubers and roots of Cyperus esculentus were found. Cores of the top 30 cm of soil were collected and cut into 0–15 and 15–30 cm sections from the low marsh, high marsh, and S. patens marsh invaded by P. australis. The soil was washed over a 1 mm sieve, and the organic matter was retained. The live material was sorted into fractions of roots and rhizomes by species. The volume of all components was measured by displacement in water before drying to obtain mass. An overall conversion factor of 8.2 g cm−3 reflects the volumetric contribution of live belowground plant components of all species studied. The relationship varies by species, with S. alterniflora contributing the greatest volume per unit mass (8.9 g cm−3), followed by S. patens (7.9 g cm−3), then P. australis and C. esculentus, both at 5.6 g cm−3. Our results not only demonstrate that the total mass and volume of each species for each component (rhizomes, roots, and dead material) were strongly correlated but that measurements of carbon density underestimate the true volumetric contributions of salt marsh plants to the soil.