Belowground plant material and its contribution to soil volume in a St. Lawrence salt marsh, QC, Canada

Abstract

El material vegetal subterráneo (raíces, rizomas y necromasa) es fundamental para la acreción vertical de marismas y para mantener su elevación a la par del nivel del mar. Muchos estudios utilizan una única relación global entre el C orgánico del suelo y el volumen subterráneo, sin incluir el espacio poroso creado por el crecimiento vegetal subterráneo, no reflejando adecuadamente su contribución a la acreción del suelo. Esta tesis presenta los resultados de una investigación que pretende resolver estos vacíos. El capítulo 1 es una revisión bibliográfica que describe la vegetación de las marismas típicas del Atlántico Noroeste. El capítulo 2 está redactado como manuscrito en donde se obtiene un factor de conversión global de 8,2 g cm⁻³ que refleja la contribución volumétrica del material vegetal subterráneo de todas las especies estudiadas, además dicha relación varía según especie. Esto demuestra que la masa total y el volumen de cada especie para cada componente están fuertemente correlacionados, y las mediciones de la densidad simple del C subestiman la contribución real de las plantas de las marismas al volumen del suelo. El capítulo 3 se presentan las conclusiones, limitaciones del estudio y se identifican ciertos criterios que pueden ser tomados para futuras investigaciones.

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. This thesis reports on research that seeks to address this gap. Chapter 1 is an introduction and literature review that describes tidal marshes vegetation typical of Northwest Atlantic tidal salt marshes. This chapter describes the importance of living biomass (roots and rhizomes) and dead material of the dominant marsh species in soil vertical accretion for marsh resiliency to keep up with sea-level changes. Chapter 2 is written in manuscript style including methods and results of a study of the relationship of the volume of live roots and rhizomes to their dry biomass. The study focuses on four species in a Quebec salt marsh. The species, 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 S. alterniflora –dominated low marsh, the S. patens–dominated high marsh, and the upper 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 roots, rhizomes, and organic matter (OM) 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. The results of this thesis not only demonstrate that the total mass and volume of each species for each component (rhizomes, roots, and dead material) are strongly correlated but that measurements of simple carbon density underestimate the actual contribution of salt marsh plants to the soil volume. Chapter 3 presents final conclusions, limitations of the study reported on in Chapter 2, and identifies directions that would be valuable for future research.