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University of Utah (2004)

BROMUS TECTORUM IMPACTS SOIL CARBON STORAGE IN SEMIARID GRASSLANDS OF CANYONLANDS NATIONAL PARK

Englund Sylvia Ruth

Titre : BROMUS TECTORUM IMPACTS SOIL CARBON STORAGE IN SEMIARID GRASSLANDS OF CANYONLANDS NATIONAL PARK

Auteur : Englund Sylvia Ruth

Université de soutenance : University of Utah

Grade : Master of Science (MS) 2004

Résumé
Invasive species reduce biodiversity, threaten native species, and can change the underlying functioning of ecosystems by modifying their environment. Bromus tectorum, an invasive annual grass, has invaded semiarid grasslands in Southern Utah, which were historically vegetated by perennial grasses such as Hilaria jamesii. Because Bromus tectorum is a short-lived annual grass with variable year-to-year productivity, inputs of plant material into the soil organic carbon (SOC) pool were expected to be different than they are in native grasslands. This thesis examines the consequences of Bromus tectorum on SOC.
Uninvaded Hilaria jamesii grasslands and mixed Hilaria jamesii-Bromus tectorum grasslands had significantly more SOC than heavily invaded Bromus tectorum grasslands (means = 0.21, 0.19, and 0.13 kg/m 2 , respectively, in the top 70 cm soil). This difference was largely driven by the accumulation of soil organic carbon directly under Hilaria jamesii plants, which were not present on heavily invaded sites. SOC was an order of magnitude lower than the average reported for other desert biomes, There were significant differences in root biomass between Hilaria, HilariaBromus and Bromus sites (means = 0.46, 0.26, and 0.07 kg/m 2 in the top 70 cm soil), reflecting the difference in belowground inputs to SOC in a year of low Bromus tectorum productivity. Multiple regression analysis showed that whereas an increase in perennial cover resulted in increased in root biomass and soil organic carbon storage, an increase in annual cover resulted in a decrease in these parameters. This suggests that a shift in vegetation from perennial to annual grasses, which have less allocation of resources belowground, can reduce SOC and can affect nutrient cycling as well as other ecosystem properties. Since Bromus tectorum has C3 photosynthesis and Hilaria jamesii C4 photosynthesis, I was able to use carbon isotopes to quantify roots and SOC derived from either the invasive or the native species. Independent contrasts showed that the isotopic values of Bromus tectorum –invaded soils were lower than uninvaded soils. The significant changes in δ 13 C of SOC with invasion showed that soil carbon turnover is rapid in these grasslands.

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