Using biogeochemical tracing and ecohydrological monitoring to increase understanding of water, sediment and carbon dynamics across dryland vegetation transitions

Using biogeochemical tracing and ecohydrological monitoring to increaseunderstanding of water, sediment and carbon dynamics across drylandvegetation transitionsAlan Puttock (1), Jennifer Dungait (2), Kit Macleod (3), Roland Bol (4), and Richard Brazier (1)(1) University of Exeter, CLES, Geography, Exeter, UK (ap267@ex.ac.uk), (2) Sustainable Soils and Grassland SystemsDepartment Rothamsted Research-North Wyke Okehampton Devon, UK, (3) The James Hutton Institute Craigiebuckler,Aberdeen, AB15 8QH, Scotland, UK, (4) Forschungszentrum Juelich GmbH 52425 Juelich Sitz der Gesellschaft: Juelich,GermanyDrylands worldwide have experienced rapid and extensive environmental change, which across large areas hasbeen characterised by the encroachment of woody vegetation into grasslands. Woody encroachment leads tochanges in the abiotic and biotic structure and function of dryland ecosystems and has been shown to result inaccelerated soil erosion and loss of soil nutrients.The relationship between environmental change, soil erosion and the carbon cycle in dryland environmentsremains uncertain. Covering over 40 % of the terrestrial land surface, dryland environments are of significantglobal importance, both as a habitat and a soil carbon store. Thus, there is a clear need to further our understandingof dryland vegetation change and impacts on carbon dynamics. Here, grama grass to creosote shrub and gramagrass to piñon-juniper woodland; two grass-to-woody ecotones that occur across large swathes of the semi-aridSouthwestern United States are investigated.This study combines an ecohydrological monitoring framework with a multi-proxy biogeochemical approachusing stable carbon isotope and n-alkane lipid biomarkers to trace the source of organic carbon. Resultswill be presented showing that following woody encroachment into grasslands, there is a transition to a moreheterogeneous ecosystem structure and an increased hydrological connectivity. Consequentially, not only dodrylands lose significantly more soil and organic carbon via accentuated fluvial erosion, but this includessignificant amounts of legacy organic carbon which would previously have been stable under the previous grasscover. Results suggest that dryland soils may therefore, not act as a stable organic carbon pool and that acceleratedfluvial erosion of carbon, driven by vegetation change, has important implications for the global carbon cycle