title: Stable carbon isotope analysis of fluvial sediment fluxes over two contrasting C(4) -C(3)  semi-arid vegetation transitions.
abstract: BACKGROUND
  Globally, many drylands are experiencing the encroachment of woody vegetation into grasslands. These changes in ecosystem structure and processes can result in increased sediment and nutrient fluxes due to fluvial erosion. As these changes are often accompanied by a shift from C(4) to C(3) vegetation with characteristic δ(13) C values, stable isotope analysis provides a promising mechanism for tracing these fluxes.
  
  
  METHODS
  Input vegetation, surface sediment and fluvially eroded sediment samples were collected across two contrasting C(4) -C(3) dryland vegetation transitions in New Mexico, USA. Isotope ratio mass spectrometric analyses were performed using a Carlo Erba NA2000 analyser interfaced to a SerCon 20-22 isotope ratio mass spectrometer to determine bulk δ(13) C values.
  
  
  RESULTS
  Stable isotope analyses of contemporary input vegetation and surface sediments over the monitored transitions showed significant differences (p <0.05) in the bulk δ(13) C values of C(4) Bouteloua sp. (grama) grassland, C(3) Larrea tridentata (creosote) shrubland and C(3) Pinus edulis/Juniperus monosperma (piñon-juniper) woodland sites. Significantly, this distinctive δ(13) C value was maintained in the bulk δ(13) C values of fluvially eroded sediment from each of the sites, with no significant variation between surface sediment and eroded sediment values.
  
  
  CONCLUSIONS
  The significant differences in bulk δ(13) C values between sites were dependent on vegetation input. Importantly, these values were robustly expressed in fluvially eroded sediments, suggesting that stable isotope analysis is suitable for tracing sediment fluxes. Due to the prevalent nature of these dryland vegetation transitions in the USA and globally, further development of stable isotope ratio mass spectrometry has provided a valuable tool for enhanced understanding of functional changes in these ecosystems.
authors.affiliation: Geography, College of Life and Environmental Sciences, University of Exeter, Exeter, EX4 4RJ, UK. ap267@ex.ac.uk
authors.forename: Alan
authors.initials: A
authors.lastname: Puttock
authors.forename: Jennifer A J
authors.initials: JA
authors.lastname: Dungait
authors.forename: Roland
authors.initials: R
authors.lastname: Bol
authors.forename: Elizabeth R
authors.initials: ER
authors.lastname: Dixon
authors.forename: Christopher J A
authors.initials: CJ
authors.lastname: Macleod
authors.forename: Richard E
authors.initials: RE
authors.lastname: Brazier
mesh_terms: D002244:Carbon / Q000032:analysis*; D002247:Carbon Isotopes / Q000032:analysis; D017753:Ecosystem; D019015:Geologic Sediments / Q000032:analysis*; D018653:Juniperus / Q000378:metabolism*; D032442:Larrea / Q000378:metabolism*; D013058:Mass Spectrometry; D009516:New Mexico; D028223:Pinus / Q000378:metabolism*; D006109:Poaceae / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D002244:Carbon