Document: SHA-3-40-56

Differences in carbon isotope values between above and belowground plant material for 15 semidesert grassland species.

BIEDENBENDER, S.H.* ¹, M.P.MCCLARAN ², J.QUADE ² and M.A.WELTZ ³

USDA-ARS, Tucson, AZ 85719 USA ¹
University of Arizona , Tucson, AZ USA ²
USDA-ARS, Fort Collins, CO USA ³

Abstract:
Consistent lifeform-specific differences in the magnitude of the range in d¹³ values between above and belowground plant material would have serious implications for the interpretation of the commonly observed pattern of less negative d¹³C values of soil organic carbon with increasing soil depth, and the reconstruction of the contributions of C₃ and C₄ components of past vegetation. In this study, the d¹³C values for above and belowground plant biomass were compared for 15 semidesert grassland species. The d¹³C values for belowground material were significantly less negative than the aboveground material for nearly 50% of the species, but only 1 of 5 grass species had different values, compared to 6 of 8 C₃ woody species. The direction of the difference (belowground was less negative) was consistent with the limited number of previous reports, and the magnitude of the difference (1-3) was also largely consistent for the C₃ shrubs and forbs, but our result for the C₄ grasses was smaller than expected from previous reports. Based on our results, where belowground material was 1-3 less negative than aboveground material for C₃ shrubs, but only 0.5 less negative for C₄ grasses, commonly used mixing formulas based on aboveground d¹³C values would overestimate the contribution of C₄ grasses to past vegetation. However, more field research examining the d¹³C values of soil organic carbon must be completed to confirm the life-form specific variation found in the plant material reported in this study.

Keywords: 13C natural abundance, soil organic matter, vegetation change, Semidesert Grassland, above/belowground biomass

This abstract is being presented at: 8:45 AM in session:
Oral Session #9: Respiration and Isotopes.