Wednesday, August 8, 2007

PS 40-6: Spatial patterns of soil δ13C in a subtropical woodland

E. Bai1, Thomas W. Boutton1, Feng Liu1, X. Ben Wu1, and Steven R. Archer2. (1) Texas A&M University, (2) University of Arizona

δ13C of soil organic carbon (SOC) is commonly used to reconstruct plant community history, determine sources of SOC, and quantify SOC turnover rates.  To optimize this methodology, it is critical to understand the biotic and edaphic mechanisms that influence spatial patterns of δ13Csoc.  In this study, we examined spatial patterns of δ13Csoc in a C3 woodland that developed on a site that was once C4 grassland in southern Texas. Soil cores (0-15cm; n=208) were collected, georeferenced, and analyzed for δ13C and soil texture.  Plant community characteristics and NDVI were quantified. Variogram analysis indicated that δ13Csoc was heterogeneous (nugget=0.82‰, sill=1.54‰, range=7.5m).  Neither NDVI nor the distribution of mesquite trees (previously shown to initiate woody invasion in this region) influenced this spatial pattern.  However, δ13Csoc was positively correlated with % clay.  Cross-variogram analysis also indicated these two variables were spatially correlated at distances < 10.1m.  Kriged maps of δ13Csoc and % clay illustrated that areas with higher δ13Csoc corresponded with areas with higher % clay.  Clay is critical for the formation of organomineral complexes and microaggregates that stabilize SOC and protect it from decay.  Thus, woodland stands with high % clay should have relatively slow SOC turnover rates, and should retain more C4-derived SOC from the grassland that once dominated this site.  This study reveals the importance of soil texture in controlling spatial patterns of δ13Csoc in areas with C3-C4 vegetation change, and highlights the need to consider this edaphic property in isotopic studies of vegetation history or SOC dynamics.

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