Spatial patterns of soil δ13C in a subtropical woodland

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 δ¹³C_soc.  In this study, we examined spatial patterns of δ¹³C_soc 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 δ¹³C_soc 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, δ¹³C_soc was positively correlated with % clay.  Cross-variogram analysis also indicated these two variables were spatially correlated at distances < 10.1m.  Kriged maps of δ¹³C_soc and % clay illustrated that areas with higher δ¹³C_soc 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 δ¹³C_soc 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.