Abstract:

Understanding the patterns and drivers of carbon isotope discrimination (13Δ) in C₃ and C₄ functional groups is critical for predicting C₃/C₄ vegetation ratio from the isotopic composition of soil organic matter. In this study, we aimed to evaluate how intraspecific variation will modify functional group-level 13Δ values and the associated prediction of C₃/C₄ vegetation ratio.

We investigated 13Δ of 726 individual plants (96 species; C₃ and C₄ functional groups) and topsoil organic matter in 26 grassland communities along an aridity gradient in northern China. The fraction of C₄ contribution was calculated with mixing models that considered: (i) both intra- and interspecific effects on the 13Δ values of C₃ and C₄ functional groups; (ii) only interspecific effects; or (iii) none of these effects.

We found divergent responses of plant 13Δ at the intraspecific level to the changes of aridity across the gradient. The 13Δ of both C₃ and C₄ functional groups was negatively correlated with an aridity index, with higher sensitivity for C₃ than for C₄ functional groups. Intraspecific 13Δ variation played a key role in driving the total 13Δ variations of C₃ plants. Overlooking such intraspecific effect in mixing models led to a greatly increased fraction of C₄ contribution to soil organic carbon. A correction for the effects of intraspecific variation is therefore essential for correctly inferring C₃/C₄ vegetation ratio in the past. Our findings provide basic information for the reconstruction of past vegetation change from bulk materials in arid and semiarid biomes.

Key words: arid and semiarid grasslands, interspecific variation, intraspecific variation, soil organic matter, two-member mixing models, vegetation ratio