Effects of plant intraspecific variation on the prediction of C3/C4 vegetation ratio from carbon isotope composition of topsoil organic matter across grasslands

Abstract Aims 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. Methods 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. Important Findings 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.