Potassium Isotopes in Herbaceous Plants: A Potential New Tool for C3 and C4 Plant Research

Plants play a critical role in the surficial environment, influencing energy transfer, the global carbon cycle, and climate change. Potassium (K) controls many essential plant physiological features like photosynthesis, cell osmosis adjustment, and enzyme activity. However, few plant species of K isotopes have been investigated and knowledge regarding the mechanism of isotopic fractionations in biological processes is still limited. This study investigated foliar K isotopic compositions of C3 and C4 vegetation and soil in the typical karst region of Guizhou Province, Southwest China. The results showed that: (a) The δ41K of soil ranged from −0.04‰ to −0.08‰, with no K isotope ratios change between topsoil and subsoil. (b) Foliar δ41K of C3 vegetation ranged from −1.06 ± 0.06‰ to −0.15 ± 0.06‰ and foliar δ41K of C4 vegetation varied from 0.28 ± 0.05‰ to 1.15 ± 0.06‰. (c) Foliar δ41K was negatively related to foliar K concentration (R2 = 0.78; p < 0.01); The significant K isotope differences between C3 and C4 vegetation are likely attributed to their different mechanisms for drought stress or their different foliar K concentration. This study firstly documented the potential of K isotopes as a novel tool for biological processes to distinguish between C3 and C4 plants and also shed light on the potential future application of K isotope signatures in soil‐plant interactions.