This is because carbon-isotope fractiontion of soil carbonate is mainly controlled by the composition of soil CO2, which is determined by the biomass of C3 and C4 plants.

• 这是因为土壤碳酸盐中的碳稳定同位素的分馏过程主要取决于土壤当中的CO2气体的碳同位素分馏状况，而后者又主要与成土时上覆植被的类型（C3、C4型植物）有关。