Stable C and N Isotopes: A Tool to Interpret Interacting Environmental Stresses on Soil and Plant

Abstract

Natural abundances of stable isotopes of nitrogen and carbon (δ15N and δ13C) are being widely used to study N and C cycle processes in plant and soil systems. Variations in δ15N of the soil and the plant reflect the potentially variable isotope signature of the external N sources and the isotope fractionation during the N cycle process. N2 fixation and N fertilizer supply the nitrogen, whose δ15N is close to 0‰, whereas the compost as an organic input generally provides the nitrogen enriched in 15N compared to the atmospheric N2. The isotope fractionation during the N cycle process decreases the δ15N of the substrate and increases the δ15N of the product. N transformations such as N mineralization, nitrification, denitrification, assimilation, and the NH3 volatilization have a specific isotope fractionation factor (α) for each N process. Variation in the δ13C of plants reflects the photosynthetic type of plant, which affects the isotope fractionation during photosynthesis. The δ13C of C3 plant is significantly lower than, whereas the δ13C of C4 plant is similar to that of the atmospheric CO2. Variation in the isotope fractionation of carbon and nitrogen can be observed under different environmental conditions. The effect of environmental factors on the stomatal conductance and the carboxylation rate affects the carbon isotope fractionation during photosynthesis. Changes in the environmental factors such as temperature and salt concentration affect the nitrogen isotope fractionation during the N cycle processes; however, the mechanism of variation in the nitrogen isotope fractionation has not been studied as much as that in the carbon isotope fractionation. Isotope fractionation factors of carbon and nitrogen could be the integrated factors for interpreting the effects of the environmental factors on plants and soils.