δ13C values of plants as indicators of soil water content in modern ecosystems of the Chinese Loess Plateau

We selected typical restored grassland along a chronosequence of natural vegetation restoration on the Chinese Loess Plateau (CLP) and determined the soil water content once each month from April to October as well as the stable carbon isotope composition of six plant species (C3), for both the aboveground (δ13CAP) and belowground (δ13CBP) portions, to examine the sensitivity of plant δ13C values to soil water content in modern ecosystems. The study sites include three restored grasslands, which have been restored for 3 years (RG1), 13 years (RG2) and 23 years (RG3) since farmland abandonment. Results showed that the soil water content increased significantly with vegetation restoration, with means of 7.3%, 10.7% and 13.1% for RG1, RG2 and RG3, respectively. The multispecies mean δ13CAP and δ13CBP values were −27.1‰, −27.5‰, −28‰ and −26.9‰, −27.4‰, −27.6‰ in RG1, RG2, RG3, respectively, decreasing significantly with vegetation restoration. Further analysis indicates that the multispecies mean δ13CAP and δ13CBP values were negatively correlated with soil water content, with r2 values of 0.72 and 0.98, respectively. The δ13CAP and δ13CBP values of all individual species were also significantly and negatively correlated with soil water content, except for Saussurea japonica, for which the δ13CBP value increased with soil water content. Of all the plant species, the responses of both the δ13CAP and δ13CBP values to soil water content were more sensitive for Potentilla bifurca. We suggest that the δ13CAP and δ13CBP values of individual plant species are likely reliable indicators of the soil water content of the specific year of sampling and a longer-term (several years) integrated signal, respectively.