Carbon isotope discrimination as a surrogate for soil available water capacity in rainfed areas: A study in the Languedoc vineyard plain

Soil available water capacity (SAWC) is a key soil function for plant growth. Classical SAWC characterization requires time consuming determinations of bulk density and specific soil moisture contents. Consequently, these data are extremely sparse in existing soil databases. Using surrogates of the vegetal response to characterize SAWC across a great number of sites constitutes a promising perspective. The carbon isotope ratio (delta C-13) measured in a plant organ is largely known as an indicator of plant water status. The aim of the paper is to test delta C-13 as an indicator of SAWC in rainfed vineyard. delta C-13 values of grapes at harvest time were measured at 21 sites on the Languedoc vineyard plain with contrasting SAWC (33 to 308 mm) for four years (2015 to 2018) with contrasting annual precipitation (from 390 to 715 mm). The relationships between delta C-13 and SAWC determined using a classical approach (soil description, soil sampling and laboratory methods) were satisfactory for all years (RMSEs from cross validation were between 35 and 61 mm). Better relationships were obtained between delta C-13 and SAWC for years that showed a full winter recharge of SAWC (2015, 2017 and 2018). Averaging the delta C-13 measurements over such years provided an even better relationship (R-2 = 0.85; RMSE 32 mm), which was successfully validated in distant sites on the Languedoc vineyard plain. This work demonstrated that delta C-13 can be considered as a simple and inexpensive surrogate for estimating SAWC. In addition to considerably increasing the density of SAWC measurements, the use of delta C-13 would lead to better consideration of the contribution of deep horizons in the case of perennial plants. Application of this isotopic technique to other agro-systems is required to better define the potential areas of use of delta C-13.