Monitoring the triple oxygen isotope composition of water and biogenic silica at the soil-plant-atmosphere interface: benefits for investigating West African present and past water cycles

Quantitative data are needed to constrain vegetation-hydroclimate in water cycle modelling. Here, we use the triple oxygen isotope composition (δ'¹⁸O and ¹⁷O-excess) of water compartments to track water transfers and mixing within the soil-plant-atmosphere continuum. At three AMMA-CATCH sites in Benin and Senegal we monitored the δ'¹⁸O and ¹⁷O-excess of precipitation, groundwater, soil water and plant water, as well as the ¹⁷O-excess of phytoliths, an indicator of atmospheric relative humidity. We found that : 1) the ¹⁷O-excess in precipitation is very stable over several years; 2) groundwater has δ'¹⁸O and ¹⁷O-excess values consistent with a multi-year recharge by modern precipitation; 3) the ¹⁷O-excess in soil water shows a limited contribution of evaporated water, despite high evaporation conditions, which has important implications for our knowledge of water transfers within soils; 4) extrapolating linear relationships between δ'¹⁸O and excess ¹⁷O-excess of leaf and stem water allows us to determine the origin of the water absorbed by the roots. At the savanna and dry forest sites, during the rainy season, grasses absorb soil water supplied by precipitation. In contrast, during the dry season, trees reach the perennial groundwater recharge. 5) the ¹⁷O-excess of grass and tree leaf water follow the dynamics of relative humidity; 6) the ¹⁷O-excess of grass phytoliths records daily relative humidity during the growing season. These results provide a solid basis for using the triple oxygen isotope composition of water and phytoliths to trace present and past water cycles at the soil-plant-atmosphere interface.

This study was conducted in the framework of the HUMI-17 and PAST-17 projects supported by the ANR (ANR-17-CE01-0002-01 and ANR-22-CE01-0027-01), JA and CV have benefited from a Marie Sklodowska-Curie grant from the European Union (n°101063961 for JA and 101063961 for CV). TT acknowledge funds from FORMAS (Dnr 2021-00644), and the European Union under the Development Smart Innovation through Research in Agriculture (DeSIRA) Initiative (FOOD/2019/410-169).