This dataset contains carbon-13 (13C) and nitrogen-15 (15N) isotopic signatures of belowground root biomass samples from herbaceous wetlands in the Atchafalaya and Terrebonne basins of the Mississippi River Delta in coastal Louisiana, U.S., during August 2021. The data were collected during the Delta-X Fall deployment. Field measurements were conducted at three sites in the Atchafalaya basin and three sites in the Terrebonne basin. Five of the sites are adjacent to sites from the Coastwide Reference Monitoring System (CRMS). The other site is located in Wax Lake Delta (WLD) without appropriate adjacent CRMS sites. Herbaceous wetland sites in both basins were chosen to represent a salinity gradient including freshwater, brackish, and saline ecosystems. Root biomass samples were collected using a gouge soil auger. Bulk isotopic signatures in living fine roots were measured with a mass spectrometer coupled to an elemental analyzer.