This dataset is from the Delta-X campaign that was conducted during the period of March 19 to April 2, 2021. Belowground biomass (BGB) and necromass (BGN) samples for herbaceous wetlands were collected in the Atchafalaya and Terrebonne basins in southeastern coastal Louisiana. Field measurements were conducted at 6 sites in the Atchafalaya (N = 1) and Terrebonne (N = 5) basins in coastal Louisiana. Five of the sites are adjacent to sites from the Coastwide Reference Monitoring System. The other site is in the 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.