COS 60-5 - Seasonal variation of root biomass and necromass in herbaceous wetlands across distinct salinity and hydrogeomorphic gradients in the Atchafalaya and Terrebonne Basins (Louisiana, USA)
The Atchafalaya and Terrebonne Basins in the Mississippi River Deltaic Plain are examples of coastal basins that have experienced natural and anthropogenic changes over time and are increasingly vulnerable to global climate change. Sea level rise and reduced riverine sediment supply have altered coastal wetland vegetation dynamics (above- and belowground), distribution and composition of plant species, as well as wetland elevation capital, threatening their long-term sustainability. We investigated root biomass and necromass dynamics in response to seasonality and soil properties across different hydrogeomorphic (HGM) zones (supratidal, intertidal) and along a salinity gradient in two coastal basins – Atchafalaya (Sediment Rich) and Terrebonne Basin (Sediment Poor) – with contrasting sediment delivery and hydrological regimes during the early growing season (March 2021) and peak biomass season (August 2021). Root dynamics were evaluated at all sites by collecting root cores (0-50 cm depth) using a PVC coring device (10.2 cm diameter). Root cores were sectioned into 10 cm intervals, processed separately following standard protocols, and sorted into biomass and necromass categories based on their buoyancy, turgor, and color. Additional soil cores (50 cm depth) were collected at all sites and zones and sectioned into 10 cm intervals to evaluate soil properties.
Results/Conclusions
Total root biomass increased from freshwater to saline sites along both basins and was on average 1.2-1.8 times higher in Atchafalaya relative to Terrebonne Basin across seasons. Root biomass ranged from 74 to 2394 g m-2 across HGM zones and sites, with estimates 1-2.3 times higher during peak biomass season compared to early growing season. Root biomass decreased with soil depth at all sites and zones, with higher allocation in the 0-10 cm interval relative to other depths (10-50 cm), particularly during the peak biomass season. Total root necromass was 2-35 times higher than biomass across all sites and zones, with estimates ranging from 1663 to 7698 g m-2. Soils in Atchafalaya sites had higher bulk density and lower organic matter content compared to sites in Terrebonne, suggesting greater influence of riverine mineral inputs in the Atchafalaya system. Differences in root biomass estimates across sites and basins likely suggest the regulatory effect of salinity and other environmental conditions (e.g., soil nutrient availability, hydroperiod) on carbon allocation and partitioning. This research will contribute to a better understanding of how root dynamics respond to environmental gradients and are likely to be influenced by changing ecosystems across deltaic basins.
