Greenhouse Gas Flux, Soil Microbiome Diversity, and Dominant Vegetation in an Area of Cyclical Surface Water Loss on the Middle Rio Grande Riparian Corridor

The Middle Rio Grande experiences significant water conveyance loss and surface water drying in the reach between the San Acacia Diversion Dam and Fort Craig. This area is important for transporting water from northern New Mexico to Elephant Butte Reservoir and Texas. Sources of water conveyance loss are well studied and include a hydraulic gradient draining surface water into groundwater, river channel aggradation to a higher elevation than the low flow conveyance channel after decades of flood prevention, and evapotranspiration. Lesser known is how this cycle of local surface water drying and re-wetting affects riparian corridor vegetation community composition, greenhouse gas flux from groundwater, and functional diversity of the soil microbiome.

This study investigates 4 distinct vegetation communities subject to fluxes in available groundwater: invasive stands of mature tamarisk (Tamarix chinensis), cottonwood (Populus fremontii) galleries, native grassland, and burned tamarisk stands. Field methods utilized a novel method for in situ groundwater greenhouse gas flux sampling, water table monitoring, vegetation surveys, remote sensing, and community level physiological profile analysis of soil microbiota from deep and shallow soil samples. Results show vegetation communities are predominantly phreatophytic and display a north-south gradient mediated by burn scars and land use. Despite this gradient and different vegetation communities, mean daily water table flux at each site was comparable between sites, with no significant difference in greenhouse gas flux from groundwater, leaving questions regarding drivers in differences of functional soil microbial diversity between sites.

Results indicate groundwater greenhouse gas flux trends are possibly influenced by watershed-level processes. With these results, future workers investigating the relationship between water table levels, surface water availability, and greenhouse gas flux on the Middle Rio Grande may do so by devoting their field efforts and funding to one riparian corridor site instead of many for more efficient monitoring. These results can yield a better understanding of riparian corridor ecology and nutrient cycles in a pulse-driven, arid ecosystem.