Factors influencing groundwater microbial communities across an intensive agricultural landscape

Groundwater is the world’s largest freshwater biome, supporting unique biota, and supplying a critical water source. In Australia, groundwater is critical to rural communities, particularly in agricultural areas like the Murray Darling Basin (MDB). However, agricultural activities, including irrigation and use of nitrogenous fertilisers, threaten groundwater quality, with likely impacts to groundwater biota and the ecosystem services they provide. The aim of this project was to investigate nitrogen contamination in shallow groundwaters in the Lachlan, Murrumbidgee and Murray catchments of the southern MDB and its relationship to microbial assemblages, biomass and activity. It was expected that nitrate concentrations would vary in relation to land use, and microbial assemblages would differ with nitrate concentrations. Forty bores were sampled during two time periods. Site attributes, groundwater quality, microbial cell biomass and activity were recorded, and microbial communities where characterised using DNA metabarcoding. Although nitrate concentrations and the abundance of nitrifying bacteria varied with land use, electrical conductivity (EC) was the primary driver of microbial assemblages at the regional scale. Nitrogen species were overall lower than expected, a likely impact of drought, but were correlated with microbial assemblages within each catchment, highlighting the importance of analysing patterns at multiple spatial scales.