Already the DOE Watershed SFA research team has begun to cast light on the influence that these deviations from past trends can have on downgradient water quantity and quality. Snowmelt, for example, is occurring about a month earlier than it did a quarter century ago. This change in timing could impact the synchrony that currently exists between when snowmelt water is delivered to soil microbes and plants, and how those communities use and cycle water and nutrients. It could also impact the stage of a river, and associated biogeochemical-hydrological interactions that occur across terrestrial-aquatic interfaces.
Watershed SFA project researchers are exploring how natural variability as well as artificially induced snowmelt dynamics influence these complex interactions. For example, they have deployed black tarps to change the radiation and thus snowmelt timing relative to adjacent plots along an elevation gradient. They are monitoring vegetation, soil, and bedrock hydro-biogeochemical processes in accelerated and controlled plots to document the impact of snowmelt timing on exports.
Their measured observations and computer simulations show that the soil microbial community beneath snowpack contributes to an earlier and larger peak of nitrate export with early snowmelt compared to a more typical snowmelt scenario. Moreover, the study revealed that the water-table fluctuations associated with snowmelt infiltration led to biogeochemical reactions in the deeper Mancos Shale bedrock, which contributed a significantly larger pulse of bedrock nitrogen export than has been documented before.
The work depicts how different compartments of the watershed contribute differently to early snowmelt, and underscores the importance of understanding such response at both the local and aggregated scales The work has recently led to a ‘two nitrogen world’ hypothesis that is being tested in the new phase, focused on soil-microbe-plant interactions that contribute to a shallow nitrogen export and a deeper microbe-mineral bedrock interaction that contributes to a deeper export.
According to SFA team member Nick Bouskill, “Within mountainous ecosystems, the lack of available nitrogen can limit plant growth and photosynthesis, and microbial activity. Coarse soils, sparse vegetation, and strong hydrological events, such as snowmelt and monsoonal precipitation, can flush nitrogen prior to it being retained by plants and microbes. Predicting how nitrogen cycles throughout this ecosystem necessitates a greater understanding of these two nitrogen worlds, and the contributions that they make to productivity and downstream exports.
SFA team member Heidi Steltzer also describes how in both scenarios of early snowmelt and decreased snowpack, shrubs have replaced grasses and wildflowers as the dominant vegetation. Those grasses and wildflowers rapidly take up nitrogen and other elements from water within snowmelt. It’s not yet clear if these new plants can quickly assume the roles of their predecessors and prevent nitrates or other elements from entering the river and traveling downstream.
The researchers are also investigating how hydro-biogeochemical exchanges between river corridor elements (hyporheic zones, off-channel wetlands, and floodplains) alter riverine chemical signatures, as a function of river stage and river order. SFA team member Dipankar Dwivedi leads a team in using high-performance computing to predict flow and reactions through and out of river meanders.
“Development, application, and validation of advanced, high performance computational models that can predict hyporheic zone processes in high resolution have enabled us to explore how exports of carbon, iron, and other geochemical species vary–from a single meander to larger reaches, and across changing river stage conditions,” Dwivedi says. “An accurate model for exploring these hydro-biogeochemical interactions is important because the hyporheic zones play an outsized role in linking watershed hillslopes with rivers.”