Nitrogen cycling within mountainous watersheds: The headwaters of mountainous regions are important sources of nutrients to downstream ecosystem. However, characteristic traits of mountainous ecosystems, including sparse vegetation, coarse soils, and strong hydrological pulses from snowmelt and monsoonal rains, renders these ecosystems deficient in nutrients that limit primary productivity, specifically nitrogen. The retention and release of nitrogen occurs due to complex, multiscale biological-environmental interactions within and across bedrock-to-canopy compartments, terrestrial to aquatic interfaces, and watershed-scale hydrobiogeochemical gradients. Working across this full range of scales, we are seeking to capture the ecosystem traits, from bedrock lithology to topography and aspect, through to microbial metabolic diversity and interactions within vegetation, that mechanistically determine how mountainous ecosystems retain and release nitrogen We’re using a combination of field and lab measurements and watershed-scale modeling to identify hot spots and hot moments of nitrogen cycling, and to quantify the input, transformation, and export of different organic and inorganic nitrogen species around the catchment across different seasons.

Watershed retention and release of nitrogen: Using concentration-discharge relationships of different nitrogen species, and the dual isotopes of nitrate (Bouskill et al. 2019, Bouskill et al., 2024), and causality analysis, we can better understand how nitrogen is retained and released from watersheds of contrasting ecosystems 'traits' (e.g., bedrock, aspect, slope, and vegetation coverage).  

High-Altitude Nitrogen Suite Of Models (HAN-SoMo): The fate of nitrogen within terrestrial ecosystems is generally dependent on subsurface water residences times. Short residence times can promote a flushing behavior, whereby, biological nitrogen is lost from the system. Alternatively, under longer residence times nitrogen undergoes various biogeochemical reactions (see figure below) that can lead to it's retention or loss in gaseous forms (e.g., nitrogen/ nitrous oxide). We recently built a new model (Maavara et al. 2021) that depicts how nitrogen is cycled and lost from mountainous watersheds as a function of subsurface hydrology. Through this model we can estimate the degree to which rock-derived nitrogen might contribute to loss from the system. While high in representative hillslopes (see Wan et al. 2021), its importance drops when the heterogeneous bedrocks of the watershed are represented. 

Funding: Watershed Function Scientific Focus Area, funded by DOE, Biological and Environmental Research Program.