Effects of lateral nitrate flux and instream processes on dissolved inorganic nitrogen export in a forested catchment: A model sensitivity analysis

Abstract

The importance of terrestrial and aquatic ecosystems in controlling nitrogen dynamics in streams is a key interest of ecologists studying dissolved inorganic nitrogen (DIN) export from watersheds. In this study, we coupled a stream model with a terrestrial ecohydrological model and conducted a global sensitivity analysis to evaluate the relative importance of both ecosystems to nitrogen export. We constructed two scenarios (''normal'' and high nitrate loads) to explore conditions under which terrestrial (lateral nitrate flux) or aquatic ecosystems (instream nutrient processes) may be more important in controlling DIN export. In a forest catchment, although the forest ecosystem controls the nitrogen load to streams, sensitivity results suggested that most nitrogen output from the terrestrial ecosystem was taken up by instream microbial immobilization associated with benthic detritus and retained in detritus. Later the immobilized nitrogen was remineralized as DIN. Therefore, the intra-annual pattern of DIN concentration in the stream was low in fall and became high in spring. Not only was instream microbial immobilization saturated with the high nitrogen load scenario, but also the net effect of immobilization and mineralization on DIN export was minimized because nitrogen cycling between organic and inorganic forms was accelerated. Overall, our linked terrestrial-aquatic model simulations demonstrated that stream process could significantly affect the amount and timing of watershed nitrogen export when nitrogen export from the terrestrial system is low. However, when nitrogen export from the terrestrial system is high, the effect of stream processes is minimal.

Ecosystem processes at the watershed scale: Influence of flowpath patterns of canopy ecophysiology on emergent catchment water and carbon cycling

“…RHESSys couples elements of the ecosystem models BIOME‐BGC (Running & Hunt, ) and CENTURY (Parton, Schimel, Cole, & Ojima, ), with distributed watershed models to derive the coupling between ecosystem water use, carbon and nitrogen cycling with lateral soil water redistribution. RHESSys has been widely used to estimate spatially distributed soil moisture, ET, surface and subsurface run‐off, carbon and nitrogen cycling in different biomes and under different climate and land use change scenarios (e.g., Band, Patterson, Nemani, & Running, ; Bart, Tague, & Moritz, ; Garcia, Tague, & Choate, ; Hanan, Tague, & Schimel, ; Hwang et al, ; Lin, ; Lin, Webster, Hwang, & Band, ; Miles & Band, ; Tague & Band, ). RHESSys uses a landscape hierarchical structure over nested patch (Figure ), hillslope and watershed scales (Figure ).…”

Warming‐Induced Earlier Greenup Leads to Reduced Stream Discharge in a Temperate Mixed Forest Catchment

“…RHESSys integrates a number of biotic and abiotic processes from several submodels: MT‐Clim to extrapolate plot‐scale climate measurements to the landscape based on elevation, aspect, and slope (Running et al, ), Biome‐BGC to account for plot‐scale carbon and water cycles (Running & Hunt Jr., ; White et al, ), CENTURY NGAS to simulate long‐term soil carbon and nitrogen processes (Parton et al, , ), and a Distributed Hydrology Soil Vegetation Model (DHSVM) to explicitly simulate the lateral hydrologic flows following topographic gradients over landscapes (Wigmosta et al, ). Besides the aboveground vertical fluxes (e.g., GPP and ET), carbon and nitrogen are also transported laterally with both surface and subsurface lateral flows between adjacent patches (the simulation units) as forms of dissolved organic and inorganic matter (Lin et al, ). RHESSys has been successfully applied to simulate biogeochemical fluxes in a wide range of ecosystems and climate regions (Band et al, ; Christensen et al, ; Hwang et al, ; Hwang et al, ; Tague & Grant, ; Tague et al, ; Vicente‐Serrano et al, ; Zierl et al, ).…”

Reservoirs are hotspots of nitrogen cycling in peatland catchments

“…The Kinder Reservoir switched from being a sink of TDN in the biologically dormant winter season, to being a source of N in the spring season, with ~237 kg N of DIN exported in excess of fluvial DIN input (Table ). Although immobilization of DIN was slightly higher than mineralization in winter, in spring, mineralization is as much or greater than immobilization, thus allowing for the direct export of DIN from the catchment (Lin et al, ). Whilst a 42 ± 5% decrease in the annual average DON concentration was observed in the reservoir outlet, relative to fluvial inputs (Figure ), there was little or no difference between the average annual concentrations of DIN in the Kinder Reservoir, and its inlet and outlet (Figure b).…”

“…Errors for extrapolated fluxes are reported as one standard error. Errors for interpolated fluxes were calculated based on the method described by Hope et al the direct export of DIN from the catchment (Lin et al, 2015). Whilst a 42 ± 5% decrease in the annual average DON concentration was observed in the reservoir outlet, relative to fluvial inputs ( Figure 6), there was little or no difference between the average annual concentrations of DIN in the Kinder Reservoir, and its inlet and outlet (Figure 5b).…”