Towards closing the watershed nitrogen budget: Spatial and temporal scaling of denitrification

Abstract

[1] Enhanced consideration of the hydrogeomorphic template of watersheds is critical to understanding watershed nitrogen budgets. We developed a framework to estimate the spatial distribution and temporal dynamics of soil moisture and soil oxygen in surficial soils to scale nitrogen transformations for a forested watershed (Pond Branch) in Maryland, USA. We sampled soil cores in upland, hillslope hollow, riparian hollow, and riparian hummock landscape positions in different seasons for biogeochemical fluxes including measurement of N 2 gas produced via denitrification. We extrapolated these rates in space and time with information derived from in situ soil oxygen and soil moisture probes to scale fluxes from plots to the catchment level. We addressed three questions: (1) How important are seasonal, daily, and storm event variations in soil oxygen for denitrification? (2) How is denitrification spatially distributed through the watershed? (3) How important is denitrification to the watershed nitrogen budget? We found that microtopography within the riparian zone is a significant influence on soil oxygen dynamics and therefore redox-sensitive biogeochemical processes such as denitrification. Riparian zone hollows (lower topographic positions) represented 0.5%-1.0% of the catchment area, but accounted for >99% of total denitrification. Interestingly, topography was a much stronger controller of oxygen than rainfall, which had little influence on temporal variation in soil oxygen levels. Spatial and temporal extrapolations of measured rates suggest that a minimum of 16%-27% of atmospheric nitrogen deposition is lost to denitrification. These results suggest that the importance of denitrification in the nitrogen budget of forested watersheds depends fundamentally on the presence of landscape elements, such as riparian hollows that function as "hot spots" of activity.Citation: Duncan, J. M., P. M. Groffman, and L. E. Band (2013), Towards closing the watershed nitrogen budget: Spatial and temporal scaling of denitrification,

Landscape analysis of soil methane flux across complex terrain

“…Consistent with previous research on CO 2 fluxes at TCEF (Riveros-Iregui and McGlynn, 2009) and other studies (Duncan et al, 2013;Vidon et al, 2014), our regression model results suggest that the topographic redistribution of water and the frequency distribution of relevant functional landscape elements should be considered in scaling exercises. These approaches may better reflect CH 4 dynamics in a variety of watersheds, such as locations where the riparian extent is proportionally larger and potentially offsets the upland CH 4 sink to a greater degree (Sakabe et al, 2016).…”

“…Functional landscape elements and terrain metrics that represent topographically driven hydrologic gradients have been used to analyze and scale biogeochemical cycles (e.g., carbon : Creed et al, 2002;Riveros-Iregui and McGlynn, 2009;Pacific et al, 2011;nitrogen: Hedin et al, 1998b;Creed and Beall, 2009;Duncan et al, 2013;Anderson et al, 2015;phosphorus: Devito et al, 2000;sulfate: Welsch et al, 2004), but limited analogous work has been done for CH 4 consumption. The importance of soil moisture in mediating CH 4 fluxes has been shown across ecosystems von Fischer and Hedin, 2007), but studies of how this influence is related to, or predictable from, landscape characteristics have been limited (Boeckx et al, 1997;Creed et al, 2013).…”

Mechanisms driving the seasonality of catchment scale nitrate export: Evidence for riparian ecohydrologic controls

“…This phenomenon has been documented in Ontario, Canada [Hill, 2011]. These deeper riparian soils tend to be in areas with higher topographic wetness index values and are more likely to have higher connectivity to the stream [Duncan et al, 2013]. A third source is the stream channel, bed, and banks, a portion of which are exposed with groundwater fluctuations and could produce NO 2 3 which is then transported during the day as groundwater and runoff levels decline.…”

“…As the spatial extent In Pond Branch, stream channels and portions of the near-stream zone including many riparian hollows are on a continuum of the aquatic-terrestrial nexus that varies in extent seasonally, and daily, with storms and diel evapotranspiration. Previously, Duncan et al [2013] have estimated that 98-99% of the entire watershed denitrification in Pond Branch is occurring in riparian hollows, which comprise less than 1% of the watershed area. During summer dry downs when riparian hollows drain, there is a large increase in denitrification as more NO 2 3 , which is in limited supply in this environment, becomes available, presumably via coupled nitrification-denitrification [Duncan et al, 2013].…”

“…Previously, Duncan et al [2013] have estimated that 98-99% of the entire watershed denitrification in Pond Branch is occurring in riparian hollows, which comprise less than 1% of the watershed area. During summer dry downs when riparian hollows drain, there is a large increase in denitrification as more NO 2 3 , which is in limited supply in this environment, becomes available, presumably via coupled nitrification-denitrification [Duncan et al, 2013]. However, a fraction of that remaining NO 2 3 is available for transport to the stream.…”