Drivers of atmospheric nitrate processing and export in forested catchments

Abstract

Increased deposition of reactive atmospheric N has resulted in the nitrogen saturation of many forested catchments worldwide. Isotope-based studies from multiple forest sites report low proportions (mean 5 10%) of unprocessed atmospheric nitrate in streams during baseflow, regardless of N deposition or nitrate export rates. Given similar proportions of atmospheric nitrate in baseflow across a variety of sites and forest types, it is important to address the postdepositional drivers and processes that affect atmospheric nitrate transport and fate within catchments. In a meta-analysis of stable isotope-based studies, we examined the influence of methodological, biological, and hydrologic drivers on the export of atmospheric nitrate from forests. The d values tended to increase with increasing baseflow discharge at all sites examined. To explain these differences, we present a conceptual model of hydrologic flowpath characteristics (e.g., saturation overland flow versus subsurface stormflow) that considers the influence of topography on landscape-stream hydrologic connectivity and delivery of unprocessed atmospheric nitrate to streams. Methodological biases resulting from differences in sampling frequency and stable isotope analytical techniques may further influence the perceived degree of unprocessed atmospheric nitrate export. Synthesis of results from numerous isotope-based studies shows that small proportions of unprocessed atmospheric nitrate are common in baseflow. However, hydrologic, topographic, and methodological factors are important drivers of actual or perceived elevated contributions of unprocessed atmospheric nitrate to streams.

Export flux of unprocessed atmospheric nitrate from temperate forested catchments: A possible new index for nitrogen saturation

“…By determining both concentration and the 17 O-excess of stream nitrate, we can quantify unprocessed NO 3 − atm in stream water accurately and precisely. Recent studies on nitrate including unprocessed NO 3 − atm exported from forested catchments via streams during base flow period have revealed that the export flux of unprocessed NO 3 − atm increases in accordance with increases in the stream nitrate concentration (Rose et al, 15 2015a; Rose et al, 2015b;Tsunogai et al, 2014). These results implied that unprocessed NO 3 − atm exported from forested catchments can be used as a tracer to identify the biological processing of nitrate in each catchment area and to clarify the processes regulating nitrate concentrations in stream water.…”

AirSWOT measurements of river water surface elevation and slope: Tanana River, AK

“…Several types of active radars overcome the drawbacks of optical sensors, although all current systems also have limitations of their own [Smith, 1997;Calmant et al, 2008;Schumann et al, 2009]. Radar altimetry measurements from sensors such as Jason-2 and Envisat provide point-based measurements of river WSEs, which can be used to interpolate slopes and develop stage-discharge relationships [O'Loughlin et al, 2013;Paris et al, 2016]. However, altimeter observations have low spatial and temporal resolutions, with vertical errors of decimeters to meters [Calmant et al, 2008;Biancamaria et al, 2016a;O'Loughlin et al, 2016].…”

Episodic, seasonal, and annual export of atmospheric and microbial nitrate from a temperate forest

“…Two recent studies using Δ 17 data reported that NO 3 -N Atm made up 5-10% of total NO 3 -N in streams, groundwater, and/or soils within temperate forest ecosystems [Costa et al, 2011;Tsunogai et al, 2010], whereas Rose et al [2015aRose et al [ , 2015b reported up to~50% NO 3 -N Atm in a stream draining a conifer-dominated watershed. These studies, along with qualitative evidence for NO 3 -N Atm in base flow based on δ 18 O-NO 3 À data [Rose et al, 2015b;Sebestyen et al, 2014], have suggested that the loss of unprocessed, deposited N can occur outside of direct runoff producing events [Curtis et al, 2011]. However, prior studies were not specifically designed to quantify intra-annual and inter-annual variation in the export of NO 3 -N Atm and NO 3 -N Micro in streams during a range of hydrological conditions.…”