The influence of climate on average nitrogen export from large watersheds in the Northeastern United States

Abstract

The flux of nitrogen in large rivers in North America and Europe is well explained as a function of the net anthropogenic inputs of nitrogen to the landscape, with on average 20 to 25% of these inputs exported in rivers and 75 to 80% of the nitrogen retained or denitrified in the landscape. Here, we use data for average riverine nitrogen fluxes and anthropogenic inputs of nitrogen over a 6-year period (1988)(1989)(1990)(1991)(1992)(1993) for 16 major watersheds in the northeastern United States to examine if there is also a climatic influence on nitrogen fluxes in rivers. Previous studies have shown that for any given river, nitrogen fluxes are greater in years with higher discharge, but this can be interpreted as storage of nitrogen in the landscape during dry years and flushing of this stored nitrogen during wet years. Our analyses demonstrate that there is also a longer-term steady-state influence of climate on riverine nitrogen fluxes. Those watersheds that have higher precipitation and higher discharge export a greater fraction of the net anthropogenic inputs of nitrogen. This fractional export ranges from 10 to 15% of the nitrogen inputs in drier watersheds in the northeastern United States to over 35% in the wetter watersheds. We believe this is driven by lower rates of denitrification in the wetter watersheds, perhaps because shorter water residence times do not allow for as much denitrification in riparian wetlands and low-order streams. Using mean projections for the consequences of future climate change on precipitation and discharge, we estimate that nitrogen fluxes in the Susquehanna River to Chesapeake Bay may increase by 3 to 17% by 2030 and by 16 to 65% by 2095 due to greater fractional delivery of net anthropogenic nitrogen inputs as precipitation and discharge increase. Although these projections are highly uncertain, they suggest a need to better consider the influence of climate on riverine nitrogen fluxes as part of management efforts to control coastal nitrogen pollution.

Application of watershed deposition tool to estimate from CMAQ simulations the atmospheric deposition of nitrogen to Tampa Bay and its watershed

“…Chloride was assumed to be a conservative tracer for atmospheric deposition, such that the expected concentration of N ions in stream water was calculated from the product of the observed stream water chloride ion concentration and the ratio of atmospheric N flux to chloride flux. The ratio of observed to expected N ion concentrations in stream water yielded a minimum N retention of 82% or maximum export of 18% for the watershed, which is lower than the estimate of 26% N export for northeastern U.S. watersheds (Howarth et al, 2006) and higher than the estimate of 6% N export for North Carolina's Neuse River Estuary (Whitall et al, 2003 Figure 1) (Strayer et al, 2007). From the Gandy Bridge data, wet deposition rates were calculated according to eq 1, where C rain is the daily rainfall concentration of either NO 3 À -N or NH 4 þ -N and has units of mg L À1 , D is the rainfall depth and has units of cm d À1 , and F wet is the deposition rate (i.e., flux) and has units of kg N ha À1 d À1 .…”

Transformation of the Nitrogen Cycle: Recent Trends, Questions, and Potential Solutions

“…Ultimately, the fate of Nr that enters terrestrial systems appears to be under strong climatic control (25), an interaction that helps explain regional differences in N export and that should be considered in forecasts of future Ncycle dynamics. The fate and impacts of Nr are also often dependent on its chemical form, further highlighting the need to better resolve changing inputs of oxidized versus reduced forms of Nr.…”

The nitrogen cascade from agricultural soils to the sea: modelling nitrogen transfers at regional watershed and global scales

“…By contrast, in the Seine, Scheldt and Mississippi river basins, a much higher fraction of NANI is delivered to the sea, causing severe marine eutrophication in the receiving coastal waters [23,24]. Although the processes responsible for the 'retention' of N along the N cascade are numerous and complex, Howarth et al [17,25,26] have shown that their overall effect in terms of the fraction of net total nitrogen inputs delivered to the sea is highly correlated to climate, varying between 100 per cent in cold and humid regions to less than 10 per cent under arid and warm climate conditions. Billen et al [14] have proposed a sigmoid relationship to represent the effect of specific run-off on the fraction of net total nitrogen input (NTNI ¼ NANI þ natural inputs) delivered to the sea (see the electronic supplementary material, figure S3).…”