Two adjacent forested catchments: Dramatically different NO₃⁻ export

Abstract

[1] Two adjacent catchments with similar temperate forest cover and podzolic soils have annual nitrate (NO 3 -) export that differs by a factor of 10. Monthly rates of mineralization and nitrification measured by the buried bag technique, soil C/N ratios, and the contribution of microbial NO -during NO 3 -loss do not show evidence of denitrification, although denitrification proceeding to completion in isolated pockets followed by mixing with higher NO 3 -groundwaters would yield the same result. Alternatively, active uptake of NO 3 -by vegetation following spring melt will also deplete the groundwater NO 3 -in the shallow soil depths without isotopic fractionation. The low NO 3 -catchment also has lower NO 3 -in shallow soil waters during spring melt. Shallower slopes promote near-surface flow paths in organic-rich soil horizons which may facilitate denitrification during spring melt. Although the catchment with low NO 3 -export has a large wetland near the catchment outlet, the NO 3 -attenuating capacity of this wetland is largely unused except in the late fall because growing season groundwater concentrations of NO 3 -are undetectable and the wetland is frozen during snowmelt. In the high NO 3 -catchment, organic-rich soils and vegetation in the riparian zone cannot completely attenuate high NO 3 -in discharging groundwaters. In our study, factors controlling NO 3 -in groundwater such as slope, stratigraphy, and hydraulic conductivity can play a larger role than riparian zones in controlling differences in annual NO 3 -export observed between catchments.

Spatial distribution of soils determines export of nitrogen and dissolved organic carbon from an intensively managed agricultural landscape

“…NO − 3 leached to shallow groundwater may be steadily released through time periods of a year or more (Martin et al, 2004;Molénat et al, 2002;Ruiz et al, 2002b). As shown by Schiff et al (2002), NO − 3 concentrations can be traced back to groundwater charges and might give a correct basis to explain the almost constant NO − 3 -N concentrations measured at sampling point 1 over the whole sampling period.…”

“…As summarized in Table 1, there are more organic soils in the contributing area of the southern catchment than in the northern catchment. These soil properties imply a partial immobilization of NO − 3 (Randall and Mulla, 2001;Schiff et al, 2002) and therefore lower concentrations in stream water. In the same area with organic soils along the southern branch, Schelde et al (2012) observed that ammonia was more abundant than NO − 3 in the organic soils, and they found emissions of nitrous oxide to be relatively low at the riparian organic soils.…”

Factors controlling nitrogen release from two forested catchments with contrasting hydrochemical responses

“…For example, Schiff et al (2002) noted an order of magnitude difference in the NO 3 export from two catchments within the Harp Lake watershed in Ontario, Canada. They attributed high NO 3 concentrations in streamflow to very steep hillslope gradients that expedited the movement of NO 3 -enriched groundwater downslope to the stream.…”

“…Furthermore, below a threshold deposition value of [N] ³ 0Ð5 to 0Ð6 kmol ha 1 year 1 , little export of NO 3 occurs and no relation with atmospheric N deposition is evident. Hence, other factors, such as tree species composition (Lovett and Mitchell, 2004;Christopher et al, 2006), land-use history (Compton and Boone, 2000;Goodale and Aber, 2001), climatic variables (Mitchell et al, 1996;Groffman et al, 2001;Goodale et al, 2003;Park et al, 2003) and hydrological regime (Creed and Band, 1998;Inamdar et al, 47 2000;Schiff et al, 2002;Ito et al, 2005), contribute to the heterogeneous spatial pattern of NO 3 export in eastern catchments of North America and other temperate forested regions.…”

Global warming triggers the loss of a key Arctic refugium

“…As a consequence, nitrogen retention in the terrestrial system and nitrate export to a lake or stream may vary considerably between catchments [53]. For example, Sphagnum peat provides a natural trap for inorganic nitrogen (and the removal of nitrogen through denitrification), effectively reducing nitrogen inputs into freshwater systems [50,54,55]. Although measured data on atmospheric nitrogen deposition and export are lacking for the HBL (as they are in almost all Arctic regions), it is certainly a region that experiences low concentrations of atmospheric nitrogen deposition [51] as well as low fluxes of nitrogen to regional freshwater lotic systems [50,52].…”