Searching for similarity in topographic controls on carbon, nitrogen and phosphorus export from forested headwater catchments

Topography influences hydrological processes that in turn affect biogeochemical export to surface water on forested landscapes. The differences in long-term average annual dissolved organic carbon (DOC), organic and inorganic nitrogen [NO3--N, dissolved organic nitrogen (DON)], and phosphorus (total dissolved phosphorus, TDP) export from catchments in the Algoma Highlands of Ontario, Canada, with similar climate, geology, forest and soil were established. Topographic indicators were designed to represent topographically regulated hydrological processes that influence nutrient export, including (1) hydrological storage potential (i.e. effects of topographic flats/depressions on water storage) and (2) hydrological flushing potential (i.e. effects of topographic slopes on potential for variable source area to expand and tap into previously untapped areas). Variations in NO3--N export among catchments could be explained by indicators representing both hydrological flushing potential (91%, p<0.001) and hydrological storage potential (65%, p<0.001), suggesting the importance of hydrological flushing in regulating NO3--N export as well as surface saturated areas in intercepting NO3--N-loaded runoff. In contrast, hydrological storage potential explained the majority of variations among catchments in DON (69%, p<0.001), DOC (94%, p<0.001) and TDP (82%, p<0.001) export. The lower explanatory power of DON (about 15% less) compared with that of DOC and TDP suggests another mechanism influencing N export, such as controls related to alternative fates of nitrogen (e.g. as gas). This study shows that simple topographic indicators can be used to track nutrient sources, sinks and their transport and export to surface water from catchments on forest landscapes. Copyright (c) 2013 John Wiley & Sons, Ltd.