Hierarchical regulation of nitrogen export from urban catchments in central Arizona-Phoenix

In urban catchments of arid central Arizona, we investigate how the export of
mineral and organic nitrogen (N) in storm runoff is regulated by interactions
between local landscape characteristics and broader scale storm features. First, we
test whether N export is more a function of (1) processes that affect N
concentration in runoff or (2) the propensity of the catchment to convey rainfall as
runoff. With data pooled across catchments, the mass of N in export (load) is
determined by processes regulating runoff N concentration. There are exceptions when
catchments are examined individually, where N load from some catchments is
determined by the hydrologic responsiveness of the catchment. Second, we investigate
the relationship between N export and catchment features. Loads per catchment area
were greater from more impervious catchments, probably because impervious catchments
held more N in a mobilizable phase and conveyed more rainfall as overland flow.
Loads per area were lower from larger catchments, possibly owing to more N-retention
hot spots in larger catchments. Catchments with the greatest N exports were those
with commercial land use, and loads decreased as development became less prevalent
or as residential replaced industrial land use. Third, we investigated how catchment
features moderated direct responses of N export to storms. Export was less
correlated with storm features in catchments that were larger, more pervious, and
less industrial. Results support an N build and flush hypothesis, which purports
that there is little biotic processing of N deposited to arid, urban surfaces with
little organic matter. The rate and duration of deposition determine the size of the
mobile N pool. Any amount of rainfall capable of generating overland flow would
entrain nearly all mobilizable N and export it from the catchment. Nonetheless,
these results suggest that, even with daunting seasonal and interannual variability
in storm conditions, material export can be reduced by managing intrinsic catchment
features.