Riparian Carbon and Nitrogen Cycling: Influences of Spatial Heterogeneity and Hydrologic Vectors

Terrestrial and aquatic ecosystem ecologists study the same phenomena
(e.g., nutrient cycling, energy flow, succession) but they do so within separate
conceptual frameworks. Terrestrial frameworks address state factors of climate,
organisms, parent material, topography, and time to understand ecosystem
processes within a location, but less often consider how ecosystems are
connected. Aquatic ecologists, especially those studying flowing waters,
emphasize the role of water as a means of propagating processes and materials
through space and time. Both terrestrial and aquatic ecosystems contain
hydrologic vectors as well as soils or sediments that record the influences of
state factors. This dissertation is an investigation of carbon and nitrogen
cycling in the stream-riparian corridor of the San Pedro River, a large desert
river, and tests the relative effects of soil characteristics and hydrologic
vectors. Riparian zones are well suited to such investigations because they are
spatially heterogeneous and subject to material and water inputs from multiple
hydrologic vectors. Spatially explicit methods showed that flood vectors
homogenized soil characteristics and denitrification along a stream reach with
frequent overbank floods, whereas soils of a riparian site that was infrequently
inundated by floods were heterogeneous. Legacies of previous floods appeared to
determine the spatial locations of denitrification at the latter site. At a plot
scale, manipulation of precipitation and flood vectors across a gradient of soil
types showed that water functioned as an essential resource during the dry
season, resulting in increased emissions of trace gases following simulation of
floods. Following inundation of riparian soils by several monsoon floods,
however, further addition of water appeared to suppress biogeochemical activity
by decreasing oxygen availability. Thus desert riparian ecosystems appear to
shift seasonally along a terrestrial-aquatic continuum, functioning similarly to
terrestrial ecosystems during dry seasons, but like wetlands following
inundation by floods. Finally, analyses encompassing several years of
observations revealed strong correlations between regimes of hydrology and
resource availability. Overall, this study demonstrates that the hydrologic
regime drives spatial and temporal patterns in biogeochemical processes, but
that soil characteristics may modify the duration and magnitude of biological
responses to hydrologic vectors in desert riparian ecosystems.