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.