Nitrogen transport and processing in the intermittent drainage network: Linking terrestrial and aquatic ecosystems.

摘要

Watersheds are highly retentive of nitrogen, with N deposition generally exceeding export in stream flow. In the Sycamore Creek Watershed, located in the Sonoran Desert of Central Arizona, approximately 10% of annual atmospheric deposition of N is exported in stream flow. The fate of the remaining 90% or the location of hot spots in the landscape that are responsible for removal of N remain unknown. The question is where and when does N loss take place and what is the role of terrestrial and aquatic landscape elements in N removal? This project emphasizes the relationship between individual storm characteristics, N transport, and N loss via denitrification, along a terrestrial-aquatic continuum from hillslope patches that occupy different topographic positions, to small intermittent channel networks that hydrologically link the terrestrial uplands with larger perennial streams during storms. Results demonstrate that storm size, intensity and time since last storm influence the spatial extent of surface flow in the intermittent network. In addition, the form of N in runoff water shifts from ammonium (NH4-N) during the first storm of the summer monsoon season to nitrate (NO3-N) toward the end. Denitrification rates varied considerably amongst landscape patches, with highest values under plants and on riparian terraces and lower values in intermediate channel networks. Field and laboratory experiments demonstrate that rainfall triggers denitrification in soils, with peak rates related to carbon (C) and N availability; and that N and moisture constrain the period of biological activity following a storm, depending on spatial location. The extent and configuration of fluvial reconnection amongst patches in the landscape following long drought periods determines the fate of available N, where N is processed and removed, and may cause hot spots to shift from place to place in response to local conditions. Thus, the spatial distribution of material and movement among different landscape elements, as well as hot spots for denitrification, will depend on size, sequence and intensity of individual storms.