Capturing interactions between nitrogen and hydrological cycles under historical climate and land use: Susquehanna watershed analysis with the GFDL land model LM3-TAN

摘要

We developed a process model LM3-TAN to assess the combined effects of directhuman influences and climate change on terrestrial and aquatic nitrogen (TAN)cycling. The model was developed by expanding NOAA's Geophysical FluidDynamics Laboratory land model LM3V-N of coupled terrestrial carbon andnitrogen (C-N) cycling and including new N cycling processes and inputs suchas a soil denitrification, point N sources to streams (i.e., sewage), andstream transport and microbial processes. Because the model integratesecological, hydrological, and biogeochemical processes, it captures keycontrols of the transport and fate of N in the vegetation–soil–river system in acomprehensive and consistent framework which is responsive to climaticvariations and land-use changes. We applied the model at 1/8°resolution for a study of the Susquehanna River Basin. We simulated withLM3-TAN stream dissolved organic-N, ammonium-N, and nitrate-N loadsthroughout the river network, and we evaluated the modeled loads for1986–2005 using data from 16 monitoring stations as well as a reported budgetfor the entire basin. By accounting for interannual hydrologic variability,the model was able to capture interannual variations of stream N loadings.While the model was calibrated with the stream N loads only at the lastdownstream Susquehanna River Basin Commission station Marietta (40°02' N,76°32' W), it captured the N loads well atmultiple locations within the basin with different climate regimes, land-usetypes, and associated N sources and transformations in the sub-basins.Furthermore, the calculated and previously reported N budgets agreed well atthe level of the whole Susquehanna watershed. Here we illustrate how pointand non-point N sources contributing to the various ecosystems are stored,lost, and exported via the river. Local analysis of six sub-basins showedcombined effects of land use and climate on soil denitrification rates, withthe highest rates in the Lower Susquehanna Sub-Basin (extensive agriculture;Atlantic coastal climate) and the lowest rates in the West Branch SusquehannaSub-Basin (mostly forest; Great Lakes and Midwest climate). In the re-growingsecondary forests, most of the N from non-point sources was stored in thevegetation and soil, but in the agricultural lands most N inputs were removedby soil denitrification, indicating that anthropogenic N applications coulddrive substantial increase of N₂O emission,an intermediate of the denitrification process.