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Title:   Author  item
                                            MAGGI, F - UNIV OF CALIF BERKELEYitem
                                            GU, C - UNIV OF CALIF BERKELEYitem
                                            RILEY, W - LAWRENCE BERKELEY LABitem
                                            HORNBERGER, G - UNIVERSITY OF VIRGINIAitem
                                            Venterea, Rodney

                                                         - *Rod*item
                                            XU, T - LAWRENCE BERKELEY LABitem
                                            SPYCHER, N - LAWRENCE BERKELEY LABitem
                                            STEEFEL, C - LAWRENCE BERKELEY LABitem
                                            MILLER, N - LAWRENCE BERKELEY LABitem
                                            OLDENBURG, C - LAWRENCE BERKELEY LAB
  Submitted to: Journal of Geophysical Research-Biogeosciences
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 12/17/2007
                                    Publication  Date: 4/24/2008
                                    Citation: Maggi, F., Gu, C., Riley, W.J., Hornberger, G.M., Venterea, R.T., Xu, T., Spycher, N., Steefel, C., Miller, N.L., Oldenburg, C.M. 2008. A mechanistic treatment of the dominant soil nitrogen cycling processes: Model development, testing, and application. Journal of Geophysical Research-Biogeosciences. Available at: http://www.agu.org/journals/jg/jg0802/2007JG000578/2007JG000578.pdf.Interpretive Summary: The broad impact of NO and N2O gas emissions on climate change are widely recognized, as are the effects of NO3 water contamination on human health and eutrophication. Methods for evaluating the impacts of climate change, and fertilizer and water application techniques on N-losses in agriculture are needed for both scientific investigations and management to limit N losses. In view of the needs for future increases in crop yield for food,  fiber, and biofuel production, understanding the processes that regulate losses of solute and gaseous N-species assumes even greater importance. We adapted the coupled reactive transport model TOUGHREACT to investigate the ways in which fine-scale spatial and temporal aspects of the biogeochemical nitrogen cycle change with fertilizer amount, irrigation water amount, and fertilizer application depth. Our results indicate that responses are nonlinear both spatially and temporally, calling into question the assumptions of global assessments that invoke linear relationships between fluxes and both fertilizer and water application. TOUGHREACT-N has given promising evidence that mechanistic models can provide important tools for assessing the effects of agricultural practices on nitrogen balance and losses.  Such modeling tools can be used and interpreted by scientists, producers, and regulators in developing improved agricultural practices that both maximize production and minimize off-site N losses.
                                Technical Abstract: The development and initial application of a mechanistic model (TOUGHREACT-N) designed to characterize soil nitrogen (N) cycling and losses are described. The model couples advective and diffusive nutrient transport, multiple microbial biomass dynamics, and equilibrium and kinetic chemical reactions. TOUGHREACT-N was calibrated and tested against field measurements to assess pathways of nitrogen loss as either gas emission or solute leachate following fertilization and irrigation in a Central Valley, CA, agricultural field as functions of fertilizer application amount and depth, and irrigation water amount. Our results, relative to the period before plants emerge, show that an increase in fertilizer amount produced a nonlinear response of N-losses. An increase of irrigation water amount predominantly caused NO2- and NO3- leaching, whereas an increase in fertilization depth mainly increased leaching of all N-solutes. In addition, nitrifying bacteria largely increased in mass with increasing fertilizer amount. Increases in water application caused nitrifiers and denitrifiers to decrease and increase their mass, respectively, while nitrifiers and denitrifiers reversed their spatial stratification when fertilizer was applied below 15 cm depth. Coupling aqueous advection and diffusion, and gaseous diffusion with biological processes closely captured actual conditions and, in the system explored here, significantly clarified interpretation of field measurements.


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