A mechanistic treatment of the dominant soil nitrogen cycling processes: Model development, testing, and application

Abstract

[1] 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 N loss as either gas emission or solute leachate following fertilization and irrigation in a Central Valley, California, agricultural field as functions of fertilizer application rate and depth, and irrigation water volume. Our results, relative to the period before plants emerge, show that an increase in fertilizer rate produced a nonlinear response in terms of N losses. An increase of irrigation volume produced NO 2 À and NO 3 À 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 rate. 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.

Complexity in microbial metabolic processes in soil nitrogen modeling: a case for model averaging

“…The three models TRN1 (complex with high complexity), TRN2 (middle with intermediate complexity), and TRN3 (simple with low complexity) were comprised of mechanistic representations of physical, chemical, and biological aspects. Complex model TRN1 used here was previously presented in Maggi et al (2008) (Fig. 1).…”

“…In this work, we considered three different models of the biogeochemical N cycle; each model was derived from the underlying model TOUGHREACT-N (Maggi et al 2008;Gu et al 2009) but each of them was characterized by a different level of complexity. The three models TRN1 (complex with high complexity), TRN2 (middle with intermediate complexity), and TRN3 (simple with low complexity) were comprised of mechanistic representations of physical, chemical, and biological aspects.…”

“…In the last few decades, much effort has been invested in developing biogeochemical models with different complexity that capture nutrient cycling across the scales (Li et al 1992;Parton et al 2001;Maggi et al 2008). Regardless of the detail at which the chemical, biological and physical processes are taken into account, all these models are, in the best case, selective mathematical approximations of processes that are essentially complex.…”

“…Therefore, the second objective focuses on addressing the model predictive uncertainty in biogeochemical models by using Bayesian Model Averaging approach. We developed three different versions of biogeochemical N cycle model THOUGHREACT-N (Maggi et al 2008;Gu et al 2009;Xu 2008). Each version of the model presents different complexity levels in representing the N-cycling in our study field.…”

“…It was outside the limits of this study to investigate all the possible model complexity mentioned previously. The process used here represents only a subset of N transformation processes, but arguably most dominant process that controls soil nitrogen fate (Maggi et al 2008). In the current study, we described the nitrogen microbial metabolic processing (1) lumped microbial kinetics or (2) disaggregating into functional groups (e.g., denitrifiers and ammonia oxidizers).…”