Agricultural Lands Assessment of Nitrogen (ALAN) is a one-dimensional process-oriented model designed to describe in reasonable detail the fate of water and nitrogen (N) in croplands at short time-steps (i.e. minutes to hours) using easy-to-obtain input data.  ALAN integrates calculations made by eight sub-models as depicted in Figure 1. The Canopy-Transpiration-Photosynthesis (CTP) sub-model originally simulated crop transpiration, net leaf photosynthesis and stomatal conductance based on a two-leaf canopy model (Kremer et al., 2008), but was extended to include soil sensible and latent heat fluxes, crop senescence, and water interception by the crop. The crop growth sub-model simulates leaf area index and biomass accumulation based on photosynthesis and respiration rates, but restricted by shortages of soil N. Photosynthates are partitioned into leaves, stems, roots and storage organs, based on the crop development stage, which is driven by the accumulation of growing degree days. Crop N uptake is computed by a supply/demand scheme. The soil organic carbon (C) sub-model simulates the turnover of soil organic matter, manure amendments and crop residue additions, CO₂ production, and mineralization and immobilization of inorganic N using a one-pool model of soil organic matter as proposed by Kemanian and Stöckle (2010). The urea hydrolysis sub-model computes hydrolysis of urea based on soil organic carbon, soil pH and soil moisture as proposed by Goodwin and Jones (1991). The nitrification sub-model simulates oxidation of soil ammonium (NH₄⁺) with production of nitric oxide (NO) and (nitrous oxide) N₂O, based on a first-order process controlled by soil pH, soil temperature, and soil moisture. The denitrification sub-model computes reduction of soil nitrate (NO₃^-) with production of N₂O and dinitrogen (N₂). The approach is similar to that of Parton et al. (1996) and del Grosso et al. (2000), in which the degree of reduction is controlled by the soil moisture as an indicator of anoxic conditions, soil NO₃^- (electron acceptors) and soil respiration rate as an indicator of labile C levels (electron donors). 

Figure 1.  Simplified flow diagram of ALAN model

The sub-model for the transport of water, solutes, gases and heat in the soil profile solves governing partial differential equations by a fully-implicit, cell-centered finite difference formulation. Water, solutes and gases are solved together in a nested, variable-time procedure. Soil water transport is simulated by solving the Richards’ equation in a mass-conserving procedure that couples the modified Picard iteration scheme developed by Celia et al. (1990) with the internodal hydraulic conductivity algorithm developed by Romano et al. (1998). Soil runoff is computed by setting a soil surface ponding capacity, while deep percolation is calculated by tracking the water flux at the lower boundary of the soil profile. Soil NO₃^- is considered to be transported in the soil only by convection, assuming that numerical dispersion substitute, to some extent, for physical dispersion. Transport of NH₄⁺-N is simulated by a common formulation of the convection equation for aqueous NH₄⁺ coupled with a diffusion equation for gaseous ammonia (NH₃). Solution-phase and adsorbed NH₄⁺ are related by a Langmuir isotherm. Convective transport of both NO₃^- and NH₄⁺ make use of a backward-time, upwind-type scheme. Transport of gaseous NO and N₂O is simulated by a diffusion equation, using a source term for the production of NO or N₂O due to nitrification and/or denitrification processes. Gas diffusion coefficients are computed by three optional models developed and evaluated by Moldrup et al. (1997, 1999, 2004). Soil heat transport is simulated by the Fourier’s law of heat conduction according the procedure developed by Campbell (1985).

The NH₃ exchange sub-model uses information from other sub-models to calculate the network of NH₃ fluxes in the soil-plant-atmosphere system as shown in Figure 2. Sub-stomatal NH₃ concentration is calculated according to Farquhar et al. (1980) by relating the ratio of apoplastic [NH₄⁺]/[H⁺] to the plant N concentration, based on studies by Husted et al. (1996), Schjoerring et al. (1998), and Mattsson and Schjoerrinig (2002). NH₃ is allowed to be deposited on the leaf cuticle based on the model of Sutton and Fowler (1993), but is washed off by precipitation and irrigation events. Finally, ALAN is driven by meteorological variables and management practices. Daily meteorological variables are distributed into short-term values according to the procedure described by Kremer (2006). Daily rainfall, however, is disaggregated into short-time intensity values using a modification of the model developed by Arnold and Williams (1989). Management practices considered are tillage operations, manure amendments, and applications of water and N-fertilizers. Inorganic N can be applied as urea, NH₄⁺- and NO₃^--based fertilizers, on the soil surface, incorporated into the soil, or distributed by the irrigation water.  

Figure 2.  Electrical analog of NH₃ exchange in the soil-plant-atmosphere system as described by ALAN model

References 

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