The nitrogen submodel (Figure 3-3) has the same structure as the soil C model. The N flows follow the C flows (N flows between organic pools not shown in Figure 3-3 can be seen in Figure 3-1), and are equal to the product of the C flow and the N:C ratio of the state variable that receives the carbon. The C:N ratio of the structural pools (150) remains fixed while the N contents of the metabolic pools vary as a function of the N content of the incoming plant residue. The C:N ratio of newly formed surface microbial biomass is a function of the N content of the material being decomposed (increases for low N content). The C:N ratios of organic matter entering each of the three soil pools vary as linear functions of the size of the mineral N pool. As mineral N in the simulation soil layer increases from 0 to 2 g N m-2, the C:N ratios decrease from 15 to 3 for the active pool, from 20 to 12 for the slow pool and from 10 to 7 for the passive pool. The C:N ratio for slow material formed from surface microbial biomass is a function of C:N ratio of the surface microbe pool.
The N associated with carbon lost in respiration (30% to 80% of the carbon flow is respired) is assumed to be mineralized. Given the C:N ratio of the state variables and the microbial respiration loss for each flow, decomposition of metabolic residue, active, slow, and passive pools generally result in net mineralization of N, while decomposition of structural residue immobilizes N. Mineralized N is specified by soil layer in MINERL(*,1), and summed for the soil rooting depth in TMINRL(1)
The model uses simple equations to represent N inputs due to atmospheric wet and dry deposition, and soil and plant N fixation. Atmospheric N input (N output variable WDFXA) is a linear function of annual precipitation (output variable PRCANN), for precipitation <eq; 80 cm yr-1, set by the site parameters EPNFA(*):
WDFXA = EPNFA(1) + PRCANN * EPNFA(2)
Non-symbiotic soil N fixation, depending upon the fixed parameter NSNFIX, is calculated as a function of either the mineral N to labile P ratio (high fixation with lower ratios) or as a linear function of annual precipitation (in N output variable WDFXS), for precipitation <eq; 100 cm yr-1, set by the site parameters EPNFS(*):
WDFXS = EPNFS(1) + PRCANN * EPNFS(2)
Symbiotic plant N fixation (SNFXAC) is assumed to occur only when there is insufficient mineral N to satisfy the plant N requirement, having taken into account all possible growth reductions including P or S deficiency. Symbiotic N fixation can occur up to a maximum level of g N fixed per g C fixed (SNFXMX, crop parameters) specified for each crop type and is hence related to the plant growth rate. The model also includes fertilizer N inputs and N inputs through organic matter additions (see Fertilizer Additions and the organic matter addition parameters).
The losses of N due to leaching of NO3 are related to soil texture and the amount of water moving through the soil profile (see the description of the water flow submodel). Losses accumulate in the layer below the last soil layer (MINERL (10,1)) or are lost in the stream flow (STREAM(2)). Loss of organic N (STREAM(6)) occurs with the leaching of organic matter.
Gaseous losses of N compounds associated with mineralization /nitrification (VOLGM and VOLGMA), denitrification (VOLEX and VOLEXA), volatilization from harvesting crops (VOLPL and VOLPLA) are calculated. Losses due to burning, transfer of N in animal excreta, and soil erosion are also accounted for.