The P submodel (Figure 3-4) has the same general structure as the N submodel. The major difference is that there are five mineral P pools: labile P (PLABIL), sorbed P, strongly sorbed P (SECNDY(2)), parent P (PARENT(2), and occluded P (OCCLUD). The mineralized P pools (MINERL(*,2)) are specified by soil layer, and summed for the soil rooting depth in TMINRL(2). The labile, strongly sorbed, parent material, and occluded P pools represent the entire soil profile (SOILDEPTH). This structure is shown in the following table:
|Table: Example of the relationships between mineral P pools for a soil containing 3 layers.|
The phosphorus submodel (Figure 3-4) has been revised to give a better representation of phosphorus sorption. Because CENTURY uses a relatively long decomposition timestep (¼ month for the soil nutrient submodel) and soil solution very rapidly equilibriates with the labile fraction of adsorbed P (Cole et al., 1977) it is not appropriate to use soil solution P for the available nutrient pool. Instead, the labile P pool (PLABIL) has been defined, equivalent to resin extractable P, which is in equilibrium with a sorbed P pool ( Figure 3-5). The equilibrium between the labile and sorbed P pools is recalculated after any P additions or removals from the soil. The sum of labile P and sorbed P are represented by the state variable MINERL(*,2). Plant uptake, immobilization and leaching of P (if allowed) are controlled by the size of the labile P pool PLABIL. The fraction of labile P that is available for plant uptake varies from 0.4 to 0.8 as a function (FAVAIL(*)) of the mineral N pool size (higher fractions for high mineral N levels). As more P is removed through plant and soil microbial uptake, larger amounts become immobilized in organic matter.
The equilibrium relationship between labile P and sorbed P is defined in terms of two parameters, sorption affinity (site parameter PSLSRB) and sorption maximum (site parameter SORPMX). The sorption affinity parameter controls the fraction of the labile plus sorbed pools which is in the labile pool at low levels of P in these pools. The sorption maximum is the maximum amount of P which can be in the sorbed P pool. The sorption maximum controls the curvature of the relationship between labile P and the sum of the labile and sorbed P pools. Since these parameters control P uptake, their values should reflect the rooting depth (NLAYPG).
The sorbed P is in dynamic equilibrium (fixed parameters PSECMN(2), PMNSEC(2)) with a more strongly sorbed P pool (SECNDY(2)) which may in turn lose P (fixed parameter PSECOC) to an occluded P pool (OCCLUD). Phosphorus can enter the cycling P pools by weathering of parent material P (PARENT(2)), which is typically apatite. The rate of weathering (fixed parameter PPARMN(2)) can be a function of soil texture (fixed parameter TEXEPP(*)) (higher for fine textured soils). The rate of these P flows are all multiplied by the same moisture and temperature functions (DEFAC) that are used for organic matter decomposition.
The organic part of the P submodel operates in the same way that the N submodel works; C:P ratios of organic fractions are fixed for the structural P pool (ratio = 500) and vary as a function of the labile P pool (PLABIL) for the active, (30-80), slow (90-200), and passive (20-200) SOM pools. C:P ratios of newly formed surface microbes are functions of the P content of the material decomposing, and the C:P ratio of slow material formed from the surface microbes is a function of the C:P ratio of surface microbes. The flows for the organic P pools are calculated in exactly the same way as organic N flow.
Phosphorus losses from the system occur as result of leaching of labile P (specified as deep storage in MINERL(10,2)). P losses accumulate in the soil layer below the last layer) and organic P compounds (STREAM(7)), soil erosion, crop removal, grazing, and burning P losses. P additions come from P fertilizer and organic matter additions.