Soil Organic Matter Submodel

The SOM submodel is based on multiple compartments for SOM and is similar to other models of SOM dynamics (Jenkinson and Rayner, 1977; Jenkinson, 1990; van Veen and Paul, 1981). The pools and flows of C are illustrated in Figure 3-1. The model includes three soil organic matter pools (active, slow and passive) with different potential decomposition rates, above and belowground litter pools and a surface microbial pool which is associated with decomposing surface litter. The pool amounts are changed by transfers from other SOM pools, microbial and plant pools, decomposition, and leaching.

Above and belowground plant residues and organic animal excreta are partitioned into structural (STRUCC(*)) and metabolic (METABC(*)) pools as a function of the lignin to N ratio in the residue. With increases in the ratio, more of the residue is partitioned to the structural pools which have much slower decay rates than the metabolic pools. The structural pools contain all of the plant lignin (STRLIG(*)).

The decomposition of both plant residues and SOM are assumed to be microbially mediated with an associated loss of CO2 (RESP(*)) as a result of microbial respiration. The loss of CO2 on decomposition of the active pool increases with increasing soil sand content. Decomposition products flow into a surface microbe pool (SOM1C(1)) or one of three SOM pools, each characterized by different maximum decomposition rates.

The decomposition rate of the structural material (STRUCC(*)) is a function of the fraction of the structural material that is lignin. The lignin fraction of the plant material does not go through the surface microbe (SOM1C(1)) or active pools (SOM1C(2)) but is assumed to go directly to the slow C pool (SOM2C) as the structural plant material decomposes.

The potential decomposition rate is reduced by multiplicative functions (DEFAC) of soil moisture and soil temperature and may be increased as an effect of cultivation (CLTEFF(*), cult.100). Expected average annual values of DEFAC (ADEFAC) are 0.1 to 0.2 for C3 grasslands, 0.4 for temperate deciduous forests, and 0.9 for tropical forests. Average monthly soil temperature near the soil surface (STEMP) is the input for the temperature function. The fixed parameter IDEF specifies the kind of calculation used for the effect soil moisture upon the decomposition rate.

Effect of Soil Moisture

The IDEF values are:

  1. The soil moisture function uses the relative water content of the simulation soil layer.
  2. The soil moisture function uses the ratio of the current month's precipitation (RAIN) to potential evapotranspiration (PET).
  3. (DayCent5 only) Uses an asymmetrical bell curve based upon the water-filled pore space of the simulation soil layer.

The figure below shows the behavior of the IDEF options.

Effect of Soil Temperature

The temperature effect upon decomposition is calculated differently for monthly and daily Century. The effect has a maximum value of 1.2. The monthly version uses an exponential function:

temperature effect =  TEFF(1) + exp ( TEFF(3) * soilTemp ) * TEFF(2)

The daily version uses an arctangent function, normalized to 30 degrees C:

temperature effect = 
TEFF(2) + (TEFF(3) / PI) * ATAN( PI * TEFF(4) * [soilTemp - TEFF(1)] ) ] / value_at_30C

See this page of graphs for the behavior of the equations with various parameter values.

Turnover Rates

The active pool (SOM1C(2)) represents soil microbes and microbial products (total active pool is ~2 to 3 times the live microbial biomass level) and has a turnover time of months to a few years depending on the environment and sand content. The soil texture influences the turnover rate of the active soil SOM (higher rates for sandy soils) and the efficiency of stabilizing active SOM into slow SOM (higher stabilization rates for clay soils). The surface microbial pool (SOM1C(1)) turnover rate is independent of soil texture, and it transfers material directly into the slow SOM pool (SOM2C). The slow pool includes resistant plant material derived from the structural pool and soil-stabilized microbial products derived from the active and surface microbe pools. It has a turnover time of 20 to 50 years. The passive pool (SOM3C) is very resistant to decomposition and includes physically and chemically stabilized SOM and has a turnover time of 400 to 2000 years. The proportions of the decomposition products which enter the passive pool from the slow and active pools increase with increasing soil clay content.

Other Effects

Anaerobic conditions (high soil water content) cause decomposition to decrease. The soil drainage factor (site parameter DRAIN) allows a soil to have differing degrees of wetness (e.g., DRAIN=1 for well drained sandy soils and DRAIN=0 for a poorly drained clay soil).

The model has N, P, and S pools analogous to all of the C pools. Each SOM pool has an allowable range of C to element ratios based on the conceptual model of McGill and Cole (1981). Reflecting the concept that N is stabilized in direct association with C, C to N ratios are constrained within narrow ranges, while the ester bonds of P and S allow C to P and C to S ratios to vary widely. The ratios in the structural pool are fixed at high values, while the ratio in the metabolic pool is allowed to float in concert with the nutrient content of the plant residues. The actual ratios for material entering each SOM pool are linear functions of the quantities of each element in the labile inorganic mineral pools in the surface soil layers (MINERL(*,*)). Low nutrient levels in the labile pools result in high C to element ratios in the various SOM pools. The N, P, and S flows between SOM pools are related to the C flows. The quantity of each element flowing out of a particular pool equals the product of the C flow and the element to C ratio of the pool. Mineralization or immobilization of N, P, and S occurs as is necessary to maintain the ratios discussed above. Thus, mineralization of N, P, and S occurs as C is lost in the form of CO2 and as C flows from pools with low ratios, such as the active pool, to those with higher ratios, such as the slow pool. Immobilization occurs when C flows from pools with high ratios, such as the structural pool, to those with lower ratios, such as the active pool. The decomposition rate is reduced if the quantity of any element is insufficient to meet the immobilization demand.

The simulation pools are affected by leaching as described in the section Water Budget, Leaching and Soil Temperature.

The organic C pools in the simulation layer affect the bulk density of the corresponding soil layers, and can modify the depth distribution of organic C in the entire soil profile. See the sections Layered Soil Submodel and Exponential Depth Distribution of Organic Carbon for details on the specific algorithms and calculations. At the end of every simulation year, change in the bulk densities and thicknesses of the soil layers corresponding to the simulation depth are calculated and applied. Next, the depth distribution of organic C in the entire profile is recalculated based upon the change change in organic C. The magnitude of the change in the organic C content is the differenct in the current and previous years' mean SOMSC, the monthly soil C content of the simulation layer.

A detailed description of the structure of an earlier version of the model and the way in which model parameters were estimated is found in Parton et al. (1987).

See Also

Fixed Parameters