Most of the internal parameters in CENTURY were determined by
fitting the model to long-term soil decomposition experiments (1 to
5 year) where different types of plant material were added to soils
with a number of soil textures (Parton et al., 1987). Other more
general databases (Parton et
al., 1988;Parton et al.,
1989) were used to parameterize the P and S submodels and flows
for the formation of passive SOM. Many of the parameters such as
the plant nutrient content and lignin content were determined using
a linear equation where the slope and intercept were the input
parameters. Work in the Great Plains suggested that lignin and N
content changed as a linear function of annual precipitation. To
specify constant values for these parameters, set the slope
parameter (FLIGNI(2,*),
`crop.100`

) equal to zero and set the intercept
(FLIGNI(1,*),
`crop.100`

) equal to the desired value for the parameter.

The model includes a method for estimating steady state soil C and N levels in grassland systems which was developed for the U.S. Great Plains. If IVAUTO (site parameters) is set to 1, the model will estimate initial soil C and N levels for the different soil fractions based on the mean annual temperature, annual precipitation and soil texture of grassland (Burke et al., 1989). IVAUTO = 2 uses Burke's et al equations for cultivated fields to estimate these levels. The soil P and S levels are quite different depending on soil parent material and need to be estimated with site-specific data.

One of the most difficult parts of initializing the model is estimating the C, N, P, and S levels for the different soil fractions. However, substantial progress has been made recently in estimating the size of the soil fractions. The active soil fraction includes the live soil microbes and microbial products. This fraction can be estimated by using the microbial fumigation technique (Jenkinson and Powlson, 1976;Jenkinson et al.,1976;Jenkinson and Rayner 1977) to estimate the live microbial biomass and then doubling the live microbial biomass to account for the microbial products (active SOM = 2 to 3 times the live microbial biomass). In most soils the active soil fraction is approximately 2 to 4% of the total soil C. The slow SOM fraction is made up of lignin derived plant material and stabilized microbial products. This fraction makes up approximately 55% of the total SOM. Recent developments in SOM fractionation (Elliott and Cambardella, 1992) suggest that 40% of the total SOM in grasslands is lignin-derived plant material (referred to as POM, or "particulate organic matter" in the paper). Comparison of the size of the slow pool from C simulations with measurements of SOM indicate that the slow pool is approximately 1.6 times the amount of POM (Metherell et al., 1993b).

Unfortunately there is not a good technique for estimating the size of the stabilized microbial products pool; however, it is estimated that it is approximately 10 to 20% of the soil. The passive SOM generally makes up 30 to 40% of the total SOM and will have a higher value for high clay content soils. The best estimate of the N content of these fractions are that the slow fraction has a C:N ratio of 15 to 20, the active SOM has a C:N ratio of 8 to 12, while the passive SOM has a C:N ratio of 7 to 10. Clay soils have lower C:N ratios while silty soils have higher C:N ratios for the passive SOM. These approximations seem to work well for a large number of different soils.

The C:P and C:S ratios are not as predictable and are functions of the initial soil parent material and degree of soil weathering. The same general rules apply for C:P and C:S ratios with the active SOM having relatively low ratios (50-100), the slow SOM the highest C:P and C:S ratios (100-300), while the passive C:P and C:S ratios are fairly low (40-120). These values are appropriate for the relatively unweathered soils in the U.S. Great Plains. More weathered tropical soils have much higher C:P and C:S ratios that can be as high as 800. To use the P and S submodels, determine the organic P and S levels and it would be preferable to run full P fractionation of the soil (see citations in Hedley et al., 1982). The C:N ratio and relative size approximations are incorporated into the model when the Burke equations are used (IVAUTO=1, site parameters) to estimate initial SOM pools. For cultivated soils it is generally assumed that the size of the slow pool is lower because of cultivation (40 to 50% of the total SOM) while the size of the passive pool is increased (45 to 50%).

Decomposition and production submodels interact in ways that affect the amounts of soil C and N, soil temperature and moisture, and the rates of decomposition and production. Several parameters influence these interactions, including the "fixed" parameters IDEF, TEFF(*), DEC*, and FAVAIL(1). For example, a small increase in FAVAIL(1) increases production and decomposition rates, which can result in proportionally larger changes in soil C and N. You can tune these "fixed" parameters to match measured rates and amounts. Important production parameters are PRDX(*) and FAVAIL(1). Important decomposition parameters are IDEF, TEFF(*), DEC*.

The model has been parameterized to simulate soil organic matter dynamics in the top 20 cm of the soil. The model does not simulate organic matter in the deeper soil layers and increasing the soil depth parameter (EDEPTH, fixed parameters) does not have much impact on the model. EDEPTH is used to calculate C, N and P loss when erosion occurs. To simulate a deeper soil depth (i.e., 0-30 or 0-40 cm depth) the soil organic matter pools must be initialized appropriately. As a general rule deeper soil depths have older soil carbon dates (Jenkinson et al., 1992) and lower decomposition rates (lower temperature at deeper depths). Thus, it would be assumed that the fraction of total SOM in the passive SOM would be greater. The major change for initializing the model for deep soil depths is adjusting the fraction of SOM in the different pools (more C in passive SOM). The initial soil C levels should reflect the observed soil C levels over that depth and the decomposition rates should be decreased for all of the SOM pools (DEC3, DEC4, DEC5). To increase the soil depth from 20 cm to 30 cm, the decomposition rates should be decreased by 15%. The other adjustment would be to increase the rate of formation of passive SOM; the recommended way is to increase the flow of C from active and slow SOM to passive SOM (PS1S3 and PS2S3, fixed parameters). For example, increasing the coefficients in PS2S3 and PS1S3 will increase the amount of passive SOM formed from slow SOM and active SOM.

Site Parameters

Model Parameters

Erosion and Deposition