Agroecosystems /
Carbon Sequestration
Research Focus


Home

Who We Are

What We Do

Projects

Posters & Presentations

Carbon Sequestration

Century

Other Sites

Contact Us
  
Nebraska Phase I Progress Report
Modeling Soil Organic Matter

     The Century EcoSystem Soil Organic Matter Computer Model used in this study was first developed for grassland systems (Parton et al., 1987) but has subsequently been updated and enhanced (Metherall, et al., 1993; Parton et al., 1994; Paustian et al., In prep.) and has been used extensively to simulate organic matter and nutrient dynamics in agricultural cropping systems (e.g., Paustian et al., 1992, 1996, 2001a; 2001b; Carter et al., 1993; Parton and Rasmussen, 1994; Brenner et al., 2001a, 2001b). Century simulates long-term dynamics of carbon, nitrogen, phosphorus and sulfur in the top 20 cm of soil on a monthly basis and has proven to provide reliable estimates of soil C changes. Soil organic carbon and nitrogen stocks are represented by two plant litter pools and three soil organic matter pools (termed active, slow, and passive). The crop growth submodel simulates crop growth, dry matter production and yield to estimate the amount and quality of residue returned to the soil, as well as plant influence on soil water, nutrients and other factors affecting soil organic matter turnover. The soil water balance submodel calculates water balance components and changes in soil water availability, which influence both plant growth and decomposition/nutrient cycling processes. A variety of management options may be specified including crop type, tillage, fertilization, organic matter addition (e.g., manuring), harvest (with variable residue removal), drainage, irrigation, burning and grazing intensity. Specifying crop type and management options in the management schedule file simulates the desired cropping sequence. Figure 9 provides an overview of the Century model illustrating the main components of the model. Only carbon and nitrogen dynamics were addressed in this research. Model simulations did not include the occurrence of soil erosion.



Figure 9: Century Model

     The model was run for 6000 years to approximate steady-state conditions for pre-cultivation soil carbon contents under native vegetation using a stochastic weather generator (based on long-term mean climate) and the physical description for the region, including soil texture, soil hydric properties. The native vegetation is assumed to be a mixed warm/cool season grass prairie that was moderately grazed in the summer months with a fire frequency of three years in the eastern part of the state and fifteen years in the western part of the state.

     For the Phase I analysis, we initialized the model by first estimating pre-cultivation soil C contents followed by changes due to historical cropping practices up to and including present conditions, as described in this report.

Previous section                     Next section                    Back to Table of Contents