This forest-focused version of DayCent embodies an improved root growth model that explicitly includes fast-turnover juvenile roots and slow-turnover structural roots. Most of the carbon allocated to root growth is sent to the juvenile roots which then mature into structural roots. The death rate of juvenile and structural roots is a function of soil water and temperature. ForCent also includes growth (fixed fraction) and maintenance (function of soil water and temperature) root respiration.  The model was parameterized using observed root growth data from Oak Ridge National Laboratory (ORNL) and tested using data from the EBIS tree ¹⁴C labeling experiment.  The potential role of root exudates on soil carbon formation and the impact of soil nitrogen levels on microbial growth efficiency are being investigated as well.

Results/Conclusions

The ForCent model effectively simulates observed changes in Δ ¹⁴C values from the total soil, mineral soil, surface litter layer, and O_e layers during 6-year experiment.  Model results suggest that autotrophic soil respiration comprises 50% of the total soil respiration during the summer and less than 20% during the winter months. It is necessary to include fast turnover juvenile roots and slow turnover structural roots in the model structure in order to correctly simulate the incorporation of labeled ¹⁴C material during the growth of fine roots and the release of ¹⁴C biomass into the soil following a ¹⁴C  pulse release in 1999.