Century Modeled Organic Carbon Content in Soils with Diverse Ratios of 1500 kPa Water to Measured Clay.

Modeling of soil organic carbon (SOC) content is necessary in order to assess whether different land uses are sustainable over the long term in south central Chile, and to evaluate the potential changes of carbon storage in soils.  In general, SOC increases with increasing clay content and mean annual precipitation, decreasing mean annual temperature, and less intensive management.  Considering that the region has a variety of parent materials with varying degrees of volcanic ash, a principal consideration must first be to assess the relative effect of volcaniclastic parent materials on the accuracy of the model simulations.   The Century model was parameterized to climate, vegetation, management and soils (93) of the VIII Region of Chile that included 12 soils of the coast, 20 of the coastal mountains, 55 of the central valley, and 6 of the Andean foothills; which represented 11 Entisols, 24 Inceptisols, 13 Andisols, 24 Mollisols, 15 Alfisols, and 6 Ultisols. The ratios of 1500 kPa water/clay and cation exchange capacity (CEC)/clay at 50 cm depth were calculated for each soil to determine whether it could qualify for the isotic (volcaniclastic) mineralogy class, and to estimate clay activity, respectively. Simulations were run for periods representing 2000 years and the results were analyzed to asses whether the simulated contents of SOC coincided with the actual values in soils that have formed in diverse parent materials of volcanic and non volcanic origin. The predicted values were not significantly different from actual SOC within the Entisol, Inceptisol, Mollisol, Alfisol and Ultisol soil Orders, but were significantly lower for Andisol soils possibly because of the presence of additional clay that was not measured in particle size analysis.  However, SOC may also be stabilized in volcanic soils via the formation of Al-humus complexes. The Century model also underestimated SOC in 27 soils with increasing ratios (>1.2) of 1500 kPa water to measured clay at 50 cm soil depth, indicating isotic mineralogy that is commonly associated with non dispersed clay in soils. The CEC/clay ratio ranged from 0.44 to 7.16, and progressively increased with increasing ratios of 1500 kPa/clay, which similarly implied additional unmeasured clay that subsequently may have contributed to elevated contents of SOC compared to the Century simulations.  In regions such as south central Chile with volcanic and non volcanic parent materials across the landscape the 1500 kPa water/clay ratio can serve as an indicator of soils that have elevated values of SOC relative to the measured clay and require parameter adjustments for calibration of the Century model, such as using the % calculated clay instead of measured clay, or adjusting the parameters for degradation rates of the different fractions of soil organic matter.