Changes in soil organic matter under different tillage and rotation: Mathematical modeling in ecosys

Reductions in both tillage and fallowing may increase soil organic C. The effects of tillage and fallowing on soil C could be estimated under site- and climate-specific conditions by mathematical modeling. The simulation model ecosys predicts changes in soil C by calculating C added through fixation by plant communities and C lost through oxidation by microbial communities. During a 14-yr simulation of a field experiment in a semiarid environment, ecosys predicted that 18 and 20 g m(-2) yr(-1) more C would be sequestered in the upper 0.15 m of soil in continuous wheat (W-W; triticum aestivum L.) than in wheat-fallow (W-F) under conventional tillage (CT) and no tillage (NT), respectively, During the same period, ecosys predicted that 14 and 12 g m(-2) yr(-1) more C would be sequestered in the upper 0.15 m of soil under NT than under CT in W-W and W-F, respectively. These increases were compared with those measured from the field experiment of 9 and 19 g m(-2) yr(-1) in W-W than W-F under CT and NT, respectively, and of 13 and 4.5 g m(-2) yr(-1) under NT than CT in W-W and W-F, respectively, The sensitivity of simulated changes in soil C to doubled atmospheric CO2 and a 3 or 6 degrees C increase in air temperature was then tested. Similarity in the direction and magnitude of simulated and measured changes in soil C lend initial support to model hypotheses. However, there are some issues concerning the modeling and measurement of soil C that require further clarification.