LONG-TERM EFFECTS OF CROP MANAGEMENT IN WHEAT-FALLOW .2. CENTURY MODEL SIMULATIONS

There is increasing need to develop models to assess the long-term effects of management practices on soil and environmental quality, and to test these models across a wide range of environments. The CENTURY model was used to simulate long-term management practices for wheat (Triticum aestivum L.)-fallow agriculture. The objectives were to compare the accuracy of predicted vs. observed data and use the model to help interpret observed data and to determine the long-term impact of crop management on C and N stabilization in soil. The model simulated grain and straw yield and grain N uptake within +/-10% of the observed data, and 0- to 30-cm soil C and N within +/-5%. Improvement in performance requires addition of a dynamic plant growth submodel to represent the effect of soil N on C and N allocation to roots, straw, and grain. Soil C changes were a linear function of aboveground C inputs; an input of 200 g C m-2 yr-1 (4650 kg straw ha-1) was required to stabilize soil C at its present level. Estimates of soil C stabilization efficiency ranged from 12 to 27% and was highest for the high N fertilizer treatment. Soil N stabilization efficiency was higher for organic N additions (37-46%) than for inorganic additions (18-26%). Inorganic N treatments had similar N removal in grain, higher other nonidentifiable N losses, but lower N stabilization in soil than organic N treatments. Soil N data suggest that mineralization of N from the 30- to 60-cm layer needs to be considered in the N budget, as nearly 39 g m2 were mine between 1931 and 1986.