ORGANIC C DYNAMICS IN GRASSLAND SOILS. 2. MODEL VALIDATION AND SIMULATION OF THE LONG-TERM EFFECTS OF CULTIVATION AND RAINFALL EROSION

Abstract

The amounts of organic matter in native prairie and in an adjacent cultivated field were compared with the output from a simulation model describing organic matter dynamics. The effects of past and possible future soil management practices, and the loss of organic C through rainfall erosion were incorporated into the simulation study. Seventy years of cultivation increased the bulk density of the A horizon by an average of 16% along the catena of a Black Chernozemic soil. Organic C had decreased by 36% in the soil profile at the mid-slope position. Losses of organic N were 5–10% less. Depletion of organic C and N from the Ah horizon accounted for > 90% of the total loss from the soil profile. Therefore, extrapolation of data from surface soil, based solely on changes in the concentration of organic C and N, could result in an overestimation of organic matter losses from soils. Microbial biomass in the Ap horizon of the crop-summer-fallow site was 30% less than in the Ah horizon of the native prairie. The model predicted an immediate rise in microbial biomass C upon cultivation of the native prairie due to a large initial input of grassland litter and roots. Subsequently, the microbial biomass C decreased and approached a steady-state level which was 25% less than in the native prairie. The model indicates that large quantities of N released during the initial years of cultivation would not have been totally utilized by the cultivated crops, therefore resulting in major losses to the environment. However, now the organic matter is reaching a steady-state level and only small net release of N can be expected; external N sources are required for optimum crop production. Management practices such as straw removal and cropping sequence have short-term effects on the rate of depletion of soil organic C. Similar equilibrium levels of soil organic matter were predicted after 100 yr of cultivation in simulation studies that did not consider erosion losses. The inclusion of rainfall erosion losses indicated that major organic C and other nutrient losses will occur in management practices that include significant portions of fallow in the cropping sequence.