Simulating trends in soil organic carbon in long-term experiments using the DNDC model

Abstract

Simulations of long-term (> 20 year) soil organic carbon (SOC) dynamics by the DNDC model were compared with field observations at 11 plots in 5 field stations in Europe and Australia. The exercise was part of a NATO-sponsored workshop on long-term monitoring and modeling of soil organic matter. Eight of the eleven plots were cultivated cropland and three were grassland (harvested for hay). There were a range of fertilizer and manure treatments, as well as crop rotation sequences. Significant loss in SOC was observed at two plots in Australia where a grassland had been converted to cultivated cropland in 1925. Both field data and model simulations showed the plots reaching a new SOC equilibrium at about 44% of the 1925 levels. Equilibrium levels depended on crop rotation sequence, with higher SOC for the plot with less frequent fallowing. At one permanent grassland site at Rothamsted, UK, a large decline and recovery in SOC was observed in the field, but not in the model simulation. For all other cases, both field and model data showed relatively small changes in SOC, though field data tended to be more variable, perhaps due to variability in both crop and weed yield, and in residue management. Mean percent differences between simulated and measured SOC were 0.07% or less (as percent by weight, kilogram SOC/kilogram soil) for all but one of the plots simulated.