Simulating trends in soil organic carbon in long-term experiments using RothC-26.3

As part of a model evaluation exercise, RothC-26.3, a model for the turnover of organic carbon in non-waterlogged soils, was fitted to measurements of organic carbon from 18 different experimental treatments on 6 long-term experimental sites in Germany, England, the USA, the Czech Republic and Australia. In the fitting process, the model was first run with an annual return of plant C that had been selected iteratively to give the carbon content of the soil at the start of each experiment. This was done for the soil and climate of each site. If the radiocarbon content of the soil organic matter was known, the inert organic carbon (IOM) content of the soil was also calculated for the start of the experiment. Using these carbon and radiocarbon contents as a starting point, the model was then run for each of the experimental treatments to be fitted, using iteratively selected values for the annual return of plant materials to the soil. The value used for each treatment was selected to optimise the fit between modelled and measured data over the whole experimental period: fitting was done by eye. Thus fitted, RothC-26.3 gave an acceptable approximation to the measurements for 14 of the treatments, bearing in mind the experimental errors in measuring soil organic carbon on a per hectare basis. With four of the treatments (Highfield Bare Fallow, Park Grass plot 13d, Ruzyne farmyard manure plot and Tamworth rotation 5), the fit was less satisfactory.