The CENTURY model failed to simulate soil organic matter development in an acidic grassland

Extensive agricultural systems have been promoted all over Europe. Most studies of such systems have focused on plant species composition, while few studies have looked at soil effects. We report on an experiment at three upland sites where soil carbon, nitrogen and pH were measured twice a year for 12 years. There were four grazing treatments: no grazing, grazed to maintain a sward height of 8 cm, grazed to maintain 4 cm (all without fertiliser) and grazed to maintain 4 cm with the addition of fertiliser. There was no significant effect of treatment, but there were differences between sites over time. A simulation model, CENTURY, was adapted to the data. It also predicted little difference between treatments, but the fit to data was not very good. It was necessary to assume that the soils contained a store of undecomposed organic matter or peat that was now slowly decomposing as a result of liming. The need to modify the model to include a pH effect on decomposition in the model is highlighted.