SIMULATING TRENDS IN SOIL ORGANIC-CARBON IN LONG-TERM EXPERIMENTS USING THE ITE (EDINBURGH) FOREST AND HURLEY PASTURE ECOSYSTEM MODELS

Models are used increasingly to predict long-term changes in soil orga nic matter (SOM). Comparison with measured data is clearly desirable. We compared simulations of the mechanistic ITE (Edinburgh) Forest (EF) and Hurley Pasture (HP) ecosystem models with experimental SOM data f rom three long-term experiments: a 30 year old pine forest in South Ca rolina, USA, a 100 year old area of naturally regenerating woodland at Rothamsted in southeast England, and a 140 year old grass pasture sub jected to various input regimes also at Rothamsted. EF's model trees d ied too readily during occasional periods of drought, so we cut out th e water submodel (which includes leaching): the cut-down model simulat ed measured accumulation of C to within around 10% but greatly overest imated that of N, when leaching was in fact significant. Again, and fo r the same reason (plant death during drought), we had to cut the wate r submodel out of HP: the resulting simulations generally overestimate d SOM-N, especially in treatments receiving nitrogenous inputs, and br acketed the measured SOM-C data. Simulated SOM levels responded rapidl y to organic and inorganic inputs, however, whilst measured data did n ot. We therefore rewrote the SOM submodel to include protected and sta bilised SOM pools, in an attempt to buffer the system. The new submode l showed little effect of treatment, improved SOM-N simulations, but c onsistently overestimated SOM-C. This mismatch between measurement and model may reflect nothing more than tao shallow a sampling depth. We performed no site-specific parameter optimisation because: (1) the dat a sets are small; (2) it is not clear how much of the SOM in the syste m is contained within the experimental sampling depth; and (3) the mod els are mechanistic, with parameters reflecting real measurable proper ties of the system they represent. In the absence of such tuning, the models should simulate other relevant systems just as well as those pr esented here. (C) 1997 Elsevier Science B.V.