Simulating processes of soil phosphorus in geologically young acidic soils of Finland

In this study, the P submodel of the ICECREAM model, based on the CREAMS, GLEAMS, and EPIC models, was adapted to simulate young acidic soils, which prevail in Finland. These soils have a high P sorption capacity, plenty of inorganic P in a non-reactive apatitic form, and the organic matter content (O(r)) in the plow layer is commonly about 5%.

The model was modified mainly on the basis of 12 long-term fertilization experiments and tested on three other soils. The P pools considered were labile P (P(il)), active inorganic P (P(ia)), stable inorganic P (P(is)), fresh organic P (P(of)), and more stable organic P (P(oh)).

The major model modifications made were:

1. initialization of P(il) from the Finnish soil test results (ammonium acetate extractable P, P(AAc));
2. initialization of P(ia) and P(is) from P(il), assuming equilibrium;
3. revision of the P availability factor (F1), which regulates the flow between P(il) and P(ia);
4. calculation of the initial values of P(oh) from soil organic carbon content.

These modifications aimed to improve simulation of P(il) on the basis of data obtained from soil testing. Proper simulation of changes in P(il), reflected by the soil test P (P(AAc)), is critical since it regulates the level of dissolved P in runoff water. After the modifications, the model was able to predict changes in P(AAc) at annual P application rates of 0 and 30 kg ha(-1) in cereal cropping, but could not fully cope with a large surplus of P resulting from an annual rate of 60 kg P ha(-1).