Initialisation of the soil organic matter pools of the Daisy model

Like most other contemporary models of long-term soil organic matter (SOM) dynamics, Daisy partitions the refractory component of the SOM into two or more pools. In long-term simulations, the initial distribution of SOM between these two pools influences the simulations. However, as these pools do not correspond to any measurable entities the distribution of SOM cannot be initialised by a simple measurement. Daisy has usually been initialised by using a standard distribution, which almost corresponds to equilibrium. In applications of other models, the initial distribution between the different pools of organic matter has been chosen by assuming equilibrium at the beginning of the simulation, by using standard values or by calibrating the initial distribution to optimise simulation performance. However, as we show here the initial distribution of SOM between the different pools influences simulations notably, and the appropriate distribution is dependent on changes in management and climate at the site before the onset of a simulated experiment. This is done by simulating a scenario and initialising the Daisy model in to different ways: (1) by assuming equilibrium at the beginning of the simulation; and (2) by simulating the preexperimental management history of the site. This shows that it can be important to use plausible assumptions to initialise SOM models, and that often the only way to initialise the model is to simulate the preexperimental period of the site.