Bayesian calibration as a tool for initialising the carbon pools of dynamic soil models

The most widely applied soil carbon models partition the soil organic carbon into two or more kinetically defined conceptual pools. The initial distribution of soil organic matter between these pools influences the simulations. Like many other soil organic carbon models, the DAYCENT model is initialised by assuming equilibrium at the beginning of the simulation. However, as we show here, the initial distribution of soil organic matter between the different pools has an appreciable influence on simulations, and the appropriate distribution is dependent on the climate and management at the site before the onset of a simulated experiment. If the soil is not in equilibrium, the only way to initialise the model is to simulate the pre-experimental period of the site. Most often, the site history, in terms of land use and land management is often poorly defined at site level, and entirely unknown at regional level. Our objective was to identify a method that can be applied to initialise a model when the soil is not in equilibrium and historic data are not available, and which quantifies the uncertainty associated with initial soil carbon distribution. We demonstrate a method that uses Bayesian calibration by means of the Accept–Reject algorithm, and use this method to calibrate the initial distribution of soil organic carbon pools against observed soil respiration measurements. It was shown that, even in short-term simulations, model initialisation can have a major influence on the simulated results. The Bayesian calibration method quantified and reduced the uncertainties in initial carbon distribution. 2009 Elsevier Ltd. All rights reserved. Model initialisation can influence whether a model predicts soil at a given site to be a source or sink of CO2 (Falloon and Smith, 2000). There are many ecosystem models in use today, designed to meet different objectives. Despite their diversity, most of the models share some basic assumptions which include the representation of soil organic matter (SOM) as multiple pools with differing inherent decomposition rates governed by first order rate constants modified by climatic and edaphic reduction factors (Smith, 2001). However, as these conceptual model pools often do not correspond to measurable fractions (Smith et al., 2002), the division introduces an initialisation problem (Falloon and Smith, 2000). Incorrect initialisation of soil carbon (C) pools can cause spurious trends in output. Flawed initial conditions may produce fallacious trends as the state variables drift back towards the modelled ideal, also potentially leading to inaccurate assessment of