Carbon and decomposition model Yasso for forest soils

Models are needed to estimate dynamics of carbon in forest soils, because changes in soil carbon are laborious to measure, and future levels of soil carbon can only be predicted using models. Current process-oriented soil carbon models are not suitable to all forestry-related applications. This is because they require specific input information that is not available for all forests, and their time step is shorter than a year which is typically used in forestry. We developed a dynamic soil carbon model Yasso to be used in forestry applications. Yasso simulates the stock of soil carbon, changes in this stock and the release of carbon from soil on an annual basis. It needs estimates of litter production, information on litter quality and basic data on climate to run. Yasso consists of five decomposition compartments and two woody litter compartments. Its parameter values were determined based on measurements of litter decomposition and soil carbon. The reliability of the output of Yasso was assessed by conducting an uncertainty analysis and comparing model-calculated estimates of soil carbon to measurements taken at different forest sites in southern Finland. According to the uncertainty analysis, the estimates for the amount of soil carbon are uncertain by nature, because they depend mostly on uncertain humus parameters. Still, when linked to a forest simulator to calculate litter production, Yasso gave similar estimates for the amount of soil carbon as were measured. The estimates for changes in soil carbon, on the other hand, are more reliable by nature because they depend on more accurately known parameters. These and other tests conducted so far suggest that Yasso is applicable to forests in a wide range of environments. Further tests will increase confidence in using it for different soils. (c) 2005 Elsevier B.V All rights reserved.