Testing validity of soil carbon model Yasso07 against empirical data

Abstract

Forest soils are a heterogenic ecosystem with large carbon stock but small expected changes in relation to carbon stock size over time, and large spatial variation making it difficult to determine the changes of stock reliably with a small number of samples (Yanai et al.2003, Peltoniemi et al. 2004). Forests cover large areas of the Global land area and make them potentially significant sinks or sources of carbon over time. Reliable estimates of soil carbon stock are needed because changes of soil carbon stock are a part of the national greenhouse gas (GHG) inventories conducted under UNFCCC (United Nations Framework Convention on Climate Change) and Kyoto Protocol. Many countries have comprehensive and representative national forest inventories (NFI) that can be exploited as input data in assessing the changes in soil carbon pool by models. In between, models of biomass and turnover that convert NFI data into input data for the soil model are exploited. Therefore, soil carbon models predicting carbon stock changes are valuable tools in assessing changes in soil carbon stock. However, empirical data is necessary for verification of modelled soil carbon dynamics to achieve reliable estimates. Currently, GHG inventory of Finland uses Yasso soil carbon model to determine changes in forest soil carbon stocks and the updated version of this model Yasso07 to determine changes in soil carbon stocks after land use changes. Yasso model was selected to be used in greenhouse gas inventory inter alia due to fewer requirements in respect to input data compared to many other soil carbon models ensuring applicability to large areas (Peltoniemi et al. 2007). Yasso07 is based on a much wider range of measurements than the earlier version of Yasso and the parameter values for Yasso07 have been determined using more advanced mathematical methods (Markov chain Monte Carlo, MCMC samplings). Currently used soil carbon model does not take into consideration variation in soil moisture, although soil moisture is an important feature in many widely used soil carbon models (eg. CENTURY). Yasso07 uses annual or monthly precipitation as an approximation for soil moisture. Furthermore, in Yasso07 model the parameter uncertainties are known and can be taken into account when calculating predictions. The uncertainty estimation are required in GHG reporting and therefore updating to the Yasso07 model would facilitate that. Creation and operation of Yasso07 model is described and published in the scientific literature (Tuomi et al. 2009; Tuomi et al. 2011a; Tuomi et al. 2011b). The aim of this study was to test the validity of the soil carbon model Yasso07 against empirical data and to evaluate differences between model versions and their applicability to national green 2 house gas inventory. Validation was done following the rules given in the IPCC Good Practice Guidance (2003) in order to estimate the model’s applicability.