Soil carbon modelling as a tool for carbon balance studies in forestry

Abstract

Soils represent a remarkable stock of carbon, and forest soils are estimated to hold half of the global stock of soil carbon. Topical concern about the effects of climate change and forest management on soil carbon as well as practical reporting requirements set by climate conventions have created a need to assess soil carbon stock changes reliably and transparently. The large spatial variability of soil carbon commensurate with relatively slow changes in stocks hinders the assessment of soil carbon stocks and their changes by direct measurements. Due to these difficulties in measuring soil carbon, models widely serve to estimate carbon stocks and stock changes in soils. This dissertation aimed to develop the soil carbon model YASSO for upland forest soils. The model was aimed to take into account the most important processes controlling the decomposition in soils, yet remain simple enough to ensure its practical applicability in different applications. The model structure and assumptions were presented and the model parameters were defined with empirical measurements. The model was evaluated by studying the sensitivities of the model results to parameter values, by estimating the precision of the results with an uncertainty analysis, and by assessing the accuracy of the model by comparing the predictions against measured data and by comparing the model results to the results of an alternative model. The model was applied at the stand level to study the effects of intensified biomass extraction on the forest carbon balance. In another application, the effects of energy use of forest residues on soil carbon were quantified with the model. The model calculated soil carbon deficit was presented as an indirect CO2 emission. This emission was then compared to other emissions from the forest residue production chain and burning. Finally, the model was applied in an inventory based method to assess the national scale forest carbon balance for Finland’s forests from 1922 to 2004. According to the results of the uncertainty and sensitivity analyses, the soil carbon stock estimates of the model are uncertain, because those parameters that most strongly affect these estimates are poorly known. Carbon stock change estimates, on the other hand, are rather reliable, because the parameters determining these estimates are known better. According to a test conducted with a Canadian litterbag experiment, YASSO managed to describe sufficiently the effects of both the variable litter and climatic conditions on decomposition. When combined with the stand models or other systems providing litter information, the dynamic approach of the model proved to be powerful for estimating changes in soil carbon stocks on different scales. The climate dependency of the model, the effects of nitrogen on decomposition and forest growth as well as the effects of soil texture on soil carbon stock dynamics are areas for development when considering the applicability of the model to different research questions, different land use types and wider geographic regions. Intensified biomass extraction affects soil carbon stocks, and these changes in stocks should be taken into account when considering the net effects of forest residue utilisation as energy. On a national scale, soil carbon stocks play an important role in forest carbon balances.