Impacts of climate change on carbon and nitrogen cycles in boreal forest ecosystems

Eliasson, Peter (2007) Impacts of climate change on carbon and nitrogen cycles in boreal forest ecosystems. Doctoral diss. Dept. of Ecology, SLU. Acta Universitatis agriculturae Sueciae vol. 2007:89.

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Abstract

It is well known that soil carbon stocks decrease considerably in response to soil warming, but experimental data have shown that the loss of carbon declines within decades in apparent acclimation. An explanation to such findings is offered under the theoretical framework of ecosystem models in this thesis. Simulations of forest ecosystem responses to increased soil temperatures showed that the labile carbon in soil was reduced considerably within years after warming, although the structure and function of decomposer organisms remained intact.

Simulations of increased CO2 also confirmed positive growth response in the short term. The response of soil carbon was similar, however predicted to be less than the increase of biomass. Nitrogen availability and negative feedback mechanisms of the plant soil system were critical to the results, indicating that nitrogen progressively limited the growth response.

Assumptions concerning the response of decomposing organisms to changes in soil nitrogen are crucial to the interpretation of the above. Positive nitrogen feedback was therefore implemented to test consequences of nitrogen additions, allowing for improved decomposer efficiency when inorganic nitrogen becomes available in the soil. Responses compared well with experimental results. Simulations of moderate inputs of slowly increasing inorganic nitrogen loads in the long term revealed counteracting feedback responses: the positive nitrogen feedback resulted in increased decomposer efficiency and biomass growth also increased as expected; negative nitrogen feedback appeared in the sense that the growth response was reduced due to decreased mineralisation.

An economic estimation of the net value of carbon sink capacity was also performed. The value of the carbon sink was given in relation to conservative estimates of total net values of important stocks and flows of forest natural capital – factors such as recreational values or hydrological services omitted. The net value of the carbon sink was suggested to amount to 3-50% of the net value of Swedish forests.