PhD defence: Teresa Gómez de la Bárcena

Teresa Gómez de la Bárcena defends her thesis:

Afforestation effects on soil carbon

Principal Supervisor
Lars Vesterdal
Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen, Denmark

Co-Supervisor
Per Gundersen
Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen, Denmark

Assessment Committee
Professor Karsten Raulund-Rasmussen (chair)
Department of Geosciences and Natural Resource Management, Faculty of Science, University of Copenhagen, Denmark

Professor Bent Tolstrup Christensen
Department of Agroecology, University of Aarhus, Denmark

Dr. Axel Don
Thünen Institute, Institute of Climate-Smart Agriculture, Braunschweig, Germany

Summary
Understanding carbon (C) dynamics has become increasingly important due to the major role of C in global warming. Soils store the largest amount of organic C in the biosphere; therefore, changes in this compartment can have a large impact on the C storage of an ecosystem. Land-use change is a main driver of changes in soil organic carbon (SOC) pools worldwide. In Europe, afforestation (i.e. the establishment of new forest on non-forested land), is a major land-use change driven by economic and environmental interests due to its role as a C sequestration tool following the ratification of the Kyoto Protocol. Despite research efforts on the quantification of SOC stock change and soil C fluxes following this land-use change, knowledge is still scarce in regions where afforestation currently is and has been widespread, like Denmark and the rest of Northern Europe. This PhD thesis explored three main aspects of the impact of afforestation on soil C: i) changes in SOC stocks (in forest floors and mineral soils) on afforested cropland in Denmark and in afforested soils of Northern Europe; ii) changes in CH₄ oxidation (uptake) potential of soils; iii) changes in soil CO₂ efflux through heterotrophic respiration.

In Denmark chronosequences (i.e. space-for-time substitution) of oak and Norway spruce stands at the Vestskoven site were the tool used to explore these changes. Soil OC dynamics predicted by the chronosequence approach have often been used, however they never been validated by resampling before. According to the chronosequence approach covering a time span of 40 years, topsoils (0-25 cm soil depth including forest floors) in Vestskoven are currently neither a sink nor a source for C. The more specific decadal trends at the stand level provided by repeated sampling revealed a change in source-sink C balance between soil compartments over time, with C accumulation in the mineral soil becoming increasingly important as the sink strength of forest floors decreased within 4 decades of afforestation.

The chronosequence approach is an appropriate tool to assess SOC stock changes following

afforestation given that assumptions of similar site conditions are met. However, repeated sampling

can not only validate the chronosequence trajectories but also provide refined temporal trends.

Changes in SOC in Northern Europe investigated by meta-analysis, revealed that following afforestation, significant C sequestration in soils from this region requires at least three decades or even longer time, when afforestation is performed on former agricultural soils. The faster C accretion in forest floors compensates the initial lag-phase found in mineral soils.

The chronosequences also indicated that afforestation increases the CH₄ sink potential of soils over time due to the gradual increase in SOC and decrease of bulk density favoring CH₄-oxidation, as long as soils remain well drained. The CO₂ efflux from soils may increase, due to heterotrophic respiration of a larger SOC pool and not due to higher reactivity of the SOC. However, these C sources are likely to be compensated by the C sink in the growing forest biomass.

After the PhD defence there will be a reception at IGN, Rolighedsvej 23, canteen in the new building.  All are welcome.