Organic matter dynamics in a forest soil as affected by climate change

Large amounts of carbon are stored in boreal soils as soil organic matter. Aim of the research presented in this thesis was to quantify the effects of climate change on decomposition soil organic matter in a boreal forest ecosystem by means of field and laboratory experiments. Field experiments were carried out within the framework of the Climate Change Experiment (CLIMEX). In this project in two covered catchments in southern Norway CO 2 concentration and/or temperature were increased. An increase in temperature resulted in a higher N availability caused by increased mineralization of N. Part of the extra N was taken up by the vegetation whereas export of N in runoff increased as well. Appearantly, at least initially, the higher N availability exceeded the N demand by the vegetation.Laboratory incubations of different soil layers showed that the effect of elevated temperature on decomposition decreased with increasing depth in the soil presumably due to a decrease in substrate quality. A 14C labeling experiment in growth chambers showed that, at elevated CO 2 , more C was fixed in a plant-soil system with heather. An increase in CO 2 concentration did not change allocation to shoot, root or soil. When N supplies increased, relatively more C was fixed in shoots at the expense of roots. On the short term (days) decomposition of labile, root-derived organic matter was stimulated both by elevated CO 2 and elevated N. On the longer term, (weeks) respiration from high-CO 2 soils was lower than that of low-CO 2 soils.Simulations with the NICCCE model showed that, under normal light conditions, the forest ecosystem acts as a sink for C in the next 100 years. Especially under low light conditions, a step increase in CO 2 and temperature causes the ecosystem to become a source for C. When CO 2 and temperature are increased gradually, the ecosystem becomes a small sink for C.