The effect of granulated wood ash fertilisation on soil CO2 efflux and biomass production in two boreal peatland forests

Wood ash from bioenergy production can be recycled as a fertiliser, especially in boreal peatland forests naturally rich in nitrogen (N). We studied the effects of granulated wood ash fertilisation (5,000 kg ha–1) on soil carbon dioxide (CO2) efflux and accumulation of carbon (C) in the stem biomass of Scots pine (Pinus sylvestris L.) between a nitrogen-poor (Cladonia-type peatland forest) and a nitrogen-rich site (Vaccinium vitis-idaea-type peatland forest) located in western Finland. The CO2 efflux was measured from the bare peat surface using the manual chamber method eight years after ash fertilisation. In this case study, CO2 loss from the peat without ash application was twice as much at the nitrogen-rich site as at the nitrogen-poor site. Ash fertilisation increased tree stand biomass but also CO2 loss from the peat. At the nitrogen-rich site with higher stem volume (79–108 m3 ha–1), tree stem biomass and soil CO2 efflux increased at the same rate after ash fertilisation. On the nitrogen-poor site, where stand volume was low (7–17 m3 ha–1), soil CO2-C efflux increased more than the accumulation of C in the stem biomass, thus increasing C loss considerably. Therefore, this case study suggests that to optimise the increase of tree growth and at the same time avoid negative climate impacts, wood ash fertilisation should be used in well-stocked nitrogen-rich peatland forests. Key words: forestry; wood ash; soil respiration; Scots pine; carbon dioxide; biomass