Why granulated wood ash decreases N2O production in boreal acidic peat soil?

Increased use of bioenergy has increased the production of wood ash in power plants. Wood ash is used as a fertilizer to improve tree stand growth in nitrogen-rich peatland forests. Nutrients removed during tree harvest are recycled back to the forest by wood ash fertilization. However, there is a risk that ash enhances microbial processes and associated nitrous oxide (N2O) production in peat. Because there are difficulties in spreading loose ash, ash is nowadays granulated or hardened in other ways before it is applied as fertilizer. Granulation process changes not only the physical but also the chemical properties of ash. In previous studies wood ash has showed variable effects on N2O emissions. Nevertheless, there are indications that in laboratory conditions granulated wood ash decreases N2O production. We conducted laboratory experiments to investigate the capability of granulated wood ash to decrease N2O production in peat soils and the role of nitrification and denitrification in N2O production. Acetylene inhibition experiments showed that N2O production was dominated by nitrification over denitrification. The rate of nitrification as well as N2O production from nitrification and denitrification decreased by the addition of granulated wood ash. Wood ash addition increased peat electrical conductivity while peat pH remained unchanged. The inhibitory effect of ash was associated to ion content because isolated ion addition experiments produced a similar decline in N2O production. The inhibition of ions was more evident in acidic conditions because when peat pH was increased by addition of lime, the ions or granulated ash inhibited N2O production significantly less than they did at natural peat pH. Our results show that manipulation of ion strength could offer a new tool to slow nitrification and denitrification processes and decrease the associated N2O emissions in acidic soils.