Grey alder at the regeneration stage: Long-term effects on soil nitrogen and carbon pools and Norway spruce growth

Increasing soil N availability is an effective way to enhance soil organic carbon (SOC) sequestration and biomass production in N-limited ecosystems such as boreal Norway spruce forests. Alder has a root nodule symbiosis with Frankia, an N2-fixing bacterium, and can thus add considerable amounts of N to soil mainly through its N-rich leaf litter. The objective of this study was to determine whether the presence of grey alder at the regeneration stage had any long-term effects on soil C and N cycling and stocks and spruce growth. The study sites were two relatively fertile 40-year-old Norway spruce stands (Luhdansuo and Porkkola) in Southern Finland. At both study sites, half the spruce stand had an admixture of grey alders for approximately 10-15 years after regeneration (alder treatment), but no alder had been present in the other half (control treatment). Alders were cut from the alder treatment 25 (Luhdansuo) or 30 (Porkkola) years before the measurements. At Porkkola, alder treatment received ash fertilisation 13 years ago. We observed no significant differences in the plant-available N fluxes, measured with a microdialysis technique, or SOC stocks between treatments. The total N stock (organic horizon + 0-30 cm mineral soil) and organic horizon thickness were larger, while the microbial biomass C:N and C mineralisation rate were lower in the alder versus the control treatment at Luhdansuo. The annual ring widths of spruces were larger in the alder versus the control treatment for up to six years after alder removal at both sites, but no differences were observed in the current tree diameters or diameter increments. In conclusion, most of the current soil or tree properties did not differ significantly between treatments, possibly because the effect of alder had faded out over the 25-30 years since its removal, and because the sites were initially relatively fertile.