Biological acidification trends in Swedish forest soils

Johan Iwald1, Johan Stendahl1, Stefan Löfgren2, Erik Karltun1
1 Department of Soil Science, SLU Swedish University of Agricultural Sciences
2 Department of Aquatic Sciences and Assessment, SLU Swedish University of Agricultural Sciences

O 12.8 in Restoration and rehabilitation of ecosystems

17.07.2014, 16:10-16:30, H19

In Sweden, deposition of sulfur and nitrogen has decreased over recent decades while at the same time forest harvesting has increased. Increased harvesting of biomass causes increased biological acidification of forest soils. There is limited knowledge about the effect that tree growth has on soil acidity and how this effect is impacted by factors such as geography, temperature, deposition and buffering capacity of soils. We used data from the Swedish Forest Soil Inventory to investigate the effects of stand age and forest type (pine, spruce or deciduous forest) on soil pH-H2O and total acidity (TA). Analysis of variance was done for 8000 soil samples representing four horizons O, B, BC and C. Analyses were done for all soils in the country as well as for selections of soils, grouped by geographical region (SW southwest, SE southeast and N north) and by 10-percentiles of temperature sum (PrcTsum), sulfur deposition (PrcSdep) and exchangeable calcium (PrcCa), respectively. In most analyses, r2 values were higher for the O horizon than for the B, BC and C horizons. This is most likely due to the fact that tree roots are present mainly in the O horizon and that there is little leaching of H+ ions excreted by tree roots to lower horizons. Comparing regions, analyses pH/TA vs stand age showed the highest r2 values for region N, attributed to the low acid deposition levels in northern Sweden. Analyses pH/TA vs forest type showed the highest r2 values for region SW, which may be explained by the fast tree growth there. Analyses separately for PrcTsum showed the strongest correlations between pH/TA and stand age/forest type in the mid-temperature sum percentiles (central Sweden). This is assumed to be because of the combined effect of relatively warm climate and low deposition. Considering analyses separately for PrcSdep, r2 values became lower with increasing PrcSdep in analyses vs stand age while they became higher in analyses vs forest type. The former indicates that deposition suppresses the relationship stand age- soil pH/TA while the latter is attributed to faster tree growth and also a higher prevalence of deciduous forest in southern Sweden. The analyses separately for PrcCa all show a decrease in r2 values as PrcCa increases, indicating large buffering capacity of Ca. In conclusion, this study shows that there is ongoing acidification of forest soils as trees grow and that this acidification occurs mainly in the O horizon. The analyses also show that there are differences in soil acidity between pine, spruce and deciduous forest, particularly in southern Sweden. Finally, the clear trend with lower correlations between forest growth and soil acidity as the soil’s Ca content increases shows the importance of maintaining the buffering capacity of forest soils. This can be done, for example, by leaving the needles on-site while harvesting slash and by ash return after combustion of forest biomass for bioenergy.

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last modified 2014-06-19