Effects of forest management on soil acidification in cedar plantation

• Organic acids and plant uptake cause soil acidification in cedar plantation. • Girdling leads to soil alkalization by cation inputs by litterfall from tree dieback. • After clearcutting (cation removal in timber), soil acidification is caused by nitrification. • Net mineralization of soil organic matter neutralizes acidity generated by nitrification. • Forest management practices stimulate both proton-generating and -consuming processes. Forest management practices such as clearcutting risk increasing the leaching loss of cations and soil acidification by increasing nitrification and reducing plant uptake. To evaluate the effect of two forest management practices, clearcutting and stem girdling, on soil acidification, we quantified proton budgets by measuring ion fluxes associated with solute leaching and plant uptake in a Japanese cedar plantation in Wakayama, Japan. In the control and girdling plots, the dissociation of organic acids was a major proton-generating process that promoted cation leaching from the organic horizon. Plant uptake of mobilized cations was a dominant soil-acidifying process in the control plot, whereas litterfall from tree dieback and loss of plant uptake led to soil alkalization via cation inputs to the mineral soil in the girdling plot. In the clearcut plot, in addition to cation removal due to harvesting, nitrification was a major proton-generating process that increased aluminum (Al) leaching in the first year following clearcutting. However, proton consumption via net mineralization (loss) of soil organic matter (SOM) could neutralize the acidity generated by nitrification. Forest management practices stimulate both proton-generating and -consuming processes. Soils under cedar plantation are affected by biological acidification of surface soil by plant uptake in the rooting zone and net cation leaching by carbonic acid below the rooting zone (subsoil and bedrock), but organic matter and Al and Fe oxides accumulated in plantation could mitigate acidification induced by disturbances.