Atmospheric deposition control of soil acidification in central Italy

Acidification is a major soil-forming process, and concerns about acidifying anthropogenic atmospheric deposition make it a significant environmental issue. In the long term, the depletion of exchangeable base cations (BCEs) is the main process underpinning soil acidification. In south-central Italy, acidic soils are not rare and are often located in areas with no excess rainfall over potential evapotranspiration. Many such soils are understood to have been derived from long-term weathering, although whether acidification is an active process remains an open question. Data from the International Co-operative Programme (ICP-Forests) monitoring system revealed that BCE atmospheric deposition was high and stable in south-central Italy and allowed us to estimate the BCE budget of some acidic forest soils. We estimated the overall BCE budget of four sites in this network. Tracer approaches using conservative ions, Na+ and Cl−, were calibrated to estimate dry BCE deposition and soil water fluxes. The contribution of atmospheric deposition to the BCE budget was such that potassium was found to be regularly accumulating at all sites at the rate of 0.08 to 0.42 g⋅m−2⋅y−1, while calcium was accumulating at a rate of 1.11 g⋅m−2⋅y−1 at a central Italy site. For Ca2+ at the other sites and for magnesium at all sites, the effect of deposition was such that the rate of BCE depletion was significantly reduced. Soil acidification appeared to be a non-active process in the central, lowland and hilly areas of Italy.