Soil solution fluxes and composition trends reveal risks of nitrate leaching from forest soils of Italy

Although acidification is a natural soil formation process, atmospheric emissions of nitrogen and sulphur have exposed forest ecosystems to accelerated anthropogenic acidification for many decades. In Italy nitrogen deposition loads are high, and among the highest in Europe. Data from the Italian ICP-Forests monitoring network have shown that nitrogen deposition increases tree growth, and consequently the organic carbon sequestration. However, the accumulation of nitrogen in the ecosystem could cause nutrient imbalances and contribute to soil acidification. Within this context, we investigated the temporal changes of atmospheric deposition and soil solution concentration in the Italian ICP-Forests using the non-parametric Seasonal Mann Kendall (SMK) test. Further, we applied input-output models, to evaluate the capabilities of the soil-forest system to retain deposited nitrogen and thus protect underground waters. Increase of soil solution pH was observed in most of the sites, likely due to sulphate deposition decrease with a concomitant high and stable natural exchangeable base deposition. For the sites around the Po plain, however, high levels of nitrogen deposition impede pH increase. The site with the maximum mineral N deposition showed signs of active acidification. The analysis of the fluxes of nitrogen compounds demonstrates a complete retention of the ammonium forms, which further contribute to acidification through nitrification. Furthermore, the Italian monitoring network showed that the retention of nitrogen in the soil-forest system was effective since the observed N fluxes in mineral soil were strongly reduced compared to mineral N input in most of the plots. In spite of this, significant NO3 fluxes from the subsoil were observed in sites with high deposition, and also in the southernmost site which is exposed to relatively low mineral N loads. Drivers other than pollution should also be considered since N can follow different pathways at different time scales, influencing N leaching independently from the amount of deposition.