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 ${\mathit{NO}}_3^{-}$ 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.

尽管酸化是自然的土壤形成过程，但数十年来，大气中氮和硫的排放使森林生态系统遭受了加速的人为酸化。在意大利，氮沉降量很高，在欧洲是最高的。意大利ICP-Forests监测网络的数据表明，氮的沉积增加了树木的生长，从而增加了有机碳的固存。但是，生态系统中的氮积累可能导致养分失衡并促进土壤酸化。在此背景下，我们使用非参数季节性Mann Kendall（SMK）试验研究了意大利ICP-森林中大气沉积和土壤溶液浓度的时间变化。此外，我们应用了投入产出模型，

在大多数地点观察到土壤溶液pH值的升高，可能是由于硫酸盐沉积的减少以及随之而来的高而稳定的自然可交换基础沉积所致。但是，对于Po平原周围的地点，高水平的氮沉积会阻止pH值升高。矿物氮沉积最多的部位显示出活性酸化的迹象。对氮化合物通量的分析表明，铵形式完全保留，从而进一步通过硝化作用促进了酸化。此外，意大利的监测网络表明，土壤氮素在土壤-森林系统中的保留是有效的，因为与大多数样地中输入的矿物氮相比，观察到的矿物土壤中的氮通量大大降低。尽管如此，重要的是${\mathit{没有}}_3^{--}$在沉积量高的地方以及暴露于相对较低的矿物氮负荷的最南端的地方都观察到了来自地下土壤的通量。还应考虑除污染外的其他驱动因素，因为氮可以在不同的时间尺度上遵循不同的路径，从而独立于沉积量而影响氮的浸出。