Forest soils further acidify in core Natura 2000 areas amongst unaware government policy

The intensification of agriculture and livestock husbandry has led to increasing atmospheric deposition of nitrogenous compounds and soil acidification. We field measured extremely acidic soils with pH < 3 (i.e., soils with the acidity of domestic vinegar) over extensive areas of the forested national parks on sandy soils in the Netherlands. These areas show stress from the negative impacts of increased soil acidity on forest health and biodiversity. We demonstrate that soil acidity has worsened from an average pH of approximately 4.5 to the current average pH = 3.2 over the last 22 years for extensive areas of Natura 2000 forest soils in the Netherlands. Current government policy has been guided without knowledge of such extreme acidity because the field data sampling does not cover Natura 2000 areas, and soil acidification was estimated based on poorly calibrated atmospheric nitrogen deposition models. The policy challenge of soil acidification in Natura2000 areas is solvable with the following recommendations: • Implement regulatory action to biennially field sample soil pH across Natura 2000 forest parks, focusing on sandy soils with limited buffering capacity. • To include in models of nitrogen deposition all sources of nitrogen, including for example off-leash dog walking areas in Natura 2000 forest areas. • To use these soil pH field samples to regularly recalibrate estimates of soil pH from atmospheric nitrogen deposition models to better inform government, industry, and the agricultural sector about the ongoing impact of N deposition on already severely acidic soils. • To implement further significant reductions in the deposition of all nitrogen compounds on Natura 2000 areas.