The role of atmospheric n deposition in soil acidification in forest ecosystems

The emissions of NOx and NH3 into the atmosphere are on the rise, and the emitted N compounds are distributed globally, causing concerns over possible effects on human health and the environment. The emission of N occurs primarily from agricultural activities and fossil-fuel combustion (e.g., automobile and industrial boilers). Atmospheric N deposition has become an important factor affecting forest health and sustainability in recent decades, partly as a result of decreasing S emissions. Nitrogen compounds that fall onto soil and water bodies with precipitation and dry deposition can cause damage to ecological systems. Chronic excessive N input can lead to base cation depletion, nutrient imbalances, eutrophication, soil acidification, and forest dieback. Leaching losses of NO₃-gradually decrease the acid buffering capacity of forest soils. Moreover, releases of Al and Mn induced by soil acidification cause toxicity to trees. Biological processes play a crucial role in soil acidification, as N compounds originated from atmospheric deposition undergo nutrient cycling processes. However, the effects of N transformation on soil acidification in forest ecosystems are often overlooked. Thus, an improved understanding of the impact of atmospheric N deposition on N cycling processes is needed to better predict the responses of forest ecosystems to such depositions. In this paper, we explore the linkage between N cycling and soil acidification in forest ecosystems, and evaluate the role of N cycling and atmospheric N deposition on soil acidification in forest ecosystems by discussing: 1) the N cycle and its effect on soil acidification in forest ecosystems, 2) the effect of N deposition on forest soils and trees, in terms of base cation depletion, soil acidification, and aluminum toxicity, 3) leaching losses of N from forest soils, and 4) critical loads of atmospheric N deposition in forest ecosystems.