[2] Sulfur and nitrogen, as acidifying agents, may change soil nutrient content by removing calcium and releasing toxic aluminum, further impacting plants and animals.
The deposition of nitrogen contributes to nutrient enrichment in freshwater, coastal, and estuarine ecosystems, which may cause toxic algal blooms, fish kills, and loss of biodiversity.
Some components of an ecosystem are more sensitive to deposition than others; therefore, critical loads can be developed for a variety of ecosystem components and responses, including (but not limited to) shifts in diatoms, increases in invasive grass species, changes in soil chemistry, decreased forest health, altered and reduced biodiversity, and lake and stream acidification.
The next step is to link the status of that element to some chemical criterion (e.g. the base cation to aluminium ratio, Bc/Al) and a critical limit (e.g. Bc/Al=1) which should not be violated.
Fish and Wildlife Service, use critical loads to: identify resources at risk, focus research and monitoring efforts, inform planning and other land management activities, evaluate potential impacts of emission increases, and develop pollution reduction strategies.
[6][7] Empirical critical loads were simply determined as the deposition levels with reported field occurrence of detrimental ecological effects.
Moreover, the values of the critical loads can vary remarkably when based on different biological or chemical response of an ecosystem, such as physiological variation, reduced biodiversity, elevated nitrate leaching, and changes in soil microorganisms.
In South and East Asia, comprising China, Korea, Japan, the Philippines, Indo-China, Indonesia, and the Indian subcontinent, critical loads were first computed and mapped as part of the impact module of the Asian version of the Regional Air pollution INformation and Simulation model (RAINS-Asia) based on the steady-state mass balance approach.
[8] Thereafter, critical loads with higher resolution were calculated in many Asian countries such as Japan, Russia, South Korea, India, and China.
[7] Although similar methods were applied in Asia as in Europe, the steady state mass balance approach has been improved by considering base cation deposition.