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Northern Research Station
11 Campus Blvd., Suite 200
Newtown Square, PA 19073
(610) 557-4017
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Providing Clean Air and Water

Critical Loads Glossary

Base Cations
Calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium (K+) are base cations. Base cations are elements or ions with a positive charge (cations) that can neutralize acids.
Base Saturation, BS
Base saturation is a way of measuring the base cations are available to plants. Base saturation is given as the percentage of potential cation exchange sites that actually have exchangeable base cations on them. It is expressed as a percentage of the total cation exchange capacity.
Categorical Approach
A categorical approach is used when there are very few or no data available at a site. It is an empirical approach which classifies the site into a one of several categories associated with a value or range of values for each parameter in the critical load equation (for example base cation weathering, nutrient removal, etc.).
Cation Exchange
Cation Exchange is the interchange between a cation (positively charged ion) in solution and another cation on the surface of soil particles.
Cation Exchange Capacity, CEC
Cation exchange capacity is a measure of the total exchangeable cations that a soil can absorb.
Critical load­
The critical load is the level of deposition below which significant harmful ecological effects do not occur.
Critical load for acidity
The critical load for acidity this is the total deposition of acidity inputs to the ecosystem (S and N combined); it is sometime referred to as critical load for S+N.
Critical load for N
The critical load for N is the total deposition of N to the ecosystem. The critical load for N may be simply the critical load for N effects from acidification. Preferably, a critical load N nutrient is calculated in addition, and the critical load N used will be the lower of the two.
Critical load for N nutrient
The critical load for N nutrient is the deposition of N to an ecosystem below which no harmful effects of N saturation occur.
Critical load for S
The critical load for S is the total deposition of S to the ecosystem; when no critical load N is calculated, the critical load S is equal to the critical load for acidity.
Critical threshold
A critical threshold is a chemical characteristic (usually easily measurable) that is related to the ultimate biological or ecosystem effect of concern. For example, in aquatic systems, a given ANC (critical threshold) may be related to fish mortality (the biological effect of interest).
Diameter at breast height. DBH is used to measure tree size and to estimate tree biomass.
A microbial process which converts nitrogen to a gaseous form which can then be exported from the ecosystem.
Dissolved organic carbon
Dissolved organic nitrogen
Dynamic model
Dynamic models are computer models that incorporate internal feedbacks—such as accumulation of N in the system, or exchange of base cations between soil and soil solution from year to year--and allow for the prediction of time to damage and time to recovery.
Empirical critical load
Empirical approaches are based on observations of response of ecosystem or ecosystem component (e.g., foliage, lichens, soil) to a given, observed deposition level. Empirical critical loads can be calculated for the site where the data were obtained; generally, they are applied to similar sites where such data are not available.
The exceedance of the critical load is the actual deposition minus the critical load.
Federal Land Managers Air Quality Related Values Work Group
Federal Land Manager
Gibbsite equilibrium
The relationship or exchange between aluminum and hydrogen ion concentration is based on equilibrium with the mineral Gibbsite. A fixed ratio of aluminum to hydrogen ion is used—this ratio is KGibb. Mathematically, this is expressed as:

[Al3+]crit  = KGibb x [H+]crit

The ratio, KGibb, is used to predict Al concentration based on pH. KGibb is determined observationally or from the literature.

Mean annual increment, MAI, is a measure of the net increase in biomass of a tree or forest.
Mass balance
The mass balance approach is a technique used to determine the status of an ecosystem by comparing the inputs to the system and the outputs from the system. A mass balance can be calculated for any quantity of interest, for example, water, nitrogen, etc.
Nitrogen Saturation
Nitrogen saturation is the condition when N deposition exceeds biotic (plant and microbial) demand.  Nitrogen saturation often results in elevated stream water nitrate losses.
Sensitive receptor
A sensitive receptor is a part of the ecosystem of concern to FLMs. The sensitive receptor might be a particular organism (salamander, lichen, tree species) or it might be an ecosystem compartment (soil, trees, etc.).
Simple mass balance model
Simple mass balance approaches are based on estimating the net loss or accumulation of nutrients based on inputs and outputs of the nutrient of concern (e.g., base cation, nitrogen).
Steady state
Steady state is a condition of an ecosystem where inputs are matched by output; there is no net change in a system at steady state.
Target load
The target load is the level of deposition set by policy makers to protect a given  area of sensitive ecosystem components. The target load may be higher or lower than the critical load based on considerations of economic cost of emissions reductions, timeframe, and other matters.

Last Modified: 08/19/2008