| Introduction |
Changes in Soil Chemistry and Microbial ProcessesMost nitrogen deposited through wet and dry deposition is in the form of either nitrate or ammonium, though the latter form is dominant. In the early stages of nitrogen saturation, most of the nitrogen is expected to be used by plants. In later stages of nitrogen saturation, other resources will become limiting, and more ammonium will be available to microbial populations. High concentrations of ammonium may cause the release of other cations to the soil solution by overwhelming cation exchange sites on soil particles. Microbes may use some of the excess ammonium to increase their own populations, thus increasing other microbial processes such as decomposition, but much of the ammonium will be nitrified to nitrate.In contrast to ammonium, nitrate is highly mobile in soils, as it is
not strongly adsorbed to soil particles through ion exchange. Nitrate
may be taken up by plants (though the metabolic 'cost' of nitrate uptake
is greater than for ammonium) or used by microbes in a process known as
denitrification.
Denitrification
is a facultatively anerobic process that reduces nitrate to N2 gas, thus
completing the nitrogen cycle. However,
two gases, nitric- and nitrous- oxide are intermediate products of denitrification,
and both have important roles in atmospheric chemistry. Nitrous oxide
is a strong greenhouse gas, and increased flux may affect global warming.
Nitric oxide reacts with hydrocarbons to create ozone pollution in the
troposphere. Nitrate that is not utilized by plants or microbes may
be leached to ground- and stream-water, resulting in eutrophication of
streams.
Hypothesized biogeochemical response of temperate forest ecosystems to long-term nitrogen additions (modified from Aber et al 1998). When nitrate is leached to ground- or stream-water, a loss of base cations and soil acidification may result. For each nitrate anion that is lost, a cation must be leached in order to maintain the charge balance of the soil. Therefore, as nitrogen is deposited in temperate forests, other important nutrients may be lost. However, the loss of nitrate anions often exceeds the rate of cation loss, which results in soil acidification. As the pH of the soil decreases, the buffering ability of cation exchange may be exhausted, resulting in the release of Al ions to the soils solution (van Dijk and Roelofs, 1988). Aluminum can be leached to aquatic ecosystems, where it is toxic. In addition, high Al concentrations in the soil solution can have detrimental effects on forest ecosystems. |
