Environmental Engineering Reference
In-Depth Information
area of current research is if these inherent site controls can be overwhelmed by increases in
atmospheric deposition and invasions by exotic plant species.
Agriculture
Conversion of native ecosystems to cultivated fields and pasture is one of the most
widespread and intensive global alterations of terrestrial N cycling and is a major source
of air and water N pollution. If we revisit
Figure 7.4
, it is easy to see how agriculture alters
the N cycle and increases hydrologic and gaseous N losses. Fertilizer, which usually con-
sists of ammonium salts or urea (CON
2
H
4
or (NH
2
)
2
CO, which rapidly decomposes to
ammonium), is a direct addition to the soluble, available pool, which increases the flow of
N to plants (the intended target), but also increases the potential for hydrologic and gas-
eous losses that arise from these pools. Harvest, which removes the plant community, also
increases the potential for losses as the flow of N to plants ceases. Harvest also decreases
the flow of organic matter to soil pools, which alters the balance between mineralization
and immobilization toward mineralization, leading to further increases in the soluble pool.
Soil organic matter pools are also reduced by tillage, which stimulates decomposition.
There is great interest in developing agricultural systems with “tighter” N cycles by alter-
ing management to sustain plant uptake and soil organic matter pools, for example,
through the use of winter “cover crops” to maintain plant uptake for a larger portion of
the year and to increase soil organic matter pools. There is also interest in evaluating agri-
cultural ecosystems in a landscape context, using downstream or downslope wetlands or
other ecosystem types with high capacity for absorbing excess N moving from agricultural
areas and converting this excess N into gases, plant biomass, or soil organic matter.
NITROGEN CYCLING IN AQUATIC
ECOSYSTEMS
Lakes and Oceans
In aquatic ecosystems, N is often an important limiting nutrient for both aquatic primary
production and decomposition (see Chapters 2 and 4). In lakes, there are typically distinct
zones with different forms and concentrations of N and dominant cycling pathways. Thermal
stratification of the water column creates these distinct zones; warmer water is less dense
than colder water, thereby resulting in distinct density zones within a lake that do not mix for
extended periods of time. Understanding ecosystem-level N dynamics requires exploring
these distinct zones independently and understanding when these zones mix (
Figure 7.5
). In
lakes, uptake of N by primary producers (phytoplankton) occurs in the top of the water col-
umn where light availability is highest. In this photic zone (the epilimnion), N cycling is dom-
inated by uptake by primary producers (e.g., algae, diatoms, and cyanobacteria), and a
fraction of the N is excreted as DON by primary producers. Zooplankton consume algae and
excrete dissolved inorganic N and heterotrophic bacteria mineralize particulate and dissolved
organic matter, releasing inorganic N into the epilimnion, which can be rapidly taken up by
primary producers. This can result in rapid and tight N cycling in the epilimnion. This zone is
where most consumers are found as well (e.g., zooplankton and fishes).
