Environmental Engineering Reference
In-Depth Information
to aquatic systems. This decrease in P transfer coupled with changes in P cycling within
lakes has been thought to be responsible for decreases in primary production in some
poorly buffered lakes in Europe and North America of which the watersheds are subject
to high inputs of acid in precipitation. This decline in production has been termed
oligotrophication.
The transfers of P to aquatic systems from agricultural watersheds are often accelerated
compared to transfers from intact forests or grasslands. Annual crop production can
dramatically increase erosion, primarily during periods when soils are not vegetated.
Additionally, fertilizer P is often applied in excess of P removed in crops. Over decades,
this can lead to increased P in soils that can leach or be moved by erosion of P-rich soil
particles into surface waters. In many areas these relatively diffuse (nonpoint) P inputs are
sufficient to cause eutrophication of streams, lakes, and wetlands located within agricul-
tural watersheds. To allay this increased P transfer best management practices (BMPs) can
be applied. These measures include decreasing fertilizer application, altering timing of fer-
tilizer and manure application, and constructing riparian buffer strips that can capture P
leaving croplands before reaching streams.
THE PHOSPHORUS CYCLE AT THE LOCAL
SCALE
Terrestrial Systems
Within terrestrial ecosystems, most P is in soil pools that are effectively unavailable for
plant uptake (see
Box 8.3
for a discussion of P pools throughout soil development). As a
result, plants have developed multiple strategies for accessing limited, but critical, pools
of soil P. As for nitrogen, P moves to plant roots via diffusion. Diffusion involves move-
ment along a gradient from high concentrations to low concentrations. The rate of
phosphate movement via diffusion is typically much slower than nitrate, and soil solution
concentrations of phosphate tend to be very low due to the numerous different processes,
such as soil adsorption and chemical precipitation, that take phosphate out of solution.
The most common plant strategy to access more soil P is through symbiotic relationships
with fungi. Certain types of fungi can colonize roots either by growing between root cells,
or penetrating and growing within root cells. The relationship is generally mutualistic
under conditions of low soil P availability. Plants supply carbon, an important energy
source, to fungi, and fungi increase P availability for the plant. Fungi increase P availabil-
ity by excreting phosphatase enzymes to mineralize organic P, and acids that increase
weathering and dissolution of primary and secondary mineral P (and chelated P). The
long, filamentous mycorrhizal fungal hyphae also effectively increase the root surface
area of a plant, increasing the soil volume from which P can be accessed. It is estimated
that more than 80% of terrestrial plants form mycorrhizal symbioses. The development of
this symbiosis was likely a critical step in the evolution of land plants.
Within the small number of plants that do not form mycorrhizal symbioses, many have
developed alternative strategies for accessing P. One example is the development of
cluster roots. The production of root clusters (prolific root growth) in areas of high P and
high acid exudation rates result in greater P availability and uptake. Organic acids can
