Environmental Engineering Reference
In-Depth Information
THE IMPORTANCE OF PHOSPHORUS IN
AGRICULTURAL ECOSYSTEMS
Phosphorus plays a critical role in food production. It is needed for all aspects of plant
growth, including root growth, flowering, fruiting, and seed formation ( Smil 2000 ). As
crops grow, they take up P from the soil. Increasing crop yields means that some crops
remove double or even triple the P they did 50 years ago. To maintain yields, this P must
be replenished. Before widespread use of commercial fertilizers, P was replenished by
manure from livestock grazing, flooding, or shifting cultivation. Availability of mineral P
fertilizers has enabled additions of P to come from far away. This, in turn, means that
consumption of food can happen far from where it is produced and far from local sources
of P (such as manure) since P and other minerals no longer need to be locally recycled.
This ultimately can lead to specialization and high-density production of livestock, often
beyond what the surrounding land can sustain ( Schroder et al. 2009 ). However, mineral
inputs of P have also improved human food security through the massive increases in
yield achieved through the Green Revolution.
THE IMPORTANCE OF PHOSPHORUS IN
AQUATIC ECOSYSTEMS
The Importance of Phosphorus in Freshwater Systems
Phosphorus limits primary production in many freshwater systems. Phosphorus addi-
tions in these systems can stimulate growth of aquatic primary producers, including
phytoplankton, macroalgae, and epiphytes. This limitation has been demonstrated experi-
mentally by adding P alone or in conjunction with nitrogen, which results in significant
and sometimes very large increases in production and biomass of phytoplankton
( Schindler 1977 ; Figure 8.1 ). Conversely, declines in P input can result in dramatic
declines in phytoplankton production and biomass. This control of P input on phyto-
plankton production has been particularly well demonstrated in clear, temperate lakes.
For other inland waters, the strong link between P status of the aquatic system and phy-
toplankton production is less obvious and may occur only seasonally, in some parts of
the system, or not at all. For example, in a number of estuaries P concentrations are high
relative to nitrogen concentrations (e.g., atomic N:P ratios are substantially lower than
16:1), and a substantial stimulation of primary production may occur by nitrogen addi-
tions alone.
Primary production occurs on the sediments as well as in the water column of aquatic
ecosystems. This benthic primary production includes both algal production and plant
production. Benthic primary production generally dominates total system primary produc-
tion in wetlands, shallow lakes and estuaries, and streams. Although benthic primary
producers require P to grow, P additions to the ecosystem sometimes can have negative
impacts on these producers. For well-lit streams, a number of studies have shown that
benthic algal biomass and production increase with increased P inputs. However, in
shallow lakes and some estuaries, declines in benthic production may occur with
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