Geoscience Reference
In-Depth Information
materials to form very stable compounds, making this phosphorus
unavailable to living organisms and thus moving it from the short to
the long P cycle.
The long P
cycle has a duration of hundreds of millions of years.
The sediments that accumulate on the seafloor slowly form rocks [
6
],
which are later transported onto continents by the tectonic plate
movements (orogenesis) [
7
]. The physical and chemical weathering of
these rocks, which is very slow, produces PO
4
3-
[
8
], most of which is
precipitated as chemical compounds that living organisms cannot use.
Under natural conditions, the long-term weathering of phosphate
rocks compensates for the losses from the short cycle.
Human societies have consumed much phosphorus during the last
several decades. Phosphorus is used in particular in fertilizers, which
were once in the form of naturally occurring guano (section 4.5.2) and
are today in the form of superphosphate obtained by the chemical
transformation of apatite. Phosphorus is also used to produce matches,
and as organophosphate to produce pesticides. In addition, P is used in
detergents, but concentrations of P are now regulated in many
countries to reduce eutrophication of freshwaters. Most of the
industrial phosphorus comes from apatite mines, which are under
exploitation in small number of countries.
Human interventions in the phosphorus cycle accelerate this cycle
in two major ways. The first is associated with the short P cycle,
where human activity accelerates biological production [
9
], which is
precisely what is intended when the soil is enriched with fertilizer,
making it more fertile than it would naturally be (however, fertilizers
are often used today simply to compensate for the nutrient depletion
of soils). Yet, an acceleration in biological production becomes
harmful in aquatic environments when is it due to eutrophication [
10
]
(section 4.5.2). This occurred on large scales in freshwaters in
industrial countries during the 1960-1970s, mainly due to the mass
use of detergents containing PO
4
3-
; this eutrophication has been curbed
by national regulations reducing the use of phosphate in detergents
and by the secondary treatment of wastewater. The second type of
acceleration is associated with the long P cycle. Within the last
decades, human activities have dispersed over the whole planet a
