Phosphorus (P) is foundational to all life on Earth, playing a part in the structure of DNA, RNA, and ATP. The global phosphorus cycle operates on two vastly different timescales. The rapid, biological cycle involves the uptake, use, and decay of P within ecosystems over days to years, primarily occurring in the soil where plants and microbes utilize it. The element’s journey from the terrestrial environment into rock is part of an extremely slow geological cycle that acts over millions of years. This long-term process involves physical and chemical transformations that remove P from the soil and permanently lock it away in the lithosphere.
Forms of Phosphorus in the Soil Environment
The initial state of phosphorus in the soil determines its potential for movement and transformation. Soil phosphorus is broadly classified into two main categories: organic and inorganic. Organic phosphorus comprises 30% to 65% of the total P in soil, bound within decaying plant and animal matter, microbial biomass, and humus.
Inorganic phosphorus accounts for the remaining 35% to 70% and exists primarily as mineral phosphates. This inorganic fraction includes residual particles of primary minerals like apatite from the parent rock. However, the majority of inorganic P is chemically bound, or sorbed, to the surfaces of soil particles.
This binding, or adsorption, occurs when phosphate anions attach to compounds of iron and aluminum in acidic soils, or to calcium carbonates in neutral and alkaline soils. This strong chemical attraction renders most soil P highly immobile; it does not readily dissolve in water or move through the soil profile. Therefore, the physical removal of the soil itself is necessary to begin the element’s journey toward rock formation.
The Role of Erosion and Aqueous Transport
The physical transfer of phosphorus from the land surface to aquatic systems is primarily driven by erosion and water runoff. Water overcomes the chemical stability of soil phosphorus, acting as the main transport agent. This mobilization is the intermediate step that moves P from the terrestrial cycle into the marine environment, where geological sequestration occurs.
Phosphorus travels through two distinct pathways: particulate transport and dissolved transport. Particulate phosphorus (PP) is the dominant form of loss, accounting for 60% to 90% of the total P transported from agricultural land. This occurs when soil particles, which have phosphate attached to their surfaces, are physically detached and carried away by surface runoff.
The eroded soil, rich in P-sorbed particles, is transported into streams and rivers as suspended sediment. Dissolved phosphorus (DP) is the second pathway, where soluble inorganic phosphate dissolves directly into the runoff water. While DP is a smaller percentage of the total P loss, it is immediately bioavailable in the water, contributing to the overall P load.
All transported material, including suspended particulate matter and dissolved ions, flows into lakes and, most significantly, the ocean. When the river current slows upon meeting a larger body of water, the heavier, P-rich sediment settles out in a process called sedimentation. This accumulation of phosphate-laden sediment on the continental shelf or ocean floor marks the end of the transport phase and the beginning of the long-term geological burial.
Geological Sequestration and Transformation into Rock
Once the phosphorus-rich sediment settles on the seabed, the transformation into rock begins through phosphogenesis. This process is driven by the burial of sediment layer by layer, which increases pressure and temperature over millions of years. The initial step involves microbial activity breaking down the organic matter within the accumulating sediment, releasing phosphorus back into the pore water as dissolved phosphate.
As new layers accumulate, increasing pressure leads to compaction and the expulsion of water, driving the chemical changes of diagenesis. The dissolved phosphate in the pore water, now highly concentrated in an environment often low in oxygen, begins to precipitate. This chemical reaction forms new, authigenic (newly formed) phosphate minerals, primarily carbonate fluorapatite.
This apatite mineral is the stable, long-term geological sink for phosphorus. The newly formed phosphate grains, along with phosphatic debris like fish bones or teeth, become physically concentrated through the winnowing action of bottom currents. Over vast geological time, compaction, cementation by mineral precipitation, and recrystallization of the apatite transform the soft, P-rich sediment into hard sedimentary rock.
The resulting rock, which contains a minimum of 15% to 20% phosphate by weight, is termed phosphorite or phosphate rock. This lithified rock represents the final sequestration of soil phosphorus, locking it into the Earth’s crust. The geological cycle completes when tectonic forces uplift these marine sedimentary layers, exposing the phosphorite to the surface to begin the slow process of weathering once again.