SCOPE 21 -The Major Biogeochemical Cycles and Their Interactions 

2.1 INTRODUCTION

The present chapter brings together the most recent information on reservoir sizes and fluxes for the four major cycles with which we are concerned. Internal consistency between different cycles has been aimed for to facilitate comparisons. Variability between different estimates as quoted represents uncertainty in our knowledge about the relevant reservoirs and processes.

The tables and diagrams given are not the result of re-assessment of the different cycles, but should rather be considered as reference material that might facilitate the reading of the remainder of the book. References should therefore be made to the original data sources instead of the present chapter.

This overview was not available in its present form at the time of the workshop and some of the authors of the following chapters do not explicitly refer to the present compilation but rather to the original research on which it was based. It is judged that this synthesis still will be of value.

2.2 THE CARBON CYCLE 

BERT BOLIN 

The global carbon cycle has been described in detail in SCOPE Report 13 (Bolin et al, 1979), where references to earlier attempts to present a consistent picture of this cycle are also given. Some new data have become available during the last few years, but they hardly warrant a more critical re-analysis of earlier views. There are, however, uncertainties and inconsistancies in that budgets do not always balance and further work is required to resolve some of these problems. It should also be emphasized that we need to develop methods of how to describe the carbon cycle in more detail, i.e. the way the transfer of carbon within the ocean is accomplished or the cycling of carbon within the various main biomes, including the soil. This is dealt with in several of the following chapters. The objective of this note is to give a simple view of the overall transfers that govern the global carbon cycle. We shall thus merely give a synthesis of the data given in SCOPE 13, with some minor modifications to serve as background for the discussions of biogeochemical interactions.

Table 2.1 gives the distribution of carbon between the major reservoirs. For the atmosphere a few different values are given, referring to the likely pre-industrial content of the atmosphere, the accurately known values for 1959, when careful measurements began and the most recent data from 1980.

Carbon reservoirs of the oceans are also reasonably well known, while there are considerable uncertainties about the size of the carbon reservoirs in the terrestrial biosphere, the pedosphere and the lithosphere. In some cases different estimates are quoted and a best estimate finally chosen. Data for the lithosphere are not accurate to more than one significant figure, even though sometimes two are given in the table. The values depend on the use of proper values for the average amount of carbon in different rocks, estimates of their relative occurrence and the thickness of the crust.

Table 2.2 summarizes in a similar manner the magnitude of the fluxes between the different reservoirs. Again a best estimate is given based on the determinations quoted.

Figures 2.1 and 2.2 display in schematic form the carbon cycle. The exchanges between the atmosphere, the terrestrial biomass, soil, rivers and the oceans including marine life as shown in Figure 2.1 are relatively large compared with the reservoirs involved. Characteristic turn-over times vary from a few weeks for marine biota to about one thousand years for the ocean. The circulation in the rock cycle involves reservoirs that are one to ten million times larger. Since the fluxes are one or two orders of magnitude less than those at work in air and ocean transport and in photosynthesis, the characteristic turn-over times are hundreds of millions of years.

Table 2.1 Major carbon reservoirs (when not otherwise indicated, in units of 10¹⁵ g C = PgC). References are given after Table 2.2

Atmosphere							Reference
Carbon dioxide
	Pre-industrial		265 ppm		560		(1)
			290 ppm		615		(1)
	1959		316 ppm		670		(2)
	1980		336 ppm		712		(2)
Methane			1.41 ppm		3		(3)
Carbon monoxide			0.11		0	.2	(3)
Other C-containing gases					0	.05	(3)
			Atmosphere,
			total (1980)		715
Oceans
Dissolved inorganic carbon					37,400		(4)
Dissolved organic carbon					1,000		(5)
Particulate organic carbon					30		(5)
Biota					3		(6)
			Ocean, total		38,500
Terrestrial biota and pedosphere
Phytomass short-lived			130		830		(7)
		long-lived	700		560		(1)
Animals					12		(3)
Man					0.03		(8)
Bacteria					2		(8)
Fungi					1		(8)
Standing dead organic carbon					30		(8)
Litter					60		(8)
Peat (very uncertain)					160		(1),(8)
Soil organic carbon (excluding peat)					1,500		(8),(9)
			Organic carbon in
			biota and pedo-
			sphere, total		2,3002,600
Lithosphere
Continental crust
	Sediments, carbonate				26 x 106		(10)
	Sediments, non-carbonate*				10 x 106		(10)
	Igeneous rock, non-carbonate				79 x 106		(10)
	Igeneous rock, non-carbonate				1.1 x 106		(10)
Oceanic crust
	Sediments, carbonate				14 x 106		(10)
	Sediments, non-carbonate*				6.0 x 106		(10)
	Basalt, carbonate				0.3 x 106		(10)
	Basalt, non-carbonate				0.3 x 106		(10)
		Lithosphere, total			65 x 106
*These reservoirs contain the oil, gas and coal reserves of which  510 x 103 units (510 x 1018 Pg) can possibly be recovered for use.

 Table 2.2 Major carbon fluxes (when not otherwise indicated in units of 10¹⁵ g C yr^-1 = Pg yr^-1)

Atmosphere, terrestrial biosphere							Reference
Carbon monoxide production				1.2 ± 0.6			(3)
Methane production				0.6+0.3			(3)
Photosynthesis				53			(7)
				60			(8)
Litterfall				50 ± 10			(8)
Grass and forest fires (natural; gross)				24			(4, 11)
Animal (herbitore) consumption				6			(8)
Decay of organic matter = photosynthesis
Fluxes induced by man due to:
	Fossil fuel combustion 1980			5.2			(12)
	Fossil fuel combustion 18601980			165			(12)
	Deforestation 1970 (net)			1.0 ± 0.5		2.5	(13)
	Agriculture 1970 (net)			1.5 ± 1.0
	Deforestation, agriculture 18601970			150			(13)
Atmospheresea
Air-sea exchange, average rate				60 ± 15 mmole/m2 (ppm)yr			(4)
1860 CO2 concentration 290 ppm				75 ± 20			(4)
1980 CO2 concentration 336 ppm				90 ± 20			(4)
Net flux atmosphere to sea, 1980				2.5			(4)
Oceans, marine biota
Primary production				40 ± 10			(6)
Detritus fall out from surface
	layer(~ 75 m)			4			(1),(6)
Catch of fish				0.006			(6)
Freshwater, oceans, lithosphere
River flux, inorganic, dissolved				0.5			(14)
River flux, inorganic, particulate				0.2			(14)
River flux, organic, dissolved				0.30.7			(15)
River flux, organic, particulate				0.20.5			(15)
Sedimentation in continental basins				0.05			(13)
Weathering on land				0.3			(10)
Glacial erosion				0.03			(10)
Marine Erosion				0.005			(10)
Metamorphosis (sediments
	igneous rock)			0.008			(10)
Subduction (marine sediments
igneous rock)				0.3			(10)
Sedimentation in sea, inorganic				0.15			(10)
Sedimentation in sea, organic				0.04			(10)
	(1)	Bolin et al. (1979)	(8)	Ajtay et al. (1979)
	(2)	Bacastow and Keeling (1981)	(9)	Schlesinger (1984)
	(3)	Freyer (1979)	(10)	Kempe (1979a,b)
	(4)	Bolin et al. (1981)	(11)	Seiler and Crutzen (1980)
	(5)	Mopper and Degens (1979)	(12)	Rotty (1981)
	(6)	De Vooys (1979)	(13)	Moore et al. (1981)
	(7)	Whittaker and Likens (1975)	(14)	Kempe (1979a)
			(15)	Meybeck (1981)

Figure 2.1 Size of reservoirs (in 10¹⁵ g) and fluxes (in 10¹⁵ g yr^-1) for the part of the carbon cycle that is in a state of comparatively rapid turn-over, i.e. characteristic turn-over times less than about 1000 years

Figure 2.2 Fluxes (in 10¹⁵ g yr^-1) for the carbon cycle in the earth's crust, where the characteristic turn-over times are of the order of 100 million years (based on Kempe, 1979a, b)

2.3 THE NITROGEN CYCLE

 THOMAS ROSSWALL

During the past decade there has been a rapid development towards a qualitative understanding of the global nitrogen cycle. We are, however, still far from being able to present a quantitative picture of the global nitrogen cycle. This inability is due to two main reasons. The first one is inherent in all attempts to construct models of the annual transport of elements between different reservoirs. In most cases the reported annual flows do not represent truly integrated values but are based on extrapolations to a global, yearly basis of determinations of flow rates at specific points in time and space. Secondly, there are still lacunes in our knowledge of the qualitative aspects of the biogeochemical nitrogen cycle, and certain previously neglected processes seem to be of major importance on a global scale. Examples of this are the recent rediscovery of the importance of nitrification in the production of nitrous oxide (Bremner and Blackmer, 1978; Cohen and Gordon, 1979; Crutzen, Chapter 3, this volume) and the possible production of nitric oxide also during nitrification (Lipschulz et al., 1981).

Figure 2.3 is a schematic representation of the biogeochemical nitrogen cycle. Unlike that of, for example, phosphorus, the atmosphere plays an important role in the biogeochemical nitrogen cycle on account of the importance of gaseous compounds (N₂, N₂O, NO, NH₃) all of which can be produced and consumed through biotic and abiotic processes. A more accurate understanding of the biogeochemical nitrogen cycle has become of interest in recent years as a result of the importance of processes leading to the production of nitrogen oxides in view of the importance of such gases in the regulation of the chemical composition of the atmosphere.

2.3.2 Distribution of Nitrogen

The global distribution of nitrogen is shown in Table 2.3. Although nitrogen is present in relatively large concentrations in rocks, sediments, and the atmosphere, its availability in compounds that can be utilized by most forms of life is severely restricted. This deficiency in biologically available nitrogen in terrestrial and aquatic systems makes nitrogen one of the most important limiting nutrients.

In the atmosphere a minute fraction of nitrogen occurs in forms other than N₂. The quantitatively most important form of combined nitrogen in the atmosphere is nitrous oxide (N₂O), which accounts for 99.5% of all combined nitrogen. The small amounts of nitrogen compounds occuring in the atmosphere, do however, play a major role in regulating major processes in the atmosphere (see Crutzen, Chapter 3, this volume).

Figure 2.3 A global nitrogen cycle. Units are in Tg (10¹²g) N yr^-1. From Söderlund & Rosswall (1982) based on Söderlund & Svensson (1976)

In the oceans, dimolecular nitrogen (in dissolved form) is also the most abundant form of nitrogen. Nitrate and nitrogen in dead organic matter occur in approximately equal amounts. Biomass nitrogen accounts for less than 0.001% of the total amounts of nitrogen in the hydrosphere. The ratio of plant: animal: microbial biomass-N (Table 2.3) is 15:8.5:1, which differs markedly from the ratios found in terrestrial biomass (25:0.4:1) in that animals (mainly zooplankton) contain an appreciable reservoir of nitrogen as compared to what is found in terrestrial systems.

The nitrogen in the lithosphere is inaccessible to living organisms except man, who, through the burning of coal, will release some of the bound nitrogen into the atmosphere.

Nitrogen in terrestrial systems occurs mainly in soil organic matter, litter and soil inorganic nitrogen (97% of total) with biomass accounting for only less than 3%. In the biomass, 95% occurs in the plants.

Table 2.3 Major nitrogen reservoirs (in units of 10¹⁵ g N = Pg N)

Atmosphere		% of total		Reference
Dimolecular nitrogen		3 900 000	> 99.999	(1)
Nitrous oxide		1.4	< 0.0001	(2)
Ammonia		0.0017	< 0.0001	(2)
Ammonium		0.00004	< 0.0001	(2)
Nitric oxide + Nitrogen dioxide (NOx)		0.0006	< 0.0001	(2)
Nitrate		0.0001	< 0.0001	(2)
Organic nitrogen		0.001	< 0.0001	(3)
		Total	100
Ocean
Plant biomass		0.30	0.001	(3)
Animal biomass		0.17	0.0007	(4)
Microbial biomass		0.02	0.00006	(5)
Dead organic matter (dissolved)		530	2.3	(3)
Dead organic matter (particulate)		3240	0.010.1	(3)
Dimolecular nitrogen (dissolved)		22 000	95.2	(4)
Nitrous oxide		0.2	0.009	(3)
Nitrate		570	2.5	(6)
Nitrite		0.5	0.002	(3)
Ammonium		7	0.03	(3)
		Total	100
Pedosphere including biota
Plant biomass		1114	2.6	(3)
Animal biomass		0.2	0.04	(4)
Microbial biomass		0.5	0.1	(3)
Litter		1.93.3	0.5	(3)
Soil: organic matter		300	63	(3)
	inorganic	160	34	(7)
		Total	100
Lithosphere
Rocks		190 000 000	99.8	(8)
Sediments		400 000	0.2	(8)
Coal deposits		120	0.00006	(9)
		Total	100
(1) Robinson and Robbins (1970)
(2) Galbally and Roy (in manuscript)
(3) Söderlund and Svensson (1976)
(4) Delwiche (1970)
(5) This compilation (Based on Mopper and Degens (1979) and a C/N ratio in micro-organisms of 12.5.)
(6) Emery et al. (1955)
(7) Delwiche and Likens (1977)
(8) Stevenson (1965)
(9) Donald (1960)

Simpson et al. (1977) made a compilation of data on nitrogen reservoir sizes from four different publications. Although the data in that compilation agree well with those in Table 2.3, there are estimates which differ by an order of magnitude. Galbally and Roy (in manuscript) and Sweeney et al. (1977), for example, estimated the atmospheric content of ammonia to be 1.7 and 1.5 Tg, respectively, while Delwiche and Likens (1977) gave a value of 28 Tg. The similarity between many of the published data is, however, dependent on the use of the same original source by many authors. In several instances the originally cited sources do not present the basis for the calculations and the correctness cannot be assessed. Unfortunately, there are few well documented reports on the distribution of nitrogen in different reservoirs.

2.3.3 Fluxes of Nitrogen

There is considerably uncertainty with regard to most estimates of process rates in the global biogeochemical nitrogen cycle (Table 2.4). Most estimates made in the past 10 years differ by an order of magnitude between lowest and highest values. In addition, the uncertainty is further increased because previously unidentified processes in the biogeochemical nitrogen cycle probably exist. Examples of such new processes are the mesospheric source of N₂O from excited N₂ as suggested by Zipf and Prasad (1982) and the possible production of NO during nitrification (Lipschulz et al., 1981). A further quantification of natural fluxes of nitrogen compounds between the oceanic-terrestrial systems and the atmosphere is needed in order for reliable atmospheric models involving nitrogen compounds to be developed. Here we only consider the exchanges of nitrogen compounds between the terrestrial, aquatic and atmospheric systems, while the internal transfers are not discussed. It should be realized that, for example, for the terrestrial systems, the internal transfers of nitrogen in the soilplant subsystem are one to two orders of magnitude larger than the transfers to and from the terrestrial systems (Rosswall, 1976).

Man is increasingly affecting the global biogeochemical nitrogen cycle. The industrial production of nitrogen fertilizer will, towards the end of this century, probably become as large as that produced through biological nitrogen fixation in the global terrestrial ecosystem (Söderlund and Svensson, 1976). Increased combustion temperatures increase the production of NO_x, which contributes to the acidification of rain water. If we wish to determine the possible implications of such increased amounts of fixed nitrogen for the global cycles, it is essential that they are evaluated against background knowledge of the amounts of nitrogen that are parts of the natural biogeochemical nitrogen cycle. It should be evident from the data cited in Tables 2.3 and 2.4 that we are still far from having such a quantitative knowledge.

Table 2.4 Fluxes of nitrogen (Tg N yr^-1) in the global biogeochemical nitrogen cycle. The ranges summarize rates given by the following authors: Delwiche (1970), Burns and Hardy (1975), Söderlund and Svensson (1976), McElroy et al. (1976), CAST (1976), Delwiche and Likens (1977), Liu et al. (1977), Hahn and Junge (1977), Sweeney et al. (1977), NAS (1978) and Bolin (1979). For a compilation of the individual estimates, see Rosswall (1981)

	Tg N
Biological nitrogen fixation: land	44200
Biological nitrogen fixation: ocean	1130
Atmospheric fixation (lightning)	0.530
Industrial fixation	60	(1)
Industrial combustion and fossil fuel burning
(NOx, N2O)	10-20	(2)
Fires	10200
Biogenic NOx production	090	(3)
Denitrification: land	43390
Denitrification: ocean	0330
N2O from denitrification: land	1669	(4)
N2O from denitrification: ocean	580	(5)
Nitrification N2O production: land	?
Nitrification N2O production: ocean	410	(6)
Ammonia volatilization	36250
Dry and wet deposition NH3/NH4+	110240
Dry and wet deposition NOx	40116
Dry and wet deposition organic-N	10100
River run-off (total N)	1340
(1) Data for 1979/80 (UN, 1981)
(2) Crutzen (Chapter 3, this volume)
(3) The lowest value refers to the estimated of NOx production from soils of       0.15 Tg given by Crutzen (Chapter 3, this volume).
(4) Söderlund and Svensson (1976).
(5) The lower estimate comes from Crutzen (Chapter 3, this volume) and          the upper from Söderlund and Svensson (1976).
(6) Cohen and Gordon (1979).

2.4 THE PHOSPHORUS CYCLE JEFFREY E. RICHEY

Previous evaluations of the global P cycle have identified the key fluxes and reservoirs, and have provided some estimates of their relative magnitudes (Stumm,1973; Lerman et al., 1975; Pierrou, 1976). A revision is provided here (Table 2.5; Figure 2.4). The use of phosphorus fertilizers in modern agriculture and the eutrophication of fresh waters from run-off and effluent discharge are the most visible results of human intervention in the P cycle.

The intent of this work is to summarize our current understanding of the dynamics of the phosphorus cycle by describing some of the chemical properties of P that affect its distribution, reviewing the assumptions used in the calculation of earlier P budgets and, where possible, up-dating them with new data (Table 2.5). It is becoming increasingly evident that element cycles cannot only be viewed in their global aggregate, but must be analysed on more relevant space scales. Therefore, the continental portion of the P cycle is analysed with finer resolution for 10 major geopolitical zones (Table 2.6).

Phosphate is liberated into the environment by the weathering of the apatite rocks. Because phosphate tends to precipitate to form materials of low solubility, sorb onto surfaces, and form complexes with metal ions, much of the phosphate released by weathering is immobilized (Van Wazer, 1973). As a result, of the total soil P, free phosphate is  often present in only trace quantities, with little leaching into fresh waters.

Phosphorus is present in the biota in a wide variety of organic compounds. These organic compounds are characterized by either fairly weak POC ester bonds or stable PC bonds, and undergo the hydrolytic degradation of esters and condensed polyphosphates. With these properties, P is critical as a mobile entity of cell metabolism and as a basic structural element of cell materials. Other than as an intermediate, phosphate does not participate in reductionoxidation reactions, as do C, N, and S. Because the ambient concentrations are low and the demands specific, phosphate becomes an important element for primary production in both terrestrial and aquatic environments.

The terrestrial biota contains much less P than do the source rocks and soils, with the largest reservoirs in the forests of North America, the U.S.S.R., Latin America, and Tropical Africa (450560 Tg P). Although the total dissolved reservoir in the oceans is about 77,000 Tg P, only about 50120 Tg P are contained in marine biota. This is due in large part to much of the P being below the euphotic zone.

The primary reservoirs and fluxes of P involve dissolved phosphate ion (PO₄³), dissolved organic P (DOP), and particulate inorganic P. The bulk of the P exists in the soil, marine sediments, and, of course, crustal rocks as apatite. Where the concentration of apatite is great enough, it can be commercially mined. The soil fraction is distributed approximately in proportion to the area of a region, ranging from 5900 Tg P in Europe to 29,400 Tg P in Tropical Africa, whereas the mineable rock is concentrated in North America (4,700 Tg P) and Tropical Africa (5,900 Tg P). The particulate soil fractions can be mobilized by erosional processes into rivers and subsequently to the oceans, and is estimated to be about 17 Tg P/yr. A lesser amount, about 4 Tg P/yr, is carried by the wind into the atmosphere; however, the residence time there is very short, and the atmospheric reservoir is only about 0.025 Tg P. The P cycle does not have an important atmospheric gaseous component, unlike C, N, or S.

Table 2.5 The major reservoirs and fluxes of the global phosphorus cycle. Three previous evaluations are compared, up-dated with new information, where possible, and a current estimate derived (summarized in Figure 2.4). The means of calculations and sources of each estimate are provided below. It must be remembered that all such calculations yield a value with considerable uncertainty, not a precise number. It is important to examine the underlying assumptions and sources of error for each term as given in the original reference

RESERVOIRS (Tg P)	Stumm (1973)	Lerman et al. (1975)	Pierrou (1976)	Up-date	Reference
Atmosphere
Particulates over land				0.025	(1)
Particulates over oceans				0.003	(1)
Land
Biota	1950	3,000	1,805	2,600	(2)
Soil		200,000	160,000	96,000160,000	(3)
Mineable rock	31,000	9,920	3,1409,000	19,000	(4)
Fresh-water (dissolved)			90	90	(5)
Ocean
Biota	124	138	128	50120	(6)
Dissolved (inorganic)	124,000	92,600	120128,000	80,000	(7)
Detritus (particulates)				650	(8)
Sediments	8.4 x 108	4 x 108		840,000,000	(9)

FLUXES (Tg P/yr)
Atmosphere (land)	Atmosphere (ocean)			2	1.0	(1)
Atmosphere (land)	Atmosphere (ocean)				0.3
Atmosphere	Land			3.79.3	3.2	(1)
Atmosphere	Ocean			2.63.5	1.4	(1)
Land	Atmosphere			?	4.3	(1)
Ocean	Atmosphere			?	0.3	(1)
Marine dissolved	Biota	961	9921042	9901,300	6001,000	(10)
Marine detritus	Sediment	1.9	1.7	13	213	(11)
Terrestrial biota	Soils	229	63.6	136237	200	(12)
Mineable rock	Soil	12.4	12.4	12.6	14	(14)
Soil	Fresh-water			2.512.3	47	(13)
Fresh-water (diss.)	Oceans	1.9	1.7		1.54	(14)
Fresh-water (part.)	Oceans			17.4	17	(14)

(1)	The atmospheric reservoir and fluxes are directly from Graham and Duce (1979), who summarized extensive measurements of atmospheric P concentrations and deposition rates in marine and continental regions.
(2)	Estimates of the terrestrial biota are generally calculated from estimates of C mass of 4.5 x 105 Tg C (Bolin, 1979) 8.3 x 105 Tg C (Whittaker and Likens, 1975), and C/P atomic ratios of 500 (Stumm, 1973) to 833 (Deevey, 1970), though Pierrou (1976) used dry-weight biomass conversions. These yield a most-likely value of 2,600 Tg P, with a range of 1,4004,300 Tg P.
(3)	Inorganic P in the soil has been computed from a total land area of 130 x 1012 m2 and a soil depth of 0.6 m (Lerman et al. 1975) to 1.0 m (Pierrou, 1976), with a P content of 0.100.12 % (Taylor, 1964) and a soil density of 1 kg/dm3.
	In their calculation of soil volume, Lerman et al. apparently divided by 0.6 m rather than multiplying, yielding an over-estimate. Total soil P, including about 10% organic P (Bohn, 1976), is thus 96,000160,000 Tg P, depending on soil depth.
(4)	The amount of P in mineable rock, as 30% of P2O5, has been defined as that minimum amount which is economically recoverable. As demand and technology increase, the P content of mineable rock decreases and the reservoir `increases'. The estimates of reservoir size and consumption rate are from Harris and Hare (1979) and FAO (1980).
(5)	Pierrou (1976) calculated the P in fresh-waters from a total volume of 7.2 x 1014 m3 and a mean concentration of 0.12 g P/m3. The concentration value is probably uncertain by a factor of 2.
(6)	Phosphorus in the marine biota is calculated generally from applying the Redfield ratio of C/P = 106 and carbon estimates of 2,000 Tg C (Williams, 1975) to 5,000 Tg C (Bolin, 1979). The upper estimate of Lerman et al. (1975) is based on out-dated production data.
(7)	The previous estimates of dissolved inorganic P in the oceans have used mean concentrations of 0.080.10 g P/m3 and mean depths of 3,0003,500 m. More recent GEOSECS data (Takahashi et al. 1981) suggest a concentration of 0.062 g P/m3.
(8)	The inventory of marine detrital P can be calculated as the amount of particulate carbon (3 x 104 Tg C; Mopper and Degens, 1978) times a detrital C/P atomic ratio of 120 (Broecker, 1974), or 650 Tg P.
(9)	Stumm (1973) calculated the phosphorus in sediments from a geochemical mass balance, which is based on more recent data than the value presented in Lerman et al. (1975).
(10)	Estimates of the photosynthetic uptake by marine biota have been obtained by applying the Redfield ratio to productivity data, which range from 2.5 x 104 Tg C/yr to 4 x 104 Tg C/yr (De Vooys, 1979), or 6001000 Tg P/yr. The release of P by decomposition is assumed to be equal to photosynthetic uptake.
(11)	Emery et al. (1955) assumed that the P content of sediments is 0.092% and that 1 cm of solid sediment forms every 6000 years, for a rate of 13 Tg P/yr. Assumptions of steady state, as calculated by the others, indicate that this figure might be high, and that a value of 2 Tg/yr is more appropriate.
(12)	The uptake and release of P by terrestrial biota has been estimated from productivity estimates of 3 x 104 C/yr (Bolin, 1979) to 5 x 104 Tg C/yr (Whittaker and Likens, 1975) and the C/P atomic ratios of 500822. A mean estimate of 200 Tg P/yr results.
(13)	Phosphate is introduced into fresh-waters from natural leaching processes and from various human activities it is difficult to differentiate between the sources.
	Pierrou used leaching rates from respective land-use types to estimate the total input. A similar analysis using the land-use types specified for the different geographic zones and up-dated loss rates for the different land uses (forest, grass, desert-swamp, and agriculture-pasture) from Rast and Lee (1978) gives the result 4 Tg P/yr.
(14)	The river run-off of P to the oceans includes both natural and human-influenced leaching and particulate erosion products, less that which is retained or consumed within the river. The most recent calculation of dissolved export is described in Wollast (Chapter 14, this volume). Particulate export, assuming a ratio of 0.075% for the P content of total suspended sediments (e.g. Holland, 1978), is considerably greater.

Figure 2.4 A global phosphorus cycle. Fluxes are in Tg P yr^-1 and reservoirs are in Tg P. From Table 2.5

The uptake and release of phosphate by terrestrial plants is about 200 Tg P/yr, while the equivalent marine flux is 6001000 Tg P/yr. Given the lesser marine biomass, the turnover rate in the oceans is much greater than on land. The amount finally sedimenting in the oceans is only a small fraction of the productionmineralization flux.

The natural cycle of P partly regulates the distribution of biomass because the supply and levels of phosphate are low relative to the requirements for plant and animal nutrition. Superimposed on the natural cycle is man's influence: the mining and consuming of phosphates by society, and the release of P in domestic and industrial effluents.

Table 2.6 The distribution of the terrestrial and fluvial components of the global P cycle (Table 2.5) according to major geopolitical zones. Means of calculation are described in the footnotes

	N.  America	Europe	U.S.S.R	Pac. Dev.	China	L. America	N. Africa ME	Tr. Africa	S. Asia	S.E. Asia	Reference
RESERVOIRS(TgP)
Biota	450	110	560	170	80	450	20	540	70	140	(1)
Soil	23,000	5,900	26,800	10,700	13,400	24,600	14,400	29,400	7,000	4,800	(2)
Mineable rock	4,700	50	980	280	1,600	1,260	2,900	5,900	800	530	(3)
FLUXES (TgP/yr)
Biota         Soil	28	7	35	10	5	27	2	33	4	9	(1)
Rock         Soil	2.5	4.0	2.3	1.6	0.8	1.0	0.4	0.2	0.6	0.2	(3)
Soil           Freshwater	0.7	0.3	1.0-1.8	0.3-0.6	0.4-0.6	0.5-0.8	0.2-0.3	0.6-1.0	12-0.3	0.4-0.7	(4)
Freshwater	0.20.6	0.2-0.5	0.6-1.5	0.04-0.	0.04-0.1	0.1-0.5	0.01	0.05-0.2	0.1-0.4	0.02-0.1	(5)
(1) The distribution of terrestrial biota and the uptake and release of P was calculated by determining the percent vegetation type by region        and weighting that by kg C/m2 from Whittaker and Likens (1975).
(2) The soil P distribution by region was apportioned by the percent land surface in each region.
(3) Mineable rock and fertilizer consumption were calculated by grouping the country-specific and regional data of Harris and Hare (1979) and       FAO (1980) to the appropriate zones.
(4) The relative land cover in each zone was calculated, and the run-off ratios from Rast and Lee (1978) applied.
(5) Calculated by river zone by establishing the range of P concentrations for the rivers of that zone time their total water discharge.

The annual rate of fertilizer consumption in the industrialized regions is now about 10% of the steady-state flux between the soil and biota, and approaches 50% in Europe. In the less-developed regions the rate is lower. The total input of P to fresh-waters which also includes wastes, is a considerable fraction of the fertilizer consumption. This suggests, that the mobility of this `excess' P is sufficient to be transported away from its sites of ap