The Phosphorus Cycle

Transcription

1 Lecture Outline Introduction Global Cycle Overview Major Forms Anthropogenic Influences Terrestrial Processes Freshwater Processes Ocean Processes The Phosphorus Cycle No important gaseous form, present in + 5 valence state phosphate PO 4 (3-) 1

2 Chemical info Valence state is + 5 (but depending on environment coordination can be 1,3,4,5,6) Exists as oxyanion or as an oxidized compound Form in solution varies according to ph H 3 PO 4, H 2 PO 4-, HPO 4 2-, PO 4 3- Basic acidic most soils plants uptake Introduction Phosphorus (P) is an essential nutrient for all life 12 th most abundant element in the lithosphere Major role in biological metabolism RNA ATP DNA Phospholipids 2

3 Bones are made of calcium phosphate (hydroxyapatite) and collagen (a protein) Teeth calcium hydroxyapatite, which has the formula Ca 10 (PO 4 ) 6 (OH) 2. Major Reservoirs From: Biogeochemistry of Wetlands, Reddy and Delaune (2008) 3

4 Introduction P is an essential nutrient Major mineral containing is apatite Calcium phosphate 3(Ca 3 (PO 4 ) 2 ). CaX 2 Where x is Cl- Fl- OH- CO 3 2- Marine phytoplankton: 106CO H 2 O + 16NH 3 + H 3 PO 4 + hv C 106 H 179 O 68 N 16 P + 106O 2 (organic matter) Terrestrial plants: 830CO H 2 O + 9NH 3 + H 3 PO 4 + hv C 830 H 1230 O 604 N 9 P + 830O 2 Phosphorus: forms and characteristics Phosphate: PO 4 3- Simplest form of phosphorus in the environment Only form that is readily available for biotic uptake. Highly reactive with cations (e.g. Fe, Ca) AKA: Soluble Reactive Phosphate (SRP) 4

5 The Global Phosphorus Cycle Units: metric tons Treatise on Geochemistry (2 nd Edition), Holland and Turekian Eds. Global phosphorus cycle Pools (10 15 g P) Fluxes (10 12 g P yr -1 ) ,000 15,700 available 25 Black S 1997 Red S and B 2013 Running downhill 12,800 CONCERN regarding numbers in the book 320 years based on 2011 use

6 The Global Phosphorus Cycle No significant gaseous component Phosphine gas PH 3 is made in reduced settings and has been measured over the ocean (Lightning by interaction with P containing soil dust) Produced in wastewater treatment plants Atmospheric P associated with dust and sea spray The Global Phosphorus Cycle 5 major parts: 1) Tectonic uplift 2) Erosion and chemical weathering 3) River transport to lakes and oceans 4) Sedimentation and burial 5) Eventual uplift completes the cycle 6

7 How do humans influence the P cycle? From 1961 to 2013, global phosphorus consumption increased fivefold. Only about 22% of the extracted phosphorus is actually consumed as human food. On-farm wastes have yielded to manufactured fertilizers as the primary source of phosphorus. Non-food uses of phosphorus were about five times larger in 2013 than they were in

8 Phosphorus: Anthropogenic impacts Major use of P is in agriculture also detergents Also use in home lawn/gardens Phosphorus: Anthropogenic impacts Most P mined from marine sedimentary rocks. Biggest P mine in the US is in Florida 8

9 Nature v461- Estimate reserves will last 125 years at our current rate 80% comes from sedimentary deposits Rest igneous and weathering products and guano Phosphorus: Anthropogenic impacts Deforestation increases P loading to streams can cause eutrophication Since no major atmospheric source of P, once it is removed in biomass and erosion then the land can become infertile Almost all P in rainforest is in living material Dams have stopped transport to oceans 9

10 Phosphorus: Anthropogenic impacts P from domestic sewage, human waste and detergents. Historically in the US 2 million tons of P in detergents annually (7-12% of weight of detergent was P) Phosphorus: Anthropogenic impacts Many other anthropogenic sources: plasticisers, flame-retardants, corrosion inhibitors, pesticides, food processing, garbage leachate, livestock industries--feedlots Overall: 3X increase in ocean P loading above pre industrial levels 10

11 Issues with P in fertilizers Contaminants- Cd, As, Cr, Pb, Hg, Ni, V, U, Rd, Th Mobilizing other elements Phosphorus: Terrestrial Processes Apatite mineral Ca 5 (PO 4 ) 3 OH P Crustal abundance: (0.1%) Bedrock weathering is the main P source in soils Weathered phosphate can be taken up by plants OR React with Al and Ferric oxides 11

12 Phosphorus: Terrestrial Processes Phosphorus: Terrestrial Processes P availability is controlled by precipitation of Al and Fe in low ph soils & Ca in high ph soils 5.7 is ideal ph for P availability. Book says 7 based on Figure 4.10 Organic acids can increase P availability by inhibiting the crystallization of Al and Fe hydrous oxides From Elements April

13 Phosphorus: Terrestrial Processes How plants deal with low P availability in soil: Mycorrhizae: Fungi that have a symbiotic relationship between them and the plant root Increase root volume Re-absorb P from leaves before litter fall and/or quickly recycle P in litter fall. Release enzymes and oxalic acids that can: increase weathering of apatite, solubilize Fe, Al, and Ca bound P, and remove Fe, Al, and Ca from solution through chelation and precipitation. Phosphate chemistry in soils pg 108 Apatite is weathered by carbonic acid and releases HPO 2-4 (form ph dependent) This is accumulated by organismsbecomes organic P See Figure 4.10 In soils and aquatic systems PO 2-4 becomes bound to Al and Fe oxides at high ph Ca 2+ In soils organic acids help to release 13

14 This will be impacted by geology and weathering Phosphorus: Freshwater Processes-- Rivers The main global flux of P is in rivers >90% of P in rivers is particle bound Nutrient spiraling-pg

15 Lakes-Pg 305 Allochthonous-particulate and organic bound some soluble (but not much) Autocthonous- organic matter- minor available soluble Soluble PO 2-4 forms precipitates out with Ca, Fe, and Al oxides Deposition to sediment Sediment becomes anoxic Fe released and P also. Freshwater Processes-- Lakes P is a good measure of lake productivity: <5 ug/l = low productivity >100 ug/l high productivity. Tahoe 15

16 Phosphorus: Freshwater Processes-- Lakes Phosphorus: Freshwater Processes-- Lakes Much of the hypolimnetic increase in P is near the sediments 16

17 Phosphorus: Freshwater Processes-- Lakes Under oxic conditions the P exchange equilibrium is unidirectional towards the sediments P concentration in sediments is much greater than overlying water and is bound with ferrous iron, If the sediment micro-zone is oxic, then the release of P is prevented. When it is anoxic, P is released due to ferric oxide reduction Phosphorus: Freshwater Processes-- Lakes Seasonal effect: Summer higher temperatures, more OM loading, more microbial activity = anoxic conditions During summer soluble P accumulates in the hypolimnion with fall turnover, ferrous iron is oxidized and precipitates out P as ferric phosphate. 17

18 Phosphorus: Ocean Processes River inputs are the main source of P to the oceans Some particulate bound P released as ph of water changes or if deposited under certain conditions (anoxic) Some is taken up by algae and some is directly deposited Ocean sediments are the largest store of global P Phosphorus: Ocean Processes P stimulates primary productivity in oceans Seawater: C:N:P - 1,000:15:1 Marine phytoplankton: C:N:P - 106:16:1 N and P are closely limiting N most but varies spatially 18

19 Phosphorus: Ocean Processes P limits primary productivity, which influences atmospheric CO 2 levels Interesting feedback to consider. During cold periods, sea level drops, exposing coastal sediments (high in P) to erosion remobilizing P, enhancing productivity and reducing CO2 and enhancing cooling trends. 19