the phosphorus cycle jen morse [email protected] 10 january 2013
TRANSCRIPT
Is the Phosphorus Cycle important?
● Global P cycle in Schlesinger 1997: 3 pages (vs 13 for N)
● Terrestrial P cycling in Chapin 2002: 4 pages (vs 18 for N)
● Phosphorus cycling is:
○ A) Simple
○ B) Boring
○ C) Not important
Questions to consider
● What makes phosphorus important?
● Phosphorus cycling:
○ How does global cycle P differ from N?
○ Forms, pools, fluxes
○ P cycling in soils vs. inland waters vs. marine systems
○ Controls on availability & interactions with other elements
● Why care about ecosystem P inputs and losses?
Biological importance of PEnergy and evolution
● DNA, RNA
● ATP energy transformations
● Phospholipids cell membrane structure
● Bones and teeth of vertebrates
ATP
DNA
Phosphorus basics:
● 11th most abundant element on land, 13th in seawater (Smil 2000)
● Elemental P: highly reactive
○ Isolated from urine by Hennig Brandt in 1669
○ Glows and spontaneously reacts: alchemy... matches... explosives
● Only 31P is stable; radioisotopes include 32P,33P
○ Stable isotope ecology methods don’t apply for P
Calliergon giganteum. Photo by F. R. Wesley.
P has similar oxidation states to N...
+5 +3 +1 0 -3
NO3- NO2
- N2O N2
R-NH2, NH4
+NO
+2
+5 +3 0 -3
PO43-
(inorganic),
P(=O)(OR)3
(phosphate esters)
P(OR)3
(phosphite esters)
Elemental P (highly reactive)
PH3 (phosphine)
+1+2
... but no critical redox transformations or significant gas phase.
Questions to consider
● What makes phosphorus important?
● Phosphorus cycling:
○ How does global cycle P differ from N?
○ Forms, pools, fluxes
○ P cycling in soils vs. inland waters vs. marine systems
○ Controls on availability & interactions with other elements
● Why care about phosphorus inputs and losses?
Internal cycling
Nutrient inputs
Nutrient losses
Ecosystem
Chapin et al. (2002)
Internal cycling
Nutrient inputs
Nutrient losses
• Chemical weathering of rocks• Biological fixation• Deposition from atmosphere• Fertilizers
• Transfer of nutrientsBetween plants/primary producers and soil/benthosBetween organic and inorganic forms
• Changes in ionic forms• Biological uptake• Interactions with mineral surfaces
• Leaching• Trace gas emissions• Wind and water erosion• Fire• Harvest
Ecosystem
Chapin et al. (2002)
Internal cycling
P inputs
P losses
Ecosystem
Chapin et al. (2002)
N most abundant in atmosphere...
Chapin et al. (2002) Fig. 15.4
(N fluxes in Tg/yr)
... most P stored in soils, sediments, ocean
Chapin et al. (2002) Fig 15.6
(P fluxes in Tg/yr)
Decades, Accelerated by human activities
Thousands to millions of years
P becomes available at LONG time scales
Biologically available P is limited by parent material and supply
● Relatively scarce (localized) in mineral form, low solubility in water
● Ultimately tends to limit production:
○ In aquatic systems
○ Terrestrially at long time scales
Bennett & Schipanski (2013) redrawn from Walker & Syers (1976) and Vitousek et al. (2010)
Nutrient limitation during ecosystem development
Vitousek & Farrington (1997)
Fertilization experiment:Hawai’ian tree diameter across chronosequence plots
Younger soils more N-limitedOldest soils more P-limited(but co-limitation is important)
Model applies to terrestrial ecosystems:Tropics vs. temperate zones
Atmospheric P inputs
Sources:Arid lands in Asia and N. Africa
Deposition zones:*highly weathered, humid tropical forests-Amazon-Caribbean-Congo
*Open ocean
(soil solution)
biota
Fe/Al-P
Ca-P
Active
P cycling in soils
Adapted from Brady & Weill (1999)
Low pH
High pH
INORGANIC P ORGANIC P
Passive
SOM
Mineral P forms in soils
Brady & Weill (2001)
Fixation by hydrous
oxides of Fe, Al, and Mg
Sources of P in soils: Weathering
Ca5(PO4)3 + 4H2CO3 5Ca2+ + 3HPO42- + 4HCO3
- + H2O
Apatite(mineral)
Carbonic acid(CO2 from respiration
e.g. plant roots)
Bio-available P
Weathering factors:ClimateParent materialTopographyTimeBiota (Jenny 1941)
Sources of P in soils: Mycorrhizal fungi organic and inorganic P
Brady & Weill (1999)
Species A Species DSpecies CSpecies B
Dissolved phosphate
Monoester (labile org P)
Diester Inosotol P (refractory)
Sources of P in soils: (Phosphatase) enzymes organic P
Turner (2008)
Hypothesis of increasing investment in organic P acquisition
How important are P inputs relative to internal cycles?
Chapin et al. (2002) – Table 8.1
Major Sources of Nutrients that Are Absorbed by Plantsa Source of plant nutrient (% of total) Nutrient Deposition/fixation Weathering Recycling Temperate forest (Hubbard Brook) Nitrogen 7 0 93 Phosphorus 1 < 10? > 89 Potassium 2 10 88 Calcium 4 31 65 Tundra (Barrow) Nitrogen 4 0 96 Phosphorus 4 < 1 96 a Data from (Whittaker et al. 1979, Chapin et al. 1980b)
(filter)
P cycling in water
Movement:
• water
• wind (dust)
• animals
DIP
DOP
POP
PIP
DIP(PO4
3-)
DOP
D/P = dissolved/particulateI/O = inorganic/organic
POP
PIP
uM P
TDP(Total dissolved P)
Total P
Key additional control: Redox related to element interactions
Forms of P in water:
Redox affects P via Fe: Internal eutrophication
↑ production
↑ sediment P and Fe2+ release
External P load
↑ anoxia
↓ FeOOH with associated PO43-
Mixing (without re-ppt) Sedimentation and decomposition
Fe3+ reduction in absence of DO (or NO3
-)Loss of sorption ability
Classic studies:Mortimer, EinseleCurrent Netherlands focus: Smolders et al. (2006) review
Time
Bottom water
chemistry DO DIPFe2+
As Fe increases in sediments, P may increase
Smolders et al. (2006)
... and may be released under reducing conditions.
Fe/Al-P
Sulfur can intensify internal eutrophication:
●= sulfate addition(all in waterlogged conditions)
● Alkalinity ○ Greater decay rate (acid neutralization)
○ HCO3- competes with PO4
3- for anion exchange sites
SO42- HS-
↑ HCO3-
↑ NH4+↑ PO4
3-
Smolders et al. (2006)
P like N:
● Internal cycling dominates P available for plant uptake
P unlike N:
● No P-focused oxidation-reduction reactions (redox controls are via interactions with other elements)
● Using N to obtain P: Microbes (incl. mycorrhizae) & plants produce phosphatases to access organic P
● No important gas phase
● Main pools in soils/sediments
Cycle essentially uni-directional
Questions to consider
● Why phosphorus?
● Phosphorus cycling
○ How does global cycle P differ from N?
○ Forms, pools, fluxes
○ P cycling in soils vs. inland waters vs. marine systems
○ Controls on availability & interactions with other elements
● Why care about phosphorus inputs and losses?
Humans have modified the P cycle
● Flows of P have tripled since 1960 (Milennium Ecosystem Assessment)
● P mining expected to peak ~2030 (Cordell et al. 2009)
Data from Smil (2000)
World Cropland P Balance
0
200
400
600
IN OUT
Tg
P (
1960
-199
5)
Greater accumulation of P in soils…
Fert.
Manure
Crop
LossAnimal
World Cropland P Balance
After Bennett et al. (2001)
Will long-term P-accumulation drive future exports to surface waters?
Extra P
Leads to greater streamwater P in agricultural and urban areas...
Muhller & Spahr (2006): USGS National Water-Quality Assessment Program, Scientific Investigations Report 2006–5107
Ort
hoph
osph
ate
(mg/
L)T
otal
P (
mg/
L)
A
gUrb
an
Mixe
d
Par
tial
Undev
el
Why care about nutrient inputs to aquatic systems?
● Eutrophication: “...anthropogenic nutrient loading to aquatic ecosystems (i.e., cultural eutrophication; Hasler 1947) from both point and nonpoint sources typically results in rapid increases in the rate of biological production and significant reductions in water column transparency and can create a wide range of undesirable water quality changes in freshwater and marine ecosystems.” (Smith et al. 2006)
Effects of eutrophication
● Phytoplankton blooms
● Hypoxia/anoxia
● Toxicity to wildlife
● Marine dinoflagellates: red tides (fish kills, neurotoxins in shellfish)
● Freshwater cyanobacteria (neurotoxins, hepatotoxins)
C? N? P?
Cause of eutrophication which nutrient(s)?
Classic and ongoing scientific investigations…
P linked to eutrophication in L. Washington...
Year
Nutrient diversion
Total P
Chl-a
Edmondson (1970, 1991....)
... but soap/detergent interests suggested that decreases in phytoplankton had caused the decrease in P.
Next step: Whole-lake fertilizations, Experimental Lakes Area
● C could be obtained from atmospheric inputs
Chl
orop
hyll
Total P
Schindler (1977)
● P consistently limited growth
C + N P
Why not N limitation? P
lank
toni
c N
fixa
tion
TN:TP loading ratio (molar)
Data from Howarth et al. (1988); Schindler (1977)
● N fixation greater where TN is low rel. to TP
● Cyanobacteria alleviated N limitation in lakes
Tot
al N
Total P
SEDIMENTS
0-2 m
PE
RC
EN
T 32
P
DAYS AFTER ADDITION
Why P limitation? P sediments rapidly out of water column
Levine et al. (1986)
P sediments out:● with organic matter
● as precipitates with CaCO3, Fe, Mn
Legacy effect of re-mobilization:
● Anoxic conditions release Fe-P
● Elevated CO2 release Ca-P
Schindler et al. (2008)
TP
TDP
TN
TIN
TN
:TP
TIN
:TD
P
Are estuaries and coastal zones N or P limited?
Yes: P mgmt is needed in estuaries: •evidence in some estuaries of N fixation, and of production in response to P; • need whole-ecosystem approach before making costly decisions
No: N limitation in many estuaries• low N fixers at high salinities – likely b/c SO4 inhibits N-fixer growth• mixing of low N:P waters (from offshore, & b/c of high coastal denitrification) promotes N limitation• greater P availability in estuaries than lakes• nutrient loads often at low N:P, increasing N limitation
“...controlling the eutrophication of coastal zone waters will likely require careful basin-specific management practices for both N and P.” (Smith 2006)
P limitation
N limitation
Redfield ratio (16:1 by moles)
Smith (2006)
Redfield ratio: marine algae = water column = N:P 16:1
P limitation
N limitation
[PO43-] (μmol kg-1)
[NO
3- ]
(μm
ol k
g-1)
Orig. by Redfield (1934)
Marine nutrient limitation more variable
Chapin et al.(2002) – Fig. 10.7, from Valiela (1995)
● N:P ~16:1 (molar) = Redfield ratio
o N:P < 16(-20): N limitation
o N:P > 16: P limitation
o N limitation typical in coastal zones (Howarth & Marino 2006)
o (Terrestrial: N-limited in temperate zone; P-limited on older tropical soils)
Break (15 min)
Discussion (Childers et al. 2011)and summary
P sustainability challenges: human food
Childers (2011)
Key P flowsPools and fluxes
1. P mining 2. Agricultural P use3. non agricultural P uses4. P in food5. A) P recycled in farm operations
B) P lost from farm fieldsC) P lost in food processing
/transportation
6. A) P composted in food wasteB) P in human excreta
7. P lost to landfills8. A) P from sewage P treatment recycled
as fertilizerB) P discharged in sewage treatment
P sustainability challenges: human food
● What is meant by “non-substitutability” of P resources?
● What are the prospects for increasing P availability to agriculture?
● What are benefits and obstacles of different strategies to close P cycle?
● GMO pig to reduce P in animal waste?
Species identity and the P cycle
● In what ways is species identity important to ecosystem functioning in
○ The terrestrial P cycle?
○ The aquatic P cycle?
○ The agricultural P cycle?
● Soil/sediment-focused, ~unidirectional at human time scales
● Limiting element in aquatic systems (particularly freshwater) and at long time scales
● Complex interactions with other elements
● Altered considerably by human activities (like all the cycles)
Summary of the P cycle