This document describes a study that examined the effects of salt stress and phosphorus subsidy on sawgrass peat cores in mesocosms. There were three main findings:
1. Phosphorus additions stimulated plant biomass and productivity, while salt stress had negative effects. Phosphorus partly mitigated the negative effects of salt stress.
2. However, salt stress still reduced root biomass and likely soil structure, leading to peat collapse even with phosphorus additions.
3. After freshwater restoration, initial losses in soil elevation from salt additions were not maintained, indicating the wetland's adaptive capacity. However, phosphorus additions continued stimulating aboveground plant biomass and productivity due to legacy effects.
Will freshwater restoration offset peat collapse in wetlands exposed to salt and phosphorus?
1. Will freshwater restoration offset
peat collapse in wetlands exposed
to salt and phosphorus?
Dong Yoon Lee, Sean Charles, Benjamin Wilson, Shelby Servais,
Steve Davis, Tiffany Troxler, Evelyn Gaiser, John Kominoski
Florida International University
Florida Coastal Everglades Long-Term Ecological Research
Everglades Foundation
2. Peat collapse in the River of Grass,
the Florida Everglades
Sawgrass
(Cladium jamaicense)
Oh, no!
Florida average elevation is only 1.8 m!
3. Human and climate alterations are causing
salt and phosphorus intrusion
Photo: S. Bramson
Downtown Miami
U.S. Army Corps of Engineers
Altered water flowCanals Rising sea levels in Florida Bay
Salt and
phosphorus
Original
(Data from DBHYDRO)
4. Experiments examining effects of salt stress
and phosphorus subsidy in mesocosm
• Sawgrass-peat cores from
freshwater Everglades were
manipulated with salt and
phosphorus (P) from 02/2015 to
02/2017.
• Salinity: ~10 PPT
• P: x2 ambient load
• Four treatments (n = 6) :
I. Freshwater without P (FW)
II. Freshwater with P (FWP)
III. Saltwater without P (SW)
IV. Saltwater with P (SWP)Florida Bay Interagency Science Center
8. SWFWP SWP
Ecosystem CO2 flux (µmol C m-2 s-1)
a a
b
a,b
a a
b
a,b
CO2
CO2
(⇧ or ⇩ P < 0.05; ⬀ or⬂ not significant)
(Measurement) FWP SW SWP
Soil elevation ⬀ ⬂ ⬂
Metabolism ⇧ - ⬀
Plant biomass ⬀ ⬂ ⬀
Litter breakdown ⇧ ⬂ ⇧
Root growth ⬀ ⬂ ⬀
Root breakdown ⬀ ⬂ ⇧
Root biomass ⬀ ⬂ ⬂
P > S
13. SWFWP SWPRootbiomass
(gCm-2)
ab
a
b b
FW FWP SW SWP
(⇧ or ⇩ P < 0.05; ⬀ or⬂ not significant)
(Measurement) FWP SW SWP
Soil elevation ⬀ ⬂ ⬂
Metabolism ⇧ - ⬀
Plant biomass ⬀ ⬂ ⬀
Litter breakdown ⇧ ⬂ ⇧
Root growth ⬀ ⬂ ⬀
Root breakdown ⬀ ⬂ ⇧
Root biomass ⬀ ⬂ ⬂ P < S
14. Summary: effects of P subsidy and salt stress
• There were stimulatory effects of P (FWP) and stress effects of salt
(SW).
• Effects of salt stress were mitigated by P subsidy.
(Measurement) FWP SW SWP
Soil elevation ⬀ ⬂ ⬂
Metabolism ⇧ - ⬀
Plant biomass ⬀ ⬂ ⬀
Litter breakdown ⇧ - ⇧
Root growth ⬀ ⬂ ⬀
Root breakdown ⬀ ⬂ ⇧
Root biomass ⬀ ⬂ ⬂
(⇧ or ⇩ P < 0.05; ⬀ or⬂ not significant)
15. Summary: effects of P subsidy and salt stress
• There were stimulatory effects of P (FWP) and stress effects of salt
(SW).
• Effects of salt stress were mitigated by P subsidy.
(Measurement) FWP SW SWP
Soil elevation ⬀ ⬂ ⬂
Metabolism ⇧ - ⬀
Plant biomass ⬀ ⬂ ⬀
Litter breakdown ⇧ - ⇧
Root growth ⬀ ⬂ ⬀
Root breakdown ⬀ ⬂ ⇧
Root biomass ⬀ ⬂ ⬂
(⇧ or ⇩ P < 0.05; ⬀ or⬂ not significant)
16. Summary: effects of P subsidy and salt stress
• There were stimulatory effects of P (FWP) and stress effects of salt
(SW).
• Effects of salt stress were mitigated by P subsidy.
• BUT, root biomass and likely soil
structure in the SWP were not
mitigated by P subsidy.
• As a result, there was the loss of
soil elevation (peat collapse).
(Measurement) FWP SW SWP
Soil elevation ⬀ ⬂ ⬂
Metabolism ⇧ - ⬀
Plant biomass ⬀ ⬂ ⬀
Litter breakdown ⇧ - ⇧
Root growth ⬀ ⬂ ⬀
Root breakdown ⬀ ⬂ ⇧
Root biomass ⬀ ⬂ ⬂
(⇧ or ⇩ P < 0.05; ⬀ or⬂ not significant)
19. Porewater chemistry showing salt and P
legacies
(P legacy: F(1,21) = 1.7, P = 0.20)
Freshwater restoration began
2015 2016 2017 2018
20. Soil elevation:
no effects of P and salt legacies
• P legacy: F(1,21) = 0.04, P = 0.83
• Salt legacy : F(1,21) = 1.1,P = 0.30
• P x salt: not significant
(Measurement) FWP SW SWP
Soil elevation ⬂ ⬂ ⬂
Plant biomass ⬀ ⬀ ⇧
Litter breakdown ⬀ ⬀ ⇧
GEP at max PAR ⇧ ⇧ ⇧
Respiration ⇧ ⇧ ⇧
(⇧ or ⇩ P < 0.05; ⬀ or⬂ not significant)
• Initial soil elevation loss with added salt was not maintained
as a legacy after restoration.
21. Aboveground plant biomass:
stimulatory effects of P legacy
• P legacy: F(1,21) = 27.8, P < 0.001
• Salt legacy: F(1,21) = 3.5,P = 0.07
• P x salt: not significant
(Measurement) FWP SW SWP
Soil elevation ⬂ ⬂ ⬂
Plant biomass ⬀ ⬀ ⇧
Litter breakdown ⬀ ⬀ ⇧
GEP at max PAR ⇧ ⇧ ⇧
Respiration ⇧ ⇧ ⇧
Restoration
(⇧ or ⇩ P < 0.05; ⬀ or⬂ not significant)
• Added P was removed by sawgrass and affected
aboveground biomass.
2015 2016 2017 2018
22. Sawgrass litter breakdown:
stimulatory effects of salt legacy
• P legacy: F(1,21) = 1.8,P = 0.19
• Salt legacy: F(1,21) = 12,P < 0.01
• P x salt: not significanta a a,b b
(Measurement) FWP SW SWP
Soil elevation ⬂ ⬂ ⬂
Plant biomass ⬀ ⬀ ⇧
Litter breakdown ⬀ ⬀ ⇧
GEP at max PAR ⇧ ⇧ ⇧
Respiration ⇧ ⇧ ⇧
(⇧ or ⇩ P < 0.05; ⬀ or⬂ not significant)
• Changes in biogeochemical changes drove organic matter
decomposition.
23. Gross ecosystem productivity:
stimulatory effects of P legacy
CO2
a
b
b
c
• P legacy: F(1,13) = 36, P < 0.01
• Salt legacy: F(1,13) = 3.1,P = 0.09
• P x salt: not significant
(Measurement) FWP SW SWP
Soil elevation ⬂ ⬂ ⬂
Plant biomass ⬀ ⬀ ⇧
Litter breakdown ⬀ ⬀ ⇧
GEP at max PAR ⇧ ⇧ ⇧
Respiration ⇧ ⇧ ⇧
(⇧ or ⇩ P < 0.05; ⬀ or⬂ not significant)
24. CO2
a
b
b
c
• P legacy: F(1,13) = 61, P < 0.01
• Salt legacy: F(1,13) = 7.9,P < 0.05
• P x salt: not significant
(Measurement) FWP SW SWP
Soil elevation ⬂ ⬂ ⬂
Plant biomass ⬀ ⬀ ⇧
Litter breakdown ⬀ ⬀ ⇧
GEP at max PAR ⇧ ⇧ ⇧
Respiration ⇧ ⇧ ⇧
(⇧ or ⇩ P < 0.05; ⬀ or⬂ not significant)
Ecosystem respiration:
stimulatory effects of both P and salt legacy
25. Ecosystem-scale metabolism
CO2
CO2
•Gross ecosystem productivity vs. photosynthetically
available radiation
•Ecosystem respiration vs. temperature
•Net ecosystem productivity (NEP) = GEP – ER
Full sun 70% Sun 30% Sun Dark
26. Net negative ecosystem productivity
CO2
CO2
• If NEP > 0, likely increasing
carbon sequestration and
vertical soil accretion
• If NEP < 0, likely resulting in the
loss of soil organic carbon and
elevation
2017 2018
27. Summary: legacy effects of salt and
phosphorus
• Initial soil elevation loss with added salt was not maintained as a
legacy after restoration
• Evidence of the adaptive capacity of sawgrass wetlands exposed to saltwater
intrusion.
• Legacy effects:
• Phosphorus: a stimulatory effect on plant biomass and productivity
• Salt: changes in biogeochemistry and possibly microbial communities driving
organic matter decomposition
• Freshwater restoration may not offset peat collapse in wetlands
exposed to salt water, however restoration may restore carbon
balance in wetlands with P legacies.
29. Porewater chemistry in the mesocosm
(P legacy: F(1,21) = 1.7, P = 0.20)
(Salt legacy: F(1,21) = 44, P < 0.001)
Freshwater restoration began
2015 2016 2017 2018
30. Ecosystem metabolism: P legacies stimulate
metabolism during dry season
CO2
CO2
Grossecosystemproductivity(gCm-2mo-1)
Ecosystemrespiration(gCm-2mo-1)
Wet Dry Wet Dry
31. Net ecosystem productivity
CO2
CO2
† ‡ † ‡† ‡ † ‡ † ‡ † † ‡ * † ‡ † ‡ † ‡
¥ ¢
¢ Ø
¢
¢ Ø
Tukey HSD (P < 0.05)
FW vs FWP: *
FW vs SW: †
FW vs SWP: ‡
FWP vs SW: ¥
FWP vs SWP: ¢
SW vs SWP: Ø
Wet Dry
