Turnover of wetland sediments on mineralization of carbon, nitrogen and phosphorus

Qingren Wang* and Yuncong Li

Tropical Research and Education Center

Department of Soil and Water Science

IFAS, University of Florida

ASA-CSSA-SSSA 2009, Pittsburgh, PA

Outline

• Brief introduction about the Everglades wetland

• Recycling of carbon in wetland system• Experiment setup, sampling and analysis• Results, conclusion and discussion

TREC-Homestead

Florida Bay

Natural ecosystem

National Park

Eco-Environment in south Florida

Agriculture

Natural Eco-environment in the Everglades wetland

Bulrush: Scirpus rubiginosus

Sawgrass:Cladium jamaicense

• Wetlands are an important part of the global carbon inventory: up to 1/3 of total global soil carbon is stored in wetlands

• Carbon in wetlands is relatively stable• Carbon accumulation is based on vegetation

types and soil fertility • Decomposition rate depends on water content

and temperature

Why carbon in wetlands?

Importance of wetlands

Mitra et al, 2005, Curr. Sci.

Chauhan, 2007: http://sites.google.com/site/ashvinichauhan/syntrophic-methanogenic-associations-in-florida-wetlands

Carbon cycling in wetlands

Hypothesis

Sediment mineralization was dependant on vegetation type and temperature at the same moisture and other conditions

Experiment• Intact sediment columns were collected

from wetland sites with two different main vegetations: Bulrush and Sawgrass (both are native plants).

• Kept these columns in three growth chambers with temperature at: 30, 25, and 20 oC, respectively.

• Carbon emission and nutrient mineralization were monitored through sampling without disturbing the sediments for 380 days.

Sediment collection & experiment setup

Sample analyses

• Carbon analysis: LiquiTOC• Nitrate and ammonium:

Seal AQ2+• Ortho phosphorus: Auto-

analyzer (AA-3)

• Turnover of sediments for C, N, and P with

time.

• Sediment mineralization vs. vegetation type.

• Effect of temperature on sediment turnover.

Major results

Carbon mineralization rate

0 50 100 150 200 250 300 350 4000

200

400

600

800

1000

1200

1400

1600Sawgrass Bulrush

Time (day)

CO

2-C

(mg/

kg)

Accumulative CO2-C with time

0 50 100 150 200 250 300 350 4000

2000

4000

6000

8000

10000

12000

14000

16000

f(x) = 2132.37154546737 ln(x) − 471.097993785414R² = 0.974241697877589

f(x) = 2604.96473012374 ln(x) − 651.218840685348R² = 0.972346265959714

Sawgrass Logarithmic (Sawgrass)

Bulrush Logarithmic (Bulrush)

Time (day)

Acc

umul

ated

CO

2-C

(mg/

kg)

Expressed by exponential growth model

Sawgrass Bulrusha 13285.65 11023.78b 0.0273 0.0261R2 0.977 0.969

y = a * Exp(-bx) x-time (days), and y-accumulative amount of CO2-C released from sediments

Impacted by temperature (CO2–C)

30 25 200

100200300400500600700800

aa

b

Sawgrass

CO2-

C (m

g/kg

)

Temperature (oC)

30 25 200

100200300400500600700800

ab

c

Bulrush

Nitrogen mineralization

0 50 100 150 200 250 300 350 4000

20

40

60

80

100

120 Nitrate N

SawgrassSeries3Bulrush

NO3-

N (m

g/kg

)

0 50 100 150 200 250 300 350 4000

0.2

0.4

0.6

0.8

1

1.2 Ammonium N

SawgrassSeries3Bulrush

NH4-

N (m

g/kg

)

Outlier

Time (day)

Accumulative N

0 50 100 150 200 250 300 350 4000

100

200

300

400

500

600

Sawgrass

Bulrush

Accu

mul

ativ

e N

O3-

N (m

g/kg

)

0 50 100 150 200 250 300 350 4000

1

2

3

4

5

6

7

f(x) = 1.3889456641 ln(x) − 1.9691750021R² = 0.907716209017961

f(x) = 0.4011311855 ln(x) − 0.5210457688R² = 0.952714208066964

SawgrassLoga-rithmic (Saw-grass)Bulrush

Acc

umul

ativ

e N

H4-N

(mg/

kg)

Time (day)

Phosphorus mineralization

0 50 100 150 200 250 300 350 4000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

SawgrassSeries3Bulrush

Time (day)

Orth

o-P

(mg/

kg) Outlier

Accumulative phosphorus

0 50 100 150 200 250 300 350 4000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4Sawgrass

Bulrush

Time (day)

Acc

umul

ativ

e P

(mg/

kg)

Impacted by temperature (NO3–N)

30 25 2005

101520253035404550

b

a

c

Sawgrass

NO

3-N

(mg/

kg)

30 25 200

10

20

30

40

50

b

a a

Bulrush

Temperature (o C)

Impacted by temperature (NH4-N)

30 25 200.250.260.270.280.29

0.30.310.320.33 Sawgrass

NH

4-N

(mg/

kg)

Temperature (o C)

30 25 200.250.260.270.280.29

0.30.310.320.33

a

b

aBulrush

Impacted by temperature (P)

30 25 200.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

aa

Sawgrass

Orth

o-P

(mg/

kg)

30 25 200

0.0020.0040.0060.008

0.010.0120.0140.0160.018

a

b b

Bulrush

Temperature (o C)

Conclusions (1)• The turnover of organic C was rapid in the first 100 days.

• The accumulative amount of C mineralized can be well

described by an exponential growth model.

• Nitrogen turnover was low at first 100 days and sharply

increased afterwards.

• Phosphorus mineralization was low throughout the whole

experimental period.

•

Conclusions (2) • Sawgrass had a greater turnover on C and N but lower

on P than did bulrush.

• High temperature improves the turnover of organic C

and NH4-N for both species but that of P only for bulrush.

• The turnover of NO3-N was the greatest at 25 oC,

especially for sawgrass.

• Factors in controlling the turnovers are rather

complicated, C:N ratio (18.9 in sawgrass vs. 13.8 in

bulrush) might be one of them.