moisture controls on trace gas fluxes from semiarid soils dean a. martens and jean e. t. mclain swrc...
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Moisture Controls on Trace Moisture Controls on Trace Gas Fluxes From Semiarid Gas Fluxes From Semiarid
SoilsSoils
Dean A. Martens and Jean E. T. Dean A. Martens and Jean E. T. McLainMcLain
SWRC – Tucson and Water SWRC – Tucson and Water Conservation Laboratory – Phoenix Conservation Laboratory – Phoenix
Semiarid ConceptsSemiarid Concepts• Due to limited rain and high seasonal
temperatures that limit plant productivity, semiarid systems are not important in global C dynamics?
• Semiarid ecosystems do not contribute to or mitigate atmospheric C concentrations involved in potential climate disruptions
Study site was the San Pedro Riparian zone near Tombstone AZ.
The San Pedro is the last nearly perennial, non dammed semiarid stream in the southwest.
It occupies an important avian – North – South flyway between the Rio Grande and the Colorado Rivers
Open
Mesquite
Sacaton
Vegetation Response to Vegetation Response to Seasonal MoistureSeasonal Moisture
15 mm rain in 7 months 200 mm rain in 1 month
July 2002 August 2002
Three Vegetation SitesThree Vegetation Sites
Annual grasses and forbes
Sacaton site
Mesquite Community
Instrumentation and Methods
Trace gas sampling
Ambient CO2 and soil factors
CO2 flux and isotope collection
Organic TotalSoil Depth C δ13C N C/N
(g kg-1) (‰) (g kg-1)
________________________________________________Mesquite Plant -- -27.1 -- --
O-H -- -22.7 -- -- 0-5 cm 29.9 -20.2 3.08 9.7 5-10 12.7 -18.9 1.36 9.4 10-20 10.5 -18.7 1.01 9.8
Open Plant -- -19.9 -- -- O-H -- -18.4 -- -- 0-5 cm 5.83 -18.3 0.63 9.2 5-10 6.04 -18.8 0.58 10.3 10-20 3.76 -17.1 0.37 10.3
Sacaton Plant -- -13.5 -- -- O-H -- -13.4 -- -- 0-5 cm 17.6 -16.0 1.81 9.7 5-10 11.6 -15.3 1.21 9.6 10-20 10.9 -14.5 1.23 11.0
Soil Properties
Ambient COAmbient CO22 Response to Rainfall Response to Rainfall
0
5
10
15
20
25
30
35
40
156
161
166
171
176
181
186
191
196
201
206
211
216
221
226
231
236
241
246
251
256
261
Julian Day
CO
2*10
0 (p
pm
) :
Rai
n (
mm
)
-1500
-1000
-500
0
So
il M
ois
ture
(kP
a)
CO2*100 (ppm) Rain (mm) Soil Moisture (kPa)
A
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
156
161
166
171
176
181
186
191
196
201
206
211
216
221
226
231
236
241
246
251
256
261
Julian Day
CO
2*10
0 (p
pm
) :
Rai
n (
mm
)
-1500
-1000
-500
0
So
il M
ois
ture
(kP
a)
CO2*100 (ppm) Rain (mm) Soil Moisture (kPa)
C
Open annual grass site – 697 ppm average
Mesquite site – 448 ppm average
Jan 1 – July 2002 = 15 mm; 2002 monsoon = 238 mm; Total for 2002 = 293 mm
Jan 1 – July 2003 = 40 mm; 2003 monsoon = 95 mm ; Total for 2003 = 232 mm
Date
18-J
ul
2-Aug
22-A
ug
11-S
ep9-
Oct
10-D
ec
19-F
eb
5-M
ay
16-J
ul
29-J
ul
15-A
ug
2-Sep
Ca
rbo
n D
ioxi
de
Pro
du
ctio
n (m
g C
O2 m
-2 h
-1)
0
40
80
120
160
200
240
280
320
360
400
Pre
cip
itatio
n (
mm
)
0
10
20
30
40
50
MesquiteOpenSacatonRainfall
2002 2003
Figure 3. Carbon dioxide efflux from three SPRNCA vegetation sites on measurement dates during 15-month monitoring period (July 2002 through September 2003). Symbols are averaged values from 2 or 3 flux collars installed at each site.
Carbon Dioxide Flux 2002 – 2003
2002 Monsoon = 123 - 126 mg m-2
Winter = 80 -109 mg m-2
2003 Monsoon = 72 – 105 mg m-2
2002 vs. 2003 40% reduction, yet during the 2002 season the 5X difference in soil C did not impact fluxes
Date (2002)
13 C
of
Re
sp
ire
d C
O 2 (o
/ oo)
-30
-28
-26
-24
-22
-20
-18
-16
-14
Pre
cip
ita
tio
n (
mm
)
0
10
20
30
40
50
SacatonMesquiteOpenRainfall
Figure 4. Isotopic composition of carbon dioxide respired from soil surface in three SPRNCA vegetation sites on measurement dates during monsoon and post-monsoon (July through December) 2002. Symbols are averaged values of two carbon dioxide collections per site on each sampling date, plus or minus standard deviation.
Isotopic Composition of CO2 Flux
Mesquite site = 63 to 98% C3-C
Open and Sacaton site = 50 to 65% C4-C
Date
18-J
ul
2-Aug
22-A
ug
11-S
ep9-
Oct
10-D
ec
19-F
eb
5-M
ay
16-J
ul
29-J
ul
15-A
ug
2-Sep
Met
hane
Con
sum
ptio
n ( g
CH
4 m
-2 h
-1)
0
5
10
15
20
25
30
35
40
45
50
55
Pre
cipi
tatio
n (
mm
)
0
10
20
30
40
50
MesquiteOpenSacatonRainfall
2002 2003
Figure 6. Methane consumption in three SPRNCA vegetation sites on measurement dates during 15-month monitoring period (July 2002 through September 2003). Symbols are averaged values from 2 or 3 flux collars installed at each site.
Methane Oxidation Rates 2002 – 2003
2002 Monsoon = 29 – 61 mg m-2
Winter = 118 – 160 mg m-2
2003 Monsoon = 62 – 70 mg m-2
Methane Oxidation with Soil Depth
-55
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
0 500 1000 1500 2000 2500
Porespace Methane (ppb)
De
pth
(c
m)
MonsoonTwo Weeks Post-MonsoonSix Weeks Post-MonsoonTwelve Weeks Post-Monsoon
Cool season may limit surface oxidation, but warmer temps in the subsoil continue to promote oxidation – also when surface is dry, subsoil active
Impacts of Grazing on CHImpacts of Grazing on CH44 Oxidation Oxidation
Open
Julian Day
150 160 170 180 190 200 210 220
Dep
th
0
20
40
60
80
100
Open
Julian Day
150 160 170 180 190 200 210 220
Dep
th
0
20
40
60
80
100
200 400 600 800 1000 1200 1400 1600 1800 2000
100 yr exclosure “Moderate” grazing for 50 yr
Nitrous Oxide Fluxes 2002 – 2003 Nitrous Oxide Fluxes 2002 – 2003
Date
18-J
ul
2-Aug
22-A
ug
11-S
ep9-
Oct
10-D
ec
19-F
eb
5-M
ay
16-J
ul
29-J
ul
15-A
ug
2-Sep
Nitr
ous
Oxi
de P
rodu
ctio
n (
g N
2O
m-2
h-1
)
0
10
20
30
40
50
Pre
cipi
tatio
n (m
m)
0
10
20
30
40
50
MesquiteOpenSacatonRainfall
2002 2003
Figure 5. Nitrous oxide efflux from three SPRNCA vegetation sites on measurement dates during 15-month monitoring period (July 2002 through September 2003). Symbols are averaged values from 2 or 3 flux collars installed at each site.
2002 Monsoon = 4 to 38 mg m-2
Winter = 17 to 54 mg m-2
2003 Monsoon = 8 to 20 mg m-2
2002 vs. 2003 30% reduction
δδ1515NN22O Flux From MesquiteO Flux From Mesquite
-5.00
0.00
5.00
10.00
15.00
20.00
25.00
10-Jul 20-Jul 30-Jul 9-Aug 19-Aug 29-Aug
Date (2003)
15 N
2O
Precipitation (mm)
delta 15-N2O
-6.0
-25
δ15N2O
Isotope values with dry surface soils suggests subsoil activity and during monsoon represents surface activity
Greenhouse Gas ProductionGreenhouse Gas Production
• 2002 monsoon season averaged 303 mg CO2 equivalents m-2 (57 d)
• Cool season averaged 390 mg CO2 equivalents m-2 (307 d)
• 2003 monsoon season averaged 185 mg CO2 equivalents m-2 (57 d)
• 60% reduction of warm season rain reduced CO2 equivalents by 39%
ImplicationsImplications• Recent work has emphasized the
increased contribution of terrestrial C sources to atmospheric C pools if temperatures increase – positive feedback to climate change
• For the SW region, climate change models differ on whether future climate scenarios will be wetter or drier and possible shifts from summer to winter rains
These Results SuggestThese Results Suggest
• If rainfall shifts to greater winter events, overall reductions in surface dominated CO2 and N2O fluxes and prolonged spring CH4 subsurface oxidation
• Higher warm season precipitation will increase CO2 and N2O fluxes due to rapid oxidation of labile C pools that would not be off set by higher CH4 oxidation rates