Transcript Caitlin Smith

Effects of variable rainfall and
increased nitrogen deposition on
nitrous oxide production in a
semi-arid grassland ecosystem
A forethought of global change
Global Change …

What is global climate
change?
“Global climate change
can be defined as a
considerable change from
one climatic state to
another, such as an
increase in the Earth's
temperature. An increase
in the Earth's temperature
may lead to changes in
rainfall patterns, a rise in
sea level, and a wide
range of impacts on
plants, wildlife, and
humans.”

Variable rainfall: Effects on Precipitation
in the Southwestern United States

El Nino:

Wetter years are associated
with El Nino (warm cycle)
whereas drought is associated
with La Nina (cold cycle)
Heating of the atmosphere by
global warming could increase the
frequency and intensity of El
Niño events, bringing more
precipitation to the Southwest
during winter months

Summer Monsoon:

Effects of climate change
on the North American
monsoon are poorly
understood

Likely to become more
variable
The Importance of Nitrogen
Earth’s atmosphere is
~78% N2
 Essential to many
living organisms to
produce amino acids,
proteins and nucleic
acids
 Necessary for
photosynthesis and
plant growth

Nitrogen Fixation and Cycling

Atmospheric N enters
ecosystems in 4 main
ways:




Biological fixation
(free-living or
symbiotic bacteria)
Lightning
Industrial fixation
(fertilizers)
Combustion of fossil
fuels
The Nitrogen Cycle

Process by which
atmospheric N2
is converted to
ammonium
(NH4) or nitrates
(NO3) and
ultimately back
to N2 gas and
returned to the
atmosphere.
Consequences of Anthropogenic N Addition

Ecosystems:


N addition to soil can lead to
changes that favor weeds over
native plants, reducing species
diversity
Additional N in rivers, lakes and
coastal systems stimulate blooms
of algae that deplete O2, killing
fish and other organisms resulting
in Eutrophication (Gulf of
Mexico)
“Elevated N
deposition onto arid
soils could return
relatively more
nitrogen to the
atmosphere in the
form of N2O- a
contributor to
atmospheric
warming and ozone
depletion”
Question
How
do global change factors such as
variable rainfall and increased nitrogen
deposition affect ecosystem function in a
semi-arid environment?
Hypothesis: Precipitation
 If
rainfall events occur less frequently, there will
be an initial rise in microbial activity followed by a
rapid decrease because soils are water limited.
 If
rainfall events occur more often, there will be an
initial rise in soil microbial activity which will be
sustained for a longer period of time due to higher
water availability.
Hypothesis: Increased Nitrogen
 Semi
arid ecosystems are also N limited…
So if there is an increase in N deposition, there will
be an increase in nitrous oxide (N2O) production
and community respiration (CO2) due to higher
nutrient availability.
Experimental Design



Collection of top ~5 cm of soil
from interspace
(cyanobacterial crust) soil and
from under canopy
(rhizosphere) soils
Removal of organic matter
Incubation of 100 g of each
soil type in 125 mL serum vials
for 14 days
3 Factoral Design: 12 treatment combinations

Soil type: Canopy or Interspace

Rainfall Variability:
1 x 180 mm
3 x 60 mm
6 x 30 mm




Nitrogen addition: 2 g/m2
ammonium-nitrate addition
Methods

Gas samples taken weekly
with a gas-tight syringe
and transferred to a preevacuated 5 ml vacutainer
 Samples were analyzed
within one week of
sampling to measure N2O
and CO2 on gas
chromatographs

Results!
N2O concentrations
highest in N treated
Interspace soils
 Although all soils received
the same amount of
precipitation, non-N
amended soils did not
respond as dramatically
20

18
N2O (ng/l)
16
14
12
10
8
6
4
2
0
In-N
Ca-N
In
Ca
Temporal Fluxes
No Nitrogen: No Response
Rain Events without Nitrogen Addition
14
Interspace 180mm
Canopy 60mm
Canopy 30 mm
Canopy 180mm
Interspace 60mm
Interspace 30mm
12
N20 ng/l
10
8
6
4
2
0
0
-2
5
10
Time (days)
15
Treatment 1 with N: 180 mm
Rain 1 event
100
10
N2O ng/l
Canopy
Interspace
1
0
5
10
0.1
Time (day)
15
Treatment 2 with N:60 mm
Rain 2 event
Rain 2 event
7
Interspace
Canopy
76
6
5
N2O
(ng/l)
N2O
ng/l
5
Canopy
Interspace
4
4
33
22
11
0
0
0
0
5
5
10
time (days)
Time (day)
10
15
15
Treatment 3 with N: 30 mm
Rain 3 event
Rain 3 event
7
Interspace
Canopy
7
6
6
N2O (ng/l)
N2O
ng/l
55
Canopy
Interspace
44
3
3
2
2
1
01
0
0
0
5
10
5
time (days)
Time (day)
15
10
15
What does it mean?

Increased N means more
N2O production
Even though ecosystem is
H2O limited, water alone
had no real effect on N2O
production
 They are also N limited so
they respond well to
additional nutrients


A rise in anthropogenic
N deposition could lead
to more N2O production
from soils, potentially
exacerbating global
warming and ozone
depletion
Muchos Gracias








Chelsea Crenshaw
Joe Fargione
Sevilleta LTER
US Fish and Wildlife Service
John Craig
Jennifer Johnson
NSF
The lovely ladies of House 1
Questions?