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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: Ecosystems: N addition to soil can lead to N addition to soil can lead to
changes that favor weeds over changes that favor weeds over native plants, reducing species native plants, reducing species diversitydiversity
Additional N in rivers, lakes and Additional N in rivers, lakes and coastal systems stimulate blooms coastal systems stimulate blooms of algae that deplete Oof algae that deplete O22, killing , killing fish and other organisms resulting fish and other organisms resulting in Eutrophication (Gulf of in Eutrophication (Gulf of Mexico)Mexico)
““Elevated N Elevated N deposition onto arid deposition onto arid soils could return soils could return relatively more relatively more nitrogen to the nitrogen to the atmosphere in the atmosphere in the form of Nform of N22O- a O- a
contributor to contributor to atmospheric atmospheric warming and ozone warming and ozone depletion”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 pre-evacuated 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
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In-N Ca-N In Ca
N2
O (
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Temporal FluxesNo Nitrogen: No Response
Rain Events without Nitrogen Addition
-2
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0 5 10 15
Time (days)
N2
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I nterspace 180mmCanopy 60mmCanopy 30 mmCanopy 180mmInterspace 60mmInterspace 30mm
Treatment 1 with N: 180 mmRain 1 event
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Time (day)
N2
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g/l
Canopy
Interspace
Treatment 2 with N:60 mm
Rain 2 event
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0 5 10 15time (days)
N2
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InterspaceCanopy
Rain 2 event
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0 5 10 15
Time (day)
N2
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g/l Canopy
Interspace
Treatment 3 with N: 30 mm
Rain 3 event
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0 5 10 15time (days)
N2
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InterspaceCanopy
Rain 3 event
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0 5 10 15
Time (day)
N2
O n
g/l Canopy
Interspace
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?