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

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Temporal FluxesNo Nitrogen: No Response

Rain Events without Nitrogen Addition

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Treatment 1 with N: 180 mmRain 1 event

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Treatment 2 with N:60 mm

Rain 2 event

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Rain 2 event

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Treatment 3 with N: 30 mm

Rain 3 event

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Rain 3 event

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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?