1. Effects of variable rainfall and increased nitrogen deposition on nitrous oxide production in a semi-arid grassland ecosystem A forethought of global change

2. 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.” 

3. 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

4. The Importance of Nitrogen  Earth’s atmosphere is ~78% N 2  Essential to many living organisms to produce amino acids, proteins and nucleic acids  Necessary for photosynthesis and plant growth

5. 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

6. The Nitrogen Cycle  Process by which atmospheric N 2 is converted to ammonium (NH 4 ) or nitrates (NO 3 ) and ultimately back to N 2 gas and returned to the atmosphere.

7. 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 O 2, 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 N 2 O- a contributor to atmospheric warming and ozone depletion”

8. Question  How do global change factors such as variable rainfall and increased nitrogen deposition affect ecosystem function in a semi-arid environment?

9. 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.

10. 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 (N 2 O) production and community respiration (CO 2 ) due to higher nutrient availability.

11. 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

12. 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/m 2 ammonium-nitrate addition

13. 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 N 2 O and CO 2 on gas chromatographs

14. Results!  N 2 O 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

15. Temporal Fluxes No Nitrogen: No Response

16. Treatment 1 with N: 180 mm

17. Treatment 2 with N:60 mm

18. Treatment 3 with N: 30 mm

19. What does it mean?  Increased N means more N 2 O production  Even though ecosystem is H 2 O limited, water alone had no real effect on N 2 O production  They are also N limited so they respond well to additional nutrients  A rise in anthropogenic N deposition could lead to more N 2 O production from soils, potentially exacerbating global warming and ozone depletion

20. Muchos Gracias  Chelsea Crenshaw  Joe Fargione  Sevilleta LTER  US Fish and Wildlife Service  John Craig  Jennifer Johnson  NSF  The lovely ladies of House 1

21. Questions?