1. Can’t we just plant a bunch of trees?
CO2 uptake = 50 CO2 (with 1 N)
Net atm change
= (25 – 50) = -25
+25 CO2
Plant C:N = 50
* Another idea has been that because land plants take up CO2 during photosynthesis, and CO2 stimulates their growth, we could solve the whole greenhouse gas problem by planting more trees.  Start with plant C:N average = 50, and then follow through the cycle. Once again, this solution runs up against some biogeochemical realities imposed by chemical stoichiometery (the Redfield ratio again). For example, where will the plants find the extra nitrogen in order to grow? We don't know the answer to this question, but this active area of research is gaining more knowledge on how the carbon and nitrogen cycles are linked, especially with respect to plants taking up excess CO2 in the atmosphere. In addition to planting trees to take up more CO2, there are different ways in which agriculture may also contribute to the solution of reducing the amount of CO2 in our atmosphere. 
* One idea during Reagan administration was that if the plants need nitrogen, we should allow more acid rain and nitrogen pollution – (famous for saying that trees pollute more than humans). Turns out that won’t really do it…(nitrogen cycle next lecture).
* Point is that you cannot look at just one part of the integrated system and expect to make an important change – science may not solve any of these issues per se (remember that the social-economic issues are critical), but science can tell you where to look, and how to look, and what definitely will NOT work. That is it’s role.
25 CO2
OM decomposition
Soil C:N = 25
1 N

2. “There are no magic fixes for the CO2 problem”
Take-home message:
“There are no magic fixes for the CO2 problem”
In the end, the take-home message for today's lecture, is we must recognize that "There are no magic fixes for the CO2 problem", and that a variety of solutions will be required to minimize the impacts of our altered carbon cycle on earth's ecosystems and inhabitants.

3. The GLOBAL NITROGEN CYCLE
Interactions with the C cycle, and the case of acid rain
What we wish to learn:
1. What are the major forms of nitrogen, and what microbial reactions affect these forms?
2. What are the major controls on the global nitrogen cycle and how does the N cycle impact the global carbon cycle?
3. How does “buffering” of acid rain work in soils, and how do element cycles interact to both create acid rain and to reduce its impacts on ecosystems?

4. The Global Nitrogen Cycle Accounting, Cycling, and Controls
Location Amount (1015 g)
Rocks & Sediments ,400, (unavailable)
Atmosphere ,900,000
Ocean ,348
Soils
Land Plants
Land Animals
In the atmosphere, N2 = 3,900,000
N2O =
NOx =
Going to examine the global N cycle in the same manner that we did the global C cycle – accounting, cycling, controls.

5. The Global Nitrogen Cycle – Pathways and Fluxes
Have already talked about these reactions in the lecture on microbes – review that:
1. Fixation – by microbes (main pathway)
2. Denitrification – by microbes
3. Internal cycling – nitrification and plant assimilation
4. Human activities – big flux, big changes in the global N cycle
Fluxes in 1012 g/yr

6. (2) Cycling Pathways and Reactions: 1. N2 Organic N “N-fixation”
2. Organic N NH “Mineralization”
3. NH NO “Nitrification”
4. NO3- or NH Organic N “Plant Assimilation”
5. NO N2 + N2O “Denitrification”
Note that we start with N2 gas in the atmosphere, then end with N2 gas in the atmosphere.
* Note that Mineralization is also called “decomposition” or “bacterial degradation”
* Note that Denitrification is also called “nitrate reduction”

7. Microbial reactions in the N cycle can be organized by whether they occur with or without the presence of oxygen. 4 1 2 3 5 2
* Numbers on this diagram refer to reactions described in the previous slide

8. (2) Cycling RT of N2 in the atmosphere =
Fluxes and Residence Times:
RT of N2 in the atmosphere =
= (total in atm, 1015g) / (output, 1015g/yr)
= (3,900,000) / (0.158)
= Million years
The long residence time, 24 million years, means that it is very hard to “disturb” this cycle. Need to look at it from the perspective of the atmosphere, versus the other pools – from the atm standpoint, if you shut down N-fixation (removal), who cares? But from the land perspective, that is a large amount of N coming in with a form that is useable.
Fast residence times mean that things flow through the system quickly – this could be because of fast movement of air or water, but it also means that the reaction times are fast – these materials react quickly. Short residence time means that the cycle is easy to disturb.
RT of NOX (NO + NO2) in the atmosphere
= (0.6) / (60) = year = 3.6 days

9. combination of element cycles
(3) Controls -- N deposition and Acid Rain
(A) NOx and Acid Rain
NO + O3 (ozone) NO2
NO2 + OH HNO3 (nitric acid)
HNO3 dissociates in water to form H+ and NO3-
* NO3- is a plant nutrient, stimulating CO2 uptake
(B) Sulfuric acid formation
H2SO H+ + SO4-
The H+ product in both reactions provides the “acidity”
Acid Rain is caused by a
combination of element cycles
MANY different controls, are going to focus on only two today as illustrative:
1. N deposition and interactions with C cycle (stimulation of primary production)
2. Formation of acid rain and its impacts on ecosystems

10. Nitrogen Deposition has increased Past and Present mg N/m2/yr
1860
1993
5000
2000
1000
750
500
250
100 50 25 5
N deposition has increased, almost all from human activities – adding of fertilizer is important runoff to aquatic ecosystems.
Ask is this a good thing? Remember that idea was initially we should pollute more to stimulate the plant uptake of CO2 from the atmosphere to reduce our greenhouse gas and warming problems.
Galloway and Cowling, 2002; Galloway et al., 2002

11. Will planting trees will solve all our problems?
1 N from soils
= 50 - (25 CO2 to atm)
= net 25 CO2 from atm
CO2
CO2
Vegetation
C:N = 50
Net Primary Production
Wood
C:N=200
Nitrogen deposition
Key question is how much CO2 uptake can be stimulated by N deposition?
Inorganic Nitrogen
Soil Organic Matter
C:N = 25
Nitrogen
loss
N Mineralization

12. Pg/yr
Literature source:
(Tg/yr)
(Pg/yr)
Range of estimates goes up to the 2 Pg/year, the size of the missing sink! Need to figure this out. Need to add a TRACER, use stable isotope, 15N
This wide range is due mainly to assumptions about where in forests N inputs are stored – soils, foliage, or wood.
Tg = teragram = 1012 g

13. International Experiment – 15N-Labeled Forests
Klosterhede (DK)
Gårdsjön (SW)
Speuld (NL)
Alptal (CH)
Ysselsteyn (NL)
Aber (Wales)
Forests in Europe and the U.S. were labeled with 15N to trace the fate of N deposition
Northeastern U.S.
Bear Brook
Harvard Forest
High N Deposition

14. Tree Core 15N Recovery
15N can be analyzed back through time in the tree cores, and the total amount stored is measured
15N added:
Bark
time

15. 15N Tracer Recovery in Pine Trees
1992
15N Form Applied
NH4
NO3
Mean
15N Recovery
(% of Additions) 5 10 15 20 25 30 35 40
Pine Forest
Ambient
+50 N
Fine Roots
Wood
Green Foliage
Less than 1/3 (or 5 to 30%) of N deposition is stored in trees.
Very small proportions are stored in wood.

16. Results: N Deposition and Temperate Forest CO2 Uptake
1) Most of the N deposition is stored in soils or lost in runoff.
2) Less than 1/3rd of the N deposition is stored in trees, and only ~5% is stored in wood.
3) N deposition (acid rain) does not appear to be slowing atmospheric CO2 increases, at either annual or decadal scales.
4) Although the N and C cycles are linked, adding more N to forests will not solve our CO2 problem.
So, now what about acid rain? What WILL this extra N do to our environment?
Courtesy Knute Nadelhoffer et al. 2009

17. The Ongoing Problem of Acid Rain
Harmful to:
forests lakes
fish
buildings
Acid rain has ruined nearly half of the Black Forest in southwestern Germany.

18. Sandstone portal Figure on Herten Castle in Germany – Sculpted in 1702
Photo in 1969
Photo in 1908
Sandstone portal Figure on Herten Castle in Ruhr district of Germany. Sculpted 1702; photographed in 1908.
Same sandstone portal figure photographed in 1969.

19. Acid rain formation U.S. NOx sources
“It isn’t the pollution that’s harming the environment. It’s the impurities in our air and water that are doing it.”
Dan Quayle, former Vice President
* A marketable permit system achieves an environmental standard at the aggregate level.
Some polluters might emit above the standard while others emit below the standard. The end result though is that the standard for the region is met.
A marketable permit system controls the total amount of emissions but does not control releases of every individual polluter.
The trading component is critical because polluters who can abate more efficiently than others have the incentive to sell their permits to polluters with less efficient abatement technologies.
* Cap and trade is a policy approach to controlling large amounts of emissions from a group of sources at costs that are lower than if sources were regulated individually. The approach first sets an overall cap, or maximum amount of emissions per compliance period, that will achieve the desired environmental effects. Authorizations to emit in the form of emission allowances are then allocated to affected sources, and the total number of allowances cannot exceed the cap.
Individual control requirements are not specified for sources. The only requirements are that sources completely and accurately measure and report all emissions and then turn in the same number of allowances as emissions at the end of the compliance period.
For example, in the Acid Rain Program, sulfur dioxide (SO2) emissions were 17.5 million tons in 1980 from electric utilities in the U.S. Beginning in 1995, annual caps were set that decline to a level of 8.95 million allowances by the year 2010 (one allowance permits a source to emit one ton of SO2). At the end of each year, EPA reduces the allowances held by each source by the amount of that source's emissions.

20. Chemical reactions with hydrogen ions (H+) in the soil lead to loss of buffering capacity
Soil minerals have negatively-charged surfaces, and the positively charged cations (e.g., Ca++, Na+) bind to these sites. H+ displaces the cations because it is very small.

21. H+ input increases the output of cations like Aluminum
Weathering reactions buffer some of the input of H+, reducing its output to ground and surface waters.
Some of the input of NO3- is assimilated by plants.

22. Soil buffering capacity has decreased over time from acid rain, which has increased the acidity (lowered the pH) of water flowing out of soils into streams and lakes
pH =
= pH
pH =
pH =

23. Note that organisms are differentially sensitive to low pH water
A reminder of the pH scale – lower pH means more acidic.
Note that organisms are differentially sensitive to low pH water

24. A “sensitive” catchment in the Rocky Mountains
A “sensitive” catchment in the Rocky Mountains. Note the high percentage of rock and thin soils leading to low capacity for buffering of acid rain inputs.

25. Surface water sensitivity to Acid Deposition

26. "ELEMENT CYCLES INTERACT,
Summary
1. Acid Rain is an important consequence of the nitrogen and sulfur cycles interacting, and has deleterious effects on lakes, forests, and buildings in our environment. 
2. Acid rain is produced by the interactions of these and other elements in the atmosphere, and the impacts of acid rain are controlled by other element cycles on land and in the water. 
3. Increased N deposition from acid rain does not account for ↑CO2 uptake in forests, and cannot solve our global warming problem.
4. The main take-home message for today's lecture is:
"ELEMENT CYCLES INTERACT,
and they cannot be studied in isolation”