# Presentation: 228067

## Serreze et al, 2000 Climatic Change

***Serreze et al, 2000 Climatic Change***

**Notes:**
Figure 1 shows the spatial pattern of annual mean surface air temperature trends
for the Northern Hemisphere north of 40 N over the period 1966–1995. Results
represent an update of the Chapman and Walsh (1993) analysis, based on data
from Jones (1994).

## Arctic tundra C/N model

**Arctic tundra C/N model**

**Notes:**
Carbon enters as NPP, constrained by N availability, and is eventually lost as soil respiration.  N enters through soil organic matter, and lost by leaching and lateral transport.  An increase in N addition can elicit a major change in vegetation C accumulation
-thought that carbon gains associated with increased N availability would offset losses from enhanced mineralization – if mineralized N is taken up by plants, C fixation thus stimulated should offset soil respiratory loss...if N is lost through leaching, arctic will become net CO2 source to atmosphere
At equilibrium, when a recycled element like N should be non-limiting, other resources that are not recycled (like light and water) should limit long-term C accumulation and N inputs should equal N outputs
Also at equilibrium, Internal N turnover becomes effectively uncoupled from input/output balance so N turnover is orders of magnitude greater than N inputs or outputs
if you fertilize you should be able to see where N goes
Three mechanisms that mediate change in ecosystem C and OM stocks 
•  Change in C:N ratios within OM pools
•  Redistribution of N among OM pools
•  Change in total N

## Denitrification

- Microbial-mediated, anaerobic N reduction from NO3- to N2O and N2
- Major means of N removal from soil to atmosphere
- Once thought unimportant in arctic ecosystems, but with effects of climate change could be a significant N loss pathway

**Notes:**
Why warming?  Atmospheric circulation, sea ice, cloud cover, albedo
-warming, greenhouse gases, deforestation, etc.
- Can study carbon in arctic by studying N

## •   How does this N loss pathway contribute to the amount of “missing N”?

- •**   ****Do denitrification rates differ between site and with experimental fertilization?**

**Notes:**
Some goals of this are to obtain a more complete picture of the microbial contribution to inorganic N movement – measuring forms of mineralization, nitrification, and denitrification as a way to begin characterizing the system

## Slide 5

## Denitrification Assays

***Two measurements using acetylene block:****  *
- 		Denitrification enzyme activity (DEA) - NO3-, Dextrose
- 		Unamended denitrification rates  - DI

- •** **Make systems anoxic
- •** **Add acetylene (C2H2)
- •** **Incubate at room temperature on 	a shaker table in the dark
- •** **Collect air samples at 30, 60 and 	90 minute time points
- •** **N2O analysis on GC

**Notes:**
Why warming?  Atmospheric circulation, sea ice, cloud cover, albedo
-warming, greenhouse gases, deforestation, etc.
- Can study carbon in arctic by studying N

## Slide 7

## Slide 8

## *

*****

## An Upper Bound Estimate

***Assume:***
- 	- A denitrification rate of 0.1 μg N*g soil-1*h-1 in 	fertilized plots – *Unamended CT, Moist Acidic Tundra*
- 	- Bulk density = 0.06 g/cm3, depth of denitrification 	activity ~10cm
- 	- 100 day growing season
-            This represents a loss of 1.44 g N*m-2*y-1
***The annual vegetation N uptake requirement                     is <1 g N*m******-2*******y******-1***
-      Denitrification may have considerable effects on N     	availability in these systems.

**Notes:**
Denit may be the same as plant N uptake
To a depth of 10cm, this would be 1.44 g /yr
To 5 cm, =0.72g
Do not know depth profile of NO3
Because homogenized, what integrated over soil column – warmer/cooler throughout the year, etc.