1. Chapter 4 Decomposition and Nitrogen Cycling Spring 2014

3. Nitrogen Cycle Simple Organic N NH4+ NO3- Mineralization
Different species take up NH4+, NO3-
convert to amino groups (-NH2) in plant
Both occur simultaneously
net result depends on substrate
Immobilization

4. Nitrogen Loading is already damaging the biosphere N Deposition rates ( 0 – 60kg/ha/yr )
Galloway et al Science 2008

5. Future Phosphorus Limitations ?
Cordell et al 2009.
Global Env Change 19:

6. Gulf of Mexico Dead Zone

7. PLANETARY BOUNDARIES Rockstrom et al. Nature 2009
Related to NPP

8. Precipitation Deposition 1, 10, 50 kg/yr 100 kg 100 Litter Fall kg
Fixation
30 kg/yr
Uptake
Litter Fall
Soil
Leaching
Weathering
Rock
Chemical
Reactions
Deposition
1, 10, 50 kg/yr
Decomposition
50 kg/yr
3000 kg
500 kg
100 kg
100 kg
50 kg

9. Larch
Fir
Growth N Demand:
Stem 2
Needles
100 (100% turnover)
20 (20%)
102 22
N Supply:
Retranslocation 50 10
Decomposition 51 11
Atmosphere
ANNUAL N Req. 1

10. Decomposition is the Result of 3 Simultaneous Processes
Leaching
Weathering
Biological activity
Leaching – rapid loss of soluble material from detritus by action of rain or water flow
most important in fresh litter
even complex OM may be leached
certain cations are susceptible to leaching
Na+, K+, Ca++, Mg++
Weathering
mechanical breakdown of detritus due to physical
factors such as wind abrasion, freezing/thawing
Biological Action – 2 components
1 – mechanical fragmentation by decomposer fauna
2 – microbial degradation – bacteria/fungi

11. NITROGEN Litter Microbes Humus Vegetation Soil Solution ATMOSPHERE
Gaseous losses
(e.g., respiration,
denitrification)
ATMOSPHERE
Precipitation
Fixation
SOIL
ORGANIC
Litter
Humus
Microbes
Vegetation
Decomposition
Retranslocation
Mineralization
Immobilization
Throughfall, exudation
SOIL
INORGANIC
Uptake
Soil
Solution
Similar to C cycle in some respects, differs in significant ways
Atmosphere is 79% nitrogen gas, substantial energy requirement
limits amount of fixation that can occur.
-microbes
free-living
symbiotic
N fixation, denitrification relatively low in most natural ecosystems
Precipitation – NH4+, NO3-
increasing due to human activities
Internal cycle dominated by NH4+, NO3- uptake
“closed” system
Translocation is a big difference from C
Decomposition can lead to mineralization or immobilization
(microbes competing with plants)
Only 3 forms of N that plants take up:
NH4+, NO3-, organic N
N is not a significant component of primary or
secondary minerals
Exchange
Sites
PLANTS
Exchange
Fig. 14.2
p. 255
Leaching
GROUNDWATER

12. Litter Quality & Decomposition
Type of chemical bonds and amount of energy released by decay (carbon quality)
Size and 3-D complexity of molecule
Nutrient concentration (nutrient quality)
Decomposition can take days to years

13. Climate Controls on Decomposition1 2 3 4 5
100 50 25 75
Time (years)
% Dry Weight
Tropics
Seattle
Boreal
Missoula

14. Leaf Decomposition Rates1 2 3 4 5
100 50 25 75
Time (years)
% Dry Weight
Pine (20% Lignin)
Maple (10% Lignin)
Tomato (0% Lignin)

15. Decomposition Glucose, simple sugars Cellulose* Tannins & Lignins
*Cellulose – most common molecule in plant component of terrestrial
ecosystems source of fiber for paper and paper products
(nearly pure cellulose) extracellular enzymes are required
to cleave the bonds
Tannins – polyphenols
thought to be a defence mechanism against animal consumption
Lignins
among the most complex and variable in nature
class of compounds with variable structure
2nd only to cellulose in quantitative importance in most
plant tissues – makes the plant “woody”
encrusted on and around cellulose in cells walls
to provide rigidity and strength – causes
cellulose to decay more slowly than it
otherwise would.
can’t actually perform tests to determine how much is lignin
it is what is left after a series of treatments
“proximate” analysis

16. Remaining Mass Time Fig. 13.1 p. 229 N, P, S, “HLQ” solubles “HLQ”
Nitrogen
N, P, S, “HLQ”
solubles
Remaining Mass
Phase regulated by
nutrient level and
readily available carbon
lignin decomposition rate
Fig. 13.1
p. 229

17. Fig. 13.2 p. 231 Dashed line is loss of OM
Dark line is nutrient dynamic
increasing amount of N in decomposing
litter is called net immobilization
highest in early stages of decay when the most
easily decomposed compounds are degraded
as C and energy yield from OM declines, so does
demand for nutrients -> mineralization
-> increased availability to plants
2 sources of imobilized nutrients:
throughfall (above ground)
mineralized N from older litter & soil OM (below-ground)
?fixation by free-living microbes – generally low
Fig. 13.2
p. 231

18. Decomposition of an abscised leaf with (a) high (25%) and (b) low (5%) lignin content
The decomposition of an abscised leaf in relationship to initial concentrations of protein, simple carbohydrates, hemicellulose and lignin.
The amount of leaf C remaining is controlled by the concentration and decomposition rate constant of protein and simple sugars, cellulose and hemicellulose, and lignin.
Note that differences in initial concentration have a profound influence on leaf decomposition. High lignin concentrations and low concentrations of other constituents result in relatively slow decomposition rates.
Fig
p. 555

19. Decomposition Glucose, simple sugars

20. Decomposition Cellulose

21. Decomposition Tannins & Lignins
Tannin (lhs) -
Lignin (rhs) -

23. Fig. 12.13 p. 222 Sand and loamy sand Sandy loam Loam Silt Silty
clay loam
Clay
Percentage of clay
0.24
0.20
0.16
0.12
0.08
0.04
0.096
0.080
0.064
0.048
0.032
0.016
Percentage of nitrogen
Percentage of phosphorus
Fig
p. 222

24. Humus Humus is a complex & amorphous
form of organic matter in ecosystems.
It is high in nitrogen and large polyphenolic molecules, but low in cellulose.
It can take thousands
of years to decompose.
Humus is the stable, slowly decomposing form of soil organic matter – it can
take thousands of years to decompose

25. Model of a Soil Aggregate
Clay – organic
matter complex
Open pore
Bacteria
Pore opening
Fungal hyphae
Organic matter-
Sesqui oxides
Clay domain
Closed pore
Quartz