Carbon and Nitrogen Cycling in Soils
•Weathering represented processes that mainly deplete soils in elements
relative to earth’s crust
•Biological processes differ from weathering in that they tend to enrich
soils in certain elements, most importantly C and N (soil organic matter)
•Study of soil matter has always been important:
–Organic N was main focus until 1950’s
•Maintenance of crop production (mainly N limited) until advent of
commercial N production
Still very important in countries lacking financial resources
•Soil C is now a focus:
–Conversion of tropical forests to ag (and loss of SOM) is a major reason for
increases in atm CO2
–Management of existing cropland in industrial countries a proposed way to
reduce NET CO2
Soil C Cycle
Plants + O2 = humus + CO2
Plants are equivalent of parent material (primary minerals)
Humus is equivalent of secondary minerals
Plant Organic Composition
•Plant chemistry varies greatly.
•Differences in lignin/N, ash content, etc determine how fast it is
recycled by microbes will discuss decomposition more
Plant Component
Concentration (% dry weight)1
solubles
29-56
hemi-cellulose
2-40
cellulose
13-51
lignin
4-48
protein
0.5-22
ash
4-9 Ash can be bio-minerals
1
Data compiled from: J.M. Oades, An introduction to organic matter in mineral soils,
in: Minerals in Soil Environments, 2 nd Edition, Soil Science Society of America,
Madison, WI (1989); E.A. Paul and F.E. Clark, Soil Microbiology and Biochemistry,
Academic Press, San Diego, (1996); D.M. Sylvia, J.J. F uhrmann, P.G. Hartel, and
D.A. Zuberer, Principles and Applications of Soil Microbiology, Prentice Hall, New
Jersey, (1998).
What is Soil Organic Matter?
•Contains everything from living microbes to humic compounds of great
antiquity and degree of chemical alteration
•Determining exactly what soil organic matter is made of is one of the most
challenging problems in all of soil science
–Unlike secondary mineral classification, there is no analogous approach for
organic matter
•Various methods of have devised to break total soil organic matter into
different fractions represently what is in nature:
–Chemical methods (different extractants)
–Physical methods (density, size, …)
–Combination of above
•Fractions have been chemically characterized in various ways
–C/N ratios
–Molecular structures
–14C contents
Common Soil Organic Matter Classification Scheme
SOM
Microbe biomass
plant parts
humus
(1-4%)
non-humic substance
humin
humic acid
humic subs.
fulvic acid
Property1
Humin
Humic Acid
Fulvic Acid
2
color
black
yellowish b rown
molecular weight
106 (?)
104 - >10 5
<10 3- <10 4
cation exchange
100 (?)
300-500
>500-1000
capacity (meq/100g)
carbon (%)
55
52-62
43-<52
oxygen (%)
34
29-44
>44-51
nitrogen (%)
5
3-6
2-7
hydrogen (%)
6
3-7
5
sulfur (%)
1
2
1
Data (with exception of color) derived from: Figure 3.3 i n: J.M. Oades, An
introduction to orga nic matter in mineral soils, in: Minerals in Soil Environments, 2 nd
Edition, Soil Science Society of America, Madison, WI (1989); and Table 11-5 in:
D.M. Sylvia, J.J. F uhrmann, P.G. Hartel, and D.A. Zuberer, Principles and
Applications of Soil Microbiology, Prentice Hall, New Jersey, (1998).
2
See D.G. Schulze, J.L. Nagel, G.E. VanScoyoc, T. L.Henderson, M.F. Baumgardner,
and D.E. Stott, Significance of organic matter in determining soil colors, in: J.M.
Bigham and E.J. Cio lkosz, (eds), Soil Color, Soil Science Society of America, Madison,
WI, (1993).
C/N: 11:1; 9 to 17:1; 7 to 21:1
Describing Soil C (and N) Cycling in Soils
•Except in very unusual situations, soil C and N storage (pools) are
constantly be added to and subtracted from
–Peat bogs (C loss minimal and C (peat) builds up)
–Extreme deserts (N comes in but doesn’t leave)
•The result is that the amounts change rapidly over limited spans of time and
then stabilize (steady state) at levels characteristic of climate, topography,
etc.
•The basics of this can be relatively easily described mathematically using a
mass balance (accounting) approach:…..
INPUTS= leaf litter, root
death, root exudates
LOSSES = CO2, erosion,
dissolved C
CO2
Change in soil organic matter vs time = inputs - losses
dC
IL
dt
dC
 I  kC
dt
and,afterint egration :
1
C(t)  (I  Ie kt )
k
oratsteadystate:
I
C
k
Where
•K = decomposition constant (yr-1)
•Boundary condition for integration
assumes no C at t=0
If no inputs occur (such as decomposition of a compost pile):
dC/dt = L
dC/dt = kC
C(t) = Coe-kt
where Co = starting amount
Visualization of Soil Organic Matter Buildup and Model
Some important steady state
relationships:
Non-steady state
(I>L)
k= I/C
 = C/I= residence time
Time
Steady state (I=L)
State Factors and Organic Matter Inputs
•Climate
–MAP, I  (within limits)
–MAT , I  (within limits)
•Biota
–Controls way C is added to soil (leaves vs. roots)
–Controls input quality (k)
•Topography
–Aspect, etc affect available moisture, temp etc.
•Parent Material
–Nutrients , I 
•Time
–Time , I  (over very long time spans)
•Humans
–Variable
•Decrease from crop removal
•Increase from irrigation, fertilization, etc.
State Factors and Losses (k)
•
Climate
– MAP and MAT , k  (within limits)
•
Biota
– Litter quality (lignin, C/N, etc.)affect k.
– Possible that geographic distribution of microbes varies
•
Topography
– Can cause direct erosional loss of organic matter
•
Parent Material
– clay , k decreases (chemical and physical reasons)
Time
– Effect not well known - may cause decrease in k due to clay increase
and nutrient declines
Humans
– cultivation , k  (!)
•
•
Soil organic C (to 1m), respiration=C inputs; decay rate vs. MAT
• dervied from global “Fluxnet” experiment (Sanderman et al.,
2003)
State Factors and Losses (k)
•
Climate
– MAP and MAT , k  (within limits)
•
Biota
– Litter quality (lignin, C/N, etc.)affect k.
– Possible that geographic distribution of microbes varies
•
Topography
– Can cause direct erosional loss of organic matter
•
Parent Material
– clay , k decreases (chemical and physical reasons)
Time
– Effect not well known - may cause decrease in k due to clay increase and nutrient
declines
Humans
– cultivation , k  (!)
•
•
Soil C vs. Time
40
Soil C commonly
approaches steady
state within 102 to
103 years
35
30
Soil C (kg m -2 )
Steady state value
depends on array of
other state factors…
Spodosols: (Harden et al. ,1992)
Cryosols: (Harden et al. .1992)
Mollisols: (Harden et al. ,1992)
Alfisols: (Harden et al. ,1992)
sand dune: (Syers et al. ,1970)
Hawaii: (Torn et al., 1997)
25
20
15
10
5
0
0
2000
4000
6000
Time (years)
8000
1 10
4
Soil C vs. Climate
•Soil C
increase with
MAP and
decreases
with MAT !
•Pattern is
due to
balance of
inputs and
losses and
effect of
climate on
these
Measuring Inputs and Losses
Inputs = litter (easy) + roots (difficult)
Litter measured via ‘litter traps’ (mass/area•time)
Roots not commonly measured directly except in grasslands
- common to assume root=(litter)(x) where x=1-2
Losses = soil respiraiton (easy) - root respiration (very difficult)
Soil respiration measured by surface chambers (and CO2 buildup)
- Root respiration commonly assumed = (soil respiration)(x)
where x ~ 0.5.
Soil C Concentrations vs. Soil Depth
• Discussion so far on total amounts (not how its distributed
•Inputs and in-soil redistribution processes vary greatly, resulting in 3
general depth trends:
–Exponential C decrease vs. depth (e.g. grasslands)
•Inputs decline with depth
•Transport combined with decomposition move C downward
–Erratic changes with depth (e.g. deserts)
•C inputs vary with root distribution (which is related to hydrology)
•Transport not so important (???)
–Biomodal C maxima vs. depth (e.g. sandy forest soils in temp. climates)
•Large surface inputs
•Production and transport of dissolved C
•Precipitation of dissolved C via complexation with Fe/Al

Soil C Model vs. Depth (in reader #2)
The soil C mass balance is hypo thesized to be, for grassla nd so ils, a function o f pla nt
input s (both surface and root), transpo rt, and de composition:
z
dC
dC
F L
 v
 kC 
e
dt
dz decomposition L
downward
advective
transport
Eq. 10
plant
inputs
distributed
exponentially
-1
where –v=advection rate (cm yr ), z=soil depth (cm) , F=total plant C inpu ts (g cm-2yr-1),
and L =e-folding d epth (cm) . For the bound ary condi tions that C=0 @ z=•
a nd
–v(dC/dz)=FA @ z=0 (where FA = above ground p lant C input s), the steady state solution
is:
kz
kz
kL v

FA  v
FB  v  z vL
C(z) 
e

e e
1
v
kL  v


aboveground
input / transport
root
input / transport
Eq. 11
Summary of Soil Carbon Cycle
•Soil C is controlled by inputs and losses
•Soil C strongly related to climate
•Soil C vs depth variable but somewhat predictable
•Some remaining questions:
–How important is soil C globally (and what is global
C cycle)?
–How can humans affect global soil C budget?
•Cultivation
•Global warming
–Role of soil C in international efforts to reduce
atmospheric CO2