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 Component Concentration (% dry weight)1

solubles 29-56hemi-cellulose 2-40cellulose 13-51lignin 4-48protein 0.5-22ash 4-91 Data compiled from: J.M. Oades, An introduction to organic matter in mineral soils,in: Minerals in Soil Environments, 2nd 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. Fuhrmann, P.G. Hartel, andD.A. Zuberer, Principles and Applications of Soil Microbiology, Prentice Hall, NewJersey, (1998).

•Plant chemistry varies greatly.

•Differences in lignin/N, ash content, etc determine how fast it is recycled by microbes will discuss decomposition more

Ash can be bio-minerals

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 humic subs.

humin humic acid fulvic acid

Property1 Humin Humic Acid Fulvic Acidcolor2 black yellowish brownmolecular weight 106 (?) 104 - >105 <103- <104

cation exchangecapacity (meq/100g)

100 (?) 300-500 >500-1000

carbon (%) 55 52-62 43-<52oxygen (%) 34 29-44 >44-51nitrogen (%) 5 3-6 2-7hydrogen (%) 6 3-7 5sulfur (%) 1 21Data (with exception of color) derived from: Figure 3.3 in: J.M. Oades, Anintroduction to organic matter in mineral soils, in: Minerals in Soil Environments, 2nd

Edition, Soil Science Society of America, Madison, WI (1989); and Table 11-5 in:D.M.Sylvia, J.J. Fuhrmann, P.G. Hartel, and D.A. Zuberer, Principles andApplications of Soil Microbiology, Prentice Hall, New Jersey, (1998).2See D.G. Schulze, J.L. Nagel, G.E. VanScoyoc, T.L.Henderson, M.F. Ba umgardner,and D.E. Stott, Significance of organic matter in determining soil colors, in: J.M.Bigham and E.J. Ciolkosz, (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:…..

CO2

INPUTS= leaf litter, root death, root exudates

LOSSES = CO2, erosion, dissolved C

CO2

CaCO3

AtmosphericCO2

RootInputs

LitterInputs

Organic C

Change in soil organic matter vs time = inputs - losses

€

dCdt

=I −L

dCdt

=I −kC

and,afterintegration:

C(t) =1k

(I −Ie−kt)

oratsteadystate:

C=Ik

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:

k= I/C

= C/I= residence time

Non-steady state (I>L)

Steady state (I=L)

Time

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

Soil C commonly approaches steady state within 102 to 103 years

Steady state value depends on array of other state factors…

0

5

10

15

20

25

30

35

40

0 2000 4000 6000 8000 1 10 4

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)

Time (years)

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 hypothesized to be, for grassland soils, a function of plantinputs (both surface and root), transport, and decomposition:

€

dCdt=−v

dCdz

downwardadvectivetransport

1 2 3 − kC

decomposition{ + F

Le−zL

plantinputsdistributedexponentially

1 2 3 E . 10q

wh ere– =v advecti onra (te c myr-1), z=soi l de pth( )cm, F=tot alplant C input s ( gcm-2yr-1),a nd L= -e foldi ng de pth( )cm. Fo r t he boundar y conditi onstha tC=0 @z=∞ and–v(dC/dz)=FA @ =z 0 (wher eFA = abov egr ound plan t C input)s , th este adystat esolutionis:

€

C(z)=FAve−

kzv

above−groundinput/transport1 2 3

+ FBkL−ve

−kzv ezkL−v

vL−1 ⎛ ⎝ ⎜

⎞ ⎠ ⎟

rootinput/transport

1 2 4 4 4 3 4 4 4 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