Carbon and Nitrogen Cycling in Soils

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Carbon and Nitrogen Cycling in Soils

• Weathering represented processes that mainly
  deplete soils in elements relative to earths
  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 1950s
• 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

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Soil C Cycle

• Plants O2 humus CO2
• Plants are equivalent of parent material
  (primary minerals)
• Humus is equivalent of secondary minerals

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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

Ash can be bio-minerals
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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

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Common Soil Organic Matter Classification Scheme
SOM Microbe biomass plant parts humus (1-4)
non-humic substance humic subs. humin
humic acid fulvic acid
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C/N 111 9 to 171 7 to 211
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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 doesnt 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..

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INPUTS leaf litter, root death, root
exudates LOSSES CO2, erosion, dissolved C
CO2
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Change in soil organic matter vs time inputs -
losses

• Where
• K decomposition constant (yr-1)
• Boundary condition for integration assumes no C
  at t0

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If no inputs occur (such as decomposition of a
compost pile)dC/dt LdC/dt kCC(t)
Coe-ktwhere Co starting amount
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Visualization of Soil Organic Matter Buildup and
Model
Steady state (IL)
Non-steady state (IgtL)

• Some important steady state relationships
• k I/C
• ? C/I residence time

Time
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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.

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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 ? (!)

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Soil organic C (to 1m), respirationC inputs
decay rate vs. MAT dervied from global
Fluxnet experiment (Sanderman et al., 2003)
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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 ? (!)

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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

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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

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Measuring Inputs and Losses

• Inputs litter (easy) roots (difficult)
• Litter measured via litter traps
  (mass/areatime)
• Roots not commonly measured directly except in
  grasslands
• - common to assume root(litter)(x) where x1-2
• 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.

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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

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Soil C Model vs. Depth (in reader 2)
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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