Carbon and Nitrogen Cycling in Soils - PowerPoint PPT Presentation

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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 earth s crust Biological processes differ ... – PowerPoint PPT presentation

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Title: Carbon and Nitrogen Cycling in Soils


1
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

2
Soil C Cycle
  • Plants O2 humus CO2
  • Plants are equivalent of parent material
    (primary minerals)
  • Humus is equivalent of secondary minerals

3
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
4
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

5
Common Soil Organic Matter Classification Scheme
SOM Microbe biomass plant parts humus (1-4)
non-humic substance humic subs. humin
humic acid fulvic acid
6
C/N 111 9 to 171 7 to 211
7
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..

8
INPUTS leaf litter, root death, root
exudates LOSSES CO2, erosion, dissolved C
CO2
9
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10
Change in soil organic matter vs time inputs -
losses
  • Where
  • K decomposition constant (yr-1)
  • Boundary condition for integration assumes no C
    at t0

11
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
12
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
13
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.

14
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 ? (!)

15
Soil organic C (to 1m), respirationC inputs
decay rate vs. MAT dervied from global
Fluxnet experiment (Sanderman et al., 2003)
16
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 ? (!)

17
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

18
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

19
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20
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.

21
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

22
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23
Soil C Model vs. Depth (in reader 2)
24
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
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