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“Monday is an awful way to spend 1/7 of your week…” “A clear conscience is usually a the sign of a bad memory”. U6115: Populations & Land Use Tuesday July 1, 2003. Biogeochemical Cycling on Land El Ni ño Photosynthesis and Primary Production Nutrient and Water Use Efficiency

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slide1
“Monday is an awful way to spend 1/7 of your week…”

“A clear conscience is usually a the sign of a bad memory”

u6115 populations land use tuesday july 1 2003
U6115: Populations & Land UseTuesday July 1, 2003

Biogeochemical Cycling on Land

  • El Niño
  • Photosynthesis and Primary Production
  • Nutrient and Water Use Efficiency
  • Production Fate and Detritus
  • Mass Balances of Soil OM and Nutrients
where are we now
Where are we now?
  • “Left wall” and precautionary principle
  • Human impacts on environments: “Ecological Footprint” (Quantifiable)
  • “Extinction” and “invasion” are natural processes, but the current rate of loss and of transport is, respectively:
    • Decreasing genetic/species variability
    • Homogenizing Earth’s biota
  • Biotic control over ecosystem functioning
distribution and characteristics of soils
Distribution and Characteristics of Soils
  • Podzolization: leaching of elements (Fe, Al)  Cool, moist and acidic conditions
  • Laterization: leaching of Si (not Fe, Al)  Hot and moist conditions
  • Melanization: Mixed profiles with addition of OM  Temperate conditions
  • Calcification/Salinization: low water and preservation of salts Arid conditions
biotic control over ecosystems
Biotic Control over Ecosystems

Resistance/Resilience vs. Vulnerability

  • Changes in the abundance of species that differ in ecosystem consequences should affect process rates or patterns,
  • Abundance of species with similar ecological effects should give stability (resistance and resilience)
biotic control over ecosystems1
Biotic Control over Ecosystems

Resistance/Resilience vs. Vulnerability

photosynthesis
Photosynthesis

Dual stage:

1) 6H2O  12H+ + 12e- + 3O2 (chlorophyll)

2) 6CO2 + 12H+ + 12e- C6H12O6 + 3O2

-) CO2 diffuses into plant leaves through stomata

water use efficiency in photosynthesis
Water Use Efficiency in Photosynthesis

Water use efficiency (WUE): mmole CO2 fixed/mole of H2O lost (~0.9-1.5 mmol/mol for most plants)

WUE is higher at lower conductance (stomatal aperture)

Conductance is controlled primarily by the availability of water and [CO2] inside the leaf.

Diffusion of 12CO2 is more rapid than 13CO2 (~1.1% of atmospheric CO2)

Discrimination between 12CO2 and 13CO2 is greatest when stomatal conductance is high (CO2 abundance)

when less CO2 abundance less fractionation!

nutrient use efficiency in photosynthesis
Nutrient Use Efficiency in Photosynthesis

Rate of photosynthesis is directly correlated to leaf nitrogen content (when both are expressed on a mass basis)

Most leaf N is contained in Rubisco (20-30% of leaf N). It seems thus that availability of N determines leaf enzyme content and rate of photosynthesis in land plants

Rate of Photosynthesis per unit leaf N (slope) is one measure of NUE!

Subtle variations in NUE are seen among land plants

 When leaf N increases (by fertilization), NUE declines

 When WUE increases, NUE declines

More like an index of photosynthetic potential based on leaf nutrient level!

slide14
Respiration and Net Primary Production

Net photosynthesis is the fixation of carbon in excess of simultaneous releases of CO2 by plant metabolism

Plant metabolism (respiration) is correlated with N content of plant tissues.

About ~1/2 of gross carbon fixation is used up in metabolism (respiration).

Plant respiration usually increases with stand age and with temperature

  • reduction in total plant growth rate
  • reduction NPP
slide15

NPP = Gross primary production (GPP) - Plant respiration (Rp)

NPP is, however, not directly equivalent to plant growth since some fraction of NPP is lost to grazing (herbivores) and to litterfall (nonliving organic matter - NLOM)

Photosynthesis usually captures only ~1% of total energy received in sunlight (the biosphere is fueled by a relatively inefficient process)

Allocation of NPP varies with vegetation type and age

As a result of their massive structure and high environmental T°, tropical forests expand a great amount of their gross PP in respiration and thus show lower NPP of wood biomass than boreal forests

Net Primary Production (NPP)

Net photosynthesis is the fixation of carbon in excess of simultaneous releases of CO2 by plant metabolism

slide16

Log relationship between total aboveground NPP (ANPP) and foliage biomass for a variety of plant communities in North America

Net Primary Production (NPP)

NPP = Gross primary production (GPP) - Plant respiration (Rp)

However, root systems (below ground biomass) can compose more than 1/2 of NPP and the proportional allocation of photosynthate to root growth varies as an inverse function of site fertility (availability of nutrients)

slide17

Global ranges of NPP: 45-65x1015 g C/yr

Total Biomass: ~560x1015 g C

Hence:

Mean residence time of ~9 years

Range: 4 yrs for deserts to >20 yrs for certain forests

Global estimates ofNPP and Biomass

slide18

A) These are weighed averages: different tissues have different turn over rates:

Leaves  few months

Wood  decades to centuries

B) Apparent gradient of declining NPP with increasing latitude

Global estimates ofNPP and Biomass

slide19

B) Apparent gradient of declining NPP with increasing latitude (elevation) AND decreasing moisture (precipitation)

Global estimates ofNPP and Biomass

slide20

Temperature and moisture seem to be the main factors determining NPP, with local soil conditions playing a lesser role.

Global estimates ofNPP and Biomass

Influence of these variables on microbial processes that speed up nutrient turnover in soils

Exception: Tropical rainforests where light and water are abundant

 Soil fertility becomes predominant

slide21

As communities of long-lived plants develop on land, a certain fraction of NPP is allocated to perennial, woody tissues that accumulate as biomass through time

Plant communities achieve steady-state in living biomass when allocation to woody tissues is balanced by death and loss of older parts.

Net Ecosystem Production

NEP = GPP - Rt

Where Rt = Rp + Rh + Rd

 Non living organic matter (NLOM) may still be accumulating in soils as reduced forms

FateofNPP

slide22

Increments in organic matter (OM) are possible only during the early stages of a community development.

In older communities, there is no true increment to live biomass, but there is still a delivery of NPP to soils (where it can be decomposed or stored over longer time scales)

 Fires and humans accelerate the conversion of long-term accumulation of NEP to CO2

FateofNPP

slide23

The largest fraction of NPP is delivered to soils as NLOM (plant litter).

Global patterns of NLOM deposition (plant litterfall) are similar to global patterns of NPP  Deposition of litterfall declines with increasing latitude (tropical to boreal forests).

Most detritus is delivered to upper soil layers where is is subject to decomposition. Decomposition of soil NLOM can be approximated with simple exponential decay models, where the fraction remaining after 1 year is given by:

X/Xo = e-k

Where k = Ln (X/Xo)

And t1/2 = Ln 0.5/k

Or

k = 0.693/ t1/2

Production and Fate of Detritus

slide24

An alternative, the mass balance approach, suggests that the annual decomposition should equal the annual input of fresh debris (so M remains constant)

Literfall = k x M

Where:

M  detrital mass (reservoir)

k  decay constant (flux of detrital mass in/out of reservoir)

k = Literfall/detrital mass

Values of k can provide mean residence time (1/k ) for any given system.

Production and Fate of Detritus

slide25

Since decomposition dynamics depend on environmental conditions (temperature and moisture), chemical composition of litter, and nutrient availability, residence time of NLOM in soils differ greatly from one environment to another:

Grasslands (little of aboveground NPP is contained in perennial tissues): k can range 0.2 to 0.6  1.5-5.0 years residence time

Peatlands (waterlogged, reducing environments): k are very small ~0.001  1000 years residence time

Deserts (reduced water but presence of termites and high photooxidation rates): k can reach ~1  1 year residence time (or less)

Production and Fate of Detritus

slide26

Strong correlation between evapotranspiration rates and regional patterns of decomposition for surface litter (role of water)

 however, chemical parameters and composition of litter help improve the models

Fate of Detritus

slide27

Plant litter and cellular microbes constitute the cellular fraction of soil OM. As decomposition proceeds, there is an increasing content of non-cellular OM - humus - that appears to result from microbial activity and physical processes.

Under most vegetation, the mass of humus in the soil profile exceeds the combined content of OM in the forest floor and aboveground biomass  Different k values!

Long-Term Detritus: Humus

slide28

Moisture and temperature control the balance between NPP and decomposition in surface and lower soil layers.

Accumulation of soil OM is greatest in humid cold regions whereas NPP show opposite trend.

 Accumulation of soil OM is thus largely due to differences in decomposition rather than to the NPP of terrestrial ecosystems!

Global Distribution of Soil OM

CO2 loss

Litterfall

slide29

Storage of soil OM represents the net ecosystem production (NEP) in terrestrial ecosystems..

The mass of soil OM in most upland systems is likely to have remained fairly constant before widespread disturbances of soils.

The current rate of storage in northern ecosystems (post-glaciation: 0.04x1015 g C/yr) is too small to be a significant sink for human releases of CO2 from fossil fuel burning

The terrestrial NEP for the globe is not likely to be more than 0.7% of NPP which attests to the high efficiency of decomposers (remember that only ~1% of solar energy is converted to OM)

Total storage of carbon in soil OM can only account for 0.03% of the O2 content of the atmosphere, Thus accumulation of atmospheric O2 cannot be the result of terrestrial storage of OM  dominated by accumulation in marine sediments.

Global Mass-Balance of Soil OM

slide30

Losses from many soils are typically 20-30% within the first few decades of cultivation (greater rates of decomposition under croplands)

Losses of soil carbon from agricultural soils has been a major component of the past increase in atmospheric CO2, along with forest fires and drainage of wetlands/peatlands.

Soil OM and Global Change

What does climatic change have in store for us?

Feedback mechanisms of global warming

slide31

Suggestion that change in climate and CO2 concentrations have modified the C cycle so as to render terrestrial ecosystems as substantial sinks!

Dynamic Response of Terrestrial Ecosystems

  • What is the dynamic response of ecosystem carbon fluxes to transient climate changes?
  • CO2 to 640 ppmv
  • Temp to 15.5°C
slide32

The variations in:

  • - global net primary production (NPP, Gt C/yr)
  • net ecosystem production (NEP, Gt C/yr), and
  • carbon stocks in vegetation (VGC, GtC) and soils (SOC, Gt C) resulting from atmospheric CO2 increase and climate change.

Dynamic Response of Terrestrial Ecosystems

slide33

Variations in global net ecosystem production (NEP, Gt C yr-1) responding to stabilization of atmospheric CO2 at 450, 550 and 650 p.p.m.v., or continual increase. This estimate is made with changes in atmospheric CO2 only.

Dynamic Response of Terrestrial Ecosystems

slide34

The variations in:

  • - global net primary production (NPP, Gt C/yr)
  • net ecosystem production (NEP, Gt C/yr), and
  • carbon stocks in vegetation (VGC, GtC) and soils (SOC, Gt C) resulting from atmospheric CO2 increase and climate change.

Dynamic Response of Terrestrial Ecosystems

slide35

NEP in North and tropics are of similar magnitude under contemporary climate

NEP will become much higher than in the tropics in the future (deforestation & respiration )

Seasonal amplitude of NEP is amplified  Higher NPP and lower respiration in summer

Dynamic Response of Terrestrial Ecosystems

slide36

Photosynthesis provides the energy (matter) that powers biochemical reactions of life

The terrestrial biosphere is fueled by a relatively inefficient initial process  makes it up with a highly efficient recycling process

Soil nutrients appear to be of secondary importance to NPP on land  plants have various adaptations for obtaining and recycling nutrients efficiently when they are in short supply.

Because the decomposition of NPP is extremely efficient only small amounts of NPP are added to long-term storage of soil OM.

Humans have altered the process of NPP and decomposition on land, resulting in the transfer of organic carbon to the atmosphere as CO2 , and perhaps a permanent reduction in global NPP (global change in the biogeochemical cycle of C but not in that of the O2 cycle)

Summary