““Monday is an awful way Monday is an awful way to spend 1/7 of your to spend 1/7 of your

week…”week…”

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

memory”memory”

U6115: Populations & Land UseTuesday July 1, 2003

Biogeochemical Cycling on LandBiogeochemical Cycling on Land

A)A) El NiEl Niñoño

B)B) Photosynthesis and Primary ProductionPhotosynthesis and Primary ProductionC)C) Nutrient and Water Use EfficiencyNutrient and Water Use EfficiencyD)D) Production Fate and DetritusProduction Fate and DetritusE)E) Mass Balances of Soil OM and NutrientsMass Balances of Soil OM and Nutrients

El Niño: BiogeochemistryEl Niño: Biogeochemistry

Where are we now?

A)A) ““Left wall” and precautionary principleLeft wall” and precautionary principleB)B) Human impacts on environments: “Ecological Human impacts on environments: “Ecological

Footprint” (Quantifiable)Footprint” (Quantifiable)C)C) ““Extinction” and “invasion” are natural Extinction” and “invasion” are natural

processes, but the current rate of loss and of processes, but the current rate of loss and of transport is, respectively:transport is, respectively:

i.i. Decreasing genetic/species variabilityDecreasing genetic/species variabilityii.ii. Homogenizing Earth’s biotaHomogenizing Earth’s biota

D)D) Biotic control over ecosystem functioningBiotic control over ecosystem functioning

Distribution and Characteristics of Terrestrial Ecosystems

Distribution and Characteristics of Soils

Podzolization: leaching of Podzolization: leaching of elements (Fe, Al) elements (Fe, Al) Cool, Cool, moist and acidic conditionsmoist and acidic conditions Laterization: leaching of Si Laterization: leaching of Si (not Fe, Al) (not Fe, Al) Hot and moist Hot and moist conditionsconditions Melanization: Mixed profiles Melanization: Mixed profiles with addition of OM with addition of OM Temperate conditionsTemperate conditions Calcification/Salinization: low Calcification/Salinization: low water and preservation of salts water and preservation of salts Arid conditions Arid conditions

Distribution and Characteristics of Terrestrial Ecosystems

Biotic Control over EcosystemsResistance/Resilience vs. Vulnerability

Changes in the abundance of species that differ in Changes in the abundance of species that differ in ecosystem consequences should affect process rates or ecosystem consequences should affect process rates or patterns,patterns,

Abundance of species with similar ecological effects Abundance of species with similar ecological effects should give stability (resistance and resilience)should give stability (resistance and resilience)

Physical Control over Ecosystems

Biotic Control over EcosystemsResistance/Resilience vs. Vulnerability

PhotosynthesisDual stage: Dual stage:

1) 6H1) 6H22O O 12H 12H++ + 12e + 12e-- + 3O + 3O2 2 (chlorophyll)(chlorophyll)

2) 6CO2) 6CO22 + 12H + 12H++ + 12e + 12e-- C C66HH1212OO66 + 3O + 3O22

-) CO-) CO22 diffuses into plant leaves through stomata diffuses into plant leaves through stomata

Water Use Efficiency in PhotosynthesisWater use efficiency (WUE): mmole COWater use efficiency (WUE): mmole CO22 fixed/mole of H fixed/mole of H22O lost (~0.9-O lost (~0.9-1.5 mmol/mol for most plants)1.5 mmol/mol for most plants)

WUE is higher at lower WUE is higher at lower conductance conductance (stomatal aperture)(stomatal aperture)ConductanceConductance is controlled primarily by the availability of water and is controlled primarily by the availability of water and [CO[CO22] inside the leaf.] inside the leaf.

Diffusion of Diffusion of 1212COCO22 is more is more rapid than rapid than 1313COCO22 (~1.1% of (~1.1% of atmospheric COatmospheric CO22))

Discrimination between Discrimination between 1212COCO22 and and 1313COCO22 is greatest is greatest when stomatal when stomatal conductance is high (COconductance is high (CO22 abundance)abundance)

when less COwhen less CO22 abundance abundance less fractionation!less fractionation!

Nutrient Use Efficiency in PhotosynthesisRate of photosynthesis is directly correlated to leaf nitrogen content Rate of photosynthesis is directly correlated to leaf nitrogen content (when both are expressed on a mass basis)(when both are expressed on a mass basis)Most leaf N is contained in Most leaf N is contained in RubiscoRubisco (20-30% of leaf N). It seems thus (20-30% of leaf N). It seems thus that availability of N determines leaf enzyme content and rate of that availability of N determines leaf enzyme content and rate of photosynthesis in land plantsphotosynthesis in land plants

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

Subtle variations in NUE are seen Subtle variations in NUE are seen among land plantsamong land plants When leaf N increases (by When leaf N increases (by fertilization), NUE declinesfertilization), NUE declines When WUE increases, NUE When WUE increases, NUE declinesdeclinesMore like an index of More like an index of photosynthetic potential based on photosynthetic potential based on leaf nutrient level!leaf nutrient level!

Respiration and Net Primary ProductionNetNet photosynthesis is the fixation of carbon in excess of photosynthesis is the fixation of carbon in excess of

simultaneous releases of COsimultaneous releases of CO22 by plant metabolism by plant metabolism

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

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

Plant respiration usually Plant respiration usually increases with stand age and increases with stand age and with temperature with temperature reduction in total plant growth reduction in total plant growth raterate reduction NPPreduction NPP

Net Primary Production (NPP)Net photosynthesis is the fixation of carbon in excess of

simultaneous releases of CO2 by plant metabolism

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

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

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)

Global estimates of NPP and BiomassGlobal ranges of NPP: 45-65x1015 g C/yrTotal Biomass: ~560x1015 g CHence:Mean residence time of ~9 yearsRange: 4 yrs for deserts to >20 yrs for certain forests

Global estimates of NPP and BiomassA) These are weighed averages: different tissues have different turn over rates:Leaves few monthsWood decades to centuriesB) Apparent gradient of declining NPP with increasing latitude

Global estimates of NPP and BiomassB) Apparent gradient of declining NPP with increasing latitude (elevation) AND decreasing moisture (precipitation)

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

Influence of these variables on microbial processes that speed up Influence of these variables on microbial processes that speed up nutrient turnover in soilsnutrient turnover in soilsExceptionException: Tropical rainforests where light and water are abundant: Tropical rainforests where light and water are abundant Soil fertility becomes predominantSoil fertility becomes predominant

Fate of NPPAs 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

Fate of NPPIncrements 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

Production and Fate of Detritus

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/kOr

k = 0.693/ t1/2

Production and Fate of Detritus

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 MWhere:

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 DetritusSince 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)

Fate of DetritusStrong 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

Long-Term Detritus: HumusPlant 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!

Global Distribution of Soil OMMoisture 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!

Litterfall

CO2 loss

Global Mass-Balance of Soil OM

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.

Soil OM and Global Change

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.

What does climatic change What does climatic change have in store for us?have in store for us?Feedback mechanisms of Feedback mechanisms of global warmingglobal warming

Dynamic Response of Terrestrial EcosystemsDynamic Response of Terrestrial EcosystemsSuggestion that change in climate and COSuggestion that change in climate and CO22 concentrations have concentrations have modified the C cycle so as to render terrestrial ecosystems as modified the C cycle so as to render terrestrial ecosystems as substantial sinks!substantial sinks!

What is the dynamic response What is the dynamic response of ecosystem carbon fluxes to of ecosystem carbon fluxes to transient climate changes?transient climate changes? COCO22 to 640 ppmv to 640 ppmv Temp to 15.5Temp to 15.5°C°C

Dynamic Response of Terrestrial Dynamic Response of Terrestrial EcosystemsEcosystems

The variations in:The variations in:- global net primary production (NPP, Gt C/yr)- global net primary production (NPP, Gt C/yr)- net ecosystem production (NEP, Gt C/yr), andnet ecosystem production (NEP, Gt C/yr), and- carbon stocks in vegetation (VGC, GtC) and carbon stocks in vegetation (VGC, GtC) and soils (SOC, Gt C) resulting from atmospheric soils (SOC, Gt C) resulting from atmospheric COCO

22 increase and climate change. increase and climate change.

Dynamic Response of Dynamic Response of Terrestrial EcosystemsTerrestrial Ecosystems

Variations in global net Variations in global net ecosystem production ecosystem production (NEP, Gt C yr(NEP, Gt C yr-1-1) ) responding to stabilization responding to stabilization of atmospheric COof atmospheric CO

22 at at

450, 550 and 650 450, 550 and 650 p.p.m.v., or continual p.p.m.v., or continual increase. This estimate is increase. This estimate is made with changes in made with changes in atmospheric COatmospheric CO

22 only. only.

Dynamic Response of Dynamic Response of Terrestrial EcosystemsTerrestrial Ecosystems

The variations in:The variations in:- global net primary production - global net primary production (NPP, Gt C/yr)(NPP, Gt C/yr)- net ecosystem production (NEP, net ecosystem production (NEP, Gt C/yr), andGt C/yr), and- carbon stocks in vegetation carbon stocks in vegetation (VGC, GtC) and soils (SOC, Gt C) (VGC, GtC) and soils (SOC, Gt C) resulting from atmospheric COresulting from atmospheric CO

22

increase and climate change. increase and climate change.

Dynamic Response of Dynamic Response of Terrestrial EcosystemsTerrestrial Ecosystems

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

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

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

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

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

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

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

Humans have altered the process of NPP and decomposition on land, Humans have altered the process of NPP and decomposition on land, resulting in the transfer of organic carbon to the atmosphere as COresulting in the transfer of organic carbon to the atmosphere as CO22 , , and perhaps a permanent reduction in global NPP (global change in and perhaps a permanent reduction in global NPP (global change in the biogeochemical cycle of C but not in that of the Othe biogeochemical cycle of C but not in that of the O22 cycle) cycle)