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ATMOS 397GATMOS 397GBiogeochemical Cycles and Global ChangeBiogeochemical Cycles and Global Change

Lecture 15: Biosphere and NutrientsLecture 15: Biosphere and Nutrients

Don WuebblesDon Wuebbles

Department of Atmospheric SciencesDepartment of Atmospheric Sciences

University of Illinois, Urbana, ILUniversity of Illinois, Urbana, IL

March 18, 2003March 18, 2003

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The Biosphere (AVHRR measurements)The Biosphere (AVHRR measurements)

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The Biosphere over Time

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Leafing Dates of Oak (1746–present) - This graph shows how the leafing dates of oaks in southeastern England have changed over the past 256 years.

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Terrestrial EcosystemsTerrestrial Ecosystems

C/N in leaf tissue 50 NPP globally ~ 60 x 1015 gC/yr 1.2 x 1015 gN

needed each year N and P are often limited supply of these

elements may control NPP Nutrients in greater quantities, e.g., Ca and S,

have NPP determine their rate of cycling in ecosystems and losses to streamwaters

The atmosphere is the major source of C, N, and S in terrestrial ecosystems

Rock weathering is the major source for most remaining biochemical elements, e.g., Ca, Mg, K, Fe, P

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detailed overlay of N deposition patterns with ecosystem types was key to predicting possible C storage. Where high N deposition occurs on forested systems, there is a potential for significant C storage because forest vegetation has large C:N ratios and long tissue lifetimes in wood. Thus, a map of modeled C uptake using spatially defined estimates of fossil fuel N deposition suggested potential carbon sink hotspots in the mid-latitude forests of the northern hemispere, while at the same time showed that high deposition regions over grassland or agricultural areas, such as the Great Plains, were not likely to produce much carbon storage

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Nutrient Intrasystem CyclingNutrient Intrasystem Cycling

Cycling of N within an ecosystem is often 10 to 20X greater than the amount received from outside.

Soil chemical reactions (ion exchange, mineral solubility) set constraints for plant uptake of essential elements

Plants can release organic compounds that enhance solubility

Uptake of N and P is so rapid, and soil concentration so low, that there is often none of these in the vicinity of roots.

Diffusion of P is slow and limits supply Plants respond by increasing root/shoot ratio Some plants respond by putting out enzymes to

extract nutrients

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Nutrients Needed for Growing Corn

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Nutrient Uptake by Wheat

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Nutrient BalanceNutrient Balance

Plant growth is affected by the balance of nutrients in soil

Some trees 100 N:15 P: 50 K: 5 Ca: 5 Mg: 10 S

More nutrients occur as positively charged ions in the soil solution

Plants will often release H+ to maintain balance of charge

Plants using NH4+ as N source tend to acidify the

immediate zone around their roots (NO3- uptake

has opposite effect)

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Nutrient Availability Depends on pH of the Soil

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Nitrogen Assimilation in Roots of Plants

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Nitrogen Assimilation and Nitrogen FixationNitrogen Assimilation and Nitrogen Fixation

Availability of NH4+ or NO3

- depends on environmental conditions

Waterlogged soils: NH4+

Desert conditions: NO3-

Most species show preference for NO3- even

though NH4+ is assimilated easier

NH4+ reacts easier in the soil

Rate of delivery of NO3- to roots is higher

Nitrification vs. denitrification

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Nitrogen in the Plant

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Nitrogen Fixation in Trees

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Inorganic nitrogen cycle

•no nitrogen is found in native rock

•the ultimate source of nitrogen for ecosystems is molecular nitrogen (N2) in the atmosphere (78.1% by volume)

•N2 may dissolve in water

•virtually all nitrogen would occur as N2 if not for biological processes occurring in the presence of oxygen

Molecular nitrogen enters biological pathways through nitrogen fixation by certain microorganisms:

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