Biogeochemical cycles

& global change

Despite uncertainties about details of human impact on global system, there are certainties!

-Changes in the N cycle

-Changes in atmospheric CO2

-Changes in land use

Units appropriate for global-scale studies

teragram = tg = 1012g

petagram = pg = 1015 g = 109 t = 1 gt

Humans have long sought to alter biogeochemistry. Sheep were used for this purpose in Medieval Europe.

Application of manure for over a period of centuries provided fertile lands at the expense of the rest of the landscape. Note the grass-dominated foreground.

Typical, nonfertilized Calluna heathland in The Netherlands. Photographed in 1977. Note bump of trees in background.

Repeat photograph in 1983 (six years later) with Calluna mostly gone. Note bump of trees in background.

Repeat photograph from 1987. Note bump of trees. Calluna entirely replaced by same grass as dominated on fertilized field.

The global hydrologic cycle is critical for understanding biogeochemistry because many elements move in water.

Units = km3

Units = teragrams

Global nitrate levels have increased over the last 40 years.

Changes in the global nitrogen cycle: Implications- In atmosphere NO2 NO + O; O2 + O O3

Various oxidants harm plant growthCould be responsible for tree mortality

-Dysfunctional plant carbon allocation

-NO3 is a big component of acid rain, along with SO4

Distribution – NW Europe, E North America Leaching of calcium from soilsDeath of lakesAl toxicityAppears to have killed thousands of lakes on granite areasCould have leaching effects on soilsCould be major component in tree death

- Altered community composition & loss of diversity

Experiments illustrate impact of nitrogen addition on community diversity.

Global carbon cycle – Humans are increasing global CO2

Human activity has increased atmospheric CO2 from ~280 to ~360 ppm since 1800.

Current high CO2 levels are unique in the last 160,000 years

The increase in atmospheric CO2 is derived from fossil fuels (the primary source of carbon low in 14C).

FF(6) + Veg (0.9) = Atm(3.2) + Ocean(2.0) + Unk(1.7) [PgC/yr]

Global warming is real. The most recent years have been higher than any shown on the graph.

Consequences of increased CO2

Some extreme projections show increase in temp by 5 C by 2100, with some melting of polar ice, increase in sea level by 6-8m, and increased droughts in central US, Eastern Europe, and adjacent Asia. What is certain is that we should be cautious. Do not know what we are getting in for.

- Change in rain & temp- Change in sea level- Change in species distributions- Rapid migration in a fragmented landscape

Increases contribute to acid rain.

Sodium pool sizes and transfers

Na into sea during 500 million yrs = 119.4 x 1015 t

Current dissolved Na = 14.1Deep sea sediments = 5.1Sub ocean sediments = 5.4old sedimentary rocks = 2.6Rock salt reserves = 0.4Total = 27.6

75% of 119.4 is missing

Land Use Changes

Total Terrestrial Photosynthesis 122 pg/yrNPP used by humans

Consumed by people 0.8Consumed by domestic animals 2.2Wood used 2.4Total 5.2 (4%)

NPP dominated by humans, but not usedCroplands, converted pasturesPlantations, land clearingTotal 41 (31%)

NPP lost to human activityDecreased NPP of cropland vs natural 10Desertification 4.5Occupied 2.6Total 17 (8%)

Summary Points - 1- Humans have significantly altered global biogeochemical cycles.

- Nitrogen fixation has been more than doubled by human activities. Negative impacts of enhanced atmospheric nitrogen deposition include acid precipitation, global warming, lake acidification, alteration in carbon allocation in plants, change in community composition, and loss of biodiversity.

- Atmospheric CO2 has increased from 280 ppm to 360 ppm over the last 200 years. Much of this increase is attributable to burning of fossil fuels. The increase in CO2 is causing an increase in global temperatures.

Summary Points - 2- Phosphorus mining has resulted in increases in phosphorus export in world rivers by a factor of two.

- Ratios of elements in the oceans do not match the ratios in stream water flowing into the oceans. To understand the differences it is necessary to consider, among other factors, the rates of deposition in sediments, movement as aerosols back to land, and movement and erosion of rock.

-Approximately 40% of the potential net primary photosynthesis of the earth is now within human dominated ecosystems systems.