Human Disruption of the Global Nitrogen Cycle

Alan Townsend

7 December 2007

Guest Lecture – Soils Geography

University of Colorado, Boulder

Simplified Terrestrial N Cycle

Soil Organic Matter (SOM)

Plants

NH4 NO3

Mineralization

Leaching to groundwater and streams

NO, N2O NO, N2O, N2

N2

Simplified Global N Cycle

Nitrogen Transformation Cycle: Past

N2

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ing

N-F

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Den

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Reactive N

Nitrogen Transformation Cycle: Present

N2

Lig

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ing

N-F

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Den

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Reactive N

N2+3H2 2NH3N2 + O2 NOyLe

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Nr Creation by Nature and Humans

•Since 1960:• Flows of biologically

available nitrogen in terrestrial ecosystems doubled

• > 50% of all the synthetic nitrogen fertilizer ever used has been used since 1985

• Humans produce as much biologically available N as all natural pathways and this may grow a further 65% by 2050

Human-produced Reactive Nitrogen

Timing of N Cycle Changes

A global-scale change, but not equally distributed

Annual Nitrogen Deposition(a map of fertilizer use would look about the same…)

Why is N Use Increasing?

Why is N Use Increasing?

Agricultural Use: Fertilizers (and especially N) increase yields: global use was 14 million tons in 1950 and about 135 tons now.

Fritz Haber (the Haber process) created a method for converting N2 to NH3 (won the Nobel prize in 1918). This is still how fertilizer is produced

Fertilizer Consumption - last 50 years

• Haber-Bosch has facilitated agricultural intensification

• 40% of world’s population is alive because of it

• An additional 3 billion people by 2050 will be sustained by it

• Most N that enters agroecosystems is released to the environment

N and Agricultural Ecosystems

Trends in N Deposition

Sources of N – Northeastern US

Boyer et al, 2002

Fates of N – NE US

Van Breemen et al, 2002

What goes in, comes out…NE US

N Losses Correlate with Anthropogenic N Inputs

Environmental Effects of a Changing Global N Cycle (the short list…)

• Climate

• Acid rain

• Water quality

• Coastal eutrophication

• Air quality (e.g. tropospheric O3)

• Stratospheric ozone depletion

• Species composition (including feedbacks with invasives)

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Stevens et al, Science 2004

N regulation

IPCC