Download The nitrogen (N) cycle

The nitrogen (N) cycle
Gordon Holtgrieve
C ENV 110: Food and the Environment
Oct. 12, 2014
Outline
• Take home points
• What are biogeochemical cycles?
• Importance of N to natural and managed
ecosystems
• Natural and human sources of ‘fixed’ N
• N cycling and N loses from ecosystems
• Global and local impacts of N loses
• Take home points again
Take home points
Ecosystem N sources and N losses are
increasing globally (entirely due to us).
N = N = N
NH4+ ≠ NO3- ≠ N2 ≠ N2 O ≠ Norg
It’s all about rates
Bio-Geo-Chemistry
The study of the biological, physical, and chemical
processes that govern the composition of the
natural environment.
Highly inter-disciplinary science focused on
nutrient cycling and human interactions at generally
large scales.
Biogeochemical Cycles
Conservation of mass: matter can not be created nor destroyed.
Elements reside in pools that are connected through fluxes.
Plant physiology
The primary macronutrients:
nitrogen (N),
phosphorus (P)
potassium (K)
Plants need roughly 16 times more N than P
In most ecosystems, primary production is N limited
Earth’s crust
Rank
1
2
3
4
5
8
11
17
30
Element
oxygen
silicon
aluminium
iron
calcium
potassium
phosphorus
carbon
nitrogen
Symbol
O
Si
Al
Fe
Ca
K
P
C
N
Abundance (ppm)
461,000
282,000
82,300
56,300
41,500
20,900
1,050
200
19
Atmosphere
Rank
1
2
3
Element
nitrogen
oxygen
argon
Symbol
N
O
Ar
Abundance (%)
78.1
20.9
0.96
N2
unavailable
to most
plants
N needs to be “fixed”
(NH4+, NO3-, urea)
removes fixed N
very little N
Natural N fixation is primarily from legume (N fixing) plants
• Symbiosis between rhizobium bacteria and plant
• Energetic cost to plant in exchange for access to N
Soybean
Alder
The amount of “natural” N fixation has remained
roughly constant through time
Vitousek, P. M. and P. A. Matson
(1993). In The Biogeochemistry
of Global Change: Radiative Trace
Gases. New York, Chapman and
Hall: 193-208.
Haber-Bosch process of
ammonification
Frtiz Haber
(1868 – 1934)
Carl Bosch
(1874 – 1940)
N2 + 3 H2 → 2 NH3 (ΔH = −92.4 kJ·mol−1)
J J Elser Science 2011;334:1504-1505
Published by AAAS
Humans have dramatically altered the global N cycle
Every year humans fix
more N than all natural
sources of N fixation
• Fertilizer
• Fossil fuel combustion
• Planting of N fixing crops
Vitousek, P. M. and P. A. Matson
(1993). In The Biogeochemistry
of Global Change: Radiative Trace
Gases. New York, Chapman and
Hall: 193-208.
N is fixed by the Haber-Bosch process
H-B N created the
Green Revolution and
increased World
population.
Erisman et al. 2008. How
a century of ammonia
synthesis changed the
world. Nature
Geosciences
2011 Science
“Simplified” soil nitrogen cycle
NH3
N2
NOx
N2O
N2
N2O
N2
Fertilizer
Soil
+
NH
NO
4
3
Organic
Nitrification
Matter Mineralization
Denitrification
Leaching
N losses with big negative effects
NH3
N2
NOx
N2O
N2
N2O
N2
Fertilizer
Soil
+
NO
NH
3
4
Organic
Nitrification
Matter Mineralization
Denitrification
Leaching
• ↑ N application = ↑ N loss
• As much as 60% (or more) of N applied to
agricultural fields is lost
Xiao-Tang Ju et al. 2009.
Reducing environmental risk by
improving N management in
intensive Chinese agricultural
systems. PNAS
Ammonia (NH3) emissions from legume crops
Ammonia in the atmosphere increases particulates
Lieven et al. 2009. Nature Geoscience 2, 479 - 483
Nitrogen oxides (NO & NO2)
2 NO + O2 → 2 NO2
→ Acid Rain
4 NO2 + 2H2O + O2 → HNO3
NOx + VOC → O3 + Particulates
→ Smog
Nitrous oxide (N2O)
N2O is 340 times more potent
a greenhouse gas than CO2!
Currently about 10% of total
radiative forcing (= greenhouse
effect)
Soils under fertilized agriculture
is the largest source of N2O to
the atmosphere.
Agriculture’s role in climate change
IPCC AR4 report
@#%& (aka, nitrogen) flows downhill…
N losses to aquatic ecosystems
Vitousek et al. 1997.
Ecological Applications, 7(3),
1997, pp. 737–750
Oceans downstream of
agricultural areas are
vulnerable to large
phytoplankton
blooms.
Beman et al. (2005) Nature 434, 211-214
Gulf of Mexico Hypoxic Zone
pH (also O2)
Key Early Action: “Quantify key natural and
human-influenced processes that contribute to
acidification based on estimates of sources, sinks,
and transfer rates for carbon and nitrogen.
(Action 7.2.1)”
WA Blue Ribbon Panel on Ocean Acidification
Options for controlling N losses
• Reduce N fertilization to only what is needed
• Adjust timing of N application to match plant
demand
• Use organic forms of N (urea and manure)
that take longer to become plant/microbe
available
• Grow less legumes
• Increase cost of N (N tax?)
Key messages
• Biogeochemistry is an inter-disciplinary science
focused on nutrients, humans, and ecosystems.
• N is generally the limiting nutrient in most
agricultural and natural ecosystems.
• Atmosphere is huge source of N but it needs to
be ‘fixed’.
• Humans have more than doubled the rate of N
fixation over background.
• N cycling in soils releases N in different forms
with multiple environmental consequences.
• Good fertilizer management can reduce losses.