Nitrogen deposition, elevated CO2 and forests: why pollutants are good for the trees but bad for the planet
M. Mencuccini
School of GeoSciences University of Edinburgh (UK)
OUTLINE Global
change Forest nitrogen (N) cycle and N deposition Relationships between N deposition, carbon b (C) cycle l and d other th greenhouse h gases (GHG) Implications for global change
N-deposition, year 2000
A BRIEF REVIEW ON N CYCLE
Denitrifying: y g NO3- → NO2- → NO → N2O → N2
Probably most important nutrient (with P) Ultimately derived from the atmosphere by lightning g g strikes or biological N fixation N fixation by freefree-living or symbiotic bacteria Fundamental role of microbes i b iin N cycle l Forests very good at keeping hold of NO3-
Impacts of N deposition on terrestrial ecosystems REDUCED N (NH3)
OXIDISED N (NO, (NO NO2, HNO3)
Nitrogen sources Globally 72%
of fixed N is anthropogenic Electricity generation
35%
37%
Traffic
Industry
Aber et al al., 2003
Nitrous Oxide (N2O)
Atmospheric N2 fixed to reactive nitrogen (NR)
GHG balance Particulate Matter
Nitrogen oxides ((NOx)
NR Fertilizer manufacture
Ammonium nitrate in rain ((NH4NO3)
Ammonia (NH3)
NR
Further emission of NOx & N2O carrying i on the cascade
Crops for food & animal feed
The Nitrogen cascade in agriculture
Livestock farming
Leached Nitrate (NO3-)
Terrestrial Eutrophication & Soil Acidification Natural ecosystems Aquatic Eutrophication
Nitrate in Streamwaters
Concerns about forest decline in the 1980s
ďƒ˜ Reductions
in sulphur emissions ďƒ˜ Other reasons?
Boreal forests are N N--limited as judged from these simple, early demonstrations from Sweden
(Hogberg 2007) courtesy Sune Linder, Umea Univ
Growth trends of European forests
Sparse S
d data t for f Mediterranean M dit Impact p of drought g S i k 1988 Water, Spiecker W t Air Ai & Soil Pollution
The e te terrestrial est a ‘missing ss g s sink’
28-30% of emissions stay 28in terrestrial biosphere
Partly tropical Partly temperate Partly boreal
Canadell et al al, 2007
Hypotheses to explain the increase (the anthropogenic effect paradox)
CO
2
fertilization
Warming Management N-deposition
Probable all 4 are important
POP-FACE
CO2 forcing
Young stands in temperate zone [CO2] = 550 ppm → 2050 Soil metabolism ↑ NPP ↑ Response dependent on nutrition
Norby et al al, 2005
Effects of N deposition on the global C cycle
Debate: 1) how responsive are forests to N deposition? 2) how long will the response last? 3) does the sensitivity to Ndep explain the terrestrial sink? Magnani et al, 2007; De Vries et al, 2008; Reich et al 2008; De Schriver et al, 2008; Sutton et al 2008; Magnani et al, 2008
Complete accounting for GH effects +
Atmospheric ozone
+
Aerosol, CH4 turnover
–
N use and emissions
+ + N leaching leaching, water quality Biodiversit y acidificatio n Other environmental effects
N deposition on natural ecosystems
N 2O
+
?
Climate change g
+
+ Ecosystem C sink
_ –
40 vs 200 kg C / kg N
Climate forcing
Radiative forcing (IPCC) The change in net irradiance at the top of the troposphere relative to "unperturbed" values (1750). Positive forcing (more incoming energy) Warming effect Negative forcing (more outgoing energy) Cooling effect.
For how long will the positive effect continue?
Saturation around here Aber et al (1998) Bioscience
OVERALL CONCLUSIONS Elevated CO2 and N deposition are speeding up the th C cycle l and d iincrease sequestration t ti Elevated CO2 and N deposition p are NOT helping to moderate global change Hence the title: ‘good good for trees, bad for the planet’ The biosphere is helping us; but WE need to solve our own problems
Acknowledgements F. Magnani, M. Borghetti, D. Stevenson, T. van Noije, S. Raddi, J. Grace