Human perturbations to the global Nitrogen cycle

Transcription

1 Human perturbations to the global Nitrogen cycle Lecture for Biogeochemistry and Global Change Edzo Veldkamp

2 The pace of human caused global change has increased in modern history, but none so rapidly as industrial production of nitrogen. % of extent seen in late 1980 s 75% 50% 25% deforestation CO 2 release Human population Industrial N-fertilizer (Vitousek et al, 1997)

3 Nitrogen -Essential component of proteins, genetic material, chlorophyll, etc. -Before human activities, nitrogen was scantily available -78% of earth s atmosphere is Nitrogen, but most plants and animals cannot use N 2 directly - Instead they can only use nitrogen in fixed form (NH 4 and NO 3 )

4 History of Nitrogen late 18th century: named by Jean Claude Chaptal mid 19th century: role in crop production recognized (von Liebig) late 19th century: biological N fixation discovered early 20th century: Haber-Bosch process invented

5 History of Nitrogen Haber-Bosch Process N is discovered Biological N-fixation N is nutrient Humans (billions) Haber-Bosch, Tg N (Galloway & Cowling, 2002)

6 Gaseous losses N fixation N 2 N 2 O NH 3 Industrial Symbiotic Residues, Manures NO 3 - Soil Organic Matter Organisms NO 2 - NH 3 Leaching NH 4 + Clay minerals

7 Natural Nitrogen sources -N-fixing organisms (free living and symbiotic) Tg of N / yr (1 Tg = 1 Gt = ton) Lupine is an example of a legume that form a close relationship with N-fixing bacteria such as Rhizobia

8 Natural Nitrogen sources -Lightning (Probably less than 5 Tg N)

9 Human-driven Nitrogen sources -Industrial fixation of fertilizer approx. 80 Tg / yr

10 Human-driven Nitrogen sources

11 Human-driven Nitrogen sources -Nitrogen fixing crops (legumes, but also rice) approx. 40 Tg / yr

12 Human-driven Nitrogen sources -Fossil fuel burning releases fixed N from long-term geological formations high temperature combustion also fixes small amount of atmospheric N directly more than 20 Tg / yr

13 Human-driven Nitrogen sources -Mobilization of stored N from biological pools Biomass burning: more than 40 Tg / year

14 Human-driven Nitrogen sources -drainage of wetlands, followed by oxidation of peat. More than 10 Tg / year

15 Human-driven Nitrogen sources -Land clearing for agriculture followed by oxidation of SOM. More than 20 Tg

16 Denitrification 130 Biological fixation 140 Global N-Cycle (in Tg) Human activities 90 lightning <5 River flow Internal Cycling (Schlesinger, 1994)

17 Comparison of natural and human-driven Nitrogen sources 160 Terrestrial N-fixation Tg N yr Range of estimates of natural N fixation N fertilizer Fossil fuel 20 Legume crops Year Galloway et al., (1995)

18 Global N budget in 1890 lightning rivers biomass fossil fuel natural ecosystems legumes rice animals oceans (Galloway & Cowling, 2002)

19 lightning Global N budget in 1990 rivers biomass fossil fuel natural ecosyst. Haber Bosch legumes rice animals oceans (Galloway & Cowling, 2002)

20 Impacts of increased N Global atmospheric deposition (mg N m -2 yr -1 ) of reactive nitrogen in 1993 (Galloway & Cowling, 2002)

21 Impacts on atmosphere Increased emissions of nitrogen based trace gases such as N 2 O, NO and NH 3. N 2 O: greenhouse gas, Ozone layers in stratosphere NO: precusor of acid rain and photochemical smog

22 Global N 2 O budget Tg N y Natural sources Ocean 3.0 (1-5) Tropical forest soils 3.0 ( ) Tropical savanna soils 1.0 ( ) Temperate forest soils 1.0 ( ) Temperate grasslands 1.0 ( ) Anthropogenic sources Agricultural soil 3.3 ( ) Biomass burning 0.5 ( ) Industrial Sources 1.3 ( ) Cattle and feedlots 2.1 ( ) Σ 16.2 ( ) Sinks Atmospheric Increase 3.9 ( ) Soils? Stratospheric Sink 12.3 (9-16) Σ 16.2 ( ) (IPCC, 2002)

23 Global NO budget Tg N y Natural sources Lightning <5 Tropical savanna soils 2.0 Tropical forest soils 1.0 Anthropogenic sources Biomass burning 15 Fossil fuels >20 Agricultural soils 2.3 Sum ~50

24 Nitrogen saturation Nitrogen saturation occurs when the vegetation can no longer respond to further additions of nitrogen

25 Nitrogen saturation Build-up of NH 4 + => increasing formation of NO 3 - (+H +!!) Leaching of NO 3 - leads to leaching of basic cations Progressive acidification => mobilization of Al (toxic!) Result: damage to tree roots, nutrient imbalance, fish killing

26 The Nitrogen cascade (Galloway & Cowling, 2002)

27 Effects on biodiversity and species mix Fertilization leads to dominance of fertilizer responsive species. E.g. in Netherlands, species rich heathlands are converted into species poor grasslands and forests. Species-poor ecosystems may be more vulnerable to drought, etc.

28 Example of heathland

29 Future prospects N-fertilizer use The world s growing population needs more food production which requires large quantities of N fertilizer Prediction: by 2020 global production of N fertilizer will increase from currently 80 Tg / yr to 134 Tg / yr (Smil, 2002)

30 Future prospects Reactive N created in 1995 and at time of max. human population (about 9 billion people) (Galloway & Cowling, 2002)

31 N 2 O and NO losses (% of applied fertilizer Future prospects N 2 O NO Banana Plantation Agriculture in temperate areas (Veldkamp & Keller, 1997)

32 Choice of diet Management options a= vegetarian diet; b= carnivorous diet (Galloway & Cowling, 2002)

33 Choice of diet Management options Feed conversion (FC in kg of feed/kg of edible weight) and Protein conversion efficiency (PCE in %) for different animal products: FC PCF Milk: % Carp: % Eggs: % Chicken: % Pork: % Beef: % (Smil, 2002)

34 Management options Increase efficiency of applied nitrogen Kg grain / kg N (Cassman et al., 2002)

35 Conclusions Human activities have doubled the natural rate at which N enters the land based N cycle Serious environmental consequences are already apparent (trace gases, acidification) N input can be reduced by changing diet and increasing the efficiency It is urgent that policies address the N issue, slow the pace of this global change and moderate its impacts.

36 Cited literature: Cassman KG, Dobermann A, Walters DT, (2002). Agroecosystems, nitrogen-use efficiency and nitrogen management. Ambio 31 (2): Galloway, JN and Cowling, EB (2002). Reactive Nitrogen and the world: 200 years of change. Ambio 31 (2): IPCC, Climate change 2001: the scientific basis. Contribution of working group 1 to the third assessment report of the IPCC. Cambridge University Press, Cambridge, UK and New York, 881p. Schlesinger WH, (1997). Biogeochemistry, an analysis of global change. 2nd edition Academic Press, San Diego, California, 588 p. Smil V, (2002). Nitrogen and food production: proteins for human diets. Ambio 31(2): Veldkamp E, Keller M, (1997). Nitrogen oxide emissions from a banana plantation in the humid tropics. Journal of Geophysical Research 102 (D13): Vitousek PM, Aber J, Howarth RW, Likens GE, Matson PA, Schindler DW, Schlesinger WH, Tilman D, (1997) Human alteration of the global nitrogen cycle: causes and consequences. Issues in Ecology, No 1, 15p.