Geochemical Cycles PDF

Geochemical cycles Carbon Nitrogen Phosphorous Benjamin Campforts Carbon cycle Has a geological and a biological and now also a human component Geological carbon cycle Most rocks consist of silicate minerals Silicate minerals contain oxides of silicium...

Geochemical cycles Carbon Nitrogen Phosphorous Benjamin Campforts Carbon cycle Has a geological and a biological and now also a human component Geological carbon cycle Most rocks consist of silicate minerals Silicate minerals contain oxides of silicium (you don’t have to memorize these formulas!) Minerals weather chemically http://marlimillerphoto.com/chemical.html Geological carbon cycle Weathering of silicate minerals on land (e.g. wollastonite): CaSiO3 + 3H2O + 2CO2 → Ca2+ + 2(HCO3-) + H 4SiO4 → 2 molecules of CO2 withdrawn from the atmosphere Right hand components transported in solution to the ocean, where (due to formation of shells, corals etc.): Ca2+ + 2(HCO3- ) → CaCO3 + H2O + CO 2 → 1 molecule of CO2 released to the atmosphere Note: there can be a *very* long time between wollastonite weathering and calcite (calcium carbonate) formation Thus, weathering leads to sequestration (burial, removal) of carbon from the atmosphere into calcium carbonate (shells of animals) which is finally converted to limestone. But : formation of limestone as such releases CO2 Best illustration: volcanic islands (CaSiO3) and coral reefs (CaCO3) CO2 CO2  Geological carbon cycle CO2 in deposited carbonate may be released again through metamorphism when sediments are compressed and temperature rises, e.g.: CaCO3 + SiO2 →CaSiO3 + CO2 But a lot of it also gets buried in sediments on the ocean floor and will end up being subducted: the carbon is incorporated in the astenosphere (the layer below the lithosphere) It is then released again into the atmosphere by volcanic eruptions Again time scale between deposition and re-release can be *very* long Over time considerable amounts of CO2 have been sequestered in carbonate rocks http://www.carleton.edu/departments/geol/DaveSTELLA/Carbon/long_term_carbon.htm  What about carbonate minerals ? http://academic.emporia.edu/aberjame/struc_geo/primary/prim30.jpg Geological carbon cycle While weathering of silicates leads to C sequestration, this is not the case for carbon minerals, the weathering of which is, over the long term, carbon neutral: CaCO3+CO2+H2O →Ca2++2(HCO3-) The right hand components are again transported in dissolved form to the sea where again calcium carbonate is formed by organisms and precipitation: Ca2++2(HCO3-) →CaCO3+CO2+H2O But Weathering of carbonates has not been ‘in equilibrium’ over Earth’s history We see important periods over accumulation of carbonates → periods of intense weathering and CO2 consumption and decline of CO2 concentration in the atmosphere Similarly, periods of intense volcanism lead to emission of CO2 and rise of CO2 concentration in the atmosphere Variations in weathering rates/volcanism are key driver of long- term temperature evolution of the Earth Remember wikipedia  Biological carbon cycle on land Plants take up carbon through photosynthesis which leads to production of glucose (sugar) and oxygen: energy(sunlight) + 6CO2 +6 H2O → C6H12O6 + 6O2 Memorize these formulas! Plants, animals, bacteria release carbon through respiration from the plants and from within the soil when they use the glucose to drive their metabolism C6H12O6 (glucose) + 6O2 → 6CO2 + 6 H2O +energy Respiration does not only take place by plants but also by heterotrophs (animals living from plants and animals living from animals) and decomposers (bacteria and funghi etc. that decompose dead organic matter)  Biological carbon cycle on land The total amount of photosynthesis is gross primary production (GPP) The difference between photosynthesis (GPP) and respiration by plants is Net Primary Production (NPP): NPP results in storage of C in plants The difference between GPP and respiration by plants, heterotrophs and decomposers (all elements of ecosystem) is Net Ecosystem Production (NEP): a positive NEP indicates that the ecosystem stores C. Friedlingstein et al. (2019). Careful: graph from 2019 – numbers have changed slightly since Let us make a little exercise to understand this carbon cycle better Does human breathing contribute to global warming? 8 billion people 900 g CO2 per day How much does human breathing contribute A human respires ca. 900 g of CO 2 per day Thus 8E9*9E2*3.65E2= annual g of CO 2 respired by the whole human population in a year= ca. 2.6 Pg of CO2= 2.6*12/(12+16*2)=0. 7Pg of C Question: does this contribute to global warming ? Note What is the difference between 1 Pg and Gt C? Friedlingstein et al. (2019). Careful: graph from 2019 – numbers have changed slightly since  Human impact (Le Quéré, 2018) Fluxes are relatively small compared to reservoirs Changes in fluxes are even smaller Emission of 8.9-9.9 Pg (1015g) C due to burning of fossil fuels and cement production (1Pg=1Gt) Emission of 1.4-2.0 Pg C due to land use change Carbon emissions: a global picture https://www.visualcapitalist.com/cp/mapped-carbon-dioxide-emissions-around-the-world/ Carbon emission due to land use change In some areas effect of LU change is now positive These relatively small human changes have tremendous impact ! As of September 2024: 423ppm (NOAA: (NOAA Global Monitoring Laboratory); pre-industrial: ca. 280 ppm The carbon budget is not easily closed Emission of 8.9-9.9 Pg (1015g) due to burning of fossil fuels and cement production Emission of 1.5 (0.8-2.2) Pg due to land use change Sequestration of 0.5 Pg due to boreal forest regrowth 2.4 Pg absorbed by oceans Thus theoretical net flux to the atm: 9.4+1.5-0.5-2.4=8 Pg But: net flux to the atmosphere = only ca. 4.7 Pg So, where is the rest (2.5-3.9 Pg) ?= the missing sink 1990: unidentified sink 2020: The C cycle can only be closed if a residual land sink is assumed Missing sink Implies that there is a component in the Earth System that ‘absorbs’ a significant fraction of the C that humans release into the atmosphere Is a reaction on the out-of-equilibrium situation created by human emissions Understanding it is of crucial importance to predict long-term evolution of C content of the atmosphere  Fertilisation of terrestial biota Plants provide in energy demands by conversion of CO2 to energy containing carbohydrate (glucose) through photosynthesis: 6 CO2 + 6 H2O + Energy ---> C6H12O6 + 6 O2 The plants burn some of the glucose for their own energy provision: this is called respiration C6H12O6 + 6 O2 →6 CO2 + 6 H2O + Energy The difference between photosynthesis and respiration is net primary production (NPP) Thus, if presence of CO2 increases, NPP may increase if no other elements are limiting because water can be used more efficiently (stomate need to be open for a shorter time) In reality not so simple ! FACE (Free-Air Carbon Dioxide Enrichment) experiment FACE experiment Generally, there is an increase in yield, biomass etc. but… Effect on plant growth is dependent on species and on CO2 level due to the complexity of plant life It is therefore not surprising that results also show a high variability at ecosystem level Also for crops, this yield increase is noted (although it may have been overestimated in the past: greenhouse vs. FACE exps.)  FACE experiments FACE experiments: on average a clear increase in biomass production, but results are highly variable. Effect of doubling CO2 is ca. 15-25 % on average There is also an increase in agricultural crop yields (10-15 %) CO2 effects are mainly due to indirect effect: increased water use efficiency (less stomatal opening necessary to keep desired internal CO2 concentration Response of terrestial ecosystems to increased CO2 is probably primary mechanism explaining ‘missing sink’ in carbon cycle: this is a prime example of a negative feedback cycle  Negative feedback Atm CO² - + Plant Growth What do we know until now ? Increased CO2 pressure leads to increased plant growth Plant can more easily capture CO2 it needs from the atmosphere Thus, stomata need to be opened for a shorter time period Plant has two ‘choices’: grow to the same size with less water or grow taller with the same amount of water The latter is only possible if no other factor is growth limiting (e.g. availability of nutrients: so, deposition of nitrogen may also help)  Nitrogen Life needs reactive nitrogen (Nr): nitrogen oxides and ammonium Why: building blocks of amino acids (proteins), building blocks of life Plenty of non-reactive nitrogen (N2) Production of Nr requires energy Reactive nitrogen (NOx(nitrous oxide) and NHx (ammonium (NH4+) and ammonia(NH3)) is produced by a limited number of processes: Free and symbiotic bacteria and cyanobacteria that live in the soil (free or in symbiosis with leguminous plants: ammonium production through ammonification or mineralization) Atmospheric processes (lightning, meteorite impacts…) produce a limited amount of reactive NO x The nitrogen cycle is quite complex We talk about reactive N: ammonium (NH4+), ammonia (NH3), nitrous oxides (NO, NO2, N2O) and related molecules In various compartments of the environment, nitrification (transformation of ammonium (NH x) to nitrates (NOx), oxidation) and denitrification (transformation of nitrates (NO x) to N2 and N2O, reduction) can take place Nitrification: releases energy Denitrification: takes place in hypoxic or anoxic environments: requires energy (taken by bacteria from carbohydrates, but they need the oxygen from the nitrate to ‘burn’ the carbohydrates) The nitrogen cycle The nitrogen cycle Human production of reactive nitrogen (ammonia, NH3, (Tg=1012g) has dramatically increased over last 70 yrs Drivers of the N cycle in 1850 and 1995 Grain Production Meat Production Energy Production A different representation Gruber&Galloway, Nature, 2008 Fowler et al. (2015) predict that also natural N fixation will go up this century: why ?  Most important changes Extra production of reactive N by Haber-Bosch process (now ca. 120 Tg (1012g, mainly for fertilizer) Much more production of reactive N on agricultural land (NHx) and through combustion of fossil fuels (NOx) Much more deposition of reactive N on land and on sea Much more input of reactive N by runoff into marine system Implications of changes are largely unknown ! The Fate of Haber-Bosch Nitrogen : most of the human- produced N ends up in the environment N Fertilizer N Fertilizer N N N N Produced Consumed in Crop Harvested in Food Consumed 100 94 47 31 26 14 -6 -47 -16 -5 -12 14% of the N produced in the Haber-Bosch process enters the human mouth……….if you are a vegetarian. Galloway JN and Cowling EB. 2002 The Fate of Haber-Bosch Nitrogen: most of the human- produced N ends up in the atmosphere N Fertilizer N Fertilizer N N N N Produced Applied in Crop In Feed in Store Consumed 100 94 47 31 7 4 -6 -47 -16 -24 -3 4% of the N produced in the Haber-Bosch process and used for animal production enters the human mouth. Galloway JN and Cowling EB. 2002 Geography matters Deposition of reactive N Reactive N is released in various compartments of the environment Nr and Agricultural Ecosystems 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. We do not only fertilize agricultural land: up to 30 kg/ha of dry Nr deposition in Europe http://fate.jrc.ec.europa.eu/modelling/nutrients.html Stikstofdepositie is één van de oorzaken van ‘het stikstofprobleem van Nederland (en Vlaanderen)’. https://youtu.be/UcDM80wf7-Q Nr and Terrestrial Ecosystems N is the limiting nutrient in most temperate and polar ecosystems Nr deposition alters ecosystem function by accelerating nutrient cycling: Increased nitrogen availability can lead to shifts in plant species composition, enhance soil microbial activity, and promote leaching of nutrients like nitrate, which may negatively affect water quality. Nr additions probably decrease biodiversity across the entire range of deposition Nr and Freshwater Ecosystems Surface water acidification (formation of nitric acid, HNO3) Tens of thousands of lakes and streams Biodiversity losses Nr and Coastal Ecosystems Increased algal productivity Shifts in community structure Harmful algal blooms Degradation of seagrass and algal beds Formation of nuisance algal mats Coral reef destruction Increased oxygen demand and hypoxia Increased nitrous oxide (greenhouse gas) Sybil Seitzinger, 2003 Consequences: hypoxia, algae, acidification, smog, pollution of surface waters and dead zones in oceans (see below)…. Nr and the Atmosphere NOx emissions contribute to OH, which defines the oxidizing capacity of the atmosphere NOx emissions are responsible for tens of thousands of excess-deaths per year in the United States O3 and N2O contribute to atmospheric warming N2O emissions contribute to stratospheric O3 depletion Phosphorus Often limiting factor for soil fertility Major impacts on eutrophication Phosphorus P is heavy (atomic mass: 15): no major gaseous P pool (almost no P in atmosphere) Mostly found in rocks (apatite, Ca(PO4)3(OH, Cl, F)), minor source= guano(=bird poop) Nutrient taken up by plants and also by consumers and decomposers Transported in runoff (dissolved and attached to sediments) A very important element for human life: DNA, RNA, ATP, bones, shells Phosphate mine Historic guano mining dinafem.org Phosphorous budgets and cycle (Tg) How much P do we use ? Sattari et al. : no realistic scenario points to dramatic growth of P demand Why does demand does not rise more quickly ? Slow release of P reserve in soils: that reserve is due to overfertilisation in the past Predicted P recovery Will we run out of P ? Be nice to Morocco ! Van Kauwenberghe, IDFC  a reserve base of 189 Gt (148-211 Gt) containing 36 Gt P2O5 (26-48 Gt P2O5), https://www.fertilizer.org/news/three-things-to-know-about- world-phosphate-rock-resources-and-reserves/#_ftn2  78 -100Mt 2050 42-49 Mt 2020 What does that mean? 36Gt reserve base. Current use=89Mt/yr Lifespan 36E9/89E6=ca. 405 y (!) Important elements P recovery from soils likely to rise: demand for P will therefore not rise exponentially P is, to a large extent, relatively easily recoverable: losses to environment can be drastically reduced Known reserves are still enough for many centuries to come A P crisis is not imminent but P, like N, may lead to eutrophication of surface waters Leeswijzer: Geochemical cycles Te kennen: slides Hoofdstuk 12 (355-357), 22(668-670): Niets extra te kennen, eventueel stukjes over C en P cyclus lezen ter aanvulling van slides