Settling and resuspension Energetic factors (solar radiation and temperature) Nitrogen biogeochemical cycle

Transcription

1 Settling and resuspension Energetic factors (solar radiation and temperature) Nitrogen biogeochemical cycle

2 Physical processes: settling and resuspension In the aquatic environment: physical processes that determine the transport from the water column (e.g. phytoplankton settling out from the euphotic zone) to the benthic compartment and vice versa (e.g. nutrient resuspension) In the air: deposition and erosion due to the wind (we will not talk of these)

3 Physical processes settling HP of non-interacting (dilute solution), non-aggregated particles (not always true, e.g. in wastewater treatment plants). = 0 (assuming steady state) v Fg Fb F F F f b f m t F pvg g F b 2 C A fvg v d f Ff 2 F g v g 2 C p d A f f V Spherical particles v 4g p 3C d f f d

4 Physical processes sedimentation C d Other shapes (algae): Equivalent radius or correction factor Laminar flow, sphere: C d = 24 / Re Turbulent flow, cylinders: C d = 1 g v 18 v d 2 p Re = (d ρ f v) /μ p f f f dg Stokes s law

5 Physical processes settling Simpler relationship (no constants) m t sm s v h m suspended matter h mean system depth s rate of removal by settling s 1 3 p w

6 Physical processes resuspension Factors playing a role: Wind energy (speed U and fetch F) Waves (height H s and period T s =L/v) Energy in water (depth H and stress t) Sediment (critical shear stress t c depends on characteristics of the sediment, e.g. grain-size distribution and consolidation)

7 Physical processes resuspension If e is the quantity of resuspended sediments: For shallow water : e 0 se if t t 0 e t t c td con e dyne=10-5 N 2 t 0.003u where u is speed (cm/s) at 15 cm from the bottom and t=[dyne/cm 2 ] c 3 set t, c 2 2 g m, t dyne cm, e t 7 0 d u H T s s 100 sinh 2 H L It simulates wind but not current effects

8 Physical processes resuspension The characteristics of sediments are important

9 Physical processes energetic factors Solar radiation: main energy source for ecosystems (PP). Forcing for many ecological processes (photosynthesis, photolysis, meteo, evapotranspiration, etc.) E photon energy h Planck s constant f radiation frequency λ wavelength c light speed E = h*f = h*c / λ Earth s surface lowers the quality fo energy (photons are absorbed in the visible/uv and transmitted in the IR).

10 Physical processes energetic factors

11 Physical processes energetic factors Total energy % band Wavelength (μm) 1360 W/m 2 Solar constant 9% UV <0.12 Absorbed from O 2, N 2 at 100Km O 2 at 50Km 4% absorbed and reflected O 3 at Km (CFC) Partially from O % reaches earth and is reflected 41% visibile after being used and degraded by ecosystems 50% IR % absorbed and reflected from CO 2 and N 2 O at 10 Km (greenhouse)

12 Physical processes energetic factors Currency: W/m 2 BTU/ft 2 d=0.131 W/m 2 Langley/d=1 cal/cm 2 d=0.483 W/m 2 Kcal/m 2 h=1.16 W/m 2 cal/m 2 s=4.18 W/m 2 MJ/m 2 d=86.4 W/m 2 Einstein/m 2 s=mole of photons/m 2 s Cannot be converted in the above units. Used in PAR (9 moles of photons in the visible to fix 1 mole of oxygen)

13 Physical processes energetic factors Photoperiod: (function of day and latitude) P n, Solar declination: Angle between the line linking earth-sun and the equator plane 2arccos tg tg 360 (y) = cos(y) sin(y) cos(2y) sin(2y) cos(3y) sin(3y) cos(4y) sin(4y) cos(5y) sin(5y) Yearly angle: tg max tg max 1 y n n 365 max max (tg For higher latitude values P can be also 1 or 0. max tg max )

14 Physical processes solar radiation Mean daily solar radiation short waves depends on latitude, day (Hamon, 1954). C=0 (fraction of covered sky) But solar radiation can be easily measured: regression on real data: I n a bsin y

15 Physical processes solar radiation VENEZIA MANILA Heterogeneous cloud cover over the year

16 Physical processes solar radiation For models with short time scales: I t P In n cost 0.5, P n, t can vary between 0.5-P/2 and 0.5+P/2, if the day length is normalized to 1. I is the radiation intensity. I(n) mean daily radiation given from a + b sin(y)

17 Physical processes solar radiation Q in Q sc Q sr Q lc Q lr Q br Net radiation on the surface 2 sn sc sr sc Q Q Q C Q Net (i.e. substracted cloud reflection) short wave radiation C=fraction of covered sky Q C lc T a Q br T w Long wave emission from atmosphere to the surface C=fraction of covered sky T a =dry bulb air temperaturein F Heat emission from water (back radiation) =Stefan Boltzman constant T w =water temperature, in K

18 Nitrogen cycle

19 Biogeochemical cycles Chemical elements cycle within ecosystems Elements essential for life (Bio): about 30 Resources are not unlimited, so (re)cycling is fundamental for life Cycling through the different media (land, water, air) of the planet (Geo) Cycling is sustained by solar energy

20 Cycles: gaseous (N, O, C): atmosphere is a storage/reservoir and can play an important role in controlling the cycle (e.g. CO2 diluition). Resilient (but to what degree?) sedimentary (P): land is the main storage. Less resilient. Inorganic form solar energy producers Organic form Environment air consumers water land Inorganic form decomposers Organic form Modified from Calligaro & Mantovani, 2001

21 Nitrogen cycle Diatomic gas, inert (strong bond) 78% air volume 0,03 % Earth s crust Some interesting compounds: NH3 ammonia: bacterial degradation of organic nitrogenous substance; fertilizer (only few primary producers can exploit N2); industrial utilization NH 4 + ammonium HNO3 nitric acid: strong acid N2O, NO,N2O3, NO2, N2O4, N2O5: oxides Agriculture: salts of nitric acid (nitrate NO 3- ), e.g. NaNO3 o KNO3; calcium cyanamide(cacn2) NO 2 - nitrite: salts of nitrous acid (HNO2); preservatives (e.g. ham); toxic effects (bind to hemoglobin reducing the transport of O2 blue-baby syndrome; combined with amines generate carcinogenic compounds, e.g. stomach)

22 Why the nitrogen cycle? N is found in DNA, vitamins, aminoacids, proteins, clorophyll: key molecules for biochemical and ecological processes N is abundant in the atmosphere (gaseous cycle) but fixation is difficult Consequently, N is often a limiting nutrient for PP Some compounds are toxic Eutrophication (coastal zones: toxic algae, tourism, fisheries, etc.)

23 The nitrogen cycle is complex because of the several roles that N can play: oxidation numbers from III (ammonia) to +V (nitrate) CO 2 O 2 ph Alk NO 2 NO DISSOLVED 4 H 2 CO 3 + NH 4 CO SEDIMENT 3 HCO 3 - DISSOLVED INORGANIC 4 CO 3 2- DETRITUS PARTICULATE 7 PO 4 3- DISSOLVED PHYTOPLANKTON PERIPHYTON ZOOPLANKTON 16 FISH BENTHOS 14 N 2 PRIMARY PRODUCERS 16 SECONDARY PRODUCERS PROCESSES: 1. Reareation 2. Settling 3. Burial 4. Chem. Equilibrium 5. Oxidation 6. Denitrification 7. Mineralization 8. Hydrolysis 9. Ad / De sorption 10. Uptake 11. Fixation 12. Excretion 13. Respiration 14. Essudation 15. Grazing 16. Predation ORGANIC 2 2

24 Atmospheric N: Abundant but inert: before being used by living organisms, it must be fixed. N 2

25 Biological fixation of N: bacteria, blue-green algae, bacteria in symbiosis with plants (es. legume roots), N is fixed into organic N. N2 has a very stable triple bond and symbiotic bacteria are needed to break it. The plant then absorbs ammonium or nitrates (soluble) based on the fixed ammonium. N 2

26 Abiotic N fixation: combustion in engines (high T), lightnings, industrial processes. Biological fixation is much more important than fertilizers and the abiotic fixation. Crop rotation is based on it (future: fixation also on rice, maize, etc? Self-regulating cycle?). N 2

27 Assimilation: plants assimilate (uptake) ammonium or nitrates (soluble) created based on fixed ammonium or ammonia (nitrification). N 2 NO 2 (aq) NH 4 (aq) NO 3 (aq) diffusion diffusion assimilation NH 4 (s) NO3 (s) assimilation

28 Food chain: nitrogen enters the cycle flowing from primary producers to the trophic network of consumers. N 2 NO 2 (aq) NH 4 (aq) NO 3 (aq) diffusion diffusion assimilation NH 4 (s) NO3 (s) assimilation

29 Decomposition and ammonification (mineralization): bacteria decompose organic substance (excretion/waste of living organisms, dead organisms, etc.) so that organic nitrogen is released (e.g. urea, amines, aminoacids, proteins) and then is ammonified (ammonia ammonium ion equilibrium) by bacteria or fungi N is recycled N 2 Norg(aq) NO 2 NH 4 (aq) (aq) NO 3 (aq) diffusion diffusion assimilation NH 4 (s) NO3 (s) assimilation

30 resuspension Sedimentation, resuspension e burial: in aquatic ecosystems, organic nitrogen can interact with sediments and quit from the system. The same for nitrates (burial). Also in terrestrial environments there can be exports: for example the pollution of groundwate or surface water bodies (runoff-leaching). N 2 Norg(aq) sedimentation Norg(s) assimilation NO 2 NH 4 (aq) (aq) diffusion diffusion NO 3 (aq) NH 4 (s) NO3 (s) burial assimilation

31 resuspension denitrification: nitrates are transformed into atmospheric N by denitrifier bacteria and the cycle is closed. N can also go into the atmosphere through volatilization of NH3, or in the case of combustion (fires damage forests because they reduce nutrients and fixation bacteria) N 2 N 2 O Norg(aq) sedimentation Norg(s) assimilation NO 2 NH 4 (aq) (aq) diffusion diffusion NO 3 (aq) NH 4 (s) NO3 (s) denitrification burial assimilation

32 Ammonia volatilization NH 3 (gas) + H 2 O NH 3 (aq) + H 2 O NH 4 + (aq) + OH - (aq) Also called stripping. In waters or soils with high ph (es. water bodies in summer, ph is increased by photosynthetic production). Low T slow the reaction down. Process used also in wastewater treatment (low flows), e.g. adding lime to increase ph.

33 N mineralization: ammonification Proteins and other forms of organic N are decomposed into ammonium. Bacteria degrade nitrogen compounds and incorporate the produced ammonium they need. The excess is released as ammonium ions. Norg(aq) NH 4 (aq)

34 N mineralization: nitrification oxidation of the NH 4 + ion into the NO 2 ion (nitrosomonas) and after that, in some cases, into the NO 3 ion (nitrobacter). Chemo-autotrophic bacteria (they use the exothermic reactions of oxidation of ammonium and nitrites as energy source and CO 2 as carbon source) Norg(aq) NO 2 NH 4 (aq) (aq) NO 3 (aq)

35 Biological processes N cycle Nitrification 1. Nitrosomonas NH d NO d 4 1.5O2 NO2 H 2O 2 N NH dt N NO dt 2 O k 2. Nitrobacter NO NH 4 NO 2 k 1 2 N NH NO 3 N NO 4 2 H k2 k 1 Ammonia oxidation is the limiting process, but for high temperature

36 Biological processes N cycle optimum ph for nitrification = 8-9. Strong dependence on alkalinity NH + 4 oxid. NO oxid. 2 % of maximum rate ph

37 Biological processes N cycle NITRIFICATION temperature influences the maximum reaction rate Exponential growth: C Constant: Zero: C No reaction: T>50 C Other limiting factors: moisture (bacteria need it to survive); Monod kinetics (Y cell yield coefficient gvss/gn, μ rate of bacterial growth)

38 Denitrification reduction process taking place in anoxic conditions. NO 3 acts as an electron acceptor and can be reduced to N 2 or to one of the following: N 2 O, NO, NO 2 N 2 N 2 O denitrification Norg(aq) NO 2 NH 4 (aq) (aq) NO 3 (aq) NO 3 (s)

39 Biological processes N cycle Denitrification NO 3 sost org.subst. H2O N CO 2 2 Facultative bacteria: they can adapt their metabolism to use nitrates as electron acceptors when oxygen lacks anoxic environment is needed Consumption of organic substance (several configurations of wastewater treatment plants to sustain the process without discharging ammonia): potential limiting factor

40 Biological processes N cycle Denitrification NO 3 sost org.subst. H2O N CO 2 2 Process rate increased by the kind of substrate (es. methanol vs sewage). Denitrification takes place with ph 6-10, optimum O2 inhibits the process (limiting factor Ks O2 / (Ks O2 + O2) ) but generally not a problem in wastewater treatment plants: it is consumed by facultative bacteria to oxidate organic substance

41 Phytodepuration in wetlands diffused pollution Nitro-denitro: based on N oxidation and then oxygen absence. Such contrasting conditionsare found in the roots of common reed (Phragmites). Sediments do not contain O2, roots bring O2 into the sediments N removal processes in wetlands Nitrification (rhizosphere, bacterial films, oxidized sediment zone) Denitrification (bottom) Ammonia volatilization (ph >8) Plant uptake (ammonium, nitrates: limited) Burial interactions with sediments (ion exchange)

42 NOx and atmospheric deposition Biosphere-atmosphere exchange (engine combustion, etc.) and viceversa (dry deposition, wet dep. or acid rains: NOX + H2O HNO3). Goulding (1990): deposition alone can lead to higher-than-ec-limit-for-drinkable-waters nitrate levels in drainage water in South East UK.

43 BIOGEOCHEMICAL CYCLES N 2 NO 3 - NO 2 - N in phytoplankton N in zooplankton N in fish NH 4 + N in detritus N in sediments

44 BIOGEOCHEMICAL CYCLES N 2 NO 3 - Accumulation = Input Ouput ± Reaction NO 2 - N in phytoplankton dnno 3 Q NNO in 3, in Q NNO out 3, out k NNO k NNO A den 3 nit 2 dt

45 Processes of the N cycle N fixation: soil microorganisms in symbiosis with some plants (e.g. legume) fix atmospheric nitrogen and transform it into organic nitrogen N accumulation: (bacterial uptake) heterotrophic soil organisms convert ammonia to organic compounds (proteins). ammonification: decomposition of proteins and other forms of organic N into ammonium N. nitrification: oxidation of NH 4 + ion to NO 2 ion (nitrosomonas) and then to NO 3 ion (nitrobacter). Chemotrophic bacteria using exotermhic oxidation reactions as energy source and CO 2 as carbon source. denitrification: reduction process taking place in anoxic conditions. NO 3 acts as electron acceptor and it can be reduced to N 2 or become: N 2 O, NO, NO 2. ammonia volatilization: process taking place in soils or waters with high ph NH 4 + NH 3 in atmosfera.

46 Nitrogen bioavailable forms and measures Bioavailable forms - Organic: - Dissolved - Particulate - Inorganic: - Ammonia - Nitrite - Nitrate N measures Total nitrogen o TKN (Total Kjeldhal Nitrogen) = organic N + ammonia N: Preliminary conversion of organic N to ammonia with acid digestion (H2SO4 e K2SO4) and then ammonia is measured with Nessler s reagent. Organic nitrogen. As above, but after distilling ammonia. Nitrates: spectrophotometer (for example)

47 References and further readings ANPA, 2002 Linee guida per la ricostruzione di aree umide per il trattamento di acque superficiali Burt, Heathwaite e Trudgill, 1993 Nitrate: processes, patterns and management Calligaro, L., Mantovani, A., Fondamenti di chimica per ingegneria Kadlec R.H., Knight R. L., Treatment wetlands, Lewis Publishers, CRC Press, Inc.