Biogeochemistry of C

Transcription

1 Biogeochemistry of C NREM 665 CO 2 CO 2 CH 4 diffusion & ebullition CO 2 Organic-C accretion DOC CH 4 methanogenesis

2 C Biogeochemistry I.Forms of C in WTLs & Coastal Ecosystems Molecule Oxidation State Form/Phase Carbon dioxide, CO 2 Methane, CH 4 Carbonate, CO 2-3 Bicarbonate,HCO - 3 Carbonic Acid, H 2 CO 3 Operationally-defined forms of C: Dissolved org. C (DOC) <0.7 µm Particulate t org. C (POC) >0.7 µm Plant biomass, microbial biomass, litter, soil C 2

3 Carbonate Buffer System CO 2 + H 2 O H 2 CO 3 H + + HCO 3-2H + + CO 3 2-3

4 II. Aerobic conditions: PSN & Resp. dominant rxns A. 6CO H 2 O + light C 6 H 12 O 6 + 6O 2 + 6H 2 O B. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + energy C. C storage = Production Decomposition 4

5 Ratio of aboveground to belowground biomass can range from 0.2 to 3.9 (Gopal & Massing 1990)

6 1. Decomp inhibited in flooded soils high C storage in WTLs, SGBs 2. NPP, Decomp, C storage vary across WTL types a. due to climate, hydrology, veg type, soil type, nutrient availability, microbial community/activity, etc. b. lability of FW marsh litter TSM litter 3. Litter/detritus processing in WTLs/SGBs is complex: involves microbes, shredders, deposit feeders, filter feeders, release of DOC & POC 6

7 Decomposition of Freshwater Marsh Vegetation (Mitsch & Gosselink 1993) 7

8 Sagittaria latifolia (arrowhead, duck potato) Pontederia cordata (pickerelweed) 8

9 Nuphar advena (yellow pond lilly) 9

10 Decomposition of Salt Marsh Vegetation (McKee & Seneca 1982) 10

11 Spartina alterniflora (saltmarsh cordgrass) Juncus roemerianus (black needlerush) 11

12 Plant N&P Soil organic matter accumulation CO 2 CH 4 Decomposition Bioavailable N & P Hydrology Electron acceptors Nutrients Rainfall Evapotranspiration External loading Regulators of Decomposition (Reddy & DeLaune 2008) 12

13 Detrital plant biomass Water tab ble Detritus Peat } }Decomposition } }Burial } }Compaction Decomposition & burial of OM (Reddy & DeLaune 2008) 13

14 III. Anaerobic conditions: Fermentation (Ferm) & Methanogenesis (MTG) dominant A. Ferm occurs when organic matter is TEA in anaerobic respiration 1. C 6 H 12 O 6 2CH 3 CH 2 OCOOH (Ferm) G o = 239 kj mol C 6 H 12 O 6 2CH 3 CH 2 OH + 2CO 2 (Ferm) G o = 122 kj mol -1 14

15 B. MTG occurs when methanogens use CO 2 (or other simple C compounds) as TEA in respiration & produce CH 4 1. CO 2 + 8e - + 8H + CH 4 + 2H 2 O 2. CH 3 COO + 4H 2 O 2CH 4 +2HO 2 3. MTG rates: a. summer >> winter in temperate zones, less seasonal var. in tropics b. ƒ(flood freq. & duration, pres/abs of veg) c. permanently flooded >> intermittently flooded 15

16 4. MTG in FW WTLs >> MTG in tidal WTLs? a. b. C flux in GA salt marsh vs WI lake sed (M&G 2000) 16

17 IV. CH 4 oxidation: Methanotrophs convert CH 4 to methanol, formaldehyde, & CO 2 CH 4 CH 3 OH HCHO - HCOOH CO 2 1. CH 4 depth oxidized by surface ( CH 4 flux) 17

18 2. Net CH 4 emission = ƒ(methanogenesis methane oxidation) a. rates of CH 4 emission for different wetland types 18

19 19

20 20

21 Long-term Accumulation of Organic Matter in Selected Wetlands Wetland Location C Accumulation (g m -2 yr -1 ) Reference Everglades Florida Reddy et al Typha sp. Cladium sp Salt Marsh Louisiana Hatton et al Mangrove Mexico 100 Twilley 1992 Cypress Swamp Georgia 45 Cohen 1974 Peatland Alaska Billings 1987 Bog Wisconsin Kratz & Dewitt

22 3. C sinks in WTLs a. Short-term: t immob, herb. plant & algal l production b. Long-term: woody production, C seq. in soil/sediment V. C Cyling in Tidal WTLs vs FW WTLs A. Tidal WTLs have high prod & frequent tidal exchange export higher amounts of C than FW WTLs 22

23 VI. Case Study: Are Phragmites-dominated wetlands a net source or net sink for greenhouse gases? (Brix et al. 2001) 23

24 CO 2 photosynthesis C cycling in wetlands is CO 2 CH 4 diffusion & ebullition respiration CO 2 metha thane-oxidation Organic-C accretion DOC CH 4 methanogenesis 24

25 CO 2 & CH 4 dynamics in a Phragmites- wetland Net CO 2 -assimilation CH 4 -emission CO 2 -emission (Brix et al. 2001) 25

26 CO 2 Phragmites wetland: Units: mol m -2 year -1 photosynthesis 98 s 45 4 CO 2 CH 4 diffusion & ebullition n CO 2 methane ane-oxidation 49 respiration14 DOC CH 4 methanogenesis Organic-C accretion 18 (Brix et al. 2001) 26

27 Phragmites C budget: Net CO 2 Assimilation: 98 mol m -2 yr -1 CO 2 Emission: 45 mol m -2 yr -1 CH 4molm Emission: m yr Net C Fixation: = 49 mol m -2 yr -1 Ratio CH 4 Emission:Net C Fixation:

28 The ratio of CH 4 released to annual net C fixed varies generally from 0.05 to 0.13 (Whiting & Chanton 1997) 28

29 Question: Are wetlands a net source or a net sink for greenhouse gases? Answer: Depends on the ratio of CH 4 emitted to annual net C fixed & time scale evaluated. 29

30 Greenhouse effect as a function of time scale 0.20 Ratio between CH 4 emitted & net C fixed SOURCE 0.05 SI INK Wetlands may be sources of GHGs on short time scales (<10 yr) but sinks over longer time scales (>100 yrs). Lower the ratio, sooner a site becomes a sink (Brix et al. 2001). 30

31 VII. C Biogeochemistry Conclusions A. Major processes Production: Fixation of C from atmosphere via PSN Decomposition: Conversion of complex molecules into simpler molecules via activities of aerobes & anaerobes Fermentation: Use of organic matter as TEA in anaerobic respiration Methanogenesis: Bacteria use CO 2 as TEA in anaerobic respiration CH 4 Oxidation: Methanotrophs convert CH 4 to methanol, formaldehyde, & CO 2 in aerobic surface soil zones B. Although WTLs cover ~ 7% of Earth s land area, they have a disproportionate effect on global C cycle 31