Chemical Alterations Occurring During Biomass Charring and their Impact on Char Recalcitrance

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1 Chemical Alterations ccurring During Biomass Charring and their Impact on Char Recalcitrance Heike Knicker Instituto de Recursos Naturales y Agrobiología, CSIC, Sevilla, Spain

2 Impact of fire on soil organic matter Cambisol, Central Spain Burnt site 24 years after the last fire Unburnt site - Difference in color Difference in chemistry and soil properties - Changes in C-concentration (depends on fire conditions) - C-sequestration potential of soils - Soil organic matter (SM) quality

3 Solid-State Nuclear Magnetic Resonance (NMR) Spectroscopy Carboxyl /N-Alkyl ppm Advantages: - Non-destructive - Analysis of the bulk sample Desadvantage: - Low sensitivity

4 Cross Polarization Spin system: 1 H high sensitivity high abundance short T 1 Contact time Magnetization Transfer Spin system: 13 C low sensitivity low abundance long T 1 Polarization ppm CP-efficiency depends upon: - paramagnetic compounds or radicals - dipolar interaction between 1 H and 13 C (protonation degree of 13 C)

5 Common Model for Black Carbon (soot) Barbeque charcoal H - C - H - C - H C= C C C C C Atomic H/C: 0.48 H - C - H H - C - H? H C C C C HC H 2 C Lignite coke (Bloch Decay) H/C: 0.1 C H C C H C H (CH 2 ) 13 CH C H CH 3 CH 3 C 0.29 nm (according to Sergides et al., 1987) Atomic H/C: < 0.2 Electrical conductivity disables NMR spectroscopy Atomic H/C ratio is too low for barbeque charcoal

6 Atom. H/C H/C of Pyrogenic rganic Matter (PyM) Barbeque charcoal Charcoal after a forest fire Charcoal of peat Condensate, ash (BC-Model) 0.44 H/C PyM > H/C BC-Model Benz (a) anthracene Maximal cluster size for PyM: 4-6 rings (Knicker et al., 2005)

7 Chemistry of PyM Carboxyl /N-Alkyl Carboxyl /N-Alkyl Barbeque char C / N (atom) 400 Peat-PyM (350 C, 3 min) C / N (atom) 33 Wood-PyM (350 C, 12 min) 142 Grass-PyM (350 C, 4 min) ppm ppm (Knicker et al., Geoderma, 2007) PyM PyM ccurrence of Black N

8 Conclusion H - C - H H - C - H C = C C C C C H - C - H H - C - H C C H C C HCH 2 C C H H C C H C (CH 2 ) 13 C CH CH 3 H CH 3 C 0.29 nm

9 Charring of Lignin (5-20% of plant C) 0 s, 350 C C recovery: 79% 30 s, 350 C 75% 8 min, 350 C 45% Carboxyl /N-Alkyl (C-a,b,g) 13 8 min, 450 C 16 Atm. H/C ppm R C H 2 H H H C H Dehydroxylation - Demethylation - Cleavage of side chains - Cleavage of ether bonds and dehydroxylation of ring - Formation of biphenyls

10 Charring of proteins (1-20% of plants) Casein: 13 C NMR HNC-CH-R l NH 2 Carboxyl /N-Alkyl Atomic C/N 4.0 Pyridine Amide Nitrile Pyrrole Amino -260 * -347 * 15 N NMR HNC-R 350 C/4 min C: 66% Recovery: 450 C/4 min N: 51% Recovery: C: 29% N: 23% ppm ppm Potentially highly recalcitrant N-heteroaromatics

11 New conceptual model of PyM Lignin 141 Lignin backbone Benzofuran Cellulose H Pyrrole N H Benz (a) anthracene Proteins CH 3 H Hydroxytoluene CH N H 129 R ,5-Diketopiperazine H N N Indole CH 2 N H CH 3 54 H 2 C 21 Pyridine 123 H N N Imidazole Pyranone Structure depends on: source material charring intensity Knicker et al., rg. Geochem. 2008

12 Loss (%) Chemical xidation of PyM (60 C; K 2 Cr H 2 S 4 ; 6 h): C-Loss N-Loss CREC (chemical oxidation resistant elemental carbon) is also detected in non-char samples (Knicker et al., 2007)

13 Chemical xidation of Non-PyM 0 h Vi (Luvisol) Carboxyl /N-Alkyl 23% C tot 0 h Az1 (Cambisol under pine) Carboxyl /N-Alkyl 26% C tot 0 h Pine needles Carboxyl /N-Alkyl 15% C tot 2 h 6 h 13% C tot 6 h 9% C tot CREC: 18% C tot CREC: 12% C tot ppm ppm ppm 0-100

14 Chemical xidation Resistant Elemental C (CREC) Briquette Carboxyl /N-Alkyl Grass-PyM Carboxyl /N-Alkyl Az3 (burnt) Carboxyl /N-Alkyl CREC arom : 72% C tot CREC arom : CREC arom 33% C tot 20% C tot ppm ppm ppm xidation efficiency depends on the chemistry of the source material (Knicker et al., 2007, Geoderma 142, )

15 Chemical Stability of Lignin-BC Chemical oxidation (60 C; K 2 Cr H 2 S 4 ; 6 h): Carboxyl /N-Alkyl Carboxyl /N-Alkyl 350 C/4 min 57% of C t 450 C/4 min 91% of C t ppm ppm Increasing charring temperature increases chemical stability

16 Chemical Stability of Casein-BC Chemical oxidation (60 C; K 2 Cr H 2 S 4 ; 6 h): Carboxyl /N-Alkyl Carboxyl /N-Alkyl 350 C/4 min 13% of C t 13% of N t 450 C/4 min 82% of C t 78% of N t ppm ppm - BC and BN show comparable chemical stability - Underestimation of BC of N-rich biochars

17 Aternative Concept for Char Structure: Further Implications Potential for abiotic or biotic oxidation Degradation by lignin-degraders?

18 % % Biological Degradability of PyM 13 C (13 atm%) - 15 N (50 atm%) Charring, (1 min, 4 min; 350 C) + B horizon (low C-content) Average turnover time: 20 yrs (pine: yrs) n a short term, BN has a higher stability than BC (Hilscher and Knicker., in press) C recovery 1 min. charred 4 min. charred Incubation time (months) 15 N recovery Incubation time (months)

19 Impact of Regeneration Time on SM Composition (Pine, Cambisol, Central Spain, HF-treated) Char content: (1982) Control Carboxyl /N-Alkyl Aromatic C Alkyl C mg g Carboxyl /N-Alkyl Aromatic C Alkyl Burnt % 1 year 65 24% (2005) 24 years ppm

20 Degradation of Grass Char (48 days) PyM (1 min) fresh degraded - 9% of C tot for alkyl C +4% of C tot for carboxyl C/-aryl C ppm Hilscher et al Formation of -aryl C and carboxyl C Indication for oxidation

21 A Horizon of a Neolithic Paddy Soil (China, 6000 y) Carboxyl Aromatic Fossil rice High carboxyl C content (20%) xidized char A horizon, HF-treated Depth: 110 cm Leaching? Adsorption? Further degradation? ppm (Cao et al., 2006)

22 Parent Material of a Neolithic Paddy Soil ( 13 C NMR) Aromatic C mg g -1 High aromatic C content (40-50%) supports leaching C cm C2 > 174 cm Retention by interaction of polar group with mineral phase? ppm

23 Chronosequence: Ceasing Biannual Burning (Planalto, Brazil) Time after last fire: 2 years 5 years 1 years 22 years

24 13 C NMR Spectra of the A Horizon of a Leptsol from the Planalto (Brazil) Soil (1 year after last fire) Humic Leptosol (0-5 cm): 12.1% 0-5 cm Carboxyl /N-Alkyl No clear indication for PyM Fast degradation of PyM? (15-30 cm): 6.4% (30-45 cm): 3.7% cm cm Increase of aromatic C with depth CREC + correction factor Pyrogenic C ppm 0-100

25 PyC Stocks in Comparison with non-pyc (0-30 cm) kg m Alkyl C tot Alkyl PyC years Burning leads to C increase C increase is due to addition of carbohydrates (input of unburnt necromass of dead roots) New litter input masks PyM

26 PyM in Soils Liming effect! Fertilization! PyM Litter oxidation degradation 2 deficiency (Paddy field) leaching N, P, Ca, K, Mg Masking (Brazil, Planalto) Selective enrichment of PyM (Black Soils) Absorption to mineral phase, stabilization Accumulation (Loess) (Knicker, submitted)

27 Conclusions PyM PyM PyM represents a highly heterogenic mixture including Black Nitrogen PyM is not necessarily recalcitrant. Its stability depends upon environmental conditions Concept of char as a highly refractory soil fraction has to be revised Need for a better understanding: structure of PyM sequestration of PyM in soils impact of PyM on pedogenesis

28 Thank you for your attention!!!

29 T 1C Measurements Fresh Buyo Peat: 129 ppm (Aromatic C) A: 0.2 s (55%) B: 13 s (45%) ppm -Alkyl C 0 50s 100s 150s 200s 250s delay time Charred Buyo Peat (150s) (129 ppm): A: 2 s (60%) B: 61 s (?) (40%) A: 0.6 s (72%) B: 11 s (28%) Knicker et al., 2005