Chemical Alterations ccurring During Biomass Charring and their Impact on Char Recalcitrance Heike Knicker Instituto de Recursos Naturales y Agrobiología, CSIC, Sevilla, Spain
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
Solid-State Nuclear Magnetic Resonance (NMR) Spectroscopy Carboxyl /N-Alkyl 300 200 100 0-100 ppm Advantages: - Non-destructive - Analysis of the bulk sample Desadvantage: - Low sensitivity
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 200 100 0 ppm CP-efficiency depends upon: - paramagnetic compounds or radicals - dipolar interaction between 1 H and 13 C (protonation degree of 13 C)
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
Atom. H/C H/C of Pyrogenic rganic Matter (PyM) 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 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)
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) 8 300 200 100 0-100 ppm 300 200 100 0-100 ppm (Knicker et al., Geoderma, 2007) PyM PyM ccurrence of Black N
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
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 172 146 130 114 55 22 73 (C-a,b,g) 13 8 min, 450 C 16 Atm. H/C 1.1 0.8 0.8 0.6 300 200 100 0-100 ppm 115 115 135 145 R C H 2 H H H 115 148 C H 3 56 - Dehydroxylation - Demethylation - Cleavage of side chains - Cleavage of ether bonds and dehydroxylation of ring - Formation of biphenyls
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% 156 22 5.1-233 Recovery: C: 29% N: 23% 19 5.2 300 200 100 0 ppm-100 0-100 -200-300 -400 ppm Potentially highly recalcitrant N-heteroaromatics
New conceptual model of PyM Lignin 141 Lignin backbone Benzofuran Cellulose 152 162 108 117 156 H Pyrrole N H 122 120 134 Benz (a) anthracene Proteins 130 153 CH 3 H 115 4-Hydroxytoluene 121 111 CH 3 139 137 135 N H 129 R 124 157 2,5-Diketopiperazine H N 51 166 N Indole CH 2 N H 51 136 115 123 150 148 CH 3 54 H 2 C 21 Pyridine 123 H N N 123 125 136 122 112 Imidazole 128 156 143 145 117 Pyranone Structure depends on: source material charring intensity Knicker et al., rg. Geochem. 2008
Loss (%) Chemical xidation of PyM (60 C; K 2 Cr 2 2 + H 2 S 4 ; 6 h): 120 100 80 60 40 20 0 C-Loss N-Loss CREC (chemical oxidation resistant elemental carbon) is also detected in non-char samples (Knicker et al., 2007)
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 300 200 100 ppm 0-100 300 200 100 ppm 0-100 300 200 100 ppm 0-100
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 300 200 100 ppm 0-100 300 200 100 ppm 0-100 300 200 100 ppm 0-100 xidation efficiency depends on the chemistry of the source material (Knicker et al., 2007, Geoderma 142, 178-196)
Chemical Stability of Lignin-BC Chemical oxidation (60 C; K 2 Cr 2 2 + 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 300 200 100 0-100 ppm 300 200 100 0-100 ppm Increasing charring temperature increases chemical stability
Chemical Stability of Casein-BC Chemical oxidation (60 C; K 2 Cr 2 2 + 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 300 200 100 0-100 ppm 300 200 100 0-100 ppm - BC and BN show comparable chemical stability - Underestimation of BC of N-rich biochars
Aternative Concept for Char Structure: Further Implications Potential for abiotic or biotic oxidation Degradation by lignin-degraders?
% % 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: 59-79 yrs) n a short term, BN has a higher stability than BC (Hilscher and Knicker., in press) 120 100 80 60 40 20 0 120 100 80 60 40 20 0 13 C recovery 1 min. charred 4 min. charred 0 2 4 6 8 10 12 14 16 18 20 Incubation time (months) 15 N recovery 0 2 4 6 8 10 12 14 16 18 20 Incubation time (months)
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 -1 63 Carboxyl /N-Alkyl Aromatic C Alkyl 11 21 42 25 Burnt 14 32 33 18 30% 1 year 65 24% (2005) 24 years 59 13 28 37 22 300 200 100 0-100 ppm
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 300 200 100 0-100 ppm Hilscher et al. 2009 Formation of -aryl C and carboxyl C Indication for oxidation
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? 300 200 100 0-100 ppm (Cao et al., 2006)
Parent Material of a Neolithic Paddy Soil ( 13 C NMR) Aromatic C mg g -1 High aromatic C content (40-50%) supports leaching C1 160-174 cm C2 > 174 cm 5.8 2.0 Retention by interaction of polar group with mineral phase? 300 200 100 0-100 ppm
Chronosequence: Ceasing Biannual Burning (Planalto, Brazil) Time after last fire: 2 years 5 years 1 years 22 years
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% 15-30 cm 30-45 cm Increase of aromatic C with depth CREC + correction factor Pyrogenic C 300 200 100 ppm 0-100
PyC Stocks in Comparison with non-pyc (0-30 cm) kg m -2 25 20 15 10 5 0 20.1 19.3 17.7 7.4 7.1 6.7 4.3 4.4 3.5 2.0 1.7 1.6 -Alkyl 14.8 4.5 C tot Alkyl PyC 0 5 10 15 20 25 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
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)
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
Thank you for your attention!!!
T 1C Measurements 1.0 0.8 Fresh Buyo Peat: 129 ppm (Aromatic C) A: 0.2 s (55%) B: 13 s (45%) 0.6 0.4 0.2 0.0-0.2-0.4-0.6-0.8 74 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