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Causes of bulk carbon and nitrogen isotopic fractionations in the products of vegetation burns: laboratory studies
Chemical Geology 152 Ž1998. 181–192
Causes of bulk carbon and nitrogen isotopic fractionations in the products of vegetation burns: laboratory studies Vaughan C. Turekian ) , Stephen Macko, Donna Ballentine, Robert J. Swap, Michael Garstang Department of EnÕironmental Sciences, The UniÕersity of Virginia, CharlottesÕille, VA 22903, USA
Abstract Bulk stable isotope analysis is a means for the characterization of the sources of carbonaceous and nitrogenous material aerosols derived from biogenic sources. In order to use stable isotope techniques for characterizing the products of vegetation burns the isotope effect of combustion must be known. The C 3 vegetation Colospherum mopane and Eucalyptus sp. and the C 4 vegetation Cenchris cilliarus, Antephora pubesence and Saccharum officinarum, were burned under controlled conditions in the laboratory in order to better understand how the process of combustion affects the isotopic fractionation of the produced material. Carbon isotopes for aerosol particles formed during controlled laboratory burns of C 3 vegetation were higher in d13C by 0.5‰ compared to the source vegetation. Aerosol particles captured above the controlled laboratory burns of C 4 vegetation were lower in d13C by 3.5‰ compared to the source vegetation. The proposed causes for the different isotope effects shown for C 3 and C 4 sourced products are differences in the oxidation chemistry of these two plant types. Aerosol particulate material and ashes produced during the controlled laboratory burns of the vegetation are higher in d15 N than the source vegetation by 6.6‰ and 2.5‰, respectively. Furthermore, d15 N values for the residual material produced when Eucalyptus sp. samples were heated at discrete temperatures, suggest that different pools of nitrogenous compounds are accessed at different temperatures of heating. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Bulk stable isotope analysis; Vegetation burns; Aerosol particles
1. Introduction Biomass is burned on virtually all of the continents during various times of the year ŽHao and Liu, 1994.. Burns can be started naturally, as the result of lightning strikes or by volcanic ignition, or can be anthropogenically sourced. The latter can be the
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consequence of local tribal hunting activity or may be associated with larger scale agricultural or land use policies ŽCrutzen and Andreae, 1990.. Fires release volatile organic compounds, inorganic gases and particles, all of which are either transported away from the burn site or recirculated locally. These biogenically derived compounds influence the nutrient balances of ecosystems both near to and removed from the burn region ŽCrutzen and Andreae, 1990.. An understanding of the isotopic alterations that take place during combustion is needed in order
0009-2541r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 1 0 5 - 3
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to use stable isotopes as source indicators of aerosols. This present study addresses the bulk carbon and nitrogen isotopic fractionations associated with controlled laboratory vegetation burns. The sources of organic aerosol compounds and associated gases found in the atmosphere as a result of biomass burning events can be characterized using stable isotope analyses. The application of isotope techniques to tracing the products of vegetation burns requires knowledge of the isotopic properties of the original vegetation and isotope fractionation effects of combustion on the components derived from the burn. Smith and Epstein Ž1971. have shown that there is a carbon isotopic ratio difference between C 3 and C 4 plants, with the C 4 plants being enriched in 13 C relative to C 3 plant materials. This distinction results from the characteristic pathways of carbon fixation used by these two plant types ŽO’Leary, 1988.. This isotopic separation between C 3 and C 4 vegetation provides the basis for the first order isotopic characterization of aerosol particulate matter or volatilized compounds derived from these vegetation sources. In field studies it has been shown that there are, however, carbon isotopic changes in the products of combustion ŽCachier et al., 1985.. Studies of carbonaceous particles resulting from industrial and biogenic combustion compared to wind blown biogenic material not affected by fires, indicates the utility of analyzing the d13 C values of both the aerosols as well as vegetation to asses both source and process ŽCachier et al., 1986.. Carbonaceous gases emitted during biomass burning include CO 2 , CO, methane and non-methane hydrocarbons ŽLobert and Warnatz, 1993; Ward, 1993., with the majority of the carbon released in the gaseous form as CO 2 ŽWard, 1993.. Highly efficient combustion can result in production of CO 2 which is in excess of 90% of the total carbon transformation ŽWard, 1993.. Less efficient combustion lowers the CO 2 production relative to other carbonaceous gases to as low as 50% with the other principal gas constituents becoming CO and methane, as well as other trace volatile hydrocarbons ŽWard, 1993.. Particulate matter produced during combustion consists of the recombination of volatile compounds above the burn resulting from the condensation of these compounds onto existing aerosols ŽCocks and
McElroy, 1984. as well as the elevation of thermally altered plant debris ŽCachier et al., 1985; Lobert and Warnatz, 1993; Ward, 1993.. The carbon isotopic signature of these particles will depend on the relative importance of these two sources and the physical and chemical alterations that affect these carbon pools as a result of the combustion process. Nitrogen isotope ratios in vegetation are not so much influenced by biosynthetic differences as by the environment of growth. For example, a marked difference in d15 N exists between marine and terrestrial biota. This information has been used to decipher diets of ancient humans and other organisms ŽHeaton et al., 1986; Koch et al., 1995.. In terrestrial environments arid habitat vegetation is enriched in 15 N relative to vegetation growing in habitats with normal precipitation ŽHeaton et al., 1986.. Isotopic characterization of organic nitrogenous compounds q and inorganic nitrogen ions such as NOy 3 and NH 4 , collected during studies of wet deposition of aerosols, indicate that organic nitrogen tends to be higher in 15 N compared to the inorganic nitrogen ŽCornell et al., 1995.. If partitioning occurs between volatile inorganic nitrogen-bearing compounds and residual organic compounds containing nitrogen, as can occur during the process of biomass burning and transport, the nitrogen isotope signatures of the transported components may provide an improved understanding of the origin of the aerosols. This information can lead to a better knowledge of the influence of combustion on the production and transport of nitrogenous compounds. Combustion of vegetation, and the subsequent deposition of ashes, is crucial to recycling limiting nutrients such as nitrogen, within a forested ecosystem. Many of the studies addressing the fate of nitrogen during vegetation burns have focused on the production of the residual material. Amino acids in ash samples from different intensity burns ŽDeBano et al., 1979. indicate that the relative concentrations of these compounds is diminished in the residual material with fire temperatures above 1808C. The bulk nitrogen isotopic fractionations associated with this nitrogen loss, however, have not been measured. As amino acids are the primary nitrogen containing compounds in vegetation, the partial destruction of amino acids should result in isotopic fractionation between the source vegetation and the residuum.
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2. Methods The experiments designed to answer the above questions are of two types: Ž1. C 3 and C 4 vegetation were combusted and bulk carbon and nitrogen isotope analysis were performed on the residual material and the elevated particles; Ž2. carbon and nitrogen isotopic changes during different temperatures of combustion were investigated.
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diately above the burn on a pre-ashed Ž5508C for 2 h. glass fiber filter ŽGelman Type AE. connected to a hand held filter tower coupled to a low volume pump. Ash samples represent the total residual material left on the foil following the burn. The vegetation ash and filter samples were wrapped in pre-ashed aluminum foil, placed in ultra-clean plastic bags which were then heat sealed and stored in the freezer until chemically and isotopically analyzed.
2.1. Preparation of leaf material for combustion 2.3. Protocol for CO2 release Leaf samples of C 3 and C 4 vegetation were lyopholized and ground to powder using a Wiley Mill with a a60 mesh. Eucalyptus sp. samples collected from central California and Colospherum mopane vegetation from Africa represented the C 3 vegetation for this study. The C 4 vegetation was represented by the African grasses Cenchris cilliarus and Antephora pubesence as well as the agricultural crop Saccharum officinarum Žsugar cane. from southern Africa. These species are commonly exposed to burning as a result of natural events or land clearing for agricultural use. 2.2. Combustion protocols for bulk aerosol and ash studies In order to determine the effect of combustion on the isotopic fractionation between the source material and the products, controlled laboratory burns were performed. Approximately 10 mg of the dried vegetation samples were placed on pre-ashed Ž5508C for 2 h. aluminum foil and ignited. Experiments under both flaming and smoldering conditions were performed. Oxygen gas was supplied as needed through a gas regulator in order to sustain flaming combustion. Thermocouple readings taken above the flaming combustion showed temperatures in excess of 6008C. The combustion event lasted until all the vegetation had been exposed to the flame. Smoldering burns were performed under atmospheric conditions without the addition of excess oxygen. These lower intensity smoldering combustions had temperatures below 2008C. Particulate matter from both primary emissions and rapid secondary production was sampled imme-
Incomplete combustions of vegetation were performed in order to determine if the carbon isotopic signature of CO 2 produced during burns depended on the degree of combustion and on the amount of carbon converted to CO 2 . Eucalyptus sp. and S. officinarum were weighed and placed in pre-ashed Ž8508C for 2 h. quartz tubing with copper and copper oxide. The tubes were evacuated to remove any ambient gas, and sealed. Samples were each heated at a constant temperature for 1 h. Eucalyptus sp. samples were heated at temperatures of 1508C, 3508C and 6508C and the sugar can samples were heated at 2508C, 4508C and 6508C. Samples of each vegetation were also heated at 8508C for 2 h and allowed to cool slowly following the Dumas sealed tube combustion technique described by Macko Ž1981.. These samples were assumed to represent a complete conversion of the organic carbon to CO 2 gas. In each of the lower temperature combustions, the amount of CO 2 released was compared to the potential amount of CO 2 that would form with complete oxidation. The CO 2 was purified and isolated cryogenically with liquid nitrogen. For each temperature, the amount of carbon converted to CO 2 was determined at each temperature by manometer deflection and represented as percent conversion relative to the 8508C combustion. Isotopic analysis was performed on the CO 2 gas as described below. Incomplete combustion of organic matter results in the formation of CO. The mass overlap between the CO and the N2 complicates nitrogen isotopic analysis. For this reason only the completely combusted samples were analyzed for their stable nitrogen isotopic compositions.
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2.4. Protocol for incomplete combustion: for nitrogen isotope analysis Nitrogen isotopic fractionation depends on the types of compounds accessed during heating and the isotopic distribution within these compounds. To determine how different temperatures of combustion affect the isotopic distribution of ash material produced by the combustion of organic material, Eucalyptus sp. samples were exposed to varying degrees of controlled heating in a combustion furnace. Samples were placed on pre-ashed aluminum foil boats, weighed and placed in the combustion furnace at a known temperature. After heating the sample was reweighed and stored for further analysis. Vegetation has a complex array of nitrogenous compounds. The accessing of different compounds within this array should depend on the intensity and duration of heating. In order to determine how different temperatures and times of heating affect the availability of the different nitrogen pools and the isotopic signature of the residual material, two sets of Eucalyptus sp. combustions were performed. One group was exposed to heating for 10 min per sample and the other sample set was exposed for 30 min per samples. The temperatures of combustion for both sets were 1408C, 1608C, 1858C, 2058C, 2158C and 2258C. These higher temperatures represent temperatures at which DeBano et al. Ž1979. observed a decreased amino acid content of the residual material.
200 ml glass distilled dichloromethane as the solvent. Approximately 5 ml of the dichloromethane with the dissolved lipid extract was dried over copper oxide and placed in a pre-ashed quartz combustion tube and prepared for isotopic analysis as described below. 2.6. Bulk isotope analysis For isotope analyses, samples from each type of combustion were placed in ashed quartz tubes with copper and copper oxide. The tubes were evacuated, sealed and combusted off line at 8508C for 2 h and allowed to cool slowly, using a modified Dumas technique ŽMacko, 1981.. This leads to the conversion of carbonaceous compounds to CO 2 and nitrogenous material to N2 gas. A cryogenic separation of the CO 2 and the N2 was performed by a standard laboratory technique. The d13 C and d15 N values relative to the isotopic standards Peedee Belemnite ŽPDB. and atmospheric nitrogen respectively, were obtained using a VG Prism Series II isotope ratio mass spectrometer. Isotope ratios are defined as a per-mil measurement reported in the following manner: dY s
Ž R samplerR standard . y 1
P 10 3 Ž ‰ .
where Y s13 C or 15 N with the corresponding R s13 Cr12 C or 15 Nr14 N. By definition the standards, N2 for 15 N and PDB for 13 C, have values of 0‰.
2.5. Lipid fraction extraction Lipids represent a more refractory component of vegetation ŽSun and Wakeham, 1994.. In an attempt to see if there is a relationship between photosynthetic pathway and bulk carbon isotopic signature of the lipid fraction for the vegetation used in this carbon isotopic study, isotope analysis was performed on the extracted lipid fraction and compared to the bulk vegetation. The lipid fraction from the filters was separated and also analyzed in order to determine how the process of combustion affected the isotopic signature of this labile material. The lipid fraction of S. officinarum, Cen. cilliarus, A. pubesence and Col. mopane vegetation were isolated using a Soxhlet extractor operating for 16 h using
3. Results and discussion 3.1. Carbon isotopes Two-way analysis of variance with the independent variable being the photosynthetic pathway of the source vegetation and intensity of burn indicated that the only significant parameter responsible for isotopic fractionations was the plant type ŽC 3 or C 4 .. Particles captured above controlled burns of C 3 have D13 C Ž D s d product y d source . values between y0.6‰ to 1.8‰ compared to the burned material with an average d13 C enrichment of 0.5‰ relative to the unburned plant. Controlled combustion of C 4 vegeta-
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tion results in particles with D13 C relative to the source vegetation ranging from y0.9‰ to y7.0‰. with an average d13 C depletion of y3.5‰ compared to the source vegetation ŽTable 1.. Carbon isotope values for carbonaceous particles captured during field burns of C 4 vegetation terrains ŽCachier et al., 1985. agrees with the results of this laboratory study. However, the laboratory fractionations are less pronounced than these reported in the field study. The d13 C enrichments in the products from the C 3 vegetation combusted in this present study contradict the assumption by Cachier et al. Ž1985. that the combustion of C 3 and C 4 vegetation types results in the same isotope effect, i.e., depletion. A confirmation of the different behaviors of C 3 and C 4 vegetation during burning is found in the d13 C signatures of CO 2 during incomplete combustions of Eucalyptus sp. and Saccharum officinarum ŽTable 2.. The d13 C of the CO 2 produced during different degrees of incomplete combustion of C 3 vegetation show no variation relative to the d13 C of the CO 2 produced during complete combustion ŽFig. 1.. Incomplete combustion of the C 4 vegetation shows a curvilinear relationship between percent carbon released as CO 2 compared to the total carbon in the plant. At lower levels of oxidation, more enriched 13 C is seen in the CO 2 ŽFig. 2.. The highly enriched d13 C value of y8.0‰ may be caused by Table 1 d13 C values for vegetation and the corresponding particle matter produced during controlled combustions Sample
C 3 r Flamer Vegetation d 13 C Ash C4 smolder particle
Eucalyptus a Eucalyptus b Eucalyptus b Colospherumc Colospherumc Colospherumc Saccharumd Saccharumd Cenchris e Cenchris e Antephorae Antephorae
C3 C3 C3 C3 C3 C3 C4 C4 C4 C4 C4 C4
flame flame smolder flame smolder smolder flame smolder flame smolder flame smolder
y29.2 y27.7 y27.7 y24.4 y24.4 y24.4 y12.9 y12.9 y12.9 y12.9 y13.3 y13.3
y27.4 y26.2 y27.3 y23.8 y24.6 y25.0 y16.2 y13.8 y19.9 y16.8 y15.0 y17.5
y27.9 y27.0 y27.0 y23.8 y24.4 y24.0 y12.9 y12.6 y14.7 y12.7 y13.8 y13.5
Samples collected from: a central California, b southern California, c southern Africa, d South Africa, e Namibia.
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Table 2 d13 C values for CO 2 formed during the incomplete combustion of Eucalyptus sp. and Saccharum officinarum % of total C converted to CO 2
d 13 C CO 2
d 13 C Residual Žcalculated. a
Eucalyptus sp. 0.2 47.3 88.5 100.0
y27.6 y27.5 y27.7 27.6
y27.6 y27.7 y27.7 –
Saccharum officinarum 3.0 51.5 68.0 100
y8.0 y12.1 y12.1 y12.9
y13.1 y13.2 y14.4 –
a 13
d C vegetations ŽŽ% C as CO 2 .r100.PŽ d13 C CO 2 .qŽ1yŽ% C as CO 2 .r100.PŽ d13 C residual.x.
the more efficient oxidation of a small, highly 13 Cenriched pool of carbon within the Saccharum officinarum. The d13 C values of non-oxidized, residual matter during incomplete combustion of C 4 vegetation are able to be calculated using mass balance ŽTable 2.. These different isotope effects associated with C 3 and C 4 vegetation combustion must be explained. One possible explanation is that C 3 and C 4 vegetation have distinct proportions of ‘labile’ Žoxidizable and volatile. and refractory carbon compounds, each with characteristic carbon isotopic signatures. The difference in behavior during burning, then, would depend upon the amount of labile carbon lost during combustion. An alternative possibility is that there are isotopic differences between the same constituents Žlabile and refractory. within C 3 and C 4 vegetation relative to their bulk d13 C values. Nadelhoffer and Fry Ž1988. have shown that the 13 d C of bulk leaf material is a function of the fractional contributions of refractory compounds such as lipids and lignins and more ‘labile’ Žoxidizable. starches, proteins, sugars and holocellulose Žthe acid soluble fibers of the vegetation. each with their own d13 C signals. Lipids and lignins tend to be lower in 13 C content relative to the bulk leaf whereas starches, proteins, sugars and holocellulose tend to be enriched in 13 C values relative to the bulk leaf. If the labile carbon component Ži.e., starches, proteins, sug-
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Fig. 1. d13 C of CO 2 for the incomplete combustion of Eucalyptus sp.
ars and holocellulose. represents a greater fraction of the carbon compounds in C 4 vegetation than in C 3 vegetation, then the D13 C between the CO 2 resulting from the partial combustion of the leaf material, and the source vegetation would be greater for a C 4 source than a C 3 source As a result the aerosol particulate carbon and the residual ash would be depleted in 13 C relative to the source for C 4 vegeta-
tion and enriched in 13 C relative to the source for C 3 vegetation ŽTable 1.. Fig. 3a and b present a hypothetical model illustrating the resulting differences between the fractionation processes involving C 3 and C 4 vegetation. The carbon isotopic values for the different compounds are based upon laboratory analysis of the lipid fraction combined with isotope trends observed by other
Fig. 2. d13 C of CO 2 for the incomplete combustion of S. officinarrum.
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Fig. 3. Ža. Proposed oxidation chemistry and corresponding carbon isotopic fractionations for the combustion of C 4 vegetation. Žb. Proposed oxidation chemistry and corresponding carbon isotopic fractionations for the combustion of C 3 vegetation.
authors ŽNadelhoffer and Fry, 1988.. Chemical alterations for the different compounds within the vegetation matrix that take place during combustion, will affect the isotopic signatures of the products Žgases, particulate matterial and residual matterial.. The preferential conversion of more labile compounds to CO 2 and other forms of volatile carbon that do not rapidly condense in the sample plume, would result in the residual material having an isotopic signature that represents the average of the remaining, more
refractory compounds. If the refractory nature of lipids is responsible for the less efficient volatilization and oxidation of these compounds over other oxidizeable compounds within the vegetation, the residual material and the aerosol particles will isotopically resemble the lipid compounds more than they would resemble the isotopic signal of the bulk plant material. If, however, there is volatile loss and combination of these lipid compounds into aerosol particulates during combustion, the residue of C 4
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Table 3 d13 C values for the total lipid fraction extracted from C 3 and C 4 vegetation and the corresponding aerosol particles produced during combustion Vegetation
d 13 C Vegetation
d 13 C Lipid
Aerosol particle lipid
Eucalyptus Colospherum Cenechris Saccharum Antephora
y27.6 y24.4 y12.9 y12.9 y13.3
y29.6 y24.5 y19.0 y18.4 y21.8
– y23.9 y21.9 y21.0 y21.8
burning will have higher d13 C and the aerosols on the filters will have a lower d13 C than the source vegetation. Collister et al. Ž1994. have shown that the total lipid fraction of C 4 vegetation have more depleted d13 C signatures compared to the source vegetation than is observed in C 3 vegetation. Carbon isotopic values for total lipid fraction from the vegetation in this study, also indicate a larger difference in d13 C between lipids and bulk vegetation for the C 4 pathway vegetation than for the C 3 vegetation ŽTable 3.. Additionally, Ballentine et al. Ž1996. have looked at carbon isotopic values for specific fatty acids extracted from the same vegetation used in this present study. Their findings indicate that there is greater carbon isotopic fractionation between the fatty acid fraction and the source vegetation for the C 4 vegeta-
tion of this study than is observed for the C 3 vegetation of this study. The carbon isotope analysis of total lipids extracted from the aerosol filters over these combustion experiments indicates that the isotopic signal is conserved or only slightly altered during the combustion process. Therefore, if aerosol particles from the combustion of C 4 vegetation are enriched in lipids relative to other compounds within the source vegetation there would be a decrease in d13 C in the aerosol relative to the bulk plant material. The lipid data for the C 3 Col. mopane shows little isotopic fractionation between the lipid compounds and the bulk vegetation, indicating that a preferential enrichment of the lipid fraction in the aerosol particles compared to the source vegetation would not seriously change the of the aerosol or the residue. Carbon isotope profiles ŽFigs. 4 and 5. for fresh Eucalyptus sp. exposed to varying temperatures and times of combustion show no clear isotope effect. This further supports the hypothesis that the refractory and labile carbon pools for the C 3 vegetation used in this study, have similar carbon isotopic distributions. 3.2. Nitrogen isotopes Bulk nitrogen isotope data for the aerosol particles collected over the controlled laboratory burns of the vegetation have an average d15 N enrichment of
Fig. 4. d13 C for the residual material from the step heating Ž10 min. of Eucalyptus sp.
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Fig. 5. d13 C for the residual material from the step heating Ž30 min. of Eucalyptus sp.
6.6‰ compared to the source vegetation and ranged from y1.3 to 13.1‰ different from the source plant ŽTable 4.. The residual material from these burns have a smaller d15 N enrichment than the aerosols, of 2.5‰ when compared to the parent vegetation. Further, a much smaller range in the isotopic fractionation is observed ŽTable 4.. Statistical analysis Žtwoway analysis of variance. indicates no significant difference, thus the nitrogen isotopic fractionations for both the aerosol particles and the residual material are independent of photosynthetic pathway, original d15 N of the bulk vegetation and burn intensity. A mass balance relating the nitrogen isotopic distribution of the source vegetation to the nitrogen isotopic signature of the products of combustion is given as: d15 N vegetations X d15 N residual material q Y15 N aerosol particles q Zd15 N volatile nitrogen Where X, Y and Z are the fractional contribution of the different products. In this equation, the volatile nitrogen fraction represents the nitrogenous compounds not captured on the filter, such as the gases NH 3 , NO x and N2 O which are produced during combustion of organic material ŽLogan, 1983..The
isotopic enrichment of the particles and the residual material illustrated in Table 3 must be balanced by a 15 N depletion in the volatile nitrogen. The magnitude of this 15 N depletion depends upon the combination of the d15 N enrichment of the residuum and the aerosol particles and the relative contributions of these terms to the total mass of nitrogen in the system. The depletion in 15 N of the nitrogenous gases, and the simultaneous enrichment in 15 N of the captured particulates and ash, is likely due to the preferential destruction of the bonds involving the light isotope of nitrogen including the release of nitrogenous gases during deamination of amino acids. DeNiro and Hastorf Ž1985. have proposed this mechanism to describe the results of low temperature diagenesis of organic material. They found that the residual nitrogenous material was enriched in d15 N compared to the source material. Based on this observation these authors concluded that there must be preferential loss of 15 N depleted nitrogen from the organic pool. This lost nitrogen was assumed to be derived from deamination of amino acids. Based on these above findings, it can be inferred that Ž1. ash material produced during biomass burning would have less total nitrogen compared to the source material and Ž2. the nitrogen isotopic signature of the residual material would be enriched in
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Table 4 d15 N values for vegetation and the corresponding combustion products Sample a
Eucalyptus Eucalyptus a Colospherumb Colospherumb Saccharumc Saccharumc Cenchris d Cenchris d Antephorad Antephorad
C 3rC 4
FlamerSmolder
Vegetation
C3 C3 C3 C3 C4 C4 C4 C4 C4 C4
flame smolder flame smolder flame smolder flame smolder flame smolder
6.4 6.4 3.3 3.3 6.9 6.9 10.0 10.0 6.7 6.7
d15 N Particle 13.0 19.5 2.0 4.7 12.8 15.1 22.7 15.9 17.7 9.8
Ash 8.3 8.5 8.2 5.3 9.2 6.9 14.3 12.3 9.7 8.7
Samples collected from: a southern California, b southern Africa, c South Africa, d Namibia.
15
N compared to freshly fallen leaf material. This latter statement implies that it is possible to better calculate the relative contribution of litter and burn products to the biogeochemical cycling of nutrients within forested ecosystems. Fresh Eucalyptus sp. samples exposed to varying temperatures and times of combustion show a bimodal distribution of the nitrogen isotopic signature ŽFigs. 6 and 7.. The Eucalyptus sp. samples exposed to heating for 30 min showed a greater range in the isotopic signature than the samples heated individually for 10 min. The bimodal nature of the nitrogen isotopic distribution in the residual material, points
to the accessing of different nitrogen pools during different levels of heating. The initial enrichment observed in the residual material could be caused by the volatilization of free ammonia within the vegetation ŽShelp et al., 1985., or the deamination of free amino acids. With increasing temperatures this pool of nitrogen within the vegetation, becomes enriched in 15 N as 14 N is preferentially lost through kinetic isotope effects. With increasing temperatures, the continued loss of nitrogen from the system comes preferentially from the 15 N component of this nitrogen pool. This would result in a d15 N signature of the residual material that reflects the d15 N of the
Fig. 6. d15 N for the residual material from the step heating Ž10 min. of Eucalyptus sp.
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Fig. 7. d15 N for the residual material from the step heating Ž30 min..
fresh vegetation. At higher temperatures, the second pool of nitrogen is accessed and a 15 N enrichment in the residual material is observed. This second pool of nitrogen could be representative of bound amino acids which require the increased heat in order for the combined hydrolysis, deamination reactions to take place. The range in d15 N for the residuum is greater for the samples exposed to 30-min heatings than for those samples heated for 10 min. The similar pattern of the isotopic distributions for both sample sets indicates that the availability of two or more distinct pools of nitrogenous compounds is independent of heating time. Despite the similarity in isotopic trends, the data indicate that increasing heating times results in a greater spread of the nitrogen isotopic signature of the residual material. Furthermore, increased heating times are correlated to the loss of material. Clearly the trend of enrichment of 15 N in the residue is controlled by the extent of access to the total vegetation by burning.
4. Conclusion The combustions of C 3 and C 4 vegetation yield different degrees of change in d13 C for all components of the system. Despite the differences in the
isotope effects, products of C 3 combustion are distinguishable from the products of C 4 burns through the use of bulk carbon isotopes. The combustion of C 3 vegetation yields aerosols, residue and CO 2 with d13 C values which show very small differences. The combustion of C 4 vegetation, on the other hand, results in carbon isotopic fractionation in d13 C for the three components measured. The differences in the behavior are linked to the abundances of easily oxidizable fractions relative to refractory fractions in the two vegetation types. The fractionations also are a result of the differences in the isotopic composition of lipids,and possibly other compounds within the plant matrix, relative to bulk vegetation for C 4 vegetation that is absent in the C 3 vegetation of this study. The combustion of organic material results in nitrogen in aerosol particles and residual material having higher d15 N than the source vegetation. These fractionations may allow for the isotopic characterization of combustion-derived nitrogenous material compared to elevated unburned litter. The bimodal relationship between heating temperature and d15 N of the residuum indicates that there are distinct pools of nitrogen that are accessed as a function of temperature. The evolution of burning plumes over time and space could result in the formation and scavenging
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of nitrogenous and carbonaceous compounds. Presumably any scavenging of particulate matter can lead to additional isotopic fractionations of carbon and nitrogen within the plumes. These are important considerations when extrapolating the results of this laboratory study to field observations. In conclusion, whereas carbon isotopes are unambiguously useful in characterizing the different sources of combustion-derived organic matter, nitrogen isotopes record a complex environmental and burning history and therefore may be more limited in their application to the problem of source determination in biomass burning. References Ballentine, D.C., Macko, S.A., Turekian, V.C., 1996. Variability of stable carbon isotopic compositions in individual fatty acids from combustion of C 4 and C 3 plants: implications for biomass burning. Chemical Geology, Isotope Geoscience 152, 151–161. Cachier, H., Buat-Menard, P., Fontugne, M., 1985. Source terms and source strengths of the carbonaceous aerosol in the tropics. Journal of Atmospheric Chemistry 3, 469–489. Cachier, H., Buat-Menard, P., Fontugne, M., Chesselet, R., 1986. Long-range transport of continentally-derived particulate carbon in the marine atmosphere: evidence from stable isotope studies. Tellus B 38, 161–177. Cocks, A.T., McElroy, W.J., 1984. The absorption of hydrogen chloride by aqueous aerosols. Atmospheric Environment 18, 1471–1483. Collister, J.W., Rieley, G., Stern, B., Eglinton, G., Fry, B., 1994. Compound specific d13 C analyses of leaf lipids from plants with differing carbon dioxide metabolisms. Organic Geochemistry 21, 619–627. Cornell, S., Rendell, A., Jickells, T., 1995. Atmospheric inputs of dissolved organic nitrogen to the oceans. Nature 376, 243–246. Crutzen, P.J., Andreae, M.O., 1990. Biomass burning in the tropics: impact on atmospheric chemistry and biogeochemical cycles. Science 250, 1669–1678.
DeBano, L.F., Eberlein, G.E., Dunn, P.H., 1979. Effects of burning on chaparral soils: I. Soil nitrogen. Soil Science Society of America Journal 43, 504–509. DeNiro, M.J., Hastorf, C.A., 1985. Alteration of 15 Nr14 N and 13 Cr12 C ratios of plant matter during the initial stages of diagenesis: studies utilizing archaeological specimens from Peru. Geochimica et Cosmochimica Acta 49, 97–115. Hao, W.M., Liu, M.H., 1994. Spatial and temporal distribution of tropical biomass burning. Global Biogeochemical Cycles 8, 495–503. Heaton, T.H.E., Vogel, J.C., von la Chevallerie, G., Collet, G., 1986. Climatic influence on the isotopic compositions of bone Nitrogen. Nature 322, 822–823. Koch, P.L., Heisinger, J., Moss, C., 1995. Isotopic tracking of change in diet and habitat use in African elephants. Science 267, 1340–1343. Lobert, J.M., Warnatz, J., 1993. Emissions from the combustion process in vegetation. In: Crutzen, P.J., Goldammer, J.G. ŽEds.., Fire in the Environment: The Ecological, Atmospheric and Climatic Importance of Vegetation Fires. Wiley, New York. Logan, J.A., 1983. Nitrogen oxides in the troposphere: global and regional budgets. Journal of Geophysica Research 88, 10785– 10807. Macko, S.A., 1981. Stable nitrogen isotope ratios as tracers of organic geochemical processes. PhD Thesis, University of Texas, Austin. Nadelhoffer, K.J., Fry, B., 1988. Controls on natural nitrogen-15 and carbon-13 abundances in forest soil organic matter. Soil Science Society of America Journal 52, 1633–1640. O’Leary, M.H., 1988. Carbon isotopes in photosynthesis. BioScience 38, 328–336. Shelp, B.J., Sieciecohowicz, Ireland, R.J., 1985. Determination of urea and ammonia in leaf extracts: application to Ureide metabolism. Canadian Journal of Botany 63, 1135–1140. Smith, B.N., Epstein, S., 1971. Two catagories of 13 Cr12 C ratios of higher plants. Plant Physiology 47, 380–384. Sun, M.Y., Wakeham, S.G., 1994. Molecular evidence for degradation and preservation of organic matter in the anoxic black sea basin. Geochimica et Cosmochimica Acta 58, 3407–3424. Ward, D.E., 1993. Trace gasses and particulate matter from fires —a review. In: Background Paper for Proceedings of the Victoria Falls Workshop, June 2 to 6.
Causes of bulk carbon and nitrogen isotopic fractionations in the products of vegetation burns: laboratory studies Vaughan C. Turekian ) , Stephen Macko, Donna Ballentine, Robert J. Swap, Michael Garstang Department of EnÕironmental Sciences, The UniÕersity of Virginia, CharlottesÕille, VA 22903, USA
Abstract Bulk stable isotope analysis is a means for the characterization of the sources of carbonaceous and nitrogenous material aerosols derived from biogenic sources. In order to use stable isotope techniques for characterizing the products of vegetation burns the isotope effect of combustion must be known. The C 3 vegetation Colospherum mopane and Eucalyptus sp. and the C 4 vegetation Cenchris cilliarus, Antephora pubesence and Saccharum officinarum, were burned under controlled conditions in the laboratory in order to better understand how the process of combustion affects the isotopic fractionation of the produced material. Carbon isotopes for aerosol particles formed during controlled laboratory burns of C 3 vegetation were higher in d13C by 0.5‰ compared to the source vegetation. Aerosol particles captured above the controlled laboratory burns of C 4 vegetation were lower in d13C by 3.5‰ compared to the source vegetation. The proposed causes for the different isotope effects shown for C 3 and C 4 sourced products are differences in the oxidation chemistry of these two plant types. Aerosol particulate material and ashes produced during the controlled laboratory burns of the vegetation are higher in d15 N than the source vegetation by 6.6‰ and 2.5‰, respectively. Furthermore, d15 N values for the residual material produced when Eucalyptus sp. samples were heated at discrete temperatures, suggest that different pools of nitrogenous compounds are accessed at different temperatures of heating. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Bulk stable isotope analysis; Vegetation burns; Aerosol particles
1. Introduction Biomass is burned on virtually all of the continents during various times of the year ŽHao and Liu, 1994.. Burns can be started naturally, as the result of lightning strikes or by volcanic ignition, or can be anthropogenically sourced. The latter can be the
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Corresponding author.
consequence of local tribal hunting activity or may be associated with larger scale agricultural or land use policies ŽCrutzen and Andreae, 1990.. Fires release volatile organic compounds, inorganic gases and particles, all of which are either transported away from the burn site or recirculated locally. These biogenically derived compounds influence the nutrient balances of ecosystems both near to and removed from the burn region ŽCrutzen and Andreae, 1990.. An understanding of the isotopic alterations that take place during combustion is needed in order
0009-2541r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 1 0 5 - 3
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to use stable isotopes as source indicators of aerosols. This present study addresses the bulk carbon and nitrogen isotopic fractionations associated with controlled laboratory vegetation burns. The sources of organic aerosol compounds and associated gases found in the atmosphere as a result of biomass burning events can be characterized using stable isotope analyses. The application of isotope techniques to tracing the products of vegetation burns requires knowledge of the isotopic properties of the original vegetation and isotope fractionation effects of combustion on the components derived from the burn. Smith and Epstein Ž1971. have shown that there is a carbon isotopic ratio difference between C 3 and C 4 plants, with the C 4 plants being enriched in 13 C relative to C 3 plant materials. This distinction results from the characteristic pathways of carbon fixation used by these two plant types ŽO’Leary, 1988.. This isotopic separation between C 3 and C 4 vegetation provides the basis for the first order isotopic characterization of aerosol particulate matter or volatilized compounds derived from these vegetation sources. In field studies it has been shown that there are, however, carbon isotopic changes in the products of combustion ŽCachier et al., 1985.. Studies of carbonaceous particles resulting from industrial and biogenic combustion compared to wind blown biogenic material not affected by fires, indicates the utility of analyzing the d13 C values of both the aerosols as well as vegetation to asses both source and process ŽCachier et al., 1986.. Carbonaceous gases emitted during biomass burning include CO 2 , CO, methane and non-methane hydrocarbons ŽLobert and Warnatz, 1993; Ward, 1993., with the majority of the carbon released in the gaseous form as CO 2 ŽWard, 1993.. Highly efficient combustion can result in production of CO 2 which is in excess of 90% of the total carbon transformation ŽWard, 1993.. Less efficient combustion lowers the CO 2 production relative to other carbonaceous gases to as low as 50% with the other principal gas constituents becoming CO and methane, as well as other trace volatile hydrocarbons ŽWard, 1993.. Particulate matter produced during combustion consists of the recombination of volatile compounds above the burn resulting from the condensation of these compounds onto existing aerosols ŽCocks and
McElroy, 1984. as well as the elevation of thermally altered plant debris ŽCachier et al., 1985; Lobert and Warnatz, 1993; Ward, 1993.. The carbon isotopic signature of these particles will depend on the relative importance of these two sources and the physical and chemical alterations that affect these carbon pools as a result of the combustion process. Nitrogen isotope ratios in vegetation are not so much influenced by biosynthetic differences as by the environment of growth. For example, a marked difference in d15 N exists between marine and terrestrial biota. This information has been used to decipher diets of ancient humans and other organisms ŽHeaton et al., 1986; Koch et al., 1995.. In terrestrial environments arid habitat vegetation is enriched in 15 N relative to vegetation growing in habitats with normal precipitation ŽHeaton et al., 1986.. Isotopic characterization of organic nitrogenous compounds q and inorganic nitrogen ions such as NOy 3 and NH 4 , collected during studies of wet deposition of aerosols, indicate that organic nitrogen tends to be higher in 15 N compared to the inorganic nitrogen ŽCornell et al., 1995.. If partitioning occurs between volatile inorganic nitrogen-bearing compounds and residual organic compounds containing nitrogen, as can occur during the process of biomass burning and transport, the nitrogen isotope signatures of the transported components may provide an improved understanding of the origin of the aerosols. This information can lead to a better knowledge of the influence of combustion on the production and transport of nitrogenous compounds. Combustion of vegetation, and the subsequent deposition of ashes, is crucial to recycling limiting nutrients such as nitrogen, within a forested ecosystem. Many of the studies addressing the fate of nitrogen during vegetation burns have focused on the production of the residual material. Amino acids in ash samples from different intensity burns ŽDeBano et al., 1979. indicate that the relative concentrations of these compounds is diminished in the residual material with fire temperatures above 1808C. The bulk nitrogen isotopic fractionations associated with this nitrogen loss, however, have not been measured. As amino acids are the primary nitrogen containing compounds in vegetation, the partial destruction of amino acids should result in isotopic fractionation between the source vegetation and the residuum.
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2. Methods The experiments designed to answer the above questions are of two types: Ž1. C 3 and C 4 vegetation were combusted and bulk carbon and nitrogen isotope analysis were performed on the residual material and the elevated particles; Ž2. carbon and nitrogen isotopic changes during different temperatures of combustion were investigated.
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diately above the burn on a pre-ashed Ž5508C for 2 h. glass fiber filter ŽGelman Type AE. connected to a hand held filter tower coupled to a low volume pump. Ash samples represent the total residual material left on the foil following the burn. The vegetation ash and filter samples were wrapped in pre-ashed aluminum foil, placed in ultra-clean plastic bags which were then heat sealed and stored in the freezer until chemically and isotopically analyzed.
2.1. Preparation of leaf material for combustion 2.3. Protocol for CO2 release Leaf samples of C 3 and C 4 vegetation were lyopholized and ground to powder using a Wiley Mill with a a60 mesh. Eucalyptus sp. samples collected from central California and Colospherum mopane vegetation from Africa represented the C 3 vegetation for this study. The C 4 vegetation was represented by the African grasses Cenchris cilliarus and Antephora pubesence as well as the agricultural crop Saccharum officinarum Žsugar cane. from southern Africa. These species are commonly exposed to burning as a result of natural events or land clearing for agricultural use. 2.2. Combustion protocols for bulk aerosol and ash studies In order to determine the effect of combustion on the isotopic fractionation between the source material and the products, controlled laboratory burns were performed. Approximately 10 mg of the dried vegetation samples were placed on pre-ashed Ž5508C for 2 h. aluminum foil and ignited. Experiments under both flaming and smoldering conditions were performed. Oxygen gas was supplied as needed through a gas regulator in order to sustain flaming combustion. Thermocouple readings taken above the flaming combustion showed temperatures in excess of 6008C. The combustion event lasted until all the vegetation had been exposed to the flame. Smoldering burns were performed under atmospheric conditions without the addition of excess oxygen. These lower intensity smoldering combustions had temperatures below 2008C. Particulate matter from both primary emissions and rapid secondary production was sampled imme-
Incomplete combustions of vegetation were performed in order to determine if the carbon isotopic signature of CO 2 produced during burns depended on the degree of combustion and on the amount of carbon converted to CO 2 . Eucalyptus sp. and S. officinarum were weighed and placed in pre-ashed Ž8508C for 2 h. quartz tubing with copper and copper oxide. The tubes were evacuated to remove any ambient gas, and sealed. Samples were each heated at a constant temperature for 1 h. Eucalyptus sp. samples were heated at temperatures of 1508C, 3508C and 6508C and the sugar can samples were heated at 2508C, 4508C and 6508C. Samples of each vegetation were also heated at 8508C for 2 h and allowed to cool slowly following the Dumas sealed tube combustion technique described by Macko Ž1981.. These samples were assumed to represent a complete conversion of the organic carbon to CO 2 gas. In each of the lower temperature combustions, the amount of CO 2 released was compared to the potential amount of CO 2 that would form with complete oxidation. The CO 2 was purified and isolated cryogenically with liquid nitrogen. For each temperature, the amount of carbon converted to CO 2 was determined at each temperature by manometer deflection and represented as percent conversion relative to the 8508C combustion. Isotopic analysis was performed on the CO 2 gas as described below. Incomplete combustion of organic matter results in the formation of CO. The mass overlap between the CO and the N2 complicates nitrogen isotopic analysis. For this reason only the completely combusted samples were analyzed for their stable nitrogen isotopic compositions.
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2.4. Protocol for incomplete combustion: for nitrogen isotope analysis Nitrogen isotopic fractionation depends on the types of compounds accessed during heating and the isotopic distribution within these compounds. To determine how different temperatures of combustion affect the isotopic distribution of ash material produced by the combustion of organic material, Eucalyptus sp. samples were exposed to varying degrees of controlled heating in a combustion furnace. Samples were placed on pre-ashed aluminum foil boats, weighed and placed in the combustion furnace at a known temperature. After heating the sample was reweighed and stored for further analysis. Vegetation has a complex array of nitrogenous compounds. The accessing of different compounds within this array should depend on the intensity and duration of heating. In order to determine how different temperatures and times of heating affect the availability of the different nitrogen pools and the isotopic signature of the residual material, two sets of Eucalyptus sp. combustions were performed. One group was exposed to heating for 10 min per sample and the other sample set was exposed for 30 min per samples. The temperatures of combustion for both sets were 1408C, 1608C, 1858C, 2058C, 2158C and 2258C. These higher temperatures represent temperatures at which DeBano et al. Ž1979. observed a decreased amino acid content of the residual material.
200 ml glass distilled dichloromethane as the solvent. Approximately 5 ml of the dichloromethane with the dissolved lipid extract was dried over copper oxide and placed in a pre-ashed quartz combustion tube and prepared for isotopic analysis as described below. 2.6. Bulk isotope analysis For isotope analyses, samples from each type of combustion were placed in ashed quartz tubes with copper and copper oxide. The tubes were evacuated, sealed and combusted off line at 8508C for 2 h and allowed to cool slowly, using a modified Dumas technique ŽMacko, 1981.. This leads to the conversion of carbonaceous compounds to CO 2 and nitrogenous material to N2 gas. A cryogenic separation of the CO 2 and the N2 was performed by a standard laboratory technique. The d13 C and d15 N values relative to the isotopic standards Peedee Belemnite ŽPDB. and atmospheric nitrogen respectively, were obtained using a VG Prism Series II isotope ratio mass spectrometer. Isotope ratios are defined as a per-mil measurement reported in the following manner: dY s
Ž R samplerR standard . y 1
P 10 3 Ž ‰ .
where Y s13 C or 15 N with the corresponding R s13 Cr12 C or 15 Nr14 N. By definition the standards, N2 for 15 N and PDB for 13 C, have values of 0‰.
2.5. Lipid fraction extraction Lipids represent a more refractory component of vegetation ŽSun and Wakeham, 1994.. In an attempt to see if there is a relationship between photosynthetic pathway and bulk carbon isotopic signature of the lipid fraction for the vegetation used in this carbon isotopic study, isotope analysis was performed on the extracted lipid fraction and compared to the bulk vegetation. The lipid fraction from the filters was separated and also analyzed in order to determine how the process of combustion affected the isotopic signature of this labile material. The lipid fraction of S. officinarum, Cen. cilliarus, A. pubesence and Col. mopane vegetation were isolated using a Soxhlet extractor operating for 16 h using
3. Results and discussion 3.1. Carbon isotopes Two-way analysis of variance with the independent variable being the photosynthetic pathway of the source vegetation and intensity of burn indicated that the only significant parameter responsible for isotopic fractionations was the plant type ŽC 3 or C 4 .. Particles captured above controlled burns of C 3 have D13 C Ž D s d product y d source . values between y0.6‰ to 1.8‰ compared to the burned material with an average d13 C enrichment of 0.5‰ relative to the unburned plant. Controlled combustion of C 4 vegeta-
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tion results in particles with D13 C relative to the source vegetation ranging from y0.9‰ to y7.0‰. with an average d13 C depletion of y3.5‰ compared to the source vegetation ŽTable 1.. Carbon isotope values for carbonaceous particles captured during field burns of C 4 vegetation terrains ŽCachier et al., 1985. agrees with the results of this laboratory study. However, the laboratory fractionations are less pronounced than these reported in the field study. The d13 C enrichments in the products from the C 3 vegetation combusted in this present study contradict the assumption by Cachier et al. Ž1985. that the combustion of C 3 and C 4 vegetation types results in the same isotope effect, i.e., depletion. A confirmation of the different behaviors of C 3 and C 4 vegetation during burning is found in the d13 C signatures of CO 2 during incomplete combustions of Eucalyptus sp. and Saccharum officinarum ŽTable 2.. The d13 C of the CO 2 produced during different degrees of incomplete combustion of C 3 vegetation show no variation relative to the d13 C of the CO 2 produced during complete combustion ŽFig. 1.. Incomplete combustion of the C 4 vegetation shows a curvilinear relationship between percent carbon released as CO 2 compared to the total carbon in the plant. At lower levels of oxidation, more enriched 13 C is seen in the CO 2 ŽFig. 2.. The highly enriched d13 C value of y8.0‰ may be caused by Table 1 d13 C values for vegetation and the corresponding particle matter produced during controlled combustions Sample
C 3 r Flamer Vegetation d 13 C Ash C4 smolder particle
Eucalyptus a Eucalyptus b Eucalyptus b Colospherumc Colospherumc Colospherumc Saccharumd Saccharumd Cenchris e Cenchris e Antephorae Antephorae
C3 C3 C3 C3 C3 C3 C4 C4 C4 C4 C4 C4
flame flame smolder flame smolder smolder flame smolder flame smolder flame smolder
y29.2 y27.7 y27.7 y24.4 y24.4 y24.4 y12.9 y12.9 y12.9 y12.9 y13.3 y13.3
y27.4 y26.2 y27.3 y23.8 y24.6 y25.0 y16.2 y13.8 y19.9 y16.8 y15.0 y17.5
y27.9 y27.0 y27.0 y23.8 y24.4 y24.0 y12.9 y12.6 y14.7 y12.7 y13.8 y13.5
Samples collected from: a central California, b southern California, c southern Africa, d South Africa, e Namibia.
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Table 2 d13 C values for CO 2 formed during the incomplete combustion of Eucalyptus sp. and Saccharum officinarum % of total C converted to CO 2
d 13 C CO 2
d 13 C Residual Žcalculated. a
Eucalyptus sp. 0.2 47.3 88.5 100.0
y27.6 y27.5 y27.7 27.6
y27.6 y27.7 y27.7 –
Saccharum officinarum 3.0 51.5 68.0 100
y8.0 y12.1 y12.1 y12.9
y13.1 y13.2 y14.4 –
a 13
d C vegetations ŽŽ% C as CO 2 .r100.PŽ d13 C CO 2 .qŽ1yŽ% C as CO 2 .r100.PŽ d13 C residual.x.
the more efficient oxidation of a small, highly 13 Cenriched pool of carbon within the Saccharum officinarum. The d13 C values of non-oxidized, residual matter during incomplete combustion of C 4 vegetation are able to be calculated using mass balance ŽTable 2.. These different isotope effects associated with C 3 and C 4 vegetation combustion must be explained. One possible explanation is that C 3 and C 4 vegetation have distinct proportions of ‘labile’ Žoxidizable and volatile. and refractory carbon compounds, each with characteristic carbon isotopic signatures. The difference in behavior during burning, then, would depend upon the amount of labile carbon lost during combustion. An alternative possibility is that there are isotopic differences between the same constituents Žlabile and refractory. within C 3 and C 4 vegetation relative to their bulk d13 C values. Nadelhoffer and Fry Ž1988. have shown that the 13 d C of bulk leaf material is a function of the fractional contributions of refractory compounds such as lipids and lignins and more ‘labile’ Žoxidizable. starches, proteins, sugars and holocellulose Žthe acid soluble fibers of the vegetation. each with their own d13 C signals. Lipids and lignins tend to be lower in 13 C content relative to the bulk leaf whereas starches, proteins, sugars and holocellulose tend to be enriched in 13 C values relative to the bulk leaf. If the labile carbon component Ži.e., starches, proteins, sug-
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Fig. 1. d13 C of CO 2 for the incomplete combustion of Eucalyptus sp.
ars and holocellulose. represents a greater fraction of the carbon compounds in C 4 vegetation than in C 3 vegetation, then the D13 C between the CO 2 resulting from the partial combustion of the leaf material, and the source vegetation would be greater for a C 4 source than a C 3 source As a result the aerosol particulate carbon and the residual ash would be depleted in 13 C relative to the source for C 4 vegeta-
tion and enriched in 13 C relative to the source for C 3 vegetation ŽTable 1.. Fig. 3a and b present a hypothetical model illustrating the resulting differences between the fractionation processes involving C 3 and C 4 vegetation. The carbon isotopic values for the different compounds are based upon laboratory analysis of the lipid fraction combined with isotope trends observed by other
Fig. 2. d13 C of CO 2 for the incomplete combustion of S. officinarrum.
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Fig. 3. Ža. Proposed oxidation chemistry and corresponding carbon isotopic fractionations for the combustion of C 4 vegetation. Žb. Proposed oxidation chemistry and corresponding carbon isotopic fractionations for the combustion of C 3 vegetation.
authors ŽNadelhoffer and Fry, 1988.. Chemical alterations for the different compounds within the vegetation matrix that take place during combustion, will affect the isotopic signatures of the products Žgases, particulate matterial and residual matterial.. The preferential conversion of more labile compounds to CO 2 and other forms of volatile carbon that do not rapidly condense in the sample plume, would result in the residual material having an isotopic signature that represents the average of the remaining, more
refractory compounds. If the refractory nature of lipids is responsible for the less efficient volatilization and oxidation of these compounds over other oxidizeable compounds within the vegetation, the residual material and the aerosol particles will isotopically resemble the lipid compounds more than they would resemble the isotopic signal of the bulk plant material. If, however, there is volatile loss and combination of these lipid compounds into aerosol particulates during combustion, the residue of C 4
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Table 3 d13 C values for the total lipid fraction extracted from C 3 and C 4 vegetation and the corresponding aerosol particles produced during combustion Vegetation
d 13 C Vegetation
d 13 C Lipid
Aerosol particle lipid
Eucalyptus Colospherum Cenechris Saccharum Antephora
y27.6 y24.4 y12.9 y12.9 y13.3
y29.6 y24.5 y19.0 y18.4 y21.8
– y23.9 y21.9 y21.0 y21.8
burning will have higher d13 C and the aerosols on the filters will have a lower d13 C than the source vegetation. Collister et al. Ž1994. have shown that the total lipid fraction of C 4 vegetation have more depleted d13 C signatures compared to the source vegetation than is observed in C 3 vegetation. Carbon isotopic values for total lipid fraction from the vegetation in this study, also indicate a larger difference in d13 C between lipids and bulk vegetation for the C 4 pathway vegetation than for the C 3 vegetation ŽTable 3.. Additionally, Ballentine et al. Ž1996. have looked at carbon isotopic values for specific fatty acids extracted from the same vegetation used in this present study. Their findings indicate that there is greater carbon isotopic fractionation between the fatty acid fraction and the source vegetation for the C 4 vegeta-
tion of this study than is observed for the C 3 vegetation of this study. The carbon isotope analysis of total lipids extracted from the aerosol filters over these combustion experiments indicates that the isotopic signal is conserved or only slightly altered during the combustion process. Therefore, if aerosol particles from the combustion of C 4 vegetation are enriched in lipids relative to other compounds within the source vegetation there would be a decrease in d13 C in the aerosol relative to the bulk plant material. The lipid data for the C 3 Col. mopane shows little isotopic fractionation between the lipid compounds and the bulk vegetation, indicating that a preferential enrichment of the lipid fraction in the aerosol particles compared to the source vegetation would not seriously change the of the aerosol or the residue. Carbon isotope profiles ŽFigs. 4 and 5. for fresh Eucalyptus sp. exposed to varying temperatures and times of combustion show no clear isotope effect. This further supports the hypothesis that the refractory and labile carbon pools for the C 3 vegetation used in this study, have similar carbon isotopic distributions. 3.2. Nitrogen isotopes Bulk nitrogen isotope data for the aerosol particles collected over the controlled laboratory burns of the vegetation have an average d15 N enrichment of
Fig. 4. d13 C for the residual material from the step heating Ž10 min. of Eucalyptus sp.
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Fig. 5. d13 C for the residual material from the step heating Ž30 min. of Eucalyptus sp.
6.6‰ compared to the source vegetation and ranged from y1.3 to 13.1‰ different from the source plant ŽTable 4.. The residual material from these burns have a smaller d15 N enrichment than the aerosols, of 2.5‰ when compared to the parent vegetation. Further, a much smaller range in the isotopic fractionation is observed ŽTable 4.. Statistical analysis Žtwoway analysis of variance. indicates no significant difference, thus the nitrogen isotopic fractionations for both the aerosol particles and the residual material are independent of photosynthetic pathway, original d15 N of the bulk vegetation and burn intensity. A mass balance relating the nitrogen isotopic distribution of the source vegetation to the nitrogen isotopic signature of the products of combustion is given as: d15 N vegetations X d15 N residual material q Y15 N aerosol particles q Zd15 N volatile nitrogen Where X, Y and Z are the fractional contribution of the different products. In this equation, the volatile nitrogen fraction represents the nitrogenous compounds not captured on the filter, such as the gases NH 3 , NO x and N2 O which are produced during combustion of organic material ŽLogan, 1983..The
isotopic enrichment of the particles and the residual material illustrated in Table 3 must be balanced by a 15 N depletion in the volatile nitrogen. The magnitude of this 15 N depletion depends upon the combination of the d15 N enrichment of the residuum and the aerosol particles and the relative contributions of these terms to the total mass of nitrogen in the system. The depletion in 15 N of the nitrogenous gases, and the simultaneous enrichment in 15 N of the captured particulates and ash, is likely due to the preferential destruction of the bonds involving the light isotope of nitrogen including the release of nitrogenous gases during deamination of amino acids. DeNiro and Hastorf Ž1985. have proposed this mechanism to describe the results of low temperature diagenesis of organic material. They found that the residual nitrogenous material was enriched in d15 N compared to the source material. Based on this observation these authors concluded that there must be preferential loss of 15 N depleted nitrogen from the organic pool. This lost nitrogen was assumed to be derived from deamination of amino acids. Based on these above findings, it can be inferred that Ž1. ash material produced during biomass burning would have less total nitrogen compared to the source material and Ž2. the nitrogen isotopic signature of the residual material would be enriched in
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Table 4 d15 N values for vegetation and the corresponding combustion products Sample a
Eucalyptus Eucalyptus a Colospherumb Colospherumb Saccharumc Saccharumc Cenchris d Cenchris d Antephorad Antephorad
C 3rC 4
FlamerSmolder
Vegetation
C3 C3 C3 C3 C4 C4 C4 C4 C4 C4
flame smolder flame smolder flame smolder flame smolder flame smolder
6.4 6.4 3.3 3.3 6.9 6.9 10.0 10.0 6.7 6.7
d15 N Particle 13.0 19.5 2.0 4.7 12.8 15.1 22.7 15.9 17.7 9.8
Ash 8.3 8.5 8.2 5.3 9.2 6.9 14.3 12.3 9.7 8.7
Samples collected from: a southern California, b southern Africa, c South Africa, d Namibia.
15
N compared to freshly fallen leaf material. This latter statement implies that it is possible to better calculate the relative contribution of litter and burn products to the biogeochemical cycling of nutrients within forested ecosystems. Fresh Eucalyptus sp. samples exposed to varying temperatures and times of combustion show a bimodal distribution of the nitrogen isotopic signature ŽFigs. 6 and 7.. The Eucalyptus sp. samples exposed to heating for 30 min showed a greater range in the isotopic signature than the samples heated individually for 10 min. The bimodal nature of the nitrogen isotopic distribution in the residual material, points
to the accessing of different nitrogen pools during different levels of heating. The initial enrichment observed in the residual material could be caused by the volatilization of free ammonia within the vegetation ŽShelp et al., 1985., or the deamination of free amino acids. With increasing temperatures this pool of nitrogen within the vegetation, becomes enriched in 15 N as 14 N is preferentially lost through kinetic isotope effects. With increasing temperatures, the continued loss of nitrogen from the system comes preferentially from the 15 N component of this nitrogen pool. This would result in a d15 N signature of the residual material that reflects the d15 N of the
Fig. 6. d15 N for the residual material from the step heating Ž10 min. of Eucalyptus sp.
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Fig. 7. d15 N for the residual material from the step heating Ž30 min..
fresh vegetation. At higher temperatures, the second pool of nitrogen is accessed and a 15 N enrichment in the residual material is observed. This second pool of nitrogen could be representative of bound amino acids which require the increased heat in order for the combined hydrolysis, deamination reactions to take place. The range in d15 N for the residuum is greater for the samples exposed to 30-min heatings than for those samples heated for 10 min. The similar pattern of the isotopic distributions for both sample sets indicates that the availability of two or more distinct pools of nitrogenous compounds is independent of heating time. Despite the similarity in isotopic trends, the data indicate that increasing heating times results in a greater spread of the nitrogen isotopic signature of the residual material. Furthermore, increased heating times are correlated to the loss of material. Clearly the trend of enrichment of 15 N in the residue is controlled by the extent of access to the total vegetation by burning.
4. Conclusion The combustions of C 3 and C 4 vegetation yield different degrees of change in d13 C for all components of the system. Despite the differences in the
isotope effects, products of C 3 combustion are distinguishable from the products of C 4 burns through the use of bulk carbon isotopes. The combustion of C 3 vegetation yields aerosols, residue and CO 2 with d13 C values which show very small differences. The combustion of C 4 vegetation, on the other hand, results in carbon isotopic fractionation in d13 C for the three components measured. The differences in the behavior are linked to the abundances of easily oxidizable fractions relative to refractory fractions in the two vegetation types. The fractionations also are a result of the differences in the isotopic composition of lipids,and possibly other compounds within the plant matrix, relative to bulk vegetation for C 4 vegetation that is absent in the C 3 vegetation of this study. The combustion of organic material results in nitrogen in aerosol particles and residual material having higher d15 N than the source vegetation. These fractionations may allow for the isotopic characterization of combustion-derived nitrogenous material compared to elevated unburned litter. The bimodal relationship between heating temperature and d15 N of the residuum indicates that there are distinct pools of nitrogen that are accessed as a function of temperature. The evolution of burning plumes over time and space could result in the formation and scavenging
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of nitrogenous and carbonaceous compounds. Presumably any scavenging of particulate matter can lead to additional isotopic fractionations of carbon and nitrogen within the plumes. These are important considerations when extrapolating the results of this laboratory study to field observations. In conclusion, whereas carbon isotopes are unambiguously useful in characterizing the different sources of combustion-derived organic matter, nitrogen isotopes record a complex environmental and burning history and therefore may be more limited in their application to the problem of source determination in biomass burning. References Ballentine, D.C., Macko, S.A., Turekian, V.C., 1996. Variability of stable carbon isotopic compositions in individual fatty acids from combustion of C 4 and C 3 plants: implications for biomass burning. Chemical Geology, Isotope Geoscience 152, 151–161. Cachier, H., Buat-Menard, P., Fontugne, M., 1985. Source terms and source strengths of the carbonaceous aerosol in the tropics. Journal of Atmospheric Chemistry 3, 469–489. Cachier, H., Buat-Menard, P., Fontugne, M., Chesselet, R., 1986. Long-range transport of continentally-derived particulate carbon in the marine atmosphere: evidence from stable isotope studies. Tellus B 38, 161–177. Cocks, A.T., McElroy, W.J., 1984. The absorption of hydrogen chloride by aqueous aerosols. Atmospheric Environment 18, 1471–1483. Collister, J.W., Rieley, G., Stern, B., Eglinton, G., Fry, B., 1994. Compound specific d13 C analyses of leaf lipids from plants with differing carbon dioxide metabolisms. Organic Geochemistry 21, 619–627. Cornell, S., Rendell, A., Jickells, T., 1995. Atmospheric inputs of dissolved organic nitrogen to the oceans. Nature 376, 243–246. Crutzen, P.J., Andreae, M.O., 1990. Biomass burning in the tropics: impact on atmospheric chemistry and biogeochemical cycles. Science 250, 1669–1678.
DeBano, L.F., Eberlein, G.E., Dunn, P.H., 1979. Effects of burning on chaparral soils: I. Soil nitrogen. Soil Science Society of America Journal 43, 504–509. DeNiro, M.J., Hastorf, C.A., 1985. Alteration of 15 Nr14 N and 13 Cr12 C ratios of plant matter during the initial stages of diagenesis: studies utilizing archaeological specimens from Peru. Geochimica et Cosmochimica Acta 49, 97–115. Hao, W.M., Liu, M.H., 1994. Spatial and temporal distribution of tropical biomass burning. Global Biogeochemical Cycles 8, 495–503. Heaton, T.H.E., Vogel, J.C., von la Chevallerie, G., Collet, G., 1986. Climatic influence on the isotopic compositions of bone Nitrogen. Nature 322, 822–823. Koch, P.L., Heisinger, J., Moss, C., 1995. Isotopic tracking of change in diet and habitat use in African elephants. Science 267, 1340–1343. Lobert, J.M., Warnatz, J., 1993. Emissions from the combustion process in vegetation. In: Crutzen, P.J., Goldammer, J.G. ŽEds.., Fire in the Environment: The Ecological, Atmospheric and Climatic Importance of Vegetation Fires. Wiley, New York. Logan, J.A., 1983. Nitrogen oxides in the troposphere: global and regional budgets. Journal of Geophysica Research 88, 10785– 10807. Macko, S.A., 1981. Stable nitrogen isotope ratios as tracers of organic geochemical processes. PhD Thesis, University of Texas, Austin. Nadelhoffer, K.J., Fry, B., 1988. Controls on natural nitrogen-15 and carbon-13 abundances in forest soil organic matter. Soil Science Society of America Journal 52, 1633–1640. O’Leary, M.H., 1988. Carbon isotopes in photosynthesis. BioScience 38, 328–336. Shelp, B.J., Sieciecohowicz, Ireland, R.J., 1985. Determination of urea and ammonia in leaf extracts: application to Ureide metabolism. Canadian Journal of Botany 63, 1135–1140. Smith, B.N., Epstein, S., 1971. Two catagories of 13 Cr12 C ratios of higher plants. Plant Physiology 47, 380–384. Sun, M.Y., Wakeham, S.G., 1994. Molecular evidence for degradation and preservation of organic matter in the anoxic black sea basin. Geochimica et Cosmochimica Acta 58, 3407–3424. Ward, D.E., 1993. Trace gasses and particulate matter from fires —a review. In: Background Paper for Proceedings of the Victoria Falls Workshop, June 2 to 6.