Oxygen Isotopes Signature of Phosphate in Wildfire Ash - ACS Earth

Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX

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Oxygen Isotope Signatures of Phosphate in Wildfire Ash Laura Bigio* and Alon Angert The Institute of Earth Sciences, The Hebrew University of Jerusalem, Edmond J. Safra campus, Givat Ram, Jerusalem 91904, Israel

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S Supporting Information *

ABSTRACT: Atmospheric aerosol deposition is a significant source of phosphorus (P) in many terrestrial and marine ecosystems worldwide, influencing their biogeochemistry and primary production. Particles emitted from wildfires (hereafter, ash) are the second most important source of atmospheric P after airborne dust. In this study, we aim to identify the signature of ash oxygen isotopes in phosphate. This will enable the use of this signature for the separation of ash from other atmospheric P sources. We measured P concentrations and δ18OP in ash from natural and experimental fires and also from ash heated at different temperatures. The HCl and resin P concentrations (average ± SE) were 3.15 ± 0.35 and 1 ± 0.1 mg g−1, respectively. The HCl and resin δ18OP were 15.5 ± 0.4 and 14.7 ± 0.4‰ (average ± SE), respectively. Based on previous studies, we suggest possible isotope exchange reactions during the combustion process, between oxygen in phosphate and oxygen from other probable sources (i.e., the atmosphere, and CaCO3 and CaO formed in the ash). The unique isotopic signature in the ash, ranging from 11.5 to 19.4‰ in the HCl and resin P fractions, is different from that of other atmospheric P sources such as airborne tree pollen, which has δ18OP values between 19.2‰ and 29.6‰, and Saharan-dust samples collected in Israel, which have δ18OP values ranging from 20.7‰ to 22.6‰. Thus, the δ18OP can be used as a marker for identifying atmospheric P from wildfires and for estimating its importance to the global P cycle. KEYWORDS: Ash, Combustion, Wildfires, Phosphate, Phosphorus, Isotopic signature, Oxygen isotopes, Aerosols δ18OP, provide a unique means of tracing the P sources. The use of δ18OP as a tracer of P is based on the chemical stability of the P−O bond, which at temperatures below 80 °C is broken only by biological reactions.20−24 During combustion, at temperatures above 80 °C, the original δ18OP value is erased, due to isotope exchange processes. However, the heating during wildfires is expected to cause abiotic oxygen exchange. In a previous study by Liang and Blake,25 they heated inorganic and organic P (KH2PO4 and β-glycerophosphate) to 110 and 550 °C. Heating at 110 °C for long time periods (6 days) and heating at 550 °C for short time periods (2 h) resulted in a higher δ18OP than the original value. In addition, they saw a decrease in the δ18OP values when heating organic P compounds at 550 °C for 6 h. Liang and Blake suggest in their paper25 that the δ18OP increase is promoted by oxygen isotope exchange between phosphate and atmospheric O2, and the decrease is a result of a reaction between P and SiO groups in the walls of the reaction vessels. In another study by Munro et al.,26 the δ18OP of deer bone ash, which was burned and boiled at temperatures between 25 and 900 °C, was analyzed. They found that at temperatures between 300 and 700 °C the δ18OP decreased steadily to ∼4−7‰ lower than the original compositions, and at temperatures above 725 °C, the δ18OP

1. INTRODUCTION Phosphorus (P) is an essential nutrient in many terrestrial and aquatic ecosystems.1,2 Since P is not stable in a gaseous phase, its transport in the atmosphere is limited to aerosols. The main P bearing aerosols in the atmosphere are dust particles, yet there are other important P sources, such as primary biogenic aerosols, which include pollen, and particles emitted from wildfires (hereafter, ash).3−5 Particles emitted from forests during wildfires can fertilize and contribute to soil P concentrations in plant-available P pools.3,6 However, the Amazon forest is losing P to the ocean as a result of wildfires.7,8 Much effort is invested estimating the magnitude of the atmospheric P flux to ecosystems,3,9−11 but there are still significant uncertainties regarding the contribution of wildfires to the global P budget. For example, the P concentration in ash reported by various studies varies widely.12−19 In addition, the annual global P emissions were estimated by an earlier study as 0.025 Tg of P,3 while a later study gave a higher estimate of 0.78 Tg of P.4 Moreover, there is no accepted method for the identification and differentiation between the atmospheric-P sources in atmospheric samples. Accurate estimation of the aerosol-P impacts on ecosystems productivity can be enhanced by reliable data on the contribution of the different P sources. Here, we suggest a method developed at our laboratory for tracing and identifying wildfire atmospheric P using oxygen stable isotopes in phosphate. Since the relative abundance of oxygen isotopes in phosphate varies naturally, accurate measurements of the oxygen isotope ratios in phosphate, © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

December 27, 2018 March 10, 2019 March 18, 2019 March 18, 2019 DOI: 10.1021/acsearthspacechem.8b00216 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Article

ACS Earth and Space Chemistry Table 1. Ash Sampling Locations and Vegetation fire

location

(1) Shilat

31°55′26.6″N 35°00′51.6″E

(2) Ramot forest

31°48′30.1″N 35°11′39.2″E

(3) Eshtaol forest

31°48′59.1″N 34°58′57.8″E

vegetation

(4) Zichron Yaakov

32°35′11.9″N 34°57′04.0″E and 32°35′22.7″N 34°56′58.2″E (5) Nataf 31°49′48.1″N 35°04′19.1″E (6) Ginosar 32°50′49.4″N 35°31′13.3″E (7) Luzit 31°41′14.6″N 34°53′26.4″E (8) Mariposa County, California 37°28′43.6″N 119°59′05.4″W

reached the lowest values. Bone comprises ∼70% mineral matter, which is composed primarily of calcium phosphate with a carbonated hydroxyapatite structure.27 At temperatures above 700 °C, the researchers reported that CaO was found in their samples. They suggest that the low δ18OP is a result of oxygen isotopic exchange between water vapor and bone phosphate that starts at temperatures above 300 °C, and they claim that this exchange is promoted at higher temperatures by the hygroscopic nature of CaO, which facilitates absorption of atmospheric water and forms at 700 °C. No isotopic fractionation is involved in ash transport since it is an abiotic process, which occurs at temperatures below 80 °C;21,23,28,29 therefore, δ18OP is applicable for tracing the different P sources. δ18OP has been found useful in various studies as a tracer of various phosphate sources5,30−33 and could be an effective tool for estimating ash contribution to the atmospheric P cycle. Since the frequency and intensity of wildfire and prescribed burns are expected to rise in the future,34,35 the effects of fire on the atmospheric P cycle may be of global significance. In this study, we aim to identify the signature of ash oxygen isotopes in phosphate. This will enable the use of this signature for the separation of ash from other atmospheric P sources. In addition, we evaluated ash P concentrations, which are an important parameter to estimate its importance to various ecosystems and to the global airborne-P budget.

Pinus halepensis, Cupressus sempervirens, and Sarcopoterium spinosum Pinus halepensis, Cupressus sempervirens, and Sarcopoterium spinosum Pinus halepensis, Cupressus sempervirens, and Sarcopoterium spinosum Olea europaea, Quercus calliprinos and Acacia saligna Quercus calliprinos orchard and Phragmites australis wheat field and Sarcopoterium spinosum mixed local vegetation (spruce)

2.2. Ash Color and pH. Ash color was determined according to the Munsell color chart36 using the neutral values between N9 (neutral white) and N1 (neutral black), as the ash samples were in this neutral-color range. This was done by a single person and under identical artificial light conditions. pH was measured in aqueous saturated pastes made from the ash samples (model pH 323, Xylem Analytics Germany Sales GmbH & Co. KG, WTW). 2.3. Experimental Fires. To produce ash from natural vegetation, an experimental open fire was set on a coarse mesh (∼1 cm), using pine wood and needles, 0.8 m above the ground. The ash was collected beneath the mesh, on an aluminum tray, and flying ash was collected 0.5 m downwind from the open fire on another aluminum tray. In another set of experiments, ash collected in Luzit (S. spinosum) and from Shilat (Pine) was heated in a muffle furnace. The heating was done in platinum crucibles since in glass and alumina crucibles oxygen may exchange between the samples and the vessels walls. The ash was heated to 550 °C for 3 and 24 h. Additionally, ash collected in Luzit (S. spinosum) and from Shilat (Pine) was heated for 3 h to 350, 550, 750, and 950 °C. These temperatures are within the measured range of conifer forest wildfires.37 2.4. Phosphorus Concentrations. The P concentrations in the ash samples were measured for different P pools. The resin P pool represents labile inorganic P lightly adsorbed to outer surfaces of particles,38,39 this fraction was extracted by shaking the ash with anion exchange resin membranes (BDH55164) in 5 L of deionized water for 24 h. HCl P contains the resin P fraction as well as inorganic P that is Ca-bound and some Fe-bound P.32 The HCl P pool was extracted by shaking in 100 mL of 1 M HCl for 16 h. The P concentrations were determined colorimetrically by the method of Murphy and Riley.40 2.5. Isotopic Analysis. The δ18OP values of the ash resin P were determined as described by Weiner et al.41 Briefly, the phosphate was extracted with anion exchange resin membranes, precipitated as cerium phosphate to remove other oxygen-containing compounds, and finally precipitated as silver phosphate. The HCl-extractable inorganic phosphate isotopic composition was assessed following the method of Tamburni et al.42 This method is based on extraction with 1 M HCl and purification by Superlite DAX-8 resin and through successive precipitations of ammonium phosphomolybdate and magnesium ammonium phosphate. This fraction includes the soluble P and the inorganic P content. For isotopic composition determinations, the silver phosphate is packed in silver capsules and introduced into a high temperature pyrolysis

2. MATERIALS AND METHODS 2.1. Ash Sampling. Ash samples were collected after wildfires that occurred between May and November of 2016 in the following areas (see Figure S1 in Supporting Information for a map of sampling locations in Israel): (1) a groove near Shilat, (2) Ramot Forest, (3) Eshtaol Forest, where fire retardant was applied and visible, creating a boundary between the burned and unburned vegetation, (4) two grooves in Zichron Yaakov, (5) a groove in Nataf, (6) an orchard near Ginosar, and (7) a wheat field in Luzit. At all of these sites, ash was collected as soon as possible after the fire was contained (a week at most). In addition, ash was sampled in October of 2017 in (8) Mariposa County, California, Detwiler fire. Here, the ash was collected 2 months after the fire was contained. In all the ash samples, no rain events occurred between the fire and the ash sampling. More information about the ash sampling locations and vegetation is available in Table 1. In addition, a sample of the fire retardant was analyzed. After sampling, the ash was temporarily stored in sealed plastic bags at 4 °C until analysis. Prior to analysis, ash samples were passed through a 10 mesh (2 mm) screen. B

DOI: 10.1021/acsearthspacechem.8b00216 ACS Earth Space Chem. XXXX, XXX, XXX−XXX

Article

ACS Earth and Space Chemistry

Figure 1. P concentrations in all ash samples. Black squares represent HCl-P and gray circles-resin P.

Figure 2. Ash δ18OP. Black squares markers, HCl-P; gray circles, resin P. Error bars represent standard deviation (n = 3).

when the ANOVA showed a significant difference. For the unequal variances, a Welch’s test was done and was followed by nonparametric comparisons for each pair using Wilcoxon method. Significant differences were determined at p < 0.05.

unit, where it is converted to CO in the presence of glassy carbon.43,44 The CO gas is isolated by a GC, and the oxygen isotope ratio is then measured by a continuous-flow isotope ratio mass spectrometer (IRMS). All isotopic values are given in the delta notation versus VSMOW. The average standard deviation among three replicates of the same sample was 0.3‰. All measurements were performed against two Ag3PO4 lab standards, which were calibrated against the following Ag3PO4 standards (isotopic signatures in parentheses): TU-1 (21.1‰) and TU-2 (5.4‰),44 UMCS-1 (32.6‰) and UMCS-2 (19.4‰),45 and against the IAEA-601 benzoic acid standard (23.3‰).46 2.6. Statistical Analysis. All statistical analysis was done using JMP (JMP, JMP Pro 13, SAS Institute Inc., Cary, NC, USA). The homogeneity of variances was tested using Bartlett’s test and showed equal variances in the ash from different trees and locations within all parameters tested, except for the HCl-P concentration and resin δ18OP analysis. For the parameters that showed equal variances, ANOVA test was performed and was followed by Tukey−Kramer honest significant difference test for comparison between the locations

3. RESULTS 3.1. Phosphorus Concentrations in the Resin and HCl Fractions. The HCl-P concentrations ranged from 1.4 mg g−1 in ash sampled at Ramot, pine, and cypress, to 6.7 mg g−1 in Ginosar orchard (average ± SE: 3.15 ± 0.35 mg g−1) (Figure 1). The resin-P concentrations ranged from 0.54 mg g−1 in Eshtaol pine to 2 mg g−1 in Luzit, S. spinosum (average ± SE: 1 ± 0.1 mg g −1) (Figure 1). The resin-P and HCl-P concentrations of pine and cypress ash, from the Eshtaol forest fire, sampled with proximity to the fire retardant were 3.17 and 4.62 mg g−1. The resin-P concentration of the fireretardant sample was 3.83 mg g−1. In the open-fire experiment, the ash collected near the fire had higher resin-P and HCl-P concentrations than the ash C


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ACS Earth and Space Chemistry collected beneath the fire (resin-P, 1.5 and 0.6 mg g−1; HCl-P, 4.8 mg g−1 and 2.6 mg g−1). Analysis of variance found no significant difference between the resin-P concentrations in ash sampled at different locations (P > 0.05). The HCl-P concentrations varied significantly with the location (P = 0.025), the ash collected at Ginosar had higher HCl-P concentrations than the ash collected at Shilat and Ramot. 3.2. Isotopic Values. The HCl δ18OP ranged from 12.6‰ in Luzit wheat field to 18.6‰ in Luzit S. Spinosum (average ± SE 15.5 ± 0.4‰) (Figure 2). The resin δ18OP ranged from 11.5‰ in Ginosar wild vegetation to 19.7‰ in Mariposa county ash (average ± SE 14.7 ± 0.4‰). Most of the δ18OP values were higher in the HCl fraction than in the resin P fraction. The resin δ18OP of the ash collected in the open-fire experiment were higher in the ash collected beneath the fire (20.3‰) than near the fire (14.9‰). The HCl-P δ18OP were identical within experimental uncertainty (13.5‰ and 13.6‰) in ash from beneath and near the fire. The resin δ18OP of the fire retardant was 14.3‰. Analysis of variance found no significant difference between the HCl δ18OP values in ash from different locations (P > 0.05). The resin δ18OP varied significantly with the location (P = 0.0268), the ash collected at Eshtaol had higher resin δ18OP than the ash collected at Ginosar and Zichron Ya’akov. We found a significant correlation (Figure 3) between the resin and HCl δ18OP of R2 = 0.59 with a p < 0.0001.

minimum of 12.1 ± 0.2‰ and 14.6 ± 0.5‰ in N8 in both fractions. 3.4. Ash Heating Experiments. Ash samples from Luzit (S. spinosum) and Shilat (pine) were heated to 550 °C, and the resin and HCl δ18OP values were measured after 3 and 24 h (Figure 6). The resin δ18OP values in the ash sample from Shilat increased from 14.9‰ before the experiment to 16‰ after 3 h and to 16.6‰ after 24 h. The HCl δ18OP values decreased in Shilat from 17.4‰ to 16.4‰ after 3 h and after 24 h to 17.2‰ and Luzit samples from 19.0‰ to 13.0‰ after 3 h and to 13.3‰ after 24 h. Since the experiments shown in Figure 6 show that most of the isotopic change occurs during the first 3 h, during the subsequent experiments the ash was heated to 350, 750, and 950 °C for 3 h (Figure 7). The isotopic values of the ash heated to 350 °C were lower than the initial values of the collected ash in all samples (Shilat resin and HCl δ18Op values were 5.8‰ and 9.0‰, and Luzit resin and HCl were 9.0‰ and 11.1‰). The ash heated to 550 °C had higher values (Shilat resin and HCl δ18O values were 16.0‰ and 16.4‰, and Luzit HCl was 13.0‰). The ash heated to 750 °C-Shilat HCl δ18O values were 18.9‰, and Luzit resin and HCl were 17.9‰ and 16.4‰, respectively. At 950 °C, the resin and HCl δ18O values of the Luzit sample decreased to 15.2‰ and 14.6‰, respectively.

4. DISCUSSION 4.1. High Phosphorus Concentrations in Ash. The resin P concentrations found in this study (average ± SE, 1 ± 0.1 mg g−1) were similar to the water-soluble P concentrations reported by previous studies13,19 (0.9 and 0.7 mg g−1, respectively) for ash from wood, bark, and cereal straw, from an industrial plant (Table 2). The HCl-P concentrations found in this study ranged from 1.4 to 6.7 mg g−1. The total P concentrations reported by various studies ranged from 2 to 31 mg g−1 (Table 2). Our HCl-P concentrations were similar to the minimal total P concentrations reported by other studies. The discrepancies in the P concentrations between the different studies may be a result of different extraction methods (we measured HCl-P and not total P), vegetation with different P content, and fire intensity. 4.2. Phosphorus Concentrations and Fire Severity. In order to estimate fire severity of the wild fires sampled, we classified the ash according to Munsell colors (Figure 4) using the neutral values between N9 (neutral white), to N1 (neutral black), as the ash samples were in this neutral color range. Fire severity depends not only on the amount of heat generated along the flaming front of a fire (i.e., intensity) but also on the duration of the burn.49 Ash color is often used as a measure for fire severity,13,50,51 and it depends on the combination of ash and char52 and the combustion efficiency. Dark colored char indicates incomplete combustion, while white ash indicates complete combustion.53−55 At low combustion temperatures, the organic C content is higher, thus the darker color of charred organic compounds. At higher combustion temperatures, the color is lighter (shades of light gray and white), due to a more complete combustion and a more thorough removal of organic matter and the organic darkening agents by combustion. In addition, the presence of white ash particles, consisting largely of calcite, also contributes to the lighter colors. In the sampled ash, the resin P concentrations decreased as the fire severity increased (lighter ash color, from N4 to N8) to a constant value (around 1 mg−1 g) (Figure

Figure 3. Resin and HCl δ18OP (‰) of the ash samples.

3.3. Ash Color. The resin and HCl-P concentrations of the ash samples were divided into groups according to their Munsell color (N2 to N8, higher number indicates lighter color). The average resin and HCl-P concentrations of these groups are shown in Figure 4. The average ± SE resin P concentrations increase from 1.4 ± 0.2 to 1.9 ± 0.9 mg g−1 from N2 to N3 and then decrease to 0.7 ± 0 mg g−1 in N4. The concentrations in N5, N6, N7, and N8 remain approximately 1 mg g−1. The average ± SE HCl P concentrations decrease from 3.3 ± 0.4 to 3.1 ± 1.1 mg g−1 in N2 to N3 and then, in N4, reach a minimum of 1.90 ± 0.3 mg g−1. The concentrations increase to a maximum of 4.3 ± 1 mg g−1 in N6 and then decrease to 2.6 ± 0.7 mg g−1 in N7 and remain at this value at N8. The average ± SE resin and HCl δ18OP values react similarly (Figure 5). They increase from 15.2 ± 0.6‰ and 15.3 ± 0.9‰ to a maximum of 18.9 ± 0.4‰ and 19.7 ± 0.3‰ from N2 to N3 and then decrease to a D


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Figure 4. Average P concentrations of the ash samples in the resin P fraction (gray) and the HCl fraction (black) ash against its Munsell color. Error bars represent standard error.

Figure 5. Average δ18OP of the ash samples, in the resin P fraction (gray) and the HCl fraction (black) ash against its Munsell color. Error bars represent standard error.

Figure 6. δ18OP of ash samples heated at 550 °C for 3 and 24 h. Black triangles dashed line, Shilat HCl; black squares solid line, Luzit HCl; gray circles dashed line, Shilat Resin; gray diamonds, Luzit resin. Error bars represent standard deviation (n = 3). Some error bars are smaller than the markers.

4). Fire ash is alkaline, and its pH increases with fire severity.56−58 Similar findings were reported by Pereira et al.59 and Hein-Sever et al.,56 where ash pH was fire-severity dependent, increasing from a pH of 6.9 to 7.8, and 10.3 for

high-severity fire. The solubility of P is pH dependent, and the maximum level of soluble P is found at a pH of about 7.0 in the soil solution,60 at higher pHs, P precipitates strongly with Ca and Mg.59 Indeed, our results showed that ash resin P E


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Figure 7. δ18OP of ash samples heated for 3 h at different temperatures. Black triangles dashed line, Shilat HCl; black squares solid line, Luzit HCl; gray circles dashed line, Shilat Resin; gray diamonds solid line, Luzit resin. Error bars are smaller than the markers. Dashed circles represent coal fly ash,47 and dashed black line is atmospheric O2.48

Table 2. Phosphorus Concentrations (mg g−1) Reported in the Literature source vegetation wood and grassland eucalyptus litter

type of combustion

wildfire and laboratory wildfire and laboratory tropical forest wildfire wood and bark industrial plant birch and spruce laboratory wood pine and wildfire and Douglas fir laboratory various wildfire and laboratory cereal straw and industrial wood chips plant

total P

water-soluble P

ref

1.1−31

N/A

6

0.7

12 and refs within 13

2.6−20.3 1.8−14

N/A N/A

6.2−14.5

N/A

14 15 and refs within 16

9.9−20.5

N/A

17

1−31.3

N/A

18

10.2−13.6

0.9

19

concentrations are higher in ash originated at lower fire temperatures. Ash collected in Eshtaol near the area where fire retardant was applied had the highest resin P concentration compared to the other samples (3.2 mg g−1) and a pH level of 7.2. The fireretardant pH was 6.5; thus, it is possible that the fire retardant lowered the ash pH level, leading to the high P concentration. Since the ash resin P concentration is similar to the fireretardant resin P content (3.8 mg g−1), another plausible explanation is that it is affected by leaching of the retardant into the ash. The resin P concentration of ash sampled where fire retardant was applied was ∼3 times higher than that of ash sampled without fire retardant (average ± SE: 1 ± 0.1 mg g−1 compared to 3.17 mg g−1). According to previous studies,61−63 the application of fire retardant can have long-term effects on the P concentrations in the affected ecosystem. For example, Fernandez et al.63 found that 10 years after the application of fire retardant, the soil-available P concentration was 3 times higher than that in burned soils with no fire retardant application. In a previous study by Etiegni et al.,64 the total P concentrations increased with temperature (between 500 and 1000 °C). Our results show a similar pattern of higher HCl-P concentrations with higher temperatures (lighter ash color) between N2 and N6 and then a decrease in concentrations as temperatures keep increasing. 4.3. Possible Isotopic Exchange Processes. According to previous studies25,26,65 regarding oxygen isotope exchange reactions during combustion processes, we suggest a few oxygen sources that can explain the δ18OP values in the ash sampled: (1) atmospheric O2, (2) calcite (CaCO3), and (3) atmospheric water vapor. Table 3 summarizes the probable phosphate oxygen sources in the combustion process and its δ18O. The ash resin and HCl δ18OP display a similar relationship with the Munsell color (Figure 5), with an increase from N1 to N3, and a gradual decrease in lighter colors (indicating higher combustion temperatures as discussed in section 4.2). We observed a similar behavior of temperature-dependent δ18OP in our experiments (Figure 7) of ash heated at different temperatures. The δ18OP first rises as temperature increases

concentrations increased when pH levels decreased from 12.3 to 7.2 (Figure 8), and there is a significant correlation between these two variables, R2 = 0.67, p < 0.0001. Thus, the resin-P

Figure 8. Resin P concentrations and pH levels in the ash samples. F


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original δ18OP on the ash final signature remains unclear, and future study could help to better understand it. In the experimental open fire, the resin δ18OP of ash collected beneath and near the fire was 20.3 ± 0.4‰ and 14.9 ± 0.1‰, respectively (Figure 2). The ash collected beneath the fire fell into a tray that was placed under the mesh where the fire was placed. This tray also collected small burning wood chips, which fell from the mesh, and was constantly heated by radiation from the fire. Hence, the ash collected under the fire was kept at a higher temperature for a longer time than the ash collected near the fire, which cooled off as soon as it flew away from the fire. We suggest that the δ18OP of ash beneath the fire was higher than near the fire as a result of isotopic exchange with oxygen from the atmosphere. The isotopic exchange was possible due to the continuous heating that occurred beneath the fire. In addition, we assume that the temperatures in the tray were below 500 °C; therefore, we do not expect isotopic oxygen exchange with calcite or water vapor, which would lead to lower δ18OP. In the open-fire experiment, the resin δ18OP of ash was higher (20‰ and 15‰, beneath and near the fire) than the HCl δ18OP (13‰ in both samples) (Figure 2). We suggest that the resin δ18OP, which is more liable, exchanges faster than the HCl δ18OP. In low-severity fires, such as the fire in the experiment, there is faster exchange with the atmosphere (with δ18O of 23.88 ± 0.02‰48). In ash samples from natural fires, the HCl δ18OP was higher than the resin δ18OP. The ash samples were collected after high-severity fires (compared to our open fire experiment); it is possible that in these fires the temperatures were higher than 700 °C, allowing isotopic exchange of oxygen between phosphate and water vapor, after CaO is formed. This isotopic exchange was larger in the resin pool; therefore, the δ18OP was lower in the resin fraction. 4.4. δ18OP in Ash as a Tracer. The δ18OP values in all the wildfire ash samples, from Israel and California, from different vegetation origins, were similar in both the resin pool (average ± SE 15.5 ± 0.4‰) and in the HCl pool (average ± SE 14.7 ± 0.4‰), and within a range of 11.5−19.4‰ in both fractions. In addition, there was a significant correlation between HCl and resin isotopes of R2 = 0.597, p < 0.0001 (Figure 3), suggesting similar processes affect both fractions. Since most off the ash samples were collected 1 week after the fire with no rain events in between, we assume that biological activity had no or little effect on the δ18OP. Ash sampled at California was collected 2 months after the fire (no rain events occurred between the fire and the ash sampling), yet its values are similar to the other ash samples that were collected after a week. It is possible that the ash sampled at California underwent some biological processes;71 however, it did not erase the unique δ18OP from the wildfire. Our results show that wildfire ash δ18OP is different than other atmospheric P sources, such as pollen δ18OP (19.2− 29.6‰),5 and marine sedimentary phosphorites (∼22‰),29 which have similar values to Saharan-dust samples collected in Israel (20.7−22.6‰).31,72 There is a small overlap in the pollen δ18OP and wildfire ash values between 19.2 and 19.4‰, but elemental analysis can help distinguish between the sources (Table 4), for example, wildfire ash is rich in Ca (reported values are between 94 and 317 mg g−1,15 compared with 0.45 mg g−1 in pollen). As for fly coal ash, the HCl P concentrations are significantly higher (>0.5 mg g−1) than the resin P concentrations (20‰). It is possible that part of the biomass δ18OP remains in the ash δ18OP. Weinberger et al.47 reported similar HCl δ18OP in ash with the same source country; however, no relationship between coal HCl δ18OP and fly ash δ18OP was found. The effect of the G


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ACS Earth and Space Chemistry Table 4. Major Element Concentrations in mg g−1 element

ash15

pollen5

Ca Mg Al Na Fe K

94−317 6.5−22.5 12.5−82.1 1.4−6.7 3.3−19.5 10.3−41.3

0.45 0.94 0.004 0.09 0.01 11.3

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are within a range of 17.1−20.5‰.47 These values are higher on average than the HCl δ18OP of wildfire ash (12.6−18.6‰), although there is some overlap. Still, the δ18OP is a useful trace and identification method for the different atmospheric P sources.

5. CONCLUSIONS In this study, we measured the ash-P concentrations and, for the first time, its δ18OP. This work demonstrates, based on our data set, the use of δ18OP for identifying phosphorus sources in aerosols. Wildfires leave a unique δ18O signature in ash as a result of isotopic exchange reactions between the oxygen in phosphate and oxygen from other sources (the atmosphere, water vapor, and calcite formed in the ash). This signature is different than other atmospheric P sources such as desert dust, coal ash, and pollen, making δ18OP a useful trace method in the atmospheric global P cycle.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.8b00216. Google Earth map of the ash sampling locations in Israel (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Laura Bigio: 0000-0002-7154-2523 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Avner Gross for the California ash sample. We also thank Tal Weiner for her help and advice. We thank Noa Ben Yehuda for the graphics. This research was supported by a Grant from the Israel Science Foundation (#45/14).

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REFERENCES

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