Burned phytoliths absorbing black carbon as a potential proxy for paleofire

Abstract

Developing and refining fire proxies is paramount for reliable reconstructions and the inferences that they gain about fire in the Earth System. Burned phytolith index is an important tool for fire reconstruction. However, the source of the darkened color which appears on burned phytoliths is controversial and requires additional study to understand the relationship between phytolith characteristics and fire activity. By simulating burning of six grass species under open conditions, we extracted phytoliths from the ashes using a microwave digestion method. Then, we measured the carbon content of the ashed phytolith and the unburned phytolith (from modern plant). Next, we measured the carbon content of burned phytolith when treated with bleach. Our results show that the carbon contents of ashed phytoliths are higher than phytoliths extracted from plants, and ashed phytoliths after bleaching. The increased carbon content probably resulted from adsorption of black carbon by phytoliths exposed to open flames. We conclude that phytolith- related carbon might be a potential indicator of paleofire using soils and sediments.

Keywords

black carbon burned phytolith fire indicator paleofire phytolith-occluded carbon phytolith-related carbon

Highlights

The carbon content of ashed phytolith increases during combustion.

Phytolith particles with large size and/or area are more easily darkened.

Increased carbon content of burned phytoliths derives from adsorption of black carbon.

Phytolith related carbon could be used as a potential paleofire proxy.

Introduction

Fire reconstructions facilitate the understanding of fire occurrence and causal mechanisms, the relationships between fire, climate, vegetation and human activities, and importantly can provide a scientific basis for fire prevention and ecological restoration management (Chipman et al., 2015 ; Pierce et al., 2004; Zhao et al., 2019). Exploring new potential fire proxies and indicators is therefore important for improving the insights gained by fire reconstructions.

The quantification of charcoal abundance and morphometry indices have been widely applied to soils, lakes, peats and cultural sediments to characterize paleo-wildfire, and human fire use (Ballard et al., 2017; Carracedo et al., 2018; Inoue et al., 2016; Jara et al., 2019; Lebreton et al., 2019; Luo et al., 2001; Pierce et al., 2004; Vachula et al., 2021). Charcoal in soils and sediment cores can overcome the spatiotemporal limit of fires recorded by tree scar and historical documents (Charles et al., 1998). However, using charcoal accumulation alone is not an effective approach in some vegetation communities (Cordova et al., 2011). The use of microcharcoal with other proxies, such as pollen, phytoliths, magnetic parameters, and polycyclic aromatic hydrocarbons have been successful in interpreting the multifaceted characteristics of fire (Ballard et al., 2017; Carracedo et al., 2018; Conedera et al., 2009; Cordova et al., 2011; Gedye et al., 2000; Wen et al., 2020). Phytolith particles can be deposited with charcoal and can be quantified in tandem with microcharcoal on microscope slides, so they are a promising potential tool for the study of paleofire and better understanding the relationships between fire, vegetation, and climate (Ballard et al., 2017; Carracedo et al., 2018; Cordova et al., 2011; Li et al., 2010; Li et al., 2019; Wen et al., 2020).

Phytoliths are microscopic, mineralized bodies formed in living plants. Monosilicic acid is brought into the plant through water uptake and precipitates to form opaline silica within and between cells (Das et al., 2013). Phytolith is primarily SiO₂ and H₂O, but also contains organic components, and various inorganic mineral elements (Hodson, 2016; Li et al., 2020). Cell wall phytoliths generally contain large amounts of carbon. According to Hodson’s calculation the cell wall phytoliths investigated by Perry et al. (1987) are contained around 25% carbon (Hodson, 2019). Parr and Sullivan (2014) used an indirect method and estimated %C in cell wall phytoliths was 10.12 for sugarcane and 3.37 for sorghum (Lumen phytoliths contain much lower %C; Hodson, 2016; Li et al., 2020). Meanwhile, some authors believe phytoliths contain small amounts of organic components, and have estimated the organic matter content to vary between 0.1% and 0.5% (Alexandre et al., 2015, 2016; Reyerson et al., 2016). The physical and chemical properties of phytoliths might change when burned. Some studies have reported that the color, refractive index (RI), and/or carbon composition of phytoliths could be used to derive quantitative estimates of past fire regimes (Devos et al., 2021; Evett and Cuthrell, 2017). Phytoliths that have been exposed to fire often become darkened as compared with clear, unburned phytoliths (Boyd, 2002; Morris et al., 2010; Parr, 2006; Piperno and Becker, 1996). Researchers have used the ratio of burned to unburned phytoliths (BPI – burned phytolith index) in soil and sediment profiles to infer the fire occurrence (Boyd, 2002; Gu et al., 2008; Li et al., 2010; Morris et al., 2010; Rutherford et al., 2020). However, burned phytolith coloration likely changes during weathering in the soil, making it difficult to distinguish burned from weathered but unburned phytoliths (Evett and Cuthrell, 2017). Further, only relying on color to identify burnt phytoliths was found to be unreliable (Morris et al., 2010; Rutherford et al., 2020). The RI approach to identifying burned phytolith is based on the principle that heating of a phytolith exposed to fire expels some of the water incorporated in the phytolith matrix during its formation (Evett and Cuthrell, 2017). The RI value of an individual phytolith will increase when its H₂O content is lost. If more than 50% of the phytoliths in a sample have RI values higher than 1.440, the sample is most probably burnt (Elbaum et al., 2003). However, applying the RI approach for reconstructing grassland fire regimes is problematic because the RI of phytoliths has no obvious changes unless heated above 300°C, and RI changes can occur due to rehydration and aging (Evett and Cuthrell, 2017). The systematic observation of auto-fluorescent phytoliths in contexts with unequivocal evidence for heating and burning reveals that at least some phytolith become auto-fluorescent upon heating (Devos et al., 2021). Besides these methods, an alternative approach uses the chemical composition to identify burned phytoliths. Although some research has demonstrated the content of Pb, SiO₂, and C occluded in phytoliths could change due to heating (Jones and Beavers, 1964; Nguyen and Nguyen, 2019; Wu et al., 2014), only C has been invoked as a fire indicator. The darkened color of burned phytoliths is probably derived from the oxidation of phytolith-occluded carbon (PhytOC), and is used to distinguish burned from unburned phytoliths (Gu et al., 2008; Parr, 2006). Raman peaks characteristic of graphitic carbon were proposed to be straightforward for identifying a burned phytolith (Pironon et al., 2001). However, Evett and Cuthrell (2017) hypothesized that Raman graphite peaks are not generated from the occluded carbon in phytoliths that undergoes changes with heating but rather from the black carbon adsorbed by phytoliths exposed to open flames, because Raman analysis of most visually dark burned phytoliths had the distinctive graphite peaks, and heated but clear phytoliths lacked the graphite peaks.

In addition, PhytOC ranges from 0.2%–5.8% and is an important form of long-term carbon sequestration in soils (Parr and Sullivan, 2005). Plants with high phytolith carbon sequestration rates play an important role in increasing the global carbon sink and mitigating global CO₂ emissions (Chen et al., 2019; Meng et al., 2013; Song et al., 2017). However, the combustion of plants might influence the chemical composition of phytoliths and their taphonomy in soils (Blecker et al., 2006; Nguyen and Nguyen, 2019; Nguyen et al., 2021).

In summary, there are several problems with present fire phytolith indicators, which cause uncertainties for the utilization of phytolith chemical composition as a proxy of fire activity. Further, disputes regarding the composition and cause of the darkened color of (burned) phytoliths prevents its use as a fire proxy. It is necessary and urgent to better understand fire phytolith proxy systematics, such as how the darkened color of burned phytolith forms, to enable fire reconstructions in more ecosystems of interest.

In this study, phytoliths were extracted from the ashes of several common grass species combusted in open air conditions. A subset of these ashed phytoliths were treated with bleach to determine if black carbon adsorbed by phytolith is a major carbon source in darkened phytoliths. The carbon content of the phytoliths was measured and compared between the unburned/burned samples, and the unbleached/bleached burned samples to explore the source of carbon in the burned phytoliths, and the implications of phytolith carbon content for as a proxy for fire activity.

Materials and methods

Regional setting

Samples were collected from Guilin Center for Agriculture Science and Technology Research, and Guilin Botanical Garden (N25°2′19″, E110°18′34″). Guilin City is located in southwest China. It is in the subtropical monsoon climate region, and is affected by both the SW maritime monsoon from the Indian Ocean and the SE maritime monsoon from the western Pacific Ocean. It has an annual mean temperature of 18.8°C and total annual precipitation of 1874 mm. The long-term lowest and highest monthly mean temperatures are 15.6°C for spring and 23.0°C for summer, respectively (Li et al., 2016, 2019; Yuan, 1992). Vegetation in the region is characterized by evergreen broad-leaved forest (Figure 1).

Figure 1.

Map of sampling location.

Sampling and analysis

Leaf sampling

About 500g of mature leaves of three crops (rice (Oryza latifolia Desv.), maize (Zeamays L.), and sugarcane (Saccharum officinarum Linn.)), and three common grasses (Imperata cylindrica (L.) Beauv., Miscanthus floridulus(Lab.) Warb. ex Schum et Laut., and Setaria palmifolia (Koen.) Stapf.) were collected in December, 2020. These six grasses are all common subtropical plant species. Therefore, rice, maize and sugarcane crops are widely planted and their leaves and/or stems are burned sometimes after harvest. To maintain the representativeness of sampled leaves, the leaves were obtained from more than 10 individual plants grown within 10 m² of area. The leaves sampled were put in plastic bags to prepare for laboratory processing.

Phytolith extraction and classification

All plant leaves were cleaned using ultrasonic agitation in pure water, dried in an oven at 80°C. The leaves were divided two parts. One part is used to extract phytoliths (as unburned phytolith). These leaves were cut into less than 5 mm pieces before phytolith extraction. Another part was used for the combustion simulation experiment to extract ashed phytolith. These leaves were burned on tinfoil in open air conditions to generate ash. The ashes were collected in plastic bags to extract ashed phytoliths.

To extract phytoliths, we used a microwave digestion method modified from Parr et al. (2001). In this study, we selected a relatively high temperature for the microwave digestion, and then carried out a two-time oxidation of phytolith with K₂Cr₂O₇. The microwave digestion system conditions for both digests were as follows: oven temperature was programed to increase from room temperature to 100°C within 5 min, and was held at 100°C for 10 min, and then increased from 100°C to 180°C within 5 min, and held at 180°C for 20 min, then increased from 180°C to 210°C within 5 min, and held at 210°C for 45 min. The specific steps are as follows: (1) Weigh about 1g of cut plant leaves and collected ashes, and put them into the digestion tubes, respectively; (2) Add 15 ml HNO₃ (superior grade) and put tubes in the microwave to start the first digest, (3) Add 5 ml HNO₃ (excellent grade pure) and 10 ml 30% H₂O₂ for further oxidation and digestion, then transfer samples to centrifuge tubes to be rinsed with distilled water (at least three times, 2500 rpm, 5 min); (4) Add 2.5 ml concentrated H₂SO₄ and 2.5 ml K₂Cr₂O₇ (0.8 mol/L), heat the digestion tank in a hot water bath (98°C for 90 min), then transfer samples to centrifuge tubes to rinse with ultrapure water (at least three times, 2500 rpm, 5 min) until the solutions were pH neutral. Repeated this step two times to get the unburned phytolith and ashed phytolith samples, respectively; (5) Take half of the ashed phytoliths extracted to be bleached using NaClO₃. Shake the solutions every 12 h, and let it stand for 48 h. After this process, the samples were washed and centrifuged using ultrapure water (at least three times, 2500 rpm, 5 min) until the solution were pH neutral. The phytoliths obtained were used as ashed phytolith with bleaching samples.

The respective treatment procedures for three types of samples (1: unburned, 2: ashed, and 3: ashed phytolith with bleaching) are shown in Figure 2. The extracted samples were scanned at 400× magnification using a Nikon-Eclipse 5Oipo microscope. Approximately 500 particles were counted in each sample (Figure 3).

Figure 2.

Phytolith extraction flow chart.

Figure 3.

A: Unburned phytolith; B: Ashed phytolith with bleaching; C: Ashed phytolith. a: Bulliform flabellate; b: Bilobate; c: Rondel; d: Acute bulbosus; e: Elongate entire; f: Elongate dentate; g: Spherioid psilate; h: Scale = 50 μm.

Phytoliths were classified using the methods of Wang et al. (1994), and Li et al. (2017, 2019) and named according to the International Code for Phytolith Nomenclature 1.0 and 2.0 (Madella et al., 2005; Neumann et al., 2019). In this study, the phytoliths were divided into four groups on the basis of their origin to understand the relationship of the darkened phytolith with phytolith morphology. Grass silica short cell phytoliths (GSSCP) are derived from the epidermis silica cells (short cells) (Li et al., 2017), and are generally distinguishable at the subfamily level, and thus have a certain level of taxonomic significance (Li et al., 2019). These types include Saddle (long saddle, squat saddle), Bilobate, Polylobate, and Rondel (Wen et al., 2020). Elongate phytoliths are formed in the epidermal long cells and are commonly produced in all grasses. Bulliform flabellate phytoliths, also known as cuneiform/bulliform cells (Cuneiform, square and rectangle) are produced in the epidermis bulliform cell. Other types include Acute bulbosus, Stoma, Papillate, and Spheroid psilate.

Analysis of carbon in phytolith sample

Carbon in the phytolith samples was determined by an alkali-capacity spectrophotometry method (Han et al., 2018; Yang et al., 2014). The specific steps are as follows: (1) 0.01 g of the dried phytolith powder was weighed into a 10 ml colorimetric tube, and 0.5 ml NaOH (10mol/L) was added. After standing for 12 h, the samples were transferred to centrifuge tubes and cleaned twice with 0.7 ml pure water; (2) C₆H₁₂O₆ was used to prepare an organic carbon standard solution (0.090909 g/l). About 0, 0.5, 1, 1.5, 2.0 ml aliquots were placed into colorimetric tubes, and the volume was stabilized to 2.0 ml with ultrapure water; (3) The samples prepared in steps 1 and 2 were added into 1.0 ml K₂Cr₂O₇ (0.8 mol/l) and 4.6 ml H₂SO₄. The sample was heated in a hot water bath (98°C for 1 h); (4) After cooling, the samples were stabilized to 25 ml and centrifuged (2500 r/min, 10 min); (5) The supernatant was placed in a 1 cm optical path cuvette, and the absorbance was measured at 590 nm using a photometer to generate a calibration standard curve and calculate the content of organic carbon.

Results

There were no visible plant tissues or microcharcoal particles in the phytolith slides. The phytolith occluded carbon (PhytOC) of the six modern plants varied, and ranged from 0.2% to 1.3%. The carbon content values varied between 1.1% to 2.8%, and 0.4% to 2.1% for ashed phytoliths that were unbleached and bleached, respectively. The ashed phytoliths for all of the plant species had higher carbon content than unburned phytoliths (from modern plant). The ashed phytoliths which were also bleached had lower carbon content than ashed phytoliths that were not bleached, but higher carbon content than unburned phytoliths for all of species analyzed (the carbon content of each individual plant species varied as follows: unburned phytolith (from modern plant) < ashed phytolith with bleaching < ashed phytolith; Figure 4, Supplemental material table 2).

Figure 4.

Carbon content for phytolith samples subjected to different treatments.

The plant burning process produces an abundance of phytoliths with the typical darkened phytoliths. This darkened color becomes weakened after the bleaching process (Figure 3). The relative ratios of darkened (burned) phytoliths varies between plant species, and range from 0.19 to 0.45. The Oryzalatifolia spp. and Imperata cylindrica (L.) Beauv. have lower proportions of darkened phytoliths compared to other plant species. The ratio of darkened (burned) to non-darkened (unburned) phytoliths appears to be related to phytolith morphology. In general, Bulliform flabellate and Elongate produce higher proportions of darkened phytolith than short cell and other type cell phytoliths (Figure 5, Supplemental material table 3).

Figure 5.

The ratios of burned phytolith varied with phytolith morphologies and plant species.

Discussion

The increased carbon content of ashed phytoliths originates from adsorption of black carbon

The relatively high temperature chosen for microwave digestion, and the two-time oxidation employed to extract phytoliths is useful for removing the organic matter in the matrix containing the phytoliths, termed the non-related carbon (neither phytolith occluded carbon nor extraneous organic remains on the phytolith surface). No microcharcoal or organic tissue was observed in the phytolith slides, showing that the elimination of non-related carbon was robust. Thus, the carbon content measured in our analyses is not likely to be generated from carbon isolated from phytolith particles. Although some organic matter could remain that might not be distinguishable under light microscopy (Santos and Alexandre, 2017), this study aimed to compare the carbon values between unburned (from modern plant) and ashed phytoliths, so the method of the phytolith extraction was kept the same.

Studies show that phytoliths contain 0.2%–5.8% phytolith-occluded carbon (PhytOC) (Li et al., 2014, 2020; Parr and Sullivan, 2005; Parr et al., 2009, 2010; Zuo and Lü, 2011). The PhytOC ranges from 0.1%–0.5% of the dry weight when the organic matter is oxidized with H₂SO₄, H₂O₂, HNO₃, and KClO₃, and any potential remains on the phytolith surface are dissolved using KOH (Alexandre et al., 2015, 2016; Corbineau et al., 2013, 2016; Santos et al., 2010; Yin et al., 2014). Differences in the efficiency of phytolith extraction protocols may have contributed to inconsistencies and overestimations of PhytOC quantification (Corbineau et al., 2013; Hodson, 2019). In this study, the PhytOC of 0.2%–1.3% for six modern plant species is in the low value range of previous studies (0.2%–5.8%), and the values are also relatively low when comparing the same species reported (as examples, 1.93%–2.46% for rice (Li et al., 2014), 3.88%–12.35% for sugarcane, Parr et al., 2009). We infer that this difference is due to the increased temperature of our microwave digestion and the two-time oxidation of phytoliths with K₂Cr₂O₇.

The carbon content in ashed phytolith samples is much higher than the PhytOC in modern plants, suggesting the increase of carbon values is not generated from the PhytOC that undergoes changes with heating but rather derived from the carbon adsorbed by phytoliths exposed to open flames. There are many microscopic cavities on the surface of the phytolith (Alexandre et al., 2015), and throughout cell wall types once the organic matter is removed (Nakamura et al., 2021; Zancajo et al., 2020), and black carbon has strong adsorptive character, which can make phytolith easily adsorb black carbon when it exposed to flames and/or ash. The reduced carbon content and decreased darkened color of burned phytoliths after bleaching also testifies to this viewpoint. Our results are also supported by Evett and Cuthrell (2017), who hypothesized that Raman graphite peaks of darkened burned phytoliths are not generated from the occluded carbon in phytoliths that undergoes changes with heating but rather from the carbon adsorbed by phytoliths exposed to open flames. The fact that the color of darkened phytoliths changes when subjected to weathering in the soil for several hundred years could be interpreted to mean that darkened phytolith adsorb black carbon and become darker in color. Therefore, the darkened color and increased carbon values of burned phytoliths is due to the adsorption of black carbon. Phytolith carbon might therefore be divided into occluded carbon and adsorbed black carbon which are analogously recalcitrant to oxidation and dissolution (our unpublished data), and able to be preserved in soils and sediments for long time periods.

The proportion of burned phytoliths varied with phytolith morphology and plant species

The darkened color of burned phytoliths has been deemed to originate from fire (combustion) (Gu et al., 2008; Parr, 2006). The ratio of burned to unburned phytoliths (BPI – burned phytolith index) and/or darkening to all morphologies of phytolith has been widely used for fire reconstruction (Boyd, 2002; Gu et al., 2008; Li et al., 2010; Morris et al., 2010; Rutherford et al., 2020). Higher proportions of darkened phytolith were produced from BULLIFORM FLABELLATE and ELONGATE morphologies relative to silica phytoliths, suggesting that particles with large size (surface area) more easily adsorb black carbon, and/or they contain more organic matter. Not all the ratios of burned ELONGATE are second only to the BULLIFORM FLABELLATE (Figure 5), which is probably related to how plant species influence the overall intensity of burning. Some of the leaves of Zea mays L. and Setaria palmifolia (Koen.) Stapf were not burned completely, which could cause their ratios to be decreased. This result indicates that high proportions of these burned morphological phytoliths could be found in soil and sediment samples. The proportion of darkened phytolith varying within the same plant species might be interpreted to reflect that each plant produces a different phytolith assemblage, and the combustion intensity and completeness results from each plant’s distinctive fuel character and potential combustion temperature (Hudspith et al., 2018; Lennox and Wadley, 2019). The phytolith of various morphotypes have different ratios of darkened phytolith. The ratios of darkened phytolith are influenced by the degree of plant combustion, which is related to the intensity of a fire. Meanwhile, the morphotypes exhibiting different ratios of darkened phytoliths are hypothesized to be influenced by their native and adsorbed carbon content. In consideration of the increase of carbon content of burned phytolith originating from the adsorption of black carbon, the increase was related with the individual phytolith assemblage of each plant.

Phytolith-related carbon as a potential paleofire proxy

Whereas the bulk of paleoecological research has focused on forested ecosystems, paleoenvironmental archives are increasingly useful for elucidating how fire regimes in unforested ecosystems respond to climatic and ecological change (Leys et al., 2018; Pereboom et al., 2020; Sae-Lim et al., 2018). In particular, phytoliths are proving to be a recorder of valuable information about non-forested ecosystems of the past sources, but their ability to record fire history has been questioned (Evett and Cuthrell, 2017; Morris et al., 2010; Rutherford et al., 2020). Therefore, refining phytolith-based paleofire proxies are paramount to understanding how fires affect these unforested ecosystems. Our analyses underscore the potential utility of phytoliths as proxies of fire history and examines the nuances of their reliability.

Plant combustion could cause phytoliths to adsorb black carbon and increase the phytolith-related carbon content. Thus, an increase of phytolith carbon might indicate the phytolith was burned in the absence of vegetation variations. The phytolith-related carbon value could be a potential fire (activity) indicator. Darkened and/or opaque (burned) phytolith can indicate the occurrence of fire activities (Boyd, 2002; Gu et al., 2008; Li et al., 2010; Morris et al., 2010; Rutherford et al., 2020 et al.). However, the phytolith can adsorb some colored chemicals during burial in soil and sediments. At the same time, there are likely changes that occur to burned phytolith coloration during weathering (Evett and Cuthrell, 2017). These possibilities make it difficult to distinguish between weathered, unburned phytoliths and unweathered, burned phytoliths (Evett and Cuthrell, 2017). In consideration of phytolith color influenced by adsorbing some chemical elements (such as Fe²⁺, Fe³⁺, Cu²⁺) would not cause the carbon content of phytolith changed. Using the phytolith-related carbon value as a potential index of fire activity might provide a good chance to discern the weathered and burned phytolith. In addition, the increased related carbon content could be due to the burned phytoliths which adsorb little black carbon, and do not appear obviously dark. This means the phytolith-related carbon content would more sensitively respond to fire activity than simple darkening of burned phytolith. In this way, our analysis of the phytolith-related carbon value shows that it may provide a more reliable and sensitive fire proxy than the quantification of darkened phytoliths as a relative proxy of fire.

Wildfire produces plenty of ashed phytoliths which can adsorb black carbon, undergo transport and be preserved in soil and sediments. Phytolith-related carbon content can be easily determined across a soil or sediment profile (using the same phytolith extraction process in the Methods section) to reconstruct wildfire occurrence when no obvious vegetation changes can be discerned from the phytolith assemblage as vegetation compositions can influence the PhytOC. Because fire and vegetation are often intrinsically linked (Bond et al., 2005), the ability to decipher fire and vegetation signals simultaneously is particularly important, so burned phytoliths are a very useful and robust indicator for paleo-wildfires. By pairing the burned phytolith index with the phytolith-related carbon value index might be conducive to the accurate identification of paleofire occurrence. However, the ashed phytoliths in soils and sediments could be influenced by the burned plant species, and the transportation and early diagenesis of phytolith. Thus, it is necessary to study and test whether the phytolith-related carbon could be a good paleofire indictor.

In addition, the combustion of straw crops produces plenty of burned phytolith that not only contains occluded carbon, but also adsorb black carbon. This carbon content sequestration of burned phytolith could play an important role in increasing the global carbon sink and mitigating global CO₂ emissions, although the burning of plants emits CO₂. The different phytolith morphologies show different ratios of darkened phytolith. Meaning the increase of adsorbed black carbon is related to type. To properly estimate the flux of atmospheric CO₂ sequestered by soil phytoliths, the black carbon adsorbed by phytoliths during fires should be quantified and considered. To accurately estimate the flux of atmospheric CO₂ sequestered by phytolith, the increase of carbon adsorbed by phytoliths burned from different plant species should be investigated because the amount of carbon adsorbed by phytolith is related to burning plant species and its phytolith assemblage.

Conclusions

Plant combustion could cause phytoliths to adsorb black carbon and increase the phytolith-related carbon. The carbon content decreases when the ashed phytoliths were bleached, indicating the adsorbed carbon was black carbon. The phytolith carbon can be divided into occluded carbon and adsorbed black carbon. Phytolith adsorption of black carbon together with occluded carbon can be called phytolith-related carbon. The ratio of darkened phytoliths is related with the burning plant species and its phytolith assemblage. The phytolith particles with large size and/or area are more easily darkened (burned phytolith). The increase of carbon content of burned phytoliths was related with phytolith assemblage of the burning plant. Phytolith-related carbon might be a potential paleo-wildfire indicator. The burned phytolith index incorporated with the phytolith-related carbon value index might be conducive to the accurate identification of the occurrence of past fire activity.

Combustion of plants can increase the content of phytolith-related carbon, which can increase sequestration of phytolith carbon on a long-time scale. To properly estimate the flux of atmospheric CO₂ sequestered by soil phytoliths, the carbon adsorbed by burned phytoliths should be quantified and considered in future research.

Supplemental Material

sj-docx-1-hol-10.1177_09596836221074033 – Supplemental material for Burned phytoliths absorbing black carbon as a potential proxy for paleofire

Supplemental material, sj-docx-1-hol-10.1177_09596836221074033 for Burned phytoliths absorbing black carbon as a potential proxy for paleofire by Haiyan Dong, Xiaobei Wei, Rencheng Li, Richard S Vachula, Shuhui Tan, Lintong Zhou and Tianxi Gan in The Holocene

Supplemental Material

sj-docx-2-hol-10.1177_09596836221074033 – Supplemental material for Burned phytoliths absorbing black carbon as a potential proxy for paleofire

Supplemental material, sj-docx-2-hol-10.1177_09596836221074033 for Burned phytoliths absorbing black carbon as a potential proxy for paleofire by Haiyan Dong, Xiaobei Wei, Rencheng Li, Richard S Vachula, Shuhui Tan, Lintong Zhou and Tianxi Gan in The Holocene

Footnotes

Acknowledgements

We are grateful to Wenwen Ding, Xianglong Su, and Aofeng Wang for help during the sampling and experiment.

Declaration of conflicting interests

The following statement is declared in regard to our manuscript submission of “Burned phytoliths absorbing black carbon as a potential proxy for paleofire.” All authors have read and approved this version of the article, and due care has been taken to ensure the integrity of the work. No part of this paper has been published or submitted elsewhere. No conflict of interest exists in the submission of this manuscript.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The work is supported by grants from the National Natural Science Foundation of China (Grant No. 41867058).

ORCID iD

Rencheng Li

Supplemental material

Supplemental material for this article is available online.

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