Evaluation of the occluded carbon within husk phytoliths of 35 rice cultivars

Abstract

Rice is a well-known silicon accumulator. During its periods of growth, a great number of phytoliths are formed by taking up silica via the plant roots. Concurrently, carbon in those phytoliths is sequestrated by a mechanism of long-term biogeochemical processes within the plant. Phytolith occluded C (PhytOC) is very stable and can be retained in soil for longer than a millennium. In this study, we evaluated the carbon bio-sequestration within the phytoliths produced in rice seed husks of 35 rice cultivars, with the goal of finding rice cultivars with relatively higher phytolith carbon sequestration efficiencies. The results showed that the phytolith contents ranged from 71.6 mg·g^‒1 to 150.1 mg·g^‒1, and the PhytOC contents ranged from 6.4 mg·g^‒1 to 38.4 mg·g^‒1, suggesting that there was no direct correlation between the PhytOC content and the content of rice seed husk phytoliths (R= 0.092, p>0.05). Of all rice cultivars, six showed a higher carbon sequestration efficiency in phytolith seed husks. Additionally, the carbon bio-sequestration within the rice seed husk phytoliths was approximately 0.45‒3.46 kg-e-CO₂·ha^‒1·yr^‒1. These rates indicate that rice cultivars are a potential source of carbon biosequestration which could contribute to the global carbon cycle and climate change.

Keywords

carbon sequestration / seed husks / PhytOC / phytolith / rice cultivars

Introduction

The soil carbon pool plays a vital role in the global carbon cycle ( Lal, 2004; Liu et al., 2014). The soil carbon storage capacity is influenced by many factors, such as climate (temperature and precipitation), the nature of parent geological materials (i.e., texture and mineralogy), vegetation (crop) type, and land management practices ( McCarl et al., 2007). Approximately half of all soil carbon in managed ecosystems has been lost into the atmosphere during the past two centuries due to cultivation. Thus, carbon sequestration in the agricultural ecological system is a popular subject of study ( Linquist et al., 2012). Modern crop production technologies are currently being adopted to reduce agricultural greenhouse gas emissions in these rice growing areas, such as early maturing cultivars, high use of inorganic fertilizers and pesticides, and the expansion of irrigation facilities ( Pathak et al., 2005). However, potential processes to effectively select and breed high C occlusion efficiency cultivars have been disregarded thus far. Organic carbon can be occluded within phytoliths (i.e., PhytOC) during the course of plant growth ( Parr and Sullivan, 2005). The contents of phytoliths generally constitute up to 3% of the total soil mass ( Drees et al., 1989). PhytOC has been confirmed to be an important long-term terrestrial carbon fraction in the soil ( Parr and Sullivan, 2005; Gu et al., 2012), contributes 15%‒37% of the stable soil C sink ( Parr and Sullivan, 2005), and represents up to 82% of the soil carbon in topsoils of in situ decomposition after 2000 years ( Wilding, 1967; Wilding et al., 1967; Mulholland and Prior, 1993; Parr and Sullivan, 2005).

Phytolith, the opaline amorphous silica, is absorbed by plants and is formed in the tissues of most plants ( Perry et al., 1987; Piperno, 1988; Song et al., 2012a,b), such as bamboo, rice, wheat, etc. After plants return to the soil, the straws of these plants are decomposed by microorganisms, and the phytoliths within these straws can be released directly into the soil. Phytoliths can tolerate extreme environments, such as earthquakes, dust storms, floods, forest fire erosion, etc., and are often retained in the in situ environment ( Jones and Milne, 1963; Jones and Handreck, 1967; Wilding, 1967; Wilding and Drees, 1974; Sangster and Parry, 1981; Pearsall, 1989; Hart and Humphreys, 1997; Parr, 2006; Bowdery, 2007). However, during phytolith formation in plant tissues, some elements can be occluded ( Jones and Milne, 1963), such as C, N, P, K, Ni, Al, Fe, Ca, etc. ( Wang, 1998). In this course, phytoliths can occlude 1%‒6% organic C ( Wilding et al., 1967; Parr and Sullivan, 2005; Parr et al., 2010; Song et al., 2014a). The PhytOC content of plants in different crop species ranges from 0.04% to 0.75% of the dry crop biomass (Parr and Sullivan, 2011; Li et al., 2013). However, the PhytOC content also varies significantly between different cultivars ( Parr and Sullivan, 2011; Li et al., 2013) and tissues ( Li et al., 2013) of the same crop species. Many studies demonstrate that the PhytOC content in crops depends on the C content of phytoliths ( Parr and Sullivan, 2011; Zuo and Lü, 2011) and on the phytolith content of crops ( Li et al., 2013).

Rice, which acts as a food source as well as a silica accumulator, has many phytoliths and can occlude more organic C in its organs when compared with other plants ( Song et al., 2012a,b). The global rice-planting area was approximately 1.55×10⁸ ha in 2010 (IRRI, 2011). Rice is a major crop in Asian countries, such as India and China. From 1950 to 2010, approximately 2.37 × 10⁸ Mg of CO₂ equivalents may have been sequestrated within rice phytoliths in China ( Li et al., 2013). Assuming a maximum phytolith C biosequestration flux of 0.13 Mg-e-CO₂·ha⁻¹·yr⁻¹, the global annual potential rate of CO₂ sequestrated in rice phytoliths would be approximately 1.94 × 10⁷ Mg ( Li et al., 2013), which is greater than bamboo leaf litter (1.56×10⁷ Mg-e-CO₂ per year) ( Parr et al., 2010), sugarcane leaf (0.72×10⁶ Mg-e-CO₂ per year) ( Parr et al., 2009), and millet (2.37×10⁶ Mg-e-CO₂ per year) ( Zuo and Lü, 2011).

The presence of silica phytoliths within cereal crops is well-documented ( Yoshida et al., 1962; Ball et al., 1993; Ball et al., 1996; Lux et al., 2002; Song et al., 2014a,b,). According to the correlations between phytolith content and PhytOC content in crops, results show that the PhytOC content of plants can be increased by crop species or cultivar optimization ( Li et al., 2013; Song et al., 2014a , b). However, the variability of phytolith accumulation within the many cultivars of rice has not previously been examined. In this study, we evaluated carbon biosequestration within the phytoliths produced in the rice seed husks of 35 rice cultivars. The aim of the study is to provide a choice for farmers to plant the rice cultivars with a high efficiency of carbon sequestration in phytoliths as well as high productivity.

Methods and materials

Sample preparation

Seed samples of 35 rice cultivars, which belong to seven varieties, were randomly obtained from six provinces (Jiangxi, Jiangsu, Henan, Heilongjiang, Anhui, and Zhejiang) in China in June 2013. These seed samples were derived from the conventional cultivars within the locality, as presented in Table 1. The seed samples were weighed at 5 g and placed in respective envelopes. The seed samples were then dried in an oven and removed after 8 hours for hulling. The seed husk samples were preserved for extracting phytoliths and analyzing PhytOC and PhytON.

Phytolith extraction from seed husks

The phytolith extraction method was designed to completely remove all organic material from the seed husk of each rice species ( Walkley and Black, 1934; Zuo and Lü, 2011). In this study, a revision to the wet digestion method was implemented to digest the organic matter more completely ( Walkley and Black, 1934). The detailed steps are as follows: 1) approximately 1.0 g of dry sample was weighed in a centrifuge tube (to the nearest 0.1 mg); 2) 5 mL of HNO₃ was added to the tube and heated in a water bath at 100°C until the reaction stopped; this was repeated twice, and then the sample was centrifuged at 3,000 r/min for 5 min and the supernatant was decanted; 3) the sample was rinsed with 10 mL of distilled water, centrifuged at 3,000 r/min for 5 min and decanted, followed by two additional rinses; 4) 10% HCl was then added, and the mixture was heated in a water bath at 80°C for 30 min, centrifuged at 3,000 r/min for 5 min, and the supernatant was then decanted; 5) the sample was rinsed with 10 mL of distilled water, centrifuged at 3,000 r/min for 5 min and decanted, followed by two additional rinses; 6) 5 mL HNO₃, was added, and the mixture was heated in a water bath at 100°C until the reaction stopped to ensure the removal of all organic material and then centrifuged twice at 3,000 r/min for 5 min, followed by decanting of the supernatant; 7) 5 mL of H₂SO₄ was added, and the mixture was heated in a water bath at 100°C for 1 h; 8) the mixture was cooled to room temperature, and then 30% H₂O₂ was slowly added until the liquid cleared; and 9) the mixture was centrifuged four times at 3,000 r/min for 5 min, followed by decanting of the supernatant, and finally, the phytoliths were dried in an oven at 70°C for 24 h.

PhytOC and PhytON measurements

Phytolith residue assemblages were mounted onto glass slides in a Balsam Canada mounting medium. The slides were viewed at 400 × magnification using a Jiangnan XP-213 microscope fitted with a polarizing filter and a 5.0 MP color CCD camera to ensure that no organic material exists (Fig. 1). The cross-polarized light technique was used here to indicate the absence of cellulose, calcium oxalate crystals, and starch grains, which appear bright white and blue if present ( Parr et al., 2010). The PhytOC and PhytON were determined using an Elemental Analyzer 3000 (GmbH Company, Germany).

Statistical analyses

The mean values of all parameters were taken from the measurements of three replicates, and the standard error of the means was calculated. One-way ANOVA was applied to determine the significance of the results between different varieties, and then Turkey’s multiple range tests (p<0.05) were performed. All of the statistical analyses were performed using SPSS v.13 for Windows.

Results

The results between the husk contents, phytolith contents, and PhytOC content had considerable variation among the 35 cultivars (Table 2). There was an even greater variation in the phytolith contents in the seed husks of the cultivars, ranging from 71.6 mg·g^‒1 to 150.1 mg·g^‒1. Similarly, substantial variations were observed in the PhytOC (0.64 mg·g^‒1 to 3.84 mg·g^‒1) and PhytON (0.31 mg·g^‒1 to 0.59 mg·g^‒1) contents in the seed husks of the 35 rice cultivars. Of all the rice cultivars, the highest carbon sequestration within the phytoliths was observed in the Zhonghan 35 variety, followed by Tianyou 998, Tenuo 2072, Huaidao 5, Yanjing 47‒12, and Longjing 38. Their values were (3.22, 3.46, 2.84, 2.92, 2.90, and 2.57) Mg-e-CO₂·ha^‒1·yr^‒1, respectively (Table 2).

The relationships between the seven variables of the 35 rice cultivars are shown in Table 3. No correlation existed between the phytolith content and the PhytOC content in the phytoliths of the 35 rice cultivars (R = 0.092, p>0.05). However, the phytolith content was correlated with the PhytOC content in the seed husks (R=0.380, p<0.05) and with the estimated fluxes of seed husk PhytOC (R=0.344, p<0.05). Additionally, the PhytOC content in the phytoliths exhibited a strong positive correlation with the PhytOC content in the seed husks (R=0.865, p<0.01) and with the estimated fluxes of seed husk PhytOC (R=0.888, p<0.01). There was a strong positive correlation (R=0.855, p<0.01) between the PhytOC content in the seed husks and the estimated fluxes of the seed husk PhytOC (Table 3).

The husk content of Indica varieties ranged from 21.78% to 23.24%, and that of the Japonica varieties varied between 16.38% and 19.52%. Additionally, the PhytOC content in the phytoliths of four Indica varieties varied greatly between 0.89±0.15 mg·g^‒1 and 3.84±0.44 mg·g^‒1; significant differences in the PhytOC content of the phytoliths were observed among the seven cultivars of the Indica varieties. However, the PhytOC content in the phytoliths of three Japonica varieties ranged from 2.15 mg·g^‒1 to 2.39 mg·g^‒1 (Table 4). Although the husk average content of the Indica varieties was much higher than that of the Japonica varieties, the PhytOC content in the phytoliths of Indica varieties was on average lower than that in Japonica varieties. In summary, the seed husk PhytOC content exhibited no significant differences between the two rice varieties.

The relationships between the phytolith content, the phytolith PhytOC content, the seed husk PhytOC content, the phytolith C/N, and the estimated fluxes of seed husk PhytOC were examined. The results indicated that a weak negative correlation existed between the phytolith content and the PhytOC content in the phytoliths for seven rice varieties (R=‒0.242, p>0.05), whereas the phytolith content was strongly positively correlated with the seed husk PhytOC content (R=0.855, p<0.01), with the estimated fluxes of the seed husk PhytOC (R=0.930, p<0.05), and with the phytolith C/N (R=0.966, p<0.01). In addition, the seed husk PhytOC content exhibited a strong positive correlation with the estimated fluxes of the seed husk PhytOC (R=0.840, p<0.05) and with the phytolith C/N (R=0.833, p<0.01). There was a strong positive correlation between the estimated fluxes of seed husk PhytOC and the phytolith C/N (R=0.990, p<0.01) (Table 5).

Discussion

Carbon sequestration within the phytoliths of rice seed husks

The PhytOC content in bamboo, wheat, sugarcane, and millet was found to have no direct correlation with the phytolith content formed within plants ( Parr et al., 2009, 2010; Parr and Sullivan, 2011; Zuo and Lü, 2011). Our results indicated that there was no correlation between the phytolith content or the PhytOC content in the phytoliths of the 35 rice cultivars studied (R= 0.092, p>0.05). These data indicate that for rice, as for wheat, bamboo and sugarcane ( Parr et al., 2009, 2010; Parr and Sullivan, 2011), it is the efficiency by which carbon is encapsulated by silica within the epidermal cell walls (phytoliths), rather than the actual quantity of the silica taken up by the plant, that is most important in determining the relative PhytOC yield in plant materials ( Parr and Sullivan, 2011). The finding that the carbon was occluded within rice husk phytoliths was consistent with observations made by the aforementioned researchers. In fact, the PhytOC content depends on the efficiency of the C occluded within the phytoliths during plant growth. Genetic and physiological differences within the rice cultivars may affect the formation and the efficiency of the C occluded within the phytoliths ( Parr et al., 2009,2010,; Parr and Sullivan, 2011). Therefore, it is possible to improve the carbon sequestration of the phytoliths by selecting cultivars with high phytolith content and high C occluded in phytoliths from different rice cultivars.

The efficiency of carbon sequestration within the phytoliths in rice seed husks

The long-term carbon sequestration of soil organic carbon is relatively low (only 0.7% of net primary millennia) ( Schlesinger, 1990). In addition, many carbon fractions involved in the carbon cycle, either directly or indirectly, are responsible for environmental issues. However, the occlusion of carbon within phytoliths has been retained in soils for more than a millennium resulting in the formation of a long-term, terrestrial carbon fraction ( Parr and Sullivan, 2005). In this study, we estimated that the C fluxes of rice seed husks were 0.45 to 3.46 kg-e-CO₂·ha^‒1 per year in terms of the PhytOC content of rice seed husk material on a dry weight basis ( Wang et al., 2008) and the mean annual grain production (9.3 Mg-ha^‒1) of single rice cropping systems ( Anthoni et al., 2004; Zhang et al., 2010). However, Li et al. (2013) reported that the flux of the rice PhytOC was 0.03‒0.13 t-e-CO₂·ha^‒1 per year, which is much higher than our results. The differences between this study and Li et al.’s findings may be due to the tested cultivars and the organs (i.e., this manuscript only tested the husk, and Li et al. tested four organs). This study used an organ to estimate the PhytOC flux.

The global rice-planting area was approximately 1.55 × 10⁸ ha in 2010 (IRRI, 2011,). Assuming that the largest PhytOC flux of rice seed husk was 3.46 kg-e-CO₂·ha^‒1·yr^‒1, 5.36 × 10⁵ Mg-e-CO₂ would be globally occluded within rice seed husk phytoliths per year.

Conclusions

The carbon content of phytoliths in rice seed husks depends on the efficiency of the carbon occluded within the phytoliths during rice growth periods. The PhytOC content in the husk of 35 rice cultivars ranged from 0.64 mg·g^‒1 to 3.84 mg·g^‒1. It is estimated that the flux of C occluded within the rice husk phytoliths is between 0.45 and 3.46 kg-e-CO₂·ha^‒1 per year. Of the 35 rice cultivars considered, six exhibited relatively higher phytolith and PhytOC accumulation efficiencies. Therefore, enhancing the carbon sequestration efficiency of rice cultivar phytoliths will provide us with a new approach for increasing the soil organic carbon sink, and selecting/breeding rice cultivars with high PhytOC represents critical work in this domain.

References

[1] Anthoni P M, Freibauer A, Kolle O, Schulze E (2004). Winter wheat carbon exchange in Thuringia, Germany. Agric Meteorol, 121(1‒2): 55–67

[2] Ball T B, Brotherson J D, Gardner J S (1993). A typologic and morphometric study of variation in phytoliths from einkorn wheat (Triticum monococcum). Can J Bot, 71(9): 1182–1192

[3] Ball T B, Gardner J S, Brotherson J D (1996). Identifying phytoliths produced by the inflorescence bracts of three species of Wheat (Triticum monococcum L., T. dococcon Schrank., and T. aestivum L.) using computer-assisted image and statistical analyses. J Archaeol Sci, 23(4): 619–632

[4] Bowdery D (2007). Phytolith analysis: sheep, diet and fecal material at Ambathala Pastoral Station, Queensland, Australia. In: Madella M, Débora Z, eds. Plant, People and Places–recent Studies in Phytolith Analysis. Oxford: Oxbow

[5] Drees L R, Wilding L P, Smeck N E, Senkayi A L (1989). Silica in soils: quartz and disordered silica polymorphs. In: Dixon J B, Weed S B, eds. Minerals in Soil Environments.Madison Wisconsin: Soil Society of America

[6] Gu Y S, Wang H L, Huang X Y, Peng H X, Huang J H (2012). Phytolith records of the climate change since the past 15000 years in the middle reach of the Yangtze River in China. Front Earth Sci, 6(1): 10–17

[7] Hart D M, Humphreys G S (1997). Plant opal phytoliths: an Australian perspective. Quatern Aust, 15: 17–25

[8] International Rice Research Institute (IRRI) (2011)

[9] Jones L, Handreck K (1967). Silica in soils, plants and animals. Adv Agron, 19: 107–149

[10] Jones L H P, Milne A A (1963). Studies of silica in the oat plant. Plant Soil, 18(2): 207–220

[11] Lal R (2004). Soil carbon sequestration impacts on global climate change and food security. Science, 304(5677): 1623–1627

[12] Li Z, Song Z, Parr J F, Wang H (2013). Occluded C in rice phytoliths: implications to biogeochemical carbon sequestration. Plant Soil, 370(1‒2): 615–623

[13] Linquist B, Groenigen K, Adviento‐Borbe M APittelkow CKessel C, (2012). An agronomic assessment of greenhouse gas emissions from major cereal crops. Glob Change Biol, 18(1): 194–209

[14] Liu B, Xu H, Lan J H, Sheng E G, Che S, Zhou X Y (2014). Biogenic silica contents of Lake Qinghai sediments and its environmental significance. Front Earth Sci, 8(4): 573–581

[15] Lux A, Luxová M, Hattori T, Inanaga S, Sugimoto Y (2002). Silicification in sorghum (Sorghum bicolor) cultivars with different drought tolerance. Physiol Plant, 115(1): 87–92

[16] McCarl B A, Metting F B, Rice C (2007). Soil carbon sequestration. Clim Change, 80(1–2): 1–3

[17] Mulholland S C, Prior C A (1993). AMS radiocarbon dating of phytoliths. In: Pearsall D M, Piperno D R, eds. MASCA Research Papers in Science and Archaeology. University of Pennsylvania, Philadelphia, 21–23

[18] Parr J F (2006). Effect of fire on phytolith coloration. Geoarchaeology, 21(2): 171–185

[19] Parr J F, Sullivan L A (2005). Soil carbon sequestration in phytoliths. Soil Biol Biochem, 37(1): 117–124

[20] Parr J F, Sullivan L A (2011). Phytolith occluded carbon and silica variability in wheat cultivars. Plant Soil, 342(1‒2): 165–171

[21] Parr J F, Sullivan L A, Chen B, Ye G, Zheng W (2010). Carbon bio-sequestration within the phytoliths of economic bamboo species. Glob Change Biol, 16(10): 2661–2667

[22] Parr J F, Sullivan L A, Quirk R (2009). Sugarcane phytoliths: encapsulation and sequestration of a long-lived carbon fraction. Sugar Tech, 11(1): 17–21

[23] Pathak H, Li C, Wassmann R (2005). Greenhouse gas emissions from Indian rice fields: calibration and upscaling using the DNDC model. Biogeosciences Discuss, 2: 113–123

[24] Pearsall D M (1989). Paleoethnobotany: A Handbook of PROcedures. London: Academic

[25] Perry C C, Williams R J P, Fry S C (1987). Cell wall biosynthesis during silicification of grass hairs. J Plant Physiol, 126(4‒5): 437–448

[26] Piperno D R (1988). Phytolith analysis: an archaeological and geological perspective. London: Academic

[27] Sangster A G, Parry D W (1981). Ultrastructure of silica deposits in higher plants. In: Simpson T L, Volcani B E, eds. Silicon and Siliceous Structures in Biological Systems. New York: Springer, 383–407

[28] Schlesinger W H (1990). Evidence from chronosequence studies for a low carbon-storage potential of soils. Nature, 348(6298): 232–234

[29] Song Z L, Liu H Y, Si Y, Yin Y (2012a). The production of phytoliths in China’s grasslands: implications to the biogeochemical sequestration of atmospheric CO₂. Glob Change Biol, 18(12): 3647–3653

[30] Song Z L, Müller K, Wang H L (2014a). Biogeochemical silicon cycle and carbon sequestration in agricultural ecosystems. Earth Sci Rev, 139: 268–278

[31] Song Z L, Wang H L, Strong P J, Guo F S (2014b). Phytolith carbon sequestration in China’s croplands. Eur J Agron, 53: 10–15

[32] Song Z L, Wang H L, Strong P J, Li Z M, Jiang P K (2012b). Plant impact on the coupled terrestrial biogeochemical cycles of silicon and carbon: Implications for biogeochemical carbon sequestration. Earth Sci Rev, 115(4): 319–331

[33] Walkley A, Black I A (1934). An examination of the Degtjareff method for determining soil organic matter, and a proposed modification of the chromic acid titration method. Soil Sci, 37(1): 29–38

[34] Wang S M, Zhang C H, Hu F X, Zeng K, Zhang W H, Wang W J (2008). The quantitative analysis of rice aboveground biomass and net primary productivity. Chinese Agr Sci Bull, 24: 201–205 (in Chinese)

[35] Wang Y J (1998). A study on the chemical compostion of phytolths. J Oceano Huanghai Bohai Seas, 16: 33–38 (in Chinese)

[36] Wilding L P (1967). Radiocarbon dating of biogenetic opal. Science, 156(3771): 66–67

[37] Wilding L P, Brown R E, Holowaychuk N (1967). Accessibility and properties of occluded carbon in biogenetic opal. Soil Sci, 103(1): 56–61

[38] Wilding L P, Drees L R (1974). Contributions of forest opal and associated crystalline phases to fine silt and clay fractions of soils. Clays Clay Miner, 22(3): 295–306

[39] Yoshida S, Ohnishi Y, Kitagishi K (1962). Histochemistry of silicon in rice plant II: localization of silicon within rice tissues. Soil Sci Plant Nutr, 8(1): 36–41

[40] Zhang W J, Wang X J ,Xu M G, Huang S M, Liu H, Peng C (2010). Soil organic carbon dynamics under long-term fertilizations in arable land of northern China. Biogeosciences, 7(2): 409–425

[41] Zuo X X, Lü H Y (2011). Carbon sequestration within milletphytoliths from dry-farming of crops in China. Chin Sci Bull, 56(32): 3451–3456

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg