Influence of rice growth on the fate of polycyclic aromatic hydrocarbons in a subtropical paddy field: A life cycle study

Chemosphere 119 (2015) 1233–1239

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Influence of rice growth on the fate of polycyclic aromatic hydrocarbons in a subtropical paddy field: A life cycle study Yan Wang a,b, Shaorui Wang b, Chunling Luo b,⇑, Yue Xu b, Suhong Pan c, Jun Li b, Lili Ming d, Gan Zhang b, Xiangdong Li d a

Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Guangdong Institute of Eco-environmental and Soil Sciences, Guangzhou 510650, China d Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong b c

h i g h l i g h t s  Sewage irrigation and straw burning were two main sources for PAHs in paddy fields.  The concentrations of PAHs in rice leaves decreased during the heading stage. 1

 Rice growth can reduce PAHs in surface soil at a rate of 5 ng PAHs g

a r t i c l e

i n f o

Article history: Received 24 June 2014 Received in revised form 9 September 2014 Accepted 21 September 2014

Handling Editor: X. Cao Keywords: Rice PAHs Environmental fate Soil Water Paddy field

a b s t r a c t We measured the concentrations and profiles of polycyclic aromatic hydrocarbons (PAHs) in the soil, water, and rice tissues from a typical subtropical paddy system at various stages of rice growth over two growing seasons. Rice growth had a significant impact on the distribution and dissipation of PAHs in the paddy field. While rice was growing, the concentrations of PAHs in the soils decreased at an average decline rate of 5.3 ± 2.9 ng PAHs g1 soil d1, whereas, the concentrations of PAHs in rice tissues increased with growth time. However, the concentrations of PAHs in the rice leaves decreased during the heading stage of both two growing seasons. PAH profiles in soil, water, and different rice tissues also showed different patterns with the growing time of rice. Irrigation water was a significant source of PAHs to the paddy field. Rice growth enhanced the dissipation and transport of PAHs in the paddy system, while the sewage irrigation and straw burning after harvest added or returned PAHs to the system. For food safety precaution, sewage irrigation and straw burning should be well monitored and controlled. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous semivolatile organic compounds (SVOCs) formed from the incomplete combustion of fossil fuels, coal, straw, firewood, and other materials or released during other pyrolysis processes, including industrial processing and chemical manufacturing (Peng et al., 2011). PAHs are of great environmental concern due to their toxicity and carcinogenic potential (Tao et al., 2006; Rochman et al., 2013; Shen et al., 2013). Thus, an urgent need exists to eliminate or reduce the level of PAHs in the environment.

⇑ Corresponding author. Tel.: +86 20 85290290; fax: +86 20 85290706. E-mail address: [email protected] (C. Luo). http://dx.doi.org/10.1016/j.chemosphere.2014.09.104 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.

soil d1.

The environmental fate of SVOCs in a terrestrial ecosystem is significantly influenced by plants. Plant roots can evidently degrade or take up SVOCs from the soil (Su and Zhu, 2008; Gao et al., 2010; Ma et al., 2012), while the foliage can efficiently filter or capture both vapor and particle phases of SVOCs in the air (Su et al., 2007; Choi et al., 2008; Li et al., 2009; Moeckel et al., 2009; Wang et al., 2012). This so-called ‘‘filter effect’’, which can significantly increases the air-to-soil flux of SVOCs, has been well characterized in a forest ecosystem (Terzaghi et al., 2013). Rice is one of the most widely cultivated staple foods in the world, especially in Asian countries (FAOSTAT, 2012). China is the world’s largest producer of rice, accounting for approximately 19% of the world’s rice-cultivated area and 26% of the world’s rice production (Peng et al., 2009). Thus, rice plays an important role in China’s agricultural production and food security. Unlike most

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other crops, rice is a semiaquatic plant that requires regular flooding. This flooding, in addition to the ‘‘filter effect’’ of rice on SVOCs, may dramatically influence the fate of SVOCs in paddy fields through runoff, leaching, and co-evaporation with water (Cheng et al., 2014). The microbial communities and the associated electron acceptors, which promote the degradation of SVOCs, may differ between flooded paddy field and dry farmland. Variations in the redox gradient associated with flooding patterns (Ludemann et al., 2000; Van Bodegom et al., 2001) may also influence the fate of SVOCs within a paddy field (Ma et al., 2012). The growth of rice has been shown to enhance the degradation, dissipation, and uptake of PAHs (Gao et al., 2006, 2010; Su and Zhu, 2008; Ma et al., 2012) and other SVOCs (Chu et al., 2006; Yao et al., 2007). Paddy ecosystems have received much attention recently, not only because of food safety concerns, but also because of their potential ecological functions such as phytoremediation (Li et al., 2009; Gao et al., 2010; Ma et al., 2012). However, most previous studies (Su and Zhu, 2008; Li et al., 2009) on the bioremediation potential of rice have been conducted using pots or focused on only one or two environmental matrices; few studies have comprehensively examined the environmental fate of PAHs in a rice paddy ecosystem throughout an entire growing season. The purpose of the present study was to systematically investigate the influence of rice growth on the behavior and fate of PAHs in a subtropical paddy field. Since paddy fields are one of the most widely distributed agronomic ecosystems in the world, especially in South China, the results of this study are of particular importance to food safety in China and throughout the world. 2. Materials and methods 2.1. Sampling Sampling were conducted in a paddy field located in a suburban area of Guangzhou City [23°90 5900 N, 113°220 700 E], Guangdong Province, South China. The area comprises a small farm that cultivates two crops of rice annually (different rice varieties for the two seasons). All samples were collected between May 3 and December 13, 2012, which spans two growing seasons. Rice plants (Oryza sativa L.), as well as the surface soils (0–10 cm), were collected during 4 separate growth stages: jointing (the elongation period), heading (the flowering and grain filling period), mature (the maturation period before harvest), and idle (the period after harvest). Irrigation water and field water samples were also obtained during the flooded jointing and heading stages. The irrigation was done every 4–5 d before rice mature using water from a pond nearby, if there is not enough rainfall. Each sample consisted of 5 subsamples randomly collected at 5 sites within the study area. The sampling information is shown in Table 1.

Each plant and soil samples was wrapped with aluminum foil (pre-baked at 450 °C for 4 h), put into a polyethylene ziplock bag, and transported to the laboratory. Plant samples were separated into root, stem (internode), leaf (sheath and blade), and seed (grain and hull, if any). The plant samples were washed with tap water, and rinsed 3 times with deionized water. The soil and plant samples were then freeze-dried, ground in an agate mortar, and stored at 20 °C until further chemical analysis. 2.2. Sample extraction and analysis Briefly, soil sample weighing approximately 20 g each was spiked with deuterium-labeled surrogate standards (naphthalene-D8, acenaphthene-D10, phenanthrene-D10, chrysene-D12, and perylene-D12) and then extracted with dichloromethane (DCM) and activated copper for 24 h in a Soxhlet extractor. Plant samples weighing approximately 10 g each were homogenized with 10 g baked (450 °C for 4 h) anhydrous sodium sulfate, spiked with surrogate standards, and then Soxhlet extracted with hexane/acetone (3:1, v/v) and activated copper for 48 h. Half of each resulting plant extract was used to determine the crude fat content. This extract was dried using a gentle stream of nitrogen, and the weight of residual was measured as the amount of crude fat. Water samples were first passed through baked (450 °C for 4 h) glass fiber filters (GF/F, 14.2 cm diameter). Approximately 2 L of the filtered water was spiked with surrogate standards and liquid– liquid extracted 5 times using a separatory funnel and 50 mL DCM for each extraction. The extracts were combined and then dried with baked anhydrous sodium sulfate. The details of the procedures of sample cleanup and analysis were listed in the Supporting Information (SI), S1. Totally, 13 PAH compounds, including fluorene (FLU), phenanthrene (PHE), anthracene (ANT), fluoranthene (FLA), pyrene (PYR), benzo[a]anthracene (BaA), chrysene (CHR), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), indeno[1,2,3-cd]pyrene (IcdP), dibenz[a,h]anthracene (DahA), and benzo[g,h,i]perylene (BghiP) were analyzed. 2.3. QA/QC Procedural blanks were run along with the sample extracts to assess potential contamination during the analysis. None of the target compounds were detected in the blanks. The average recoveries of the surrogate NAP-D8, ACE-D10, PHE-D10, CHR-D12, and peryleneD12 in soil and plant samples were 65 ± 17%, 79 ± 16%, 91 ± 13%, 87 ± 14%, and 94 ± 12%, respectively. In the water samples, the average recoveries were 55 ± 9%, 59 ± 6%, 76 ± 7%, 90 ± 10%, and 93 ± 9%, respectively. No corrections with respect to the surrogate recoveries

Table 1 Sampling information.a Stage

Rice

Surface soil

Irrigation water

Field water

/ June 15 July 6 /

May 31 June 15 July 6 July 18

May 31 June 15 / /

May 31 June 15 / /

/ October 15 November 6 /

September 25 October 15 November 6 December 11

September 25 October 15 / /

September 25 October 15 / /

Root

Stem

Leaf

Seed

Season 1 Jointing Heading Mature Idle 1

May 31 June 15 July 6 /

May 31 June 15 July 6 /

May 31 June 15 July 6 /

Season 2 Jointing Heading Mature Idle

September 25 October 15 November 6 /

September 25 October 15 November 6 /

September 25 October 15 November 6 /

/ indicates that the sample was not available or collected. a All samples were collected from May 3 to December 13, 2012.

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were applied to the results reported in this study. All data shown are expressed on a dry-weight basis (ng g1). 2.4. Decline rate of PAHs The decline rate of PAHs in the soil is a measure of the diurnal decrement of PAHs compared to the original quantity. It is calculated as follows:

R ¼ ðC 0  C N Þ=N;

ð1Þ 1

1

where R is the decline rate (ng g d ), C0 is the original concentration (ng g1) of PAHs in the soil, CN is the residual concentration (ng g1) of PAHs in the soil after N days of rice growth, and N is the rice growing time (d). The rice growing period used in this calculate was May 31–July 6 and September 25–November 6 2012 for the first and second growing seasons, respectively (Table 1). 3. Results and discussion 3.1. PAHs in the surface soil samples The PAH concentrations found in the soil samples are presented in Fig. 1. The total concentrations of the 13 PAHs (R PAHs) ranged from 562 ng g1 (mature stage) to 774 ng g1 (jointing stage) for the first growing season and ranged from 644 ng g1 (idle stage) to 934 ng g1 (jointing stage) for the second growing season. The total concentrations of the 7 carcinogenic PAHs (R PAHscarc: BaA, CHR, BbF, BkF, BaP, IcdP, and DahA) in the soil samples were in the range of 309–455 ng g1 for the first growing season and 350–473 ng g1 for the second growing season. In general, the PAH concentrations in the surface soils decreased with increasing rice growing time within both seasons (p 6 0.02), especially for the PAHs with relatively low volatility (>3 rings). The average decline rate for total PAHs (Fig. 1) was 5.3 ± 2.9 ng g1 d1 (i.e., 105 ± 61 ng g1 per each growing stage), which indicates that approximately 0.7% of the original amount of PAHs in the soil was removed from the soil per day of rice growth without consideration of the variation of weather conditions. The PAH with the highest average decline rate (0.7 ± 0.4 ng g1 d1, Fig. 1) was FLA, followed by PHE (0.6 ± 1.3 ng g1 d1), while FLU showed the lowest decline rate (0.1 ± 0.1 ng g1 d1). The decline rates were similar among the high molecular weight (HMW) congeners (P4 benzene rings), but varied widely among the low molecular weight (LMW) congeners (63 benzene rings), including FLU, PHE, ANT, and FLA. This variability may be attributed to new inputs of LMW PAHs from atmospheric deposition or sewage irrigation to the field. Rice growth increases the dissipation and reduction of PAHs and other organic contaminants in soils either through enhanced biodegradation by microorganisms or the plants themselves

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(Kobayashi et al., 2009; Gao et al., 2010; Ma et al., 2012), or through plant uptake and adsorption via roots and foliage (Jiao et al., 2007; Su and Zhu, 2008; Wu et al., 2011). Plants stimulate the depletion of PAHs in soils by increasing the range and populations of microbial species and communities, and by mineralization of the soil via readily available carbon species derived from root exudates (Sun et al., 2012; Wang et al., 2014). Plant roots and their exudates also mobilize or release PAHs and other chemicals from attached soil particles through dissolution and by physically perforating, incising, and squeezing the particles (Plaza et al., 2009; Wang et al., 2014), making the PAHs more readily available for microbial degradation or root uptake (Welsh et al., 2009). Plant mobilization and dissolution can also enhance the transport tendency of PAHs, including volatilization to the atmosphere and leaching to deeper soils (Gao and Zhu, 2004). Although plant roots also take up PAHs from soils and translocate them to the shoots, the relative contribution of this plant uptake is negligible compared to plant-promoted biodegradation (Su and Zhu, 2008). The frequent flooding required for rice cultivation also leads to high surface runoff, resulting in the removal of PAHs and associated particles with the irrigation water and thus decreasing the concentrations of PAHs in the surface soil. Although rice growth gradually reduced the concentrations of PAHs in the surface soil, the concentrations of PAHs in the soil samples taken during the jointing stage of the second growing season was still much greater than those in samples taken during the idle stage after the first season. This result suggests that additional inputs of PAHs occurred to the paddy ecosystem, perhaps from the open burning of rice straw and through the irrigation water used to steep the field between the two seasons. As shown in Fig. 1, PHE, FLA, and PYR were the dominant PAH compounds in the soil samples. The PAH profiles differed slightly between the different growth stages. The relative abundance of LMW PAHs gradually increased throughout the first season (jointing 21.2%, heading 25.7%, mature 25.6%, idle 27.3%) and decreased during the second season (jointing 31.7%, heading 31.0%, mature 26.5%, idle 26.4%). This trend appears to positively follow (insignificant correlation, p = 0.797) the variation with ambient temperature through the course of the seasons, suggesting that rice cultivation itself played a less dominant role on the variation of PAH profiles in this soil. The relative abundance of LMW PAHs in the soils may have been influenced by increased gaseous deposition of PAHs during the rainy season from June to October, or by the irrigation water (see section below). Phytodegradation by rice was probably not the main mechanism for the dissipation of PAH concentrations, and for the variation of PAH profiles in the soil because biological processes would likely be selective leading to a PAH profile variation that could be correlated with the rice growing time. The effect of rice cultivation on the dissipation of PAHs probably involves physical processes, such as adsorption, runoff,

Fig. 1. Concentrations of PAH congeners in the surface soils during different stages of two growing seasons. (No. 1 means the first season, and No. 2 means the second season. Value ± standard deviation means the average decline rate (ng/g/d) for each congener.)

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and leaching, which are relatively nonselective, as opposed to selective biological processes such as phytodegradation and relevant microbial degradation. 3.2. PAHs in the water samples The concentrations of PAHs in the irrigation water and field water during the two growing seasons are shown in Fig. 2. Data for the HMW congeners (e.g., BbF, BkF, etc.) that were below the detection limit are not included in the figure. The total concentrations of 7 PAHs in the irrigation water ranged from 275 ng L1 to 1470 ng L1, while the concentrations of PAHs in the paddy field water ranged from 56 ng L1 to 1490 ng L1. The PAH concentrations in the water samples varied depending on the time of year that the samples were collected. In general, the total PAH concentrations were more abundant in the irrigation water than in the field water. The relatively high PAH concentrations in the irrigation water suggest that irrigation may be one of the main inputs of PAHs in this paddy field. The greater abundance of FLU, PHE and ANT in the field water may indicate other inputs of those PAHs, such as atmospheric deposition or release from soil particles. These data are consistent with the relatively high concentrations of LMW PAHs in the contemporaneous surface soils (heading stage in the first season and jointing stage in the second season). The concentrations of PAHs in the irrigation water were greater than those reported (32.0–755 ng L1) for water from another river in the same region (An et al., 2011). 3.3. PAHs in different tissues of rice plant samples Fig. 3 shows the PAH and fat-normalized PAH concentrations found in the different rice tissues. The RPAHs were in the range of 105–1490 ng g1 (1610–25,500 ng g1 fat) for the first growing season and 33.9–207 ng g1 (288–15,700 ng g1 fat) for the second growing season. The total concentrations of the 7 carcinogenic PAHs in the various tissues were in the range of 46.0–341 ng g1

Fig. 2. Concentrations of PAH congeners in the irrigation water and field water during different stages of two growing seasons. (J means Jointing stage, while H means Heading stage)

for the first season, and 10.5–55.0 ng g1 for the second season. The concentrations of individual PAH congeners in the various rice tissues are presented in Fig. S1, SI. Generally, the PAH concentrations in the rice tissues (especially for the fat-normalized concentrations) increased with the rice growing time. The mechanisms by which PAHs accumulated in the various plant tissues may include adsorption from soil or field water, root uptake and transport via transpiration to the shoot, and absorption by the waxy cuticle or through leaf stomata from the ambient air (Wild et al., 2004; Meudec et al., 2006; Wang et al., 2012). Rice provides a carbon-rich interface, which may efficiently accumulate PAHs from the soil, water, and atmosphere. Therefore, rice growth reduces, to some extent, PAH concentrations not only in the soil and water but also throughout the whole paddy ecosystem. A slight reduction in PAH concentrations occurred in the vegetative organs (root, stem, and leaf) during the heading stage in both growing seasons (Figs. 3 and S1). Although the possibility of sampling error exists, nutrition-associated transport of PAHs from the vegetative organs to the seeds was likely responsible for the sudden decrease in PAH concentrations in the vegetative organs. Seed formation requires many nutrients, including sugar, starch, and protein. Before the heading stage, the vegetative organs of rice accumulate sugar and starch, which are transported to the reproductive organs (i.e., rice grain) at the beginning of earing (heading stage). Some of the absorbed PAHs within the plant tissues may be transported along with sugar and starch into the seeds and may explain the relatively high concentrations of PAHs observed in the new seeds, for which adsorption from the atmosphere would likely be negligible given the short time. This hypothesis suggests that possible PAH transport mechanisms associated with organic nutrients; further study would be necessary to confirm this hypothesis. Slight increasing concentrations of FLU, PHE, and ANT occurred in the roots of the first season, especially during the heading stage

Fig. 3. PAH (a) and fat normalized PAH (b) concentrations in the different tissues of rice plant during different stages of two growing seasons. (No. 1 means the first season, and No. 2 means the second season.)

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Fig. 4. PAH profiles in the water, soil, and different rice tissues. (J means Jointing stage; H means Heading stage; M means Mature stage; I means Idle stage)

(Fig. S1), and may be due to additional inputs at that time. These data are consistent with the relatively high concentrations of FLU, PHE, and ANT in the field water during the first heading stage (Fig. 2a). Among the various rice tissues, the highest PAH concentrations were found in the roots, followed by the leaves, while the concentrations in the stems and seeds were comparable and both relatively low. The concentrations and distribution trends are consistent with the studies of Tao et al. (2006). Plant roots have the ability to both absorb and adsorb PAHs from soils, with adsorption on the root surface being the most probable. As for rice, the direct contact of rice roots with water may facilitate the transport of PAHs and their adsorption onto the root surface from both soil and field water. The PAH concentrations in the shoots were relatively lower than in the roots, suggesting relatively little uptake and translocation of PAHs from the roots to the shoots. High PAH concentrations in the leaves compared to stems and grains may be due to their relatively high wax or fat content (Table S1) and their relatively large surface areas, both of which may enhance the adsorption of PAHs from the air. The fat-normalized concentrations of PAHs in the stems and leaves were comparable (Fig. 3b), supporting the idea that differing fat content may cause the relative differences in PAH concentrations in the tissues above ground. The relatively low concentrations in the seeds may be due to their short development time, allowing less adsorption compared to the other tissues. The concentrations of PAHs in the rice tissues from the first growing season were slightly higher than those from the second season, although the soil concentrations of PAHs were slightly greater in the second season. These differences may be due to different PAH concentrations in the ambient air, different

environmental conditions (e.g., temperature and rainfall), and different rice varieties for the two seasons. The fat-normalized concentrations of PAHs in the rice tissues from the two seasons were comparable (Fig. 3b), suggesting that fat content may cause the different PAH concentrations in the two rice varieties. 3.4. PAH profiles in different matrices The PAH profiles of the water, soil, and rice tissue samples differed significantly and are shown in Fig. 4. The water samples (water phase) contained greater abundances of LMW PAHs, whereas the soil samples contained greater amounts of HMW PAHs. The PAH profiles of the various plant tissues were intermediate between those of the soil and water samples, with comparable proportions of LMW and HMW compounds. These results suggest that either the PAHs in rice tissues originate from a variety of matrices, including the soil, water, and atmosphere, or that the adsorption and uptake of PAHs by rice from the soil is compound-selective. Rice growth also influenced the PAH profiles of the water, soil, and rice tissue samples, although the variations between growth stages were slight especially for the soil samples (Fig. 4). In general, the concentrations of LMW PAHs in the field water decreased with increasing the growth time of rice, whereas those in the rice tissues increased, which implies that after being introduced to the paddy ecosystem, the LMW PAHs may have a greater tendency than the HMW congeners to adsorb to soil particles and plant surfaces. However, the concentrations of LMW PAHs decreased in rice seeds with increasing growth times, inconsistent with the other rice tissues. This observation suggested that the PAHs in rice seeds may come from a different source than those in the other tissues, such

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as transport with organic nutrients from other organs, as opposed to adsorption from the surrounding environment. 3.5. Influence of rice on the fate of PAHs in the paddy ecosystem Rice growth increases the dissipation of PAHs in the paddy ecosystem via enhanced degradation, adsorption, transfer, and uptake of the PAHs. Special flooding is accompanied by variations in redox gradients within the soil, which in turn may influence the microbial degradation of PAHs for the microbial communities and electron acceptors essential for the degradation usually vary spatially along these redox gradients. Flooding can also enhance the mobility and bioavailability of SVOCs (Yao et al., 2007). Thus, both the emission of PAHs to the air and the leaching of PAHs into deeper soil may be enhanced by rice growth. Moreover, the harvesting of rice also removed PAHs from the ecosystem. The frequent use of sewage irrigation and the burning or turning over of straw after harvest, however, may add PAHs to the soil. Although, there are some benefits of sewage irrigation and straw burning, such as reuse the recycled sewage water and returning nutrients into soil, given the potential health consequences of PAHs in rice (Ding et al., 2012), sewage irrigation and straw burning after harvest should probably be strictly monitored and controlled. Although irrigation may add PAHs into the paddy ecosystem, rice growth reduces, to some extent, the PAH concentrations in the surface soil. According to the average decline rate (5.3 ± 2.9 ng g1 d1) of total PAHs in the surface soil, we can calculate the total amount of PAHs dissipated over a given surface area during the rice growing season. First, we determine the weight of the soil (wSoil) within the root penetration zone as follows:

wSoil ¼ a  d  q  ð1  wi Þ;

ð2Þ 2

where a is the soil surface area (1 m ), d is the depth of root penetration (assumed to be 10 cm), q is the soil density (assumed to be 1.5  103 kg m3), and wi is the proportion of water to soil (assumed to be 20%). Next we calculate the total amount of PAHs dissipated (wPAHs):

wPAHs ¼ r  wSoil  t;

ð3Þ 1

1

where r is the decline rate (5.3 ± 2.9 ng g d ) and t is the rice growth time (d). Thus, during a 3-month growing period (t = 90 d), the growth of rice in this paddy ecosystem reduced the amount of PAHs in the surface soil by a calculated 57.2 ± 31.3 mg m2, which accounted (62 ± 34)% of the initial amount of PAHs in the surface soil without considering any other inputs. However, we observed higher soil concentrations of PAHs prior to the second season than at the end of the first season, which indicated a significant input of PAHs to the field during the idle period, most likely from the emission of straw burning and the ash application that occurred after the rice harvest. 4. Conclusions Rice cultivation in South China has complex effects on the environmental fate of PAHs in paddy ecosystems. The growth of rice significantly enhances the dissipation of PAHs in the surface soils of paddy fields by stimulating the degradation and transfer of PAH compounds, while the rice plant itself can adsorb and absorb PAHs into its tissues from the soil and air through the roots and leaves. Unexpected, the concentrations of PAHs in the leaves decreased during the heading stage, which needs further study. The traditional farming practices, such as frequent irrigation, field steeping, and straw turnover or burning, can increase the

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