Effect of soil organic amendments on the behavior of

Science of the Total Environment 466–467 (2014) 906–913

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Effect of soil organic amendments on the behavior of bentazone and tricyclazole M. García-Jaramillo ⁎, L. Cox, J. Cornejo, M.C. Hermosín Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS-CSIC), P.O. Box 1052, 41080 Sevilla, Spain

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Dissolved organic matter can explain the different adsorption behavior observed for tricyclazole in the amended soils. • Biochar increased tricyclazole adsorption due to the nature of its dissolved organic matter and high specific surface area. • Delay of bentazone was observed in amended soils of lower dissolved organic matter content and higher specific surface area.

a r t i c l e

i n f o

Article history: Received 8 May 2013 Received in revised form 26 July 2013 Accepted 26 July 2013 Available online 25 August 2013 Editor: D. Barcelo Keywords: Olive oil residue (alperujo) Compost DOM Biochar Sorption Leaching

a b s t r a c t The effect of soil amendment with different organic residues from olive oil production on the sorption and leaching of two pesticides used in rice crops (bentazone and tricyclazole) was compared in order to understand their behavior and to improve soil properties by recycling an abundant agricultural residue in Andalucía (S. Spain). A residue from olive oil production (AJ), the organic compost derived from this organic waste (CA) and a biochar (BA) made from CA were used. A soil devoted to rice cultivation, IFAPA (I), was amended at 2% (w/w) of each amendment individually (I + AJ, I + CA and I + BA). In order to evaluate the effect of dissolved organic matter (DOM) from these amendments on bentazone and tricyclazole behavior, the DOM from the amendments was extracted, quantified and characterized by fluorescence spectroscopy and FT-IR. The affinity of DOM for soil surfaces was evaluated with (I) soil and two other soils of different physicochemical properties, ARCO (A) and GUAD (G). These studies revealed differences in DOM quantity, quality and affinity for the used soils among amendments which can explain the different sorption behavior observed for tricyclazole in the amended soils. Leaching assays under saturated/unsaturated conditions revealed a slight delay of bentazone in I + CA and I + BA soils when compared to I + AJ, that can be related to the higher DOM content and much lower specific surface area of AJ. In contrast, tricyclazole was not detected in any of the leachates during the leaching assay. Extraction of tricyclazole residues from soil columns showed that the fungicide did not move below 5 cm in the higher sorptive systems (I + CA, I + BA). The sorption of DOM from amendments on soil during the transport process can decrease the mobility of the fungicide by changing the physicochemical properties of the soil surface whose behavior may be dominated by the adsorbed DOM. © 2013 Elsevier B.V. All rights reserved.

Abbreviations: AJ, alperujo from Jaén; CA, compost of alperujo; BA, biochar of alperujo; I, IFAPA soil; A, ARCO soil; G, GUAD soil; I + AJ, IFAPA soil amended with alperujo from Jaén; I + CA, IFAPA soil amended with compost of alperujo; I + BA, IFAPA soil amended with biochar of alperujo; BTCs, breakthrough curves; OC, organic carbon; ON, organic nitrogen; OM, organic matter; DOM, dissolved organic matter; TOC, total organic carbon; Ce, equilibrium concentration; Ci, initial concentration; Cs, amount adsorbed of herbicide; Kf, sorption coefficient; nf, Freundlich coefficient and linearity parameter; OAs, organic amendments; SSA, specific surface area; BET, Brunauer, Emmett, and Teller method; FT-IR, Fourier transform infrared spectroscopy; HIX, humification index; LOD, limit of detection; LOQ, limit of quantification. ⁎ Corresponding author. Tel.: +34 954624711. E-mail address: [email protected] (M. García-Jaramillo). 0048-9697/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2013.07.088

M. García-Jaramillo et al. / Science of the Total Environment 466–467 (2014) 906–913

1. Introduction In the last years, the use of organic wastes from different origin as soil amendment has increased in southern Spain. This land management is accepted as an ecological method for the disposal of organic wastes, maintaining or increasing soil fertility at the same time (Fernandes et al., 2006). Several studies have shown the benefits of using organic amendments (OAs) to prevent losses of pesticides from runoff or leaching due to an increase in pesticide adsorption or in pesticide persistence (Cox et al., 2000; Albarrán et al., 2004; Yu et al., 2011). Organic amendments, and their hydrosoluble fraction, induce an important impact on pesticide dissipation, affecting their adsorption and transport processes through various chemical interactions (Thevenot et al., 2009). Although in most cases addition of organic amendments increases sorption, leaching of the pesticides can be either reduced or promoted. Competition between pesticides and dissolved organic matter (DOM), from the amendments, molecules for sorption sites and pesticide–DOM interactions (co-transport) can both account for enhanced pesticide leaching (Cox et al., 2007; Müller et al., 2007; Barriuso et al., 2010). On the contrary, DOM might enhance the retardation of organic pollutants through different coating processes such as cumulative sorption or cosorption (Haham et al., 2012). Because of that, their effect on pesticide behavior must be assessed in order to optimize their use. First studies about the effect of organic amendments on pesticide behavior were done in the late 70s (Doyle et al., 1978). The most studied amendments are municipal solid waste compost (Vieublé-Gonod et al., 2009; Fagnano et al., 2011), straw wastes (Houot et al., 1998), sewage sludge (Roig et al., 2012) and wine distillery wastes (Andrades et al., 2004). In our lab, the use of different solid organic wastes as soil amendment has been previously studied. Diverse residues from the olive oil industry as the liquid waste called “alpechin” (Cox et al., 1997b) and the solid waste called “alperujo”, have been applied to olive groves with very good results (Albarrán et al., 2004; Cabrera et al., 2009; Gámiz et al., 2012). The use of biochar as soil amendment is being widely extended in the last years. Several articles have already reported benefits of biochar as soil amendment, from the standpoint of carbon sequestration, reduction in greenhouse gases emission and improvement of soil fertility (Xu et al., 2012; Beesley et al., 2011; Cross and Sohi, 2011; Atkinson et al., 2010). Biochar is also an interesting material because of its ability to bind agrochemicals and prevent losses of pesticides from runoff or leaching (Cabrera et al., 2011; Martin et al., 2012; Lü et al., 2012; Ippolito et al., 2012). On the contrary, negative impacts have also been associated with biochar amending, including the reduction in pesticide plant uptake when biochar is added to the soil (Yu et al., 2009), reduced biodegradation of pesticides (Zhang et al., 2004), formation and release of potential toxicants in biomass combustion (Chagger et al., 1998) and ecotoxicological effects on soil organisms (Liesch et al., 2010). The aim of this study was to assess the effect of the addition of three different organic amendments on pesticide–soil behavior (adsorption– desorption and leaching) in a paddy soil. We also characterized the nature of the soluble organic matter added to the soil with the amendments in order to improve the understanding on how it can modify pesticide–soil interactions. The pesticides studied were bentazone and tricyclazole, which are used in rice crops at South Spain in paddy soils and their environmental behavior is of interest. Bentazone (3-isopropyl1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide) is one of the most widely used herbicides in farming worldwide for controlling sedges and broad-leaf weeds in rice paddies and other intensive crops (Romero et al., 1996). Bentazone acts as a photosynthetic electron transfer inhibitor (EPA, 1994). It is a weak acid that can be found mainly in the anionic form. Most of the available reports state that bentazone exhibits little sorption in soil and has relatively high mobility (Li et al., 2003). Hence, its potential risk of leaching and ground water contamination is very high and it is commonly detected in ground and surface waters

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at concentrations above the European threshold for drinking water (Dousset et al., 2004). In fact, slow release formulations have been proposed for decreasing the water potential contamination of this herbicide (Carrizosa et al., 2000). Tricyclazole (5-methyl-1,2,4-triazolo [3,4-b] benzo-thiazole) is a systemic fungicide, effective against Pyricularia oryzae and other fungus, applied during the transplant and direct sowing in rice crops. The potential environmental risk of this pesticide is very high because of its high persistence in the soil–water system (Padovani et al., 2006; Pareja et al., 2012). Its half life goes from 4 to 17 months in laboratory assays, and approximately 6 months in the field. Also, it does not readily hydrolyze in the environment and it is stable at 51 °C without volatilization. Furthermore, a considerable percentage of the active substance is expected to be bound to soil due to its high adsorption (Tomlin, 2006). 2. Materials and methods 2.1. Pesticides Analytical grade bentazone (≥97% purity) was supplied by BASF (Limburgerhof, Germany). Water solubility was 570 mg l−1 (20 °C), pKa = 2.3, and molecular mass 240.3 g mol−1. Tricyclazole (≥97% purity) was provided by Dr. Ehrenstorfer GmbH (Augsburg, Germany). Water solubility was 596 mg l−1 (20 °C), and molecular mass 189.24 g mol−1. These compounds were used to prepare the initial pesticide solutions use as external standards for pesticide analysis and to carry out all the experimental assays. 2.2. Soils and organic amendments Soils used were sampled from three different locations. IFAPA and GUAD are soils from Guadalquivir valley devoted to rice crop. ARCO soil, which comes also from the same valley, was chosen because of its remarkable physicochemical differences (sandy soil). Three organic amendments were selected for this study, all of them proceeding from the olive oil industry: alperujo (AJ), composted alperujo (CA) and a biochar made of the composted alperujo (BA) obtained through pyrolysis at 550 °C in a restricted oxygen atmosphere. Alperujo, obtained by a two-phase centrifugation process in the olive oil production, and composted alperujo (mixed with sheep manure) were produced in IFAPA Centro Venta del Llano, Jaén (Spain). Soils, air-dried and sieved through a 2 mm mesh, were amended at a rate of 2% (w/w) with the different amendments. 2.3. Physicochemical analysis of soils and amendments Soil texture was determined by sedimentation using the pipette method and clay mineralogy by X-ray diffraction on oriented specimens (Jackson, 1975). Soil pH were measured in a 1:2 (w/w) soil/deionised water mixture and the organic carbon content was determined by dichromate oxidation (Nelson and Sommers, 1996). Physicochemical properties of the soils and amendments are given in Table 1. The specific surface area (SSA) of the amendments was measured by nitrogen surface sorption, using a Carlo Erba Sorptomatic 1900 (Fisons Instruments) and the Brunauer, Emmett, and Teller (BET) method on 0.2 g of a sample previously degassed at 80 °C during 24 h (Table 1). 2.4. Characterization of DOM and interaction with soils For DOM extraction, soils and amendments were treated with a solution of 0.01 M CaCl2 (1:20 w/v), and shaken for 15 min at room temperature. The samples were then centrifuged at 8000 rpm for 10 min and filtered with polycarbonate filters (0.45 μm pore diameter). All the extracts were diluted 5 times to avoid organic matter precipitation. Samples were stored at 4 °C until use. The samples were measured with a Shimadzu 5050 Total Carbon Analyzer and their absorption at

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Table 1 Physicochemical properties of the soils, amendments and amended soils. Specific surface area of the studied amendments and texture and mineralogy of the selected soils. D.O.C. is referred to the grams of carbon for kilograms of soil. Soil/amendment/S + A

O.M. (%)

O.C. (%)

O.N. (%)

pH

D.O.C. (g kg−1)

S.S.A. (m2 g−1)

Sand, silt, clay (%)

Illite, kaolinite, smectite (%)

ARCO (A) IFAPA (I) GUAD (G) I + AJ (2%) I + CA (2%) I + BA (2%) Alperujo Jaén (AJ) Compost alperujo (CA) Biochar alperujo (BA)

0.5 0.9 4.7 1.3 1.1 1.0 80.8 41.6 11.9

0.3 0.5 2.7 0.8 0.6 0.6 50.2 16.4 6.9

0.04 0.05 0.24 0.07 0.07 0.06 1.50 1.30 0.30

8.5 8.3 7.2 8.3 8.2 7.8 6.7 8.3 9.9

0.53 0.27 5.01 0.32 0.25 0.11 4.77 2.09 0.35

– – – – – – b5 17 27

82, 7, 11 43, 37, 20 2, 32, 66 – – – – – –

60, 20, 20 59, 8, 33 58, 25, 17 – – – – – –

254 nm with a Perkin Elmer Lambda EZ210 Spectrophotometer. Results obtained are shown in Table 1. Fluorescence spectra of the extracts, before and after interaction with soils, were obtained under excitation at 254 nm using a Hitachi F-2500 FL-Spectrophotometer. In order to identify possible interferences due to protonated groups, fluorescence spectra were measured before and after acidification with 1 M HCl (pH = 2). The samples were also diluted with bi-distilled water to an optical density below 0.1 cm−1 to avoid possible concentration influences. A humification index (HIX) defined as the ratio between the area in the upper quarter (435–480 nm) of the fluorescence emission spectrum and the area in the lower quarter (300–345 nm) from fluorescence normalized data was calculated as an expression of the complexity and condensation of the organic molecules (Zsolnay et al., 1999; Marinari et al., 2010). DOM from the extracts (1:20 w/v) was further analyzed by Fourier transform infrared spectroscopy (FT-IR). The extracts were freezedried and analyzed directly using a Jasco FT-IR-6300 spectrometer. This instrument was fitted with a diamond crystal plate. The spectra were obtained at 8 cm−1 resolution from 650 to 4500 cm−1 using a combination of 128 scans. The FT-IR spectral peak assignments were interpreted based on characteristic vibrations for dairy manure biochars (Cao and Harris, 2010; Cantrell et al., 2012) and natural organic matter (Wen et al., 2009). In order to measure the magnitude of the interaction between the soils and the DOM from the amendments, DOM extracts were firstly diluted to the lower concentration. The interactions between soils and the DOM extracted were done in a 1:2 (w/v) ratio, as it was done in herbicide–soil sorption experiments. Suspensions were shaken at room temperature and then centrifuged and filtered as described above. 2.5. Herbicide–soil sorption experiments Adsorption isotherms of bentazone and tricyclazole on amended and non amended soils were measured using a batch equilibration method. Duplicate samples (5.0 g) of unamended and 2% (w/w) amended soils (with CA or BA) were treated with 10 ml of bentazone and tricyclazole solutions with initial concentrations (Ci) ranging from 2 to 40 mg l−1. Pesticide solutions were prepared in 0.01 M CaCl2. Suspensions were shaken at 20 ± 2 °C for 24 h and centrifuged at 8000 rpm for 10 min. Previously, it was determined that equilibrium was reached in less than 24 h and that no measurable degradation occurred during this period. Supernatants were filtered and equilibrium concentrations Ce determined by HPLC as described below. Desorption was accomplished after adsorption using the highest Ci (40 mg l−1) by replacing half of the supernatant with 0.01 M CaCl2. This cycle was repeated 3 times for each sample. Adsorption and desorption isotherms were fitted to the Freundlich equation (Cs = Kf Cenf) and the constants Kf and nf, which indicate the adsorption capacity evaluated at Ce = 1 mg l−1 and adsorption intensity respectively, were calculated. The distribution coefficient (Kd) was calculated as the ratio between the amount of pesticide adsorbed (Cs)

and the equilibrium concentration (Ce), at 5 mg l−1 (which falls within the range of pesticide concentrations studied). The hysteresis coefficient, calculated as H = nf(ads)/nf(des), provided us information about the reversibility of the adsorption. In this case, the lower values represent a more reversible process. 2.6. Leaching studies In order to simulate the leaching of these pesticides in amended and unamended soils, glass columns (20 cm length, 3 cm diameter) were packaged with 160 g of unamended soil or AJ, CA or BA (2% w/w) amended soil and leached every day with 15 ml 0.01 M CaCl2. After previous experiences in our lab with bigger columns we decided to adopt this kind of small glass columns, in order to improve the reproducibility of the lixiviation results. The top 5 cm were filled with 5 g of sea sand and the bottom 5 cm with other 5 g of sea sand plus glass wool, to minimize losses of soil and contamination of leachates with soil particles. Before pesticide application, columns were saturated with 0.01 M CaCl2 and then allowed to drain for 24 h. The difference between the amount of water applied and the amount of water recovered in this saturation step, allowed us to calculate the pore volumes for each soil column. The amount of pesticide corresponding to the application rate for bentazone and tricyclazole (1.2 kg ha−1 and 0.6 kg ha−1, respectively) were applied to the top of the columns dissolved in 2 ml of ethanol. Leachates were collected daily until no pesticide was recovered (in the case of bentazone) and during 30 days of 0.01 M CaCl2 solution application for tricyclazole. For comparison purposes with the more mobile compound bentazone, leaching experiments were only carrying out 30 days. Leachates were filtered (0.45 μm pore diameter) and analyzed directly by HPLC as described below. Experiments were run in duplicates. 2.7. Herbicide analysis and soil extraction Bentazone and tricyclazole were analyzed by HPLC using a Waters 600E chromatograph coupled to a Waters 996 diode-array detector. The column used was a Nova-Pack C18, 150 × 3.9 mm. The mobile phase for bentazone was a 40:60 acetonitrile/water (pH = 2.0 adjusted with H3PO4) mixture at a flow rate of 1 ml min−1 under isocratic conditions and UV detection at 213 nm. Tricyclazole was analyzed with a 20:80 acetonitrile/water eluent mixture at a flow rate of 1 ml min−1 under isocratic conditions and UV detection at 230 nm. Injection volume was 25 μl in all the cases. For both analytes a calibration plot was built for concentrations ranging between 0.1 and 10.0 mg l−1. A good linearity was always observed. The limits of detection (LOD) were evaluated as those concentrations which give instrumental signals (peak areas, SLOD) significantly different from the blank as SLOD = Sblank + 3σblank; σblank is the average standard deviation obtained from three scans of the mobile phase (Miller and Miller, 2000). LOD values were around 0.01 mg l−1 for the pesticides investigated.


Equally, the limits of quantification (LOQ), expressed in concentration, were calculated from the values of sensitivity (S), peak area corresponding to the concentration of 1.00 mg l−1 given by the slope of the calibration curve, and the signal SLOQ = Sblank + 10σblank. The values of LOQ (in concentration) were always around 0.05 mg l−1. Herbicides were extracted from soils with their respective mobile phase. The soil/solution ratio for the extraction of pesticides from soil was 1:2 in all the cases. Bentazone and tricyclazole recoveries were close to 85% and 100%, respectively.

3. Results and discussion 3.1. Soils and organic amendments

Intensity of fluorescence (normalized to maximum)

Table 1 summarizes the physicochemical properties of the soils and amendments studied showing important differences among them. G soil presented around 5 times higher organic matter (OM) content than I and A soils and a pH around one unit lower. Also, G had a much higher content on dissolved organic carbon (DOC) than the other soils. The lowest content of DOC was observed in I soil, may be due to its higher content in smectites where organic molecules can be retained because of internal surfaces accessible (expandable) to water and polar organic molecules (Hermosín and Cornejo, 1989; Cox et al., 1997a). Attending to the physicochemical properties of the amendments, the concentration in organic carbon (OC) and DOC presented important differences, with values of DOC from 4.77 g kg−1 (obtained for the fresh alperujo) to 0.35 g kg−1 (obtained for the biochar made from the compost of alperujo). CA presented an intermediate value between them (2.09 g kg−1). Remarkable differences in pH were also detected. Lowest value was observed in AJ (6.7), due to the presence of organic acids of low molecular weight, lipids and phenolic compounds, which confer acidic characteristic to that amendment (Alburquerque et al., 2004; Nasini et al., 2013). These compounds are partially degraded in the composting process (Alburquerque et al., 2006). Attending to the SSA, we can observe important differences among the amendments. The

most relevant is the high value measured for BA, in relation with AJ and CA. 3.2. Fluorescence and infrared spectroscopy: DOM analysis and affinity studies We characterized the nature of the soluble organic matter added to the soil with the organic amendments to improve the understanding of the way it can modify pesticide–soil interactions. Toward this end, the dissolved organic matter from the amendments and soils were characterized via fluorescence and Fourier transform infrared spectroscopy. Kalbitz and Geyer (2001) demonstrated that increasing DOM concentration resulted in a linear increase in humification indices with a sample specific slope. Therefore, we used a uniformly low DOM concentration of about 10 mg C l−1 for recording the spectra of all the samples. The normalized fluorescence spectra of the DOM extracts (Fig. 1) revealed remarkable differences among the OAs. The fluorescence spectra of the DOM from AJ amendment presented two peaks, with a maximum in the region near 300 nm, where the less complex and nonhumified molecules tend to fluoresce (Zsolnay et al., 1999; Cox et al., 2000; Nebbioso and Piccolo, 2013). The fluorescence spectra of DOM from CA and BA, with a maximum in the region near 400 nm, indicate that DOM is constituted by more complex, highly humified molecules. The peak of fluorescence spectrum of DOM from AJ in the region near 300 nm, attributed to fluorescence of small molecules, does not appear after interaction with the soils. This indicates that the less aromatic fraction of the DOM from AJ adsorbs to the soils and there is mainly humified organic material in solution. In the case of the other OAs, differences are not so high. In order to ensure comparability in the grade of adsorption of DOM from different amendments to the studied soils, AJ and CA extracts were diluted 1.7 and 1.4 times respectively, to the lowest content in DOC (BA). The percentages of adsorption (Table 2) were calculated through the difference between the DOC measured before and after the interaction of each soil with the DOM extracted from the amendments.

1,2

1,2

1,0

1,0

0,8

909

0,8

0,6

0,6

0,4 0,4 0,2 0,0

AJ CA BA

300 320 340 360 380 400 420 440 460 480

0,2

GUAD + AJ GUAD + CA GUAD + BA

0,0 300 320 340 360 380 400 420 440 460 480

1,2

1,2

1,0

1,0 0,8

0,8

0,6

0,6

0,4 0,4 0,2

ARCO+ AJ ARCO + CA ARCO + BA

0,0 300 320 340 360 380 400 420 440 460 480

Wavelength (nm)

0,2 0,0

IFAPA + AJ IFAPA + CA IFAPA + BA

300 320 340 360 380 400 420 440 460 480

Wavelength (nm)

Fig. 1. Emission fluorescence spectra of DOM (acidified, pH2) extracted from the amendments (AJ, CA, and BA), before and after their interaction with the studied soils (ARCO, IFAPA, and GUAD).

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Table 2 Sorption (%) of DOM from the amendments to the studied soils, before and after initial concentrations of DOC were adjusted to the lowest one in order to avoid effects related to differences in the initial concentration. Before

A I G

After

AJ

CA

BA

AJ

CA

BA

54.8 61.8 67.4

80.1 85.3 95.4

98.9 96.4 100

37.4 39.3 38.9

43.7 55.9 64.4

100 90 100

These results show the much higher affinity of DOM from BA for the soils studied, due to the higher complexity of its organic matter, when compared with AJ and CA, as deduced of the spectroscopic analyses. The humification index (HIX) deduced from the fluorescence spectra increases with increasing complexity of the organic compounds, e.g. increasing number of aromatic rings, degree of condensation and conjugation, as well as C/H ratio (Zsolnay, 2003). The lowest value of sorption and HIX (Tables 2 and 3) was observed for AJ, revealing that AJ contains great amounts of relatively nonhumified material. Just the opposite was observed for BA, with the highest HIX. The stronger reaction to acidification in the case of BA extract suggests a higher amount of carboxylic groups in this amendment when compared with the other amendments. FT-IR spectra of the DOM from the amendments used in this study and their spectroscopic assignment are shown in Fig. 2. The spectrum is typical for DOM systems (Nebbioso and Piccolo, 2013; Conte et al., 2011; Kovac et al., 2002; Kaiser et al., 1997). Differences in infrared spectra reflected mainly the thermal decomposition of organic matter in the pyrolysis process for the biochar production. The asymmetric (2932 cm−1) and symmetric (2856 cm−1) C–H stretching bands observed in the DOM spectra from AJ were associated with aliphatic functional groups. The decomposition of hemicelluloses during pyrolysis can be followed by the disappearance of the band at 1739 cm−1 (observed in CA), which indicate the removal of acetyl ester groups (Schwanninger et al., 2004). If we compare the BA with the CA spectrum, a marked decrease of the bands in the range between 1200 and 1000 cm−1 was observed, which indicates the loss of polysaccharides during pyrolysis. Pyrolysis also resulted in the formation of carboxylic groups (1623 cm−1), which has been attributed to the decomposition of carbohydrates (Sharma et al., 2004). Carboxylic groups contribute to negative surface charges (Sposito, 1989), which are essential for cation retention in the soil. Furthermore, pyrolysis temperatures above 400 °C enhance dehydroxylation (Bagreev et al., 2001; Rutherford et al., 2004). The loss of OH (observed at 1377 cm−1 in CA) and aliphatic groups gives rise to pore formation due to a concurrent development of fusedring structures (Kloss et al., 2012), especially at higher pyrolysis temperatures (like BA which was made at 550 °C). It was accompanied by increases of bands associated with vibrations of aromatic structures, such as at 1475 cm−1 and 1423 cm−1. It is consistent with the increasing specific surface area of the studied amendments (AJ b CA b BA,

Fig. 2. FT-IR spectra of the DOM of the organic amendments studied. The wavenumbers (cm−1) of the most relevant bands have been indicated.

Table 1) and the higher complexity of the molecules measured for this amendment.

3.3. Adsorption–desorption experiments Sorption isotherms of bentazone and tricyclazole on unamended IFAPA soil and IFAPA soil amended at 2% (w/w) with alperujo (AJ), compost (CA) and biochar (BA) of alperujo were performed. IFAPA soil was selected for the adsorption–desorption studies because it presents intermediate soil properties between ARCO (sandy) and GUAD (clayey). The very low adsorption of bentazone observed in all the conditions (isotherms not shown) did not allow us to accurately determine any sorption coefficient with the analytical technique used (HPLC). Sorption data did not fit well to the Freundlich equation, specially in the case of BA amended soil. The low adsorption of bentazone is related to its chemical structure, mainly to the prevalence of its anionic form at soil pH and to its high water solubility (Li et al., 2003), both tend to maintain the bentazone molecules or anions at the water solution phase. The low adsorption observed in the case of bentazone did not allow us to obtain any desorption result. The results observed for tricyclazole sorption (Table 4) are completely different. We can observe in Fig. 3 that soil and soil with amendment mixtures resulted basically in two levels of isotherms: high sorption in the case of unamended soil and soil amended with AJ and CA, and very high sorption in soil amended with BA. With the exception of isotherms corresponding to soil with BA (L type), all isotherms were of S type. Sorption data fit the Freundlich equation (R2 = 0.92–0.95). Slopes of sorption isotherms ŋf b 1 indicate that sorption is strongly dependent on the initial solution concentration,

Table 3 Humification index (HIX) calculated for the extracts of the amendments and after their interaction with the different soils studied. Also are shown the results of HIX obtained after the acidification of all the extracts. Amendments AJ AJ pH2 CA CA pH2 BA BA pH2

Soils 0.7 0.8 4.7 4.5 7.1 5.2

+AJ

+AJ (pH2)

+CA

+CA (pH2)

+BA

+BA (pH2)

ARCO

2.0

1.8

1.6

6.3

5.2

5.3

4.7

IFAPA

3.2

2.1

1.7

5.6

4.7

4.3

3.8

GUAD

2.1

1.9

1.7

4.7

3.9

3.4

3.0


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Table 4 Adsorption–desorption coefficients of tricyclazole in unamended soil and soil amended with AJ, CA and BA (at 2%).

I I + AJ I + CA I + BA

Kf ads (mg1 − nf Lnf kg−1)

nf ads

5.57 [4.49–6.89] 6.28 [5.05–7.82] 5.89 [4.94–7.01] 25.12 [21.88–28.71]

1.03 0.96 1.05 0.78

± ± ± ±

0.16 0.16 0.13 0.10

R2

kd 5 (Lnf kg−1)

Kf des (mg1 − nf Lnf kg−1)

nf des

0.93 0.92 0.95 0.95

5.85 5.89 6.38 17.84

6.69 [3.69–12.16] 6.19 [5.21–7.35] 15.49 [6.61–36.31] 27.29 [22.28–33.42]

1.15 1.14 0.83 0.65

with greater concentration dependence at the higher initial solution concentrations (Cox et al., 1997c). The adsorption of tricyclazole in the BA amended soil is around 4 times higher than the adsorption observed in the unamended soil and soils amended with AJ and CA. This can be related to the higher specific surface area of BA (27 m2 g−1), supporting previous results obtained with other biochars and other sorbents (Chen et al., 2008; Cabrera et al., 2011), and also to the higher coating process observed for DOM from BA (Table 2), which gives rise to more hydrophobic surfaces than in the case of AJ and CA. Desorption data of tricyclazole on the soil amended with AJ and BA fit the Freundlich equation (R2 = 0.99, 0.90), but the fit was worse on unamended soil and soil amended with CA (R2 = 0.87, 0.60). Hysteresis phenomenon (nf des value lower than nf ads) or irreversibility was observed in the case of the soils amended with CA and BA. The lowest desorption, observed in the most sorptive systems (I + BA and I + CA), indicate that hysteresis can be attributed to irreversible binding of the molecules to the amendment surfaces.

± ± ± ±

0.32 0.09 0.48 0.15

R2

H (nf ads/nf des)

0.87 0.99 0.60 0.90

0.89 0.84 1.27 1.20

studies under saturated/unsaturated flow conditions, the water moves slowly through the column and physical properties such as soil specific surface area can play an important role. The shift to the right of the maximum of bentazone BTC in CA and BA amended soils (I + CA and I + BA) agrees with the higher SSA measured for these organic amendments (17 and 27 m2 g−1, for CA and BA, respectively) compared to that of AJ (b5 m2 g−1). Bentazone molecules would move slower in these systems allowing physical sorption or entrapment to occur (Cox et al., 1997a). On the contrary, the behavior observed for I + AJ is similar to that of unamended soil, and can be attributed to the higher amount of DOC provided by this amendment. Competition between pesticides and DOM molecules for sorption sites and pesticide–DOM interactions (co-transport) can both account for enhanced pesticide leaching (Barriuso et al., 2010; Cox et al., 2007; Müller et al., 2007). The high content in NH groups observed by FT-IR spectra for DOM from AJ, may also benefit the association between the bentazone in

3.4. Leaching studies

60

50

% Bentazone leached

Bentazone was detected in almost all the leachates from the beginning of the soil column leaching experiments with the unamended I soil and I soil amended with AJ, CA and BA at 2%. Breakthrough curves (BTCs) (Fig. 4) were very similar, with the position of the maximum of all the BTCs close to one pore volume, which is typical of low sorptive systems and highly mobile compounds (Beck et al., 1993). Leaching of bentazone was completed (98 ± 2%) during the first eight days (a total of 120 ml 0.01 M CaCl2 applied) for all the treatments. However, we observed shifts of the maximum of bentazone BTCs to the right, at one pore volume, in the amended soils I + CA and I + BA despite no increase in sorption was observed. This reveals the differences between static (batch) and dynamic (soil column) studies. In the case of column

IFAPA soil I + AJ (2%) I + CA (2%) I + BA (2%)

40

30

20

10

0 0

2

3

4

5

6

120

Cumulative % bentazone leached

80

60

Cs (ppm)

1

40

IFAPA soil I + AJ (2%) I + CA (2%) I+ BA (2%)

20

0 0

2

4

6

8

10

Ce (ppm) Fig. 3. Tricyclazole adsorption–desorption isotherms calculated for unamended soil and soil amended with AJ, CA and BA at 2%.

100

80

60

40

IFAPA soil I + AJ (2%) I + CA (2%) I + BA (2%)

20

0

0

1

2

3

4

5

6

Pore volume Fig. 4. Breakthrough curves (BTCs) of bentazone (a) leaching and cumulative bentazone (b) leaching in the unamended and amended soil hand packed columns.


% tricyclazole extracted from the soil columns

912

70

Acknowledgments

60

IFAPA soil IFAPA soil + AJ (2%) IFAPA soil + CA (2%) IFAPA soil + BA (2%)

50 40 30 20

References

10 0 0 to 5

5 to 10

10 to 15

15 to 20

Soil fractions (cm) Fig. 5. Percentage of tricyclazole extracted from the different soil fractions.

solution and DOM. Bentazone was not recovered in any soil fraction after the leaching experiment. Tricyclazole was not detected in any of the leachates along the complete assay (30 days). In soils amended with CA and with BA, tricyclazole can only be found in the first fraction of soil (5 cm) as observed in Fig. 5. The recovery of tricyclazole, with methanol, from the four soil fractions was of 59% in unamended soil and of 65, 50 and 46% in soils amended with AJ, CA and BA, respectively. These low recoveries can be attributed to the degradation of tricyclazole in the soil column. However, the very high half life reported for this compound (Tomlin, 2006) suggests that the physical entrapment in soil pores during leaching, is the main factor responsible for the low recoveries. The adsorption of DOM from amendments on soil during the transport process (as shown in Table 2) is higher for BA and CA amended soils. This enhanced adsorption in I + BA and I + CA amended soils, decreases the mobility of tricyclazole and demonstrates that the adsorbed DOM can change the physicochemical properties of the original soil surfaces which may be dominated by the adsorbed DOM. It has also been studied for other pesticides of the same family, like propiconazole (Wu et al., 2003).

4. Conclusions The DOM analysis and the affinity studies carried out with the amendments and selected soils revealed higher affinity of DOM from BA for the soils studied due to remarkable differences in the molecular structure, as it was corroborated by fluorescence and infrared spectroscopy. Sorption experiments carried out with IFAPA soil and all the amendments showed very low adsorption in unamended and amended soils for bentazone, due mainly to the anionic nature of this herbicide and its high hydro-affinity. In the case of tricyclazole, highest sorption was observed in soils amended with BA despite the lower OM content of this amendment as compared with AJ and CA. Tricyclazole was not detected in any of the leachates and was just partially recovered from the soil fractions. The lowest amount was recovered in columns handpacked with I + BA, in relation with the highest sorption observed for this soil. Thus, the complexity of the nature of the DOM from the amendment and its specific surface area, both higher in the case of BA, could be the main factors determining the behavior of tricyclazole in amended soils.

Conflict of interest None.

We wish to thank M.J. Calderon and P. Velarde for technical help. We also thank to Prof. K. Spokas for providing us the biochar we applied in this study. We appreciate the financial support of Ministerio de Economía y Competitividad with the AGL2010-21421 project, and the financial support of Junta de Andalucía through the PAIDI AGR-264 group, both cofinanced with FEDER-FSE PO2007-13. M. García-Jaramillo also thanks MEC/FECYT for a doctoral fellowship through AGL201021421-CO2-01 project of MICINN.

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