Compost and vermicompost of olive cake to bioremediate triazines-contaminated soil

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a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m

w w w. e l s e v i e r. c o m / l o c a t e / s c i t o t e n v

Compost and vermicompost of olive cake to bioremediate triazines-contaminated soil Laura Delgado-Moreno⁎, Aránzazu Peña Estación Experimental del Zaidín (CSIC), Dpto. de Geoquímica Ambiental, c/ Profesor Albareda, 1. 18008-Granada, Spain

AR TIC LE D ATA

ABSTR ACT

Article history:

The use of organic amendments to bioremediate potential organic pollutants of soil and

Received 8 November 2007

water has become an increasingly relevant issue in the last years. This strategy has been

Received in revised form 14 October 2008

applied to four triazine herbicides in a typical calcareous agricultural soil of the

Accepted 18 October 2008

Mediterranean area. The soil was amended with olive cake, compost and vermicompost

Available online 29 November 2008

of olive cake at rates four times higher than the agronomic dose in order to stimulate biodegradation of simazine, terbuthylazine, cyanazine and prometryn, added in a mixture

Keywords:

to the soils. Degradation studies were carried out in sterile and microbially active soil to

Soil

evaluate the effect of the chemical and biological degradation of triazines. The residual

Bioremediation

herbicide concentrations at the end of the degradation assay showed no significant

Herbicides

differences between non amended and amended soil. However, the addition of compost and

Olive residues

vermicompost enhanced the biological degradation rate of triazines during the first week of

Compost

incubation, with half-lives ranging form 5 to 18 days for the amended soils, whilst negligible

Vermicompost

degradation occurred in non-amended soil during this period. In contrast, olive cake did not significantly modify the degradation of triazines in spite that the addition of this amendment to soil resulted in the highest dehidrogenase activity values. In all the substrates, degradation of cyanazine and prometryn was faster (between 1.5 and two times higher) than those of terbuthylazine and simazine, without significant relationship with sorption parameters. The first order kinetic equation satisfactorily explained the experimental data for all triazines. A biphasic model, such as that proposed by Hoerl, was better to predict the very rapid triazines decay during the first week of incubation in soil amended with compost and vermicompost. © 2008 Elsevier B.V. All rights reserved.

1.

Introduction

Triazine herbicides have been largely used in agriculture for the control of different weeds in crops, such as maize or vineyards. However, due to their physicochemical properties (in particular, their relatively long persistence) there have been numerous reports of their presence in surface or ground waters (Clark and Goolsby, 2000; Blanchoud et al., 2007). Several bioremediation strategies have been proposed to reduce the presence of pesticides in soil from which they can reach groundwater, such as remediation by enhancing the

microbial population able to degrade specifically the target compounds. This strategy has been approached by addition of organic exogenous matter of different origin (Gerstl et al., 1997; Abdelhafid et al., 2000; Delgado-Moreno and Peña, 2007). Among the latter, residues from agricultural activities are being considered, since they are produced in large quantities and because its application is a way of returning some organic matter to soils whose content has been progressively depleted, such as the semiarid soils from Mediterranean areas. In Spain, and particularly in Andalusia (South of Spain), the olive culture is of high socioeconomic relevance.

⁎ Corresponding author. Tel.: +34 958 181600; fax: +34 957 129600. E-mail address: [email protected] (L. Delgado-Moreno). 0048-9697/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2008.10.047

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The production of olive oil generates huge amounts of alperujo or olive cake, ca. 800 kg per 1000 kg of olives, which is the main residue from the continuous centrifugation twophase process. This olive cake can be used for biomass or ethanol production, or to bioremediate pesticide contaminated soil. Natural olive cake has, however, some drawbacks due to its content in polyphenols and organic acids with phytotoxic and antimicrobial effects. The transformation of olive cake into more stabilized amendments, for example, by composting or vermicomposting, has become an attractive option for agricultural residues as long as negative effects on soil and ground water can be eliminated (Hartlieb et al., 2003), because degradation and transformation reactions during these processes affect both organic matter and organic contaminants present in the original substrate. In summary, the aim of the paper presented here is the evaluation of the ability of organic materials from the olive culture, either fresh (such as olive cake) or composted or vermicomposted, to modify the degradation rate of four herbicides belonging to the triazine family and differing in their physicochemical properties. Two aspects are considered in the overall process: the influence of the nature of the organic amendment and the properties of the herbicides selected.

2.

Materials and methods

2.1.

Soil and organic amendments

Soil (S) was sampled from the upper layer (0–25 cm) of an olive orchard located near Iznalloz (Granada province, Southern Spain), air-dried and passed through a 2-mm sieve. It is a Calcaric Regosol, with silty clay loam texture, 34% clay, 56% silt and 10% sand, containing 44.1% CaCO3. Total organic carbon content, determined by oxidation with K2Cr2O7 (MAPA, 1986), was 2.2% and pH (5/20 soil/water ratio) was 7.9 ± 0.1. The amendments used, air-dried and sieved b2 mm, were prepared from olive mill wastes and consisted of: olive cake from a two-stage centrifugation process (”alperujo” in Spanish, A), compost (C) and vermicompost from olive cake (V). The olive cake was obtained from an olive oil industry (Romeroliva, Deifontes, Granada, Spain). The mature compost and vermicompost were prepared as described elsewhere DelgadoMoreno et al. (2007). The main chemical properties of raw olive cake, compost and vermicompost are summarized in Nogales and Benítez (2007). The amended soil received a dose equivalent to 200 t ha− 1 of each amendment. The organic carbon content at the beginning of the study of soil amended with A, C and V was 4.9, 3.6 and 3.8%, respectively. Autoclaving soil and amendments at 103 kPa and 121 °C for 1 h d− 1 on three consecutive days was the method employed to sterilize the substrates. To verify the effectiveness of the sterilization method, 100 µL of soil and amendments suspensions were pipetted into Petri dishes containing 15 mL of agar– water minimum medium. The substrate suspensions were spread over the surface and the dishes were incubated at 22– 25 °C for two months. In this incubation time no bacterial colonies or fungi were observed.

2.2.

Herbicides

The selected herbicides were terbuthylazine (N2-tert-butyl-6chloro-N4-ethyl-1,3,5-triazine-2,4-diamine), cyanazine (2-(4chloro-6-ethylamino-1,3,5-triazin-2-ylamino)-2-methylpropiononitrile), simazine (6-chloro-N2,N4-diethyl-1,3,5-triazine2,4-diamine) and prometryn (N 2 ,N 4 -di-isopropyl-6methylthio-1,3,5-triazine-2,4-diamine), all with purity N99% (Dr. Ehrenstorfer, Germany). They are pre- or post-emergence broad-spectrum herbicides to control weeds in many crops (vines, fruit trees, olive trees, etc.) (Tomlin, 2003). They are all weakly basic compounds with pKa ranging from 0.6 to 4.1. Their water solubility and octanol/water partition coefficients, as well as their chemical structures are shown in Fig. 1 (Tomlin, 2003; Noble, 1993).

2.3.

Persistence studies

Firstly, 1 kg of the non amended and amended soil was preincubated with a humidity equivalent to 40% of water capacity at 15 °C during three days with the aim of stimulating the microbiological activity. Subsequently, a suspension of the methanol solution of the herbicide mixture was incorporated into non amended and amended soil to give a nominal initial concentration of 1.5 µg herbicide g− 1 soil (dry weight). The soil was left in a hood until the solvent had evaporated. Further water was added to adjust soil moisture content to 70% of water capacity. Each sample was thoroughly mixed by passing it several times through a 2 mm sieve and then split into two subsamples of 500 mg each, which were transferred to loosely capped glass containers. All the soil–herbicide mixtures were incubated at 15 °C and soil moisture contents were maintained

Fig. 1 – Chemical structure and selected physicochemical properties of the triazine herbicides.

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by periodic additions of Milli-Q water followed by vigorous shaking. All treatments in the subsamples were sampled per duplicate immediately after preparation and then at intervals during the subsequent 60 days. The degradation studies were also carried out in sterile non amended and amended soils. The analysis of triazine residues was carried out by ultrasound-assisted extraction. Soil samples (10 g) were extracted, per duplicate, with 50 mL of residue analysis grade methanol (Merck, Darmstadt, Germany) for 15 min. Then the samples were centrifuged at 3000 rpm and 15 °C for 15 min and the supernatant was filtered. Finally the extract was concentrated in a rotary evaporator at 60 °C. The dry residue was dissolved with 2 mL of hexane:toluene (1:1). Recoveries in non amended soil and soil amended with compost and vermicompost ranged between 81 ± 11 and 114 ± 12%, depending on the herbicide and the substrate (Delgado-Moreno et al., 2008). In soil amended with olive cake the recoveries were lower (from 54 ± 14% to 71 ± 7%) due to the interferences of co-extracted organic compounds from the olive cake.

2.4.

Analytical procedure

Samples were analysed by gas chromatography in a Varian Star 3400 CX, equipped with a thermionic specific detector and an 8200 automatic injector (all from Varian, Madrid, Spain). One mL of the sample was injected splitless, using He as the carrier gas, on a Hewlett–Packard Ultra 2 column (cross-linked 5% phenylmethylsilicone) (25 m, 0.32 mm i.d., 0.17 mm film thickness). Injector and detector temperatures were 280 and 300 °C, respectively. Oven temperature was programmed starting at 45 °C (1 min), at 25 °C min− 1 until 160 °C and at 7 °C min− 1 until 210 °C (2 min). Under these experimental conditions retention times of simazine, terbuthylazine, prometryn and cyanazine were 11.6, 12.1, 13.9 and 15.0 min, respectively.

2.5.

Dehydrogenase activity

For the determination of the dehydrogenase activity (DHA), 1 g of non amended or amended soil at different incubation times was incubated during 20 h at 25 °C with 0.2 mL of 0.4% 2-piodophenyl-3 p-nitrophenyl-5 tetrazolium chloride (INT) as a substrate. The iodonitrotetrazolium formazan (INTF) produced in the reduction of INT was extracted with a mixture of acetone: tetrachloroethylene (2:3) by shaking and measured in a spectrophotometer at 490 nm. DHA was determined in triplicate in non amended and amended nonsterile soils.

2.6.

Mathematical equations

The simple first-order equation (C = C0 × e− kt) and two biphasic models, the Hoerl function (C = C0 × ebt × tc) and a biexponential equation [C = F(C0 × e− k1t) + (1 − F)(C0 × e− k2t)], were fitted by nonlinear regression, using a Levenberg–Marquardt algorithm. In the different equations, C corresponds to the herbicide concentration at time t (d), C0 represents initial herbicide concentration and ki (d− 1) refers to the degradation constants. For ease of data treatment, C is expressed as percent of applied pesticide, such that C0 = 100. The Hoerl function includes the parameters b (similar to k in the first-order equation) and c,

which is a measurement of the deviation from the exponential behaviour (Sánchez et al., 2003). Finally in the biexponential equation, F is lower than one and represents the partitioning coefficients between pool 1 and pool 2, being k1 and k2 the rate constants for the two pools.

2.7.

Statistical analysis

Data were subjected to the statistical test of related samples using SPSS version 13.0.1 statistical software (SPSS Inc., Chicago, Illinois). The degradation curves were also compared among them with the methods of regression lines comparison using STATGRAPHICS Plus 5.1. statistical software (Statistical Graphics Corp., Princeton, NJ).

Table 1 – Kinetic parameters obtained from the simple first order equation fit for triazine residual concentration in non-amended soil (S) and soil amended with olive cake (SA), compost (SC) and vermicompost (SV) Co ±sd a

−k×102 ±sd a RSE b

t1/2

DT50 kbiot /ktotal c

Simazine S 107.6 ± 2.1 SA 100.3 ± 2.9 SC 85.1 ± 2.2 SV 80.8 ± 3.0 103.1 ± 2.7 Sster SAster 104.6 ± 1.5 91.5 ± 1.7 SCster SVster 87.6 ± 2.1

2.8 ± 0.1 3.2 ± 0.2 1.9 ± 0.1 1.9 ± 0.2 3.1 ± 0.2 2.9 ± 0.1 1.1 ± 0.1 1.5 ± 0.1

29.6 40.8 43.2 80.6 48.8 21.9 29.2 31.2

24.8 21.4 32.5 36.5 22.4 23.9 63.0 45.1

28.2 25.1 26.7 21.8 19.8 26.9 61.6 40.6

− 0.11 0.09 0.42 0.21

Terbuthylazine S 107.4 ± 2.5 SA 95.6 ± 3.8 SC 90.5 ± 1.6 SV 90.6 ± 1.8 103.1 ± 2.2 Sster SAster 103.6 ± 2.8 95.3 ± 1.6 SCster 88.5 ± 1.7 SVster

2.0 ± 0.1 2.6 ± 0.3 1.7 ± 0.1 1.5 ± 0.1 2.1 ± 0.1 2.4 ± 0.2 1.1 ± 0.1 1.1 ± 0.1

48.9 75.9 23.7 30.1 35.1 64.3 23.9 23.9

34.7 26.6 40.8 46.2 33.0 28.7 60.1 63.6

38.1 34.7 35.3 39.0 28.6 29.5 65.4 54.8

− 0.05 0.08 0.35 0.27

Prometryn S 111.7 ± 4.3 SA 100.4 ± 4.1 SC 93.7 ± 1.6 SV 92.2 ± 2.1 107.5 ± 2.2 Sster SAster 100.3 ± 4.2 94.1 ± 1.2 SCster 89.8 ± 1.8 SVster

3.4 ± 0.3 2.4 ± 0.3 3.2 ± 0.1 2.9 ± 0.1 2.2 ± 0.1 2.7 ± 0.3 0.9 ± 0.1 1.0 ± 0.1

115.3 66.5 18.3 32.1 35.0 77.4 13.9 34.4

20.4 28.9 21.7 23.9 31.5 25.1 77.0 69.3

23.0 31.9 19.3 20.7 32.8 24.9 71.9 66.6

0.35 − 0.13 0.72 0.66

Cyanazine S 101.1 ± 3.0 SC 73.3 ± 4.7 SV 75.8 ± 5.4 106.6 ± 2.3 Sster 87.8 ± 1.9 SCster 86.9 ± 2.4 SVster

4.9 ± 0.3 4.8 ± 0.6 5.8 ± 0.9 2.0 ± 0.1 1.1 ± 0.1 1.6 ± 0.1

48.7 133.0 165.6 38.5 36.7 49.1

14.1 14.4 11.9 34.7 63.0 44.2

13.2 3.4 3.5 34.0 58.7 37.1

0.59 0.77 0.72

Microbially-active and sterile soils. Standard deviation (n = 2). b Residual Standard Error. c Contribution of biotic to overall degradation. a

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3.

Results and discussion

3.1.

Triazines degradation in soils

Degradation of triazine herbicides in microbially active and sterile soil followed first-order kinetics (Table 1). The coefficients of regression were greater than 0.89 for all soil samples and highly significant (P b 0.01), thus indicating that the assumption of first-order kinetic was acceptable. A first order rate constant of cyanazine is not presented for soil amended with olive cake because of the interferences with soil matrix, which hindered the analytical determination (Delgado-Moreno et al., 2008).

3.1.1.

Microbially active and sterile non-amended soil

In microbially active non-amended soil, the remaining concentration values of cyanazine and prometryn decreased faster than those of simazine and terbuthylazine as indicated by their higher degradation rate values (Table 1). The herbicide concentration at the end of the incubation time was 27, 36, 13 and 10% of the initial amount in the case of simazine, terbuthylazine, cyanazine and prometryn, respectively. Simazine and terbuthylazine were degraded mainly by chemical processes as indicated by the absence of significant differences (P N 0.05) between degradation curves of micro-

bially active and sterile non-amended soil for both triazines (Table 1). In contrast, prometryn and cyanazine were degraded both chemically and biologically, since the k values in sterile soil were lower than in microbially active soil (Table 1). Due to this, the concentration of the latter two triazines at the end of the incubation period was approximately three times higher in sterile than in microbially active soil. Assuming that for overall degradation the rate constants of individual degradation pathways are additive, as suggested by Buser and Müller (1995) and that the contribution of biotic to overall degradation can be calculated as kbiotic / ktotal = 1 − (kabiotic / ktotal), this relationship confirms (Table 1) that the contribution of biotic pathways in non amended soil for simazine and terbuthylazine was negligible, whilst it was higher, between 35 and 60%, in the case of prometryn and cyanazine. No statistical significant relationship (P N 0.10) between the degradation rates (Table 1) and the sorption parameters (Kf) of the triazines studied (Delgado-Moreno et al., 2007) was found in this study. Thus, the Kf values for prometryn and cyanazine in the same soil were the highest (3.21) and the lowest (0.73), respectively, whilst they showed the highest degradation rates (k) (Table 1). On the other hand simazine, which was found to be only weakly sorbed on this soil (Kf = 0.72), was also one of the less degraded triazines (Table 1). These results contradict other studies (Dousset et al., 1997; Di et al., 1998) which report an inverse relationship between sorption and degradation.

Fig. 2 – Triazine degradation in non-amended soil (□) and soil amended with olive cake (Δ), compost (O) and vermicompost (X). Experimental data fitted to a first-order equation.

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The greater degradation of cyanazine and prometryn could be attributed to their chemical structure. The presence of the nitrile group in the cyanazine molecule and a methylthio group in that of prometryn (Fig. 1) could favour the degradation of these herbicides, because the nitrile group is more easily attacked than the chlorine atom (Knuesli et al., 1969; Beynon et al., 1972). In that sense, since triazines are added as a mixture to the soil, the presence of prometryn and cyanazine as a carbon and nitrogen source for microorganisms could limit terbuthylazine and simazine biological degradation. For both latter herbicides chemical degradation would be the main pathway, in agreement with the data presented. Other studies have found that cyanazine was more easily degraded in soil than atrazine and simazine (Beynon et al., 1972; Sirons et al., 1973; Blumhorst and Weber, 1992). Moreover other reports point to a limitation of atrazine degradation in bacterial communities in the presence of cyanazine (Gebendinger and Radosevich, 1999; Lin et al., 2006). The results obtained in this study suggest that cyanazine and prometryn, which have shown to be more prone to dissipation, could have also reduced simazine and terbuthylazine degradation by soil microorganisms. Abdelhafid et al. (2000) reported a decreased atrazine mineralization in the presence of other organic and mineral N compounds, inversely related to the availability of the molecules. More studies would be necessary to confirm the occurrence of phenomena of competition between the triazines studied.

3.1.2.

Microbially active and sterile amended soil

The addition of amendments to soil did not increase its overall ability to degrade the four triazines considered in this study. Thus, at the end of the incubation time, the concentration of the different herbicides in amended soils was not significantly different (P N 0.05) from that in non-amended soil (Fig. 2). Controversial results have been published since soil degradation of pesticides in the presence of organic amendments has been reported to be favoured, by stimulating microbial activity (Perruci et al., 2000; Getenga, 2003), or limited, by increasing

pesticide sorption to soil (Alvey and Crowley, 1995; Gerstl et al., 1997; Fernández et al., 2000). In spite of these previous general results, slight differences in the degradation process were observed in soil amended with compost and vermicompost with regard to non-amended soil. In the first week of incubation degradation was rapid for all triazines in soil amended with compost and vermicompost, and significant differences (P b 0.05) in herbicide concentration were found between amended and non-amended soil (Fig. 2). After seven days of incubation the residual concentration values ranged between 38% and 76% for the different herbicides in soil added with both amendments, while in non-amended soil negligible degradation occurred, except for cyanazine (Fig. 2). According to these results, in soil amended with compost and vermicompost the degradation rate of triazine herbicides could be divided into two different phases: an initially fast degradation phase (0–7 days), followed by a much slower phase (7–69 days). The rate constants calculated by fitting the data to two consecutive first-order kinetic equations are shown in Table 2. The corresponding half-lives ranged from 5 to 18 days in the first phase and from 21 to 55 days in the second one. For the overall process the range was 12–46 days. In non-amended soil the residual concentration data obtained during the first week of incubation did not fit well, except for cyanazine, to a first-order kinetics (R2 = 0.1–0.4) resulting in half-lives higher than 200 days denoting the absence of degradation of terbuthylazine, simazine and prometryn during this period (Table 2). These results indicate that, although organic amendments had a negligible influence on the behaviour of triazines in soil when the whole incubation time is considered, compost and vermicompost enhanced triazine degradation in the first days. Similar findings were reported by Navarro et al. (2003) for simazine and terbuthylazine. The higher values of degradation rates for all triazines during the first week of incubation in soil amended with compost and vermicompost could be related with the increase

Table 2 – Kinetic parameters corresponding to the fitting to a first-order equation of triazine residual concentration in nonamended soil (S) and soil amended with compost (SC) and vermicompost (SV) for the initially fast degradation phase (<7 d) and the slow phase (>7 d) Simple first order equation C0 ± sd a

− k × 102 ± sd a

Simple first order equation

t1/2

R2

C0 ± sd a

− k × 102 ± sd a

t1/2

R2

Simazine

Terbuthylazine Sb7 d SN7 d SC b 7 d SC N 7 d SV b 7 d SV N 7 d

102.9 ± 5.2 102.8 ± 3.7 96.8 ± 3.5 85.4 ± 3.1 97.9 ± 4.5 89.6 ± 3.9

0.3 ± 0.7 1.9 ± 0.1 3.9 ± 0.4 1.5 ± 0.1 4.0 ± 0.5 1.5 ± 0.1

216.6 36.6 18.0 46.5 17.3 44.9

0.44 0.99 0.93 0.99 0.89 0.98

Sb7 d Sb7 d SC b 7 d SC N 7 d SV b 7 d SV N 7 d

100.5 ± 2.1 99.3 ± 7.1 92.1 ± 5.6 73.7 ± 5.2 94.9 ± 9.1 63.8 ± 4.9

0.3 ± 0.3 2.4 ± 0.1 4.6 ± 0.6 1.5 ± 0.1 6.5 ± 1.0 1.3 ± 0.1

219.3 28.6 15.1 46.4 10.7 54.7

0.16 0.98 0.86 0.95 0.83 0.96

Prometryn Sb7 d SN7 d SC b 7 d SC N 7 d SV b 7 d SV N 7 d

99.3 ± 11.4 136.8 ± 24.0 98.2 ± 2.6 97.9 ± 13.4 99.7 ± 6.3 96.0 ± 9.2

0.3 ± 1.5 4.4 ± 0.2 4.9 ± 0.3 3.3 ± 0.1 5.7 ± 0.7 3.1 ± 0.1

225.3 15.7 14.3 21.2 12.1 22.3

0.07 0.97 0.97 0.96 0.90 0.98

Cyanazine Sb7 d SN7 d SC b 7 d SC N 7 d SV b 7 d SV N 7 d

101.4 ± 5.7 77.6 ± 9.1 86.8 ± 15.9 53.7 ± 7.7 89.2 ± 21.0 39.7 ± 4.5

5.9 ± 0.7 3.5 ± 0.1 13.0 ± 1.9 3.2 ± 0.1 13.9 ± 3.0 2.4 ± 0.1

11.8 19.5 5.3 22.0 5.0 29.3

0.93 0.98 0.85 0.96 0.78 0.96

a

Standard deviation (n = 2).

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Fig. 3 – Dehydrogenase activity (DHA) in non-amended soil (□) and soil amended with olive cake (Δ), compost (O) and vermicompost (X) along the incubation time. The vertical lines represent the standard deviation in each sample (n = 3). in DHA observed in these substrates (Fig. 3). This parameter has been considered as an indicator to estimate the total microbial activity of soil, compost and vermicompost (Dungan et al., 2003; Benítez et al., 2005) and could be related with pesticide degradation. The higher triazine concentration at the end of the incubation period (data not shown), together with the lower k values of triazines in sterile soil amended with compost and vermicompost with regard to microbially active substrates (Table 1), show the role of the microorganisms in the degradation of the triazines studied. For compost- and vermicompost-amended soils, a higher contribution of biological degradation processes was taking place as shown by higher parameters kbiotic / ktotal (Table 1). On the contrary, no significant differences (P N 0.05) in the concentration of each triazine at each incubation time were observed among soil amended with olive cake and the rest of substrates. Neither the degradation rate values for triazines in soil amended with olive cake were, in general, significantly

different (P N 0.05) from the corresponding values in nonamended soil, in spite that soil amended with olive cake showed the highest DHA during the incubation period (Fig. 3). On the other hand, no significant differences (P N 0.05) were found between the degradation curves of terbuthylazine, prometryn and simazine in sterile and microbially active soil amended with olive cake (Table 1). The values kbiotic/ktotal were in all cases close to zero, a further support of the absence of biological contribution to triazines degradation. This is an indication that, although olive cake stimulated microbial activity, it did not enhance herbicide degradation. The microorganisms may have preferred the more easily degradable organic compounds of this amendment as a carbon and/ or nitrogen source instead the triazine herbicides. Other authors have considered this hypothesis previously for olive cake and different organic amendments (Moorman et al., 2001; Sánchez et al., 2003; Delgado-Moreno and Peña, 2007). However it has to be taken into account that in olive cake-amended soil the variability in the determination of pesticide residues was high, ranging between 4 and 20%, which could make difficult the observation of statistical differences. Although the first order approach produced high linear correlation coefficients, a difference between the t1/2 and DT50 values was observed mainly in the cases of a rapid decrease of herbicide concentration during the first week of incubation, that is, in soil amended with compost and vermicompost (Table 1). Therefore for both substrates the degradation rate was faster than predicted from the first-order kinetic fit. To explain this pattern, two more complex models, the Hoerl and biexponential equations were used to fit the residual triazine concentrations (Table 3). As it was expected, both the biexponential and the Hoerl equations explain better than the simple first order equation the very rapid triazines decay during the first week of incubation in soil amended with compost and vermicompost since they provide in general lower residual standard errors (Tables 1 and 3). Both models have been satisfactorily used to describe the degradation of different compounds (Cumming et al., 2002; Sánchez et al., 2003; Delgado-Moreno and Peña, 2007).

Table 3 – Kinetic parameters corresponding to the fitting to biexponential and Hoerl equations of the residual triazine concentration in soil amended with compost (SC) and vermicompost (SV) Biexponential equation F ± sd

a

2

−1

k1 × 10 ± sd (d )

2

Hoerl equation −1

b

k2 × 10 ± sd (d )

RSE

1.6 ± 0.1

69.0 ± 22.0

14.21

a ± sd

SC Simazine Terbuthylazine Prometryn Cyanazine

0.88 ± 0.03 0.50 ± 0.03

2.9 ± 0.2 2.8 ± 0.3

52.4 ± 37.9 88.0 ± 18.2

14.36 22.60

82.5 ± 1.3 89.3 ± 1.3 92.4 ± 1.5 61.6 ± 2.2

SV Simazine Terbuthylazine Prometryn Cyanazine

0.64 ± 0.03 0.85 ± 0.02 0.84 ± 0.04 0.45 ± 0.03

1.2 ± 0.1 1.3 ± 0.1 2.6 ± 0.2 2.7 ± 0.3

38.4 ± 9.1 67.5 ± 37.2 49.0 ± 30.5 81.8 ± 15.9

21.18 16.99 20.82 23.65

77.1 ± 1.6 89.5 ± 1.4 90.1 ± 1.8 59.1 ± 1.9

a b c

0.76 ± 0.02 c

− b × 102 ± sd

−c × 102 ± sd

RSE

1.5 ± 0.1 1.5 ± 0.1 3.0 ± 0.1 2.8 ± 0.3

3.2 ± 0.4 1.7 ± 0.4 1.1 ± 0.4 7.7 ± 0.7

14.28 15.02 15.70 28.11

1.3 ± 0.1 1.3 ± 0.1 2.6 ± 0.1 2.7 ± 0.3

4.6 ± 0.5 1.9 ± 0.4 1.9 ± 0.5 9.2 ± 0.6

23.20 18.99 22.17 20.16

Standard deviation (n = 2). Residual Standard Error. Calculated k1 and F values not shown because of too large standard errors. Detailed information about the equations in M&M.

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4.

Conclusions

The addition of exogenous organic matter, proceeding from the olive agroindustry, has a differentiated effect on the degradation of various triazines depending on the nature of the amendment. The addition of compost and vermicompost to soil caused a faster decrease in the herbicides concentration during the first week of incubation due to the stimulation of microorganisms which enhanced triazines degradation. In contrast, the use of olive cake as amendment stimulated general microbial population and activity without concurrent increases in herbicide degradation, suggesting that the specific microbial populations responsible for degrading the herbicides were not stimulated. Thus, it is not possible to generalize, as is often the case, by simply considering that organic matter addition will enhance herbicide degradation, because the degradation rate will depend on the type of exogenous amendment and on the properties of the herbicides involved. In the last case, it is important to take into account the bioremediation of herbicide mixtures. Although more studies are necessary, the presence in soil of cyanazine and prometryn could limit the biological degradation of terbuthylazine and simazine, what could have negative consequences in the environment.

Acknowledgements We thank the Spanish Ministry of Education for a FPU research grant and the CSIC for a postgraduate grant received by LDM. The Project CAO001-007 from Junta de Andalucía partially financed this study. The SIC helped in the determination by GC–MS of some samples.

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