Water Air Soil Pollut (2014) 225:1935 DOI 10.1007/s11270-014-1935-8
The Role of Sugarcane Residues in the Sorption and Leaching of Herbicides in Two Tropical Soils Fabrício Garcia Giori & Valdemar Luiz Tornisielo & Jussara Borges Regitano
Received: 11 November 2013 / Accepted: 19 March 2014 / Published online: 1 April 2014 # Springer International Publishing Switzerland 2014
Abstract Sugarcane is a major crop in Brazil that generates huge amounts of organic residues that are usually left deposited in, or applied to the soil, and thereby affect the behavior of herbicides. This study assessed the effects of sugarcane residues (straw, ash, and compost) and residence time (“aging”) on the sorption of alachlor and diuron in two contrasting soils (LVd and LVAd), as well as the effects of these residues on the leaching of alachlor. Adding straw and compost had no effect, whereas adding ash significantly increased sorption of both herbicides. Aging (28 days) increased apparent sorption distribution coefficients (Kd,app values) by 1.2 to 2.3 times. Straw and ash amendments resulted in less leaching of alachlor (<1.0 % of the applied amount) than compost or control soil (~6 % of the applied amount). The straw retained ~80 % of the applied alachlor during leaching. Although this may be overrated due to an artifact of the methodology adopted, alachlor retention in the straw could not be predicted by the use of Kd,app. The transport potential of alachlor may be overestimated if aging and sugarcane straw management are not factored into the models.
F. G. Giori : J. B. Regitano (*) Department of Soil Science, College of Agriculture “Luiz de Queiroz”, University of São Paulo - ESALQ/USP, P.O. Box 9, 13418-900 Piracicaba, SP, Brazil e-mail:
[email protected] V. L. Tornisielo Ecotoxicology Laboratory, Center for Nuclear Energy in Agriculture, University of São Paulo - CENA/USP, P.O. Box 96, 13400-970 Piracicaba, SP, Brazil
Keywords Diuron . Alachlor . Straw . Ash . Compost . Transport
1 Introduction Sugarcane is one of the most important crops in Brazil, occupying more than 8.5 million ha, producing 40 million Mg of sugar and 24 billion L of ethanol (harvest of 2011/2012). The sector moves more than US$ 50 billion, which corresponds to nearly 2 % of Brazil’s gross income and generates 1.3 million jobs (UNICA 2012). In recent years, sugarcane burning practices have been phased out giving rise to 8 to 20 Mg ha−1 of straw that enhances soil organic fraction and leads to improvements in chemical, physical, and biological soil attributes (Ceddia et al. 1999; Pinheiro et al. 2010). Previously, straw burning resulted in the accumulation of ash and released large amounts of greenhouse gases and ashes into the atmosphere. Furthermore, other sugarcane industry wastes, such as filter cake, vinasse, and boiler ash (which corresponds to the bagasse burnt in the boiler), as well as their composts are now applied to soils in order to reduce the cost of mineral fertilizers. In 2011, nearly 11 million Mg of filter cake, 4 million Mg of boiler ash, and 380 million m3 of vinasse were yielded, which correspond to 630,000 Mg of urea, 225,000 Mg of MAP, and 1,800,000 Mg of KCl (about 2.6 million Mg of fertilizers) (Luz and Korndörfer 2011). These practices may reduce pesticide contamination of surface and ground waters due to either sorption enhancement in the residues or transport hindrance since
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there is no ash to co-transport sorbed pesticides to the bodies of water (Langenbach et al. 2008; Dal Bosco et al. 2012). Additionally, the remaining straw is capable of reducing erosion by up to 32 % (Rossetto 2010). The availability of herbicides in the soil solution is the primary factor that dictates transport and degradation of pesticides and effectiveness of weed control, and it is inversely related to its sorption potential. Soil sorption of non-polar or low-polarity herbicides is usually related to hydrophobic partition and organic carbon content (Dorado et al. 2005; Liu et al. 2010). The use of reliable sorption distribution coefficients (Kd values) in models to predict either the transport or the environmental fate of pesticides is an important tool that provides useful information at a lower cost than laboratory or field studies. However, in order to generate reliable predictions, the parameters must be calibrated to reflect each individual set of soil management conditions and physical-chemical attributes of the soil and molecules under study. Furthermore, the residence time of a herbicide in soil also affects its sorption potential, which is known as the aging effect. Increased Kd values with aging have been reported for various classes of herbicides (Regitano et al. 2006; Regitano and Koskinen 2008; Martin et al. 2012), but it was not significant in the case of dicamba (Menasseri et al. 2003). The environmental fate of pesticides already been evaluated under many scenarios, but with little regard to the effects of using sugarcane residues. However, such use would represent a suitable solution for the disposal of wastes generated by the sugarcane industry in Brazil. Interception of herbicides by sugarcane straw or other residues has been examined only recently focusing mostly on weed control effects (Correia et al. 2013). The literature shows that crop residues and other organic wastes may enhance sorption, reduce leaching and surface runoff (Selim et al. 2003; Yang et al. 2006; Langenbach et al. 2008), but the magnitude of these effects will depend on environmental factors (e.g., occurrence, intensity, and duration of rainfall) and on the herbicides properties (e.g., their solubility in water). Alachlor (2-chloro-2',6'-diethyl-Nmethoxymethylacetanilide) and diuron (3-(3,4dichlorophenyl)-1,1-dimethylurea) are herbicides used on a large scale in Brazil. They are registered for preemergence application (diuron can be used as a postemergent as well) to various commercially important crops, including sugarcane (Rodrigues and Almeida 2011). Despite their importance for agriculture, these
Water Air Soil Pollut (2014) 225:1935
molecules have been detected in both surface and ground waters (Vryzas et al. 2012a) and may thereby impact human health and the equilibrium of aquatic ecosystems. Understanding how these herbicides behave and where they ultimately end up in the environment under the new no-burn sugarcane cropping regime in Brazil is critical to both the controlling of weeds and the minimizing of negative environmental impacts. Thus, the aim of this study was to assess how sugarcane residues and soil residence time (“aging”) affect sorption of alachlor and diuron in soils as well as the effects of these organic residues on the leaching of alachlor due to its high mobility.
2 Materials and Methods 2.1 Herbicides and Soils Analytical (Fluka Analytical, Seelze, Germany) and radiolabeled (14C-uniform-ring-labeled, Ciba Geigy, Delhi, India) standards of the herbicides alachlor (specific activity=3.70 MBq mg−1 and purity >98 %) and diuron (specific activity=2.43 MBq mg−1 and purity >98 %) were used. Soils (LVAd and LVd) were classified as typic hapludoxes (Soil Survey Staff 1999) and collected from the surface layer (0–10 cm) of sugarcane-growing areas (located in the small towns of Novo Horizonte and Pradópolis in the state of São Paulo, Brazil). Sugarcane has been grown in these towns for approximately 50 years, and in the last 5–6 years, they have adopted a no-burn harvest policy. These soils were selected because they are representative of sugarcane areas, but differ in texture and organic carbon contents (Table 1). Soil samples were air-dried, sieved through 2-mm mesh, and stored at ambient temperature. Texture was quantified via densimetry. Acidity (pH) was determined in 0.01 mol L−1 CaCl2. Calcium, potassium, and magnesium were extracted using the ion-exchange resin method and Ca+2 and Mg+2 determined by atomic absorption spectrophotometry (Perkin Elmer, AAnalyst 400) and K+ with flame photometry (Digimed, DM-62). Potential acidity (H+Al) was determined via pH-SMP, and the concentration of total soil organic carbon via dry combustion in an elemental autoanalyzer (Leco, TruSpec CN). Cation-exchange capacity (CEC) was calculated as the ∑Ca+2, Mg+2, K+, and H+Al. Poorly crystallized (“amorphous”) iron, aluminum, and manganese oxides
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were determined using an acid solution of ammonium oxalate and “free” or crystalline iron, aluminum, and manganese oxides using a dithionite-citrate-bicarbonate (DCB) solution (Camargo et al. 2009) (Table 1). 2.2 Residues The straw (SP91-1285 variety) was collected after mechanized harvesting, oven-dried (65 °C for 3 days), and either sieved through 8-mm mesh for the sorption experiment or cut into ~2.5-cm fragments for the leaching experiment. Ash was obtained by burning straw in a partially closed container for 8 min (~280 °C). The particle size distributions of the ash were 43, 36, 9, and 12 % for mesh sizes of <0.30, 0.30–0.84, 0.84–1.19, and 1.19–2.50 mm, respectively. Compost was obtained by composting filter cakes and boiler ash in a 2:1 proportion (v/v) for 80 days, and then sieved through 2-mm mesh. The chemical and physical attributes of the organic residues were determined according to Alcarde (2009) and are presented in Table 2. 2.3 Soil Amendments and Aging on the Sorption of Alachlor and Diuron Aliquots of 5 g of air-dried soils were placed into centrifuge tubes (Teflon, 50 mL) and then amended with 5.3 and 14 Mg ha−1 of ash and straw, respectively, which corresponds to the mean values remaining in the field after the sugarcane had been harvested with and without burning, respectively, and with 10 Mg ha−1 of compost which corresponds to the mean value applied to the field. It was estimated assuming soil density equals to 1.2 g cm−3 and soil depth equals to 10 cm. The amendments were not mixed, but layered on the surface of the soils. Then, 50 μL of the herbicide solutions (280 and 267 μg mL−1 for alachlor and diuron, respectively, having radioactive concentrations of ~35 kBq mL−1) were applied to reach the field rates (= 3.36 and
3.20 kg a.i. ha−1 for alachlor and diuron, respectively). Afterwards, soil moisture contents were adjusted and maintained at 75 % of field capacity for 0 (t0), 7 (t7), and 28 days (t28), in a semi-dark room at 25±2 °C (the tubes were kept partially open to avoid anaerobiosis). After each incubation period, replicate soil-residue samples were extracted with 10 mL of 0.005 mol L−1 CaCl2 solution by shaking in a horizontal shaker at 160 rpm for 24 h. The tubes were centrifuged at 3,000 rpm (830g) for 15 min and 1-mL aliquots of the supernatants analyzed for 14C by liquid scintillation counting (LSC) (Packard, Tri-carb 1500). The supernatants were discarded, and the remaining slurries were dried in a ventilation oven (40 °C, 48 h) and macerated, and triplicates of 0.20 g subsamples were combusted using a biological oxidizer (900 °C, for 3 min) (Harvey Instruments, OX500) to determine the sorbed concentrat i o n s . T h e r e s ul t i n g 1 4 C O 2 w a s tr a p p e d i n monoethanolamine-basis scintillation solution and the 14 C quantified by LSC. Recovery of the applied radioactivity ranged from 94 to 110 % (data not shown). To determine the identity of the 14C, the supernatants were concentrated in a ventilation oven (40 °C, 48 h) and identified via thin layer chromatography (TLC) using a radio scanner (Berthold GmbH & Co.) . The solvent systems were isopropyl alcohol, dichloromethane, and formic acid (4:5:1v/v/v) for alachlor and hexane and acetone (3:2 v/v) for diuron. For all treatments, 14C corresponded exclusively to the parent molecules (data not shown). To calculate apparent sorption distribution coefficients (Kd,app, L kg−1) after different aging periods, it was assumed that the CaCl2-extractable fraction represented the solution phase concentration (Ce’), the oxidated fraction represented the sorbed phase concentration (S’), and that Kd,app =S’/Ce’ (Regitano et al. 2006). We use the word apparent because the coefficients were not estimated according to traditional sorption experiments, but the literature considers this approach
Table 1 Chemical and physical attributes of soils Soils
pH
CEC
C
mmolc dm−3
g kg−1
FeDCB
FeOx
AlDCB
AlOx
MnDCB
MnOx
Sand
Silt
Clay
LVAd
6.4
58.9
6.8
11.9
0.7
3.8
0.5
0.4
0.05
768
30
202
LVd
5.1
84.5
20.0
117.5
4.8
20.5
5.6
1.4
0.6
98
218
684
Fe, Al, and MnDCB crystalline oxides, Fe, Al, and MnOx poorly crystallized oxides
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Table 2 Chemical and physical attributes of the straw, ash, and compost pH
K2O
CaCl2
g kg−1
Straw
5.6
2.1
1.9
1.1
18.2
1.7
6.4
446.0
69:1
250
0.23
Ash
8.0
9.5
5.7
2.9
32.3
3.6
10.1
316.0
31:1
340
0.23
Compost
7.9
3.7
10.4
3.6
21.7
1.5
7.2
83.5
12:1
150
0.66
Wastes
P2O5
Mg
Ca
reasonable since Kd,app attained at time zero usually generates similar values to the traditional Kd values (Regitano et al. 2006; Regitano and Koskinen 2008). 2.4 Soil Amendments on the Leaching of Alachlor Alachlor and the LVAd soil were selected because they represented the worst scenario for leaching due to the lowest sorption of alachlor in this soil. The study was carried out in accordance with OECD protocol (OECD 2004), with minor adaptations. Glass columns (length=30 cm, diameter=5 cm, and a conical end) were used to pack the soils. Fiberglass and sterilized sand (with HCl) were added to the conical portion to serve as a support. Soil packing was done manually up to a height of 20 cm, with a density of ~1.48 g cm−3. The residues (15.0 g of straw, 8.0 g of compost, and 3.5 g of ash, which corresponded to the same application rates of the previous assay considering soil weight) were added to the top of the soils, and a fine layer of glass wool was used to ensure the homogeneous dispersion of the water and to avoid surface disturbance. Soil columns were then saturated by capillarity with a 0.005 mol L−1 CaCl2 solution and excess water drained by gravity. Immediately afterwards, 200 μL of the 14Calachlor solution at 8,125 μg a.i. mL−1 (radioactive concentration of~429 kBq mL−1) were added to the columns to represent field rate (3.36 kg a.i. ha−1). The columns were stored in a semi-dark room at a temperature of 25±2 °C. The water (170 mm of 0.005 mol L−1 CaCl2 solution) was applied using a peristaltic pump with continuous flow for 48 h. The leachate was collected at 12-h intervals and 1-mL aliquots analyzed via LSS. After 48 h, soil samples were removed from the inside of the columns with pressurized air in 5-cm sections, placed in aluminum containers, air-dried, macerated, and homogenized for posterior oxidation of subsamples (0.2 g), as described in the sorption experiment. The straw was removed
S
N
C
C:N
CEC
Density
mmolc kg−1
g cm−3
separately from the 0–5 cm layer, air-dried, macerated, and oxidized to determine herbicide concentration. Ash and compost could not be separated from the top layer. Recovery of the applied radioactivity varied from 94.9 to 108.2 %, with a mean value of 104.5±6.8 % (data not shown). The percentages of the herbicide leached or retained in each soil layer were subjected to variance analysis and to Tukey’s test (p<0.05).
3 Results and Discussion 3.1 Soil Amendments and Aging on the Sorption of Alachlor and Diuron The retention of the studied herbicides varied strongly according to soil type, residence time (aging), and organic residue amendments (Table 3). Initially (at t0), alachlor showed a low to moderate sorption potential for the soils and their respective treatments (Kd,app < 1.6 L kg−1 in the LVAd soil and <4.0 L kg−1 in the LVd soil), except for the ash (Kd,app =5.4–7.6 L kg−1). This is in line with the results of previous studies using batch sorption Kd values (Laabs and Amelung 2005; Dal Bosco et al. 2012). By contrast, the sorption potential of diuron was consistently high at t 0 (K d,app = 5.5– 94.6 L kg−1) (Table 3). These values are also similar to those reported in the literature for batch experiments (Inoue et al. 2006; Liu et al. 2010). Therefore, the use of Kd,app seems reasonable since the values attained here at t0 (Kd,app for alachlor=1.2 and 3.8 and for diuron=5.5 and 50.4 L kg−1 in the LVAd and LVd, respectively) were close to the batch Kd values obtained in our previous work (Kd for alachlor=1.0 and 3.2 and for diuron= 5.9 and 52.1 L kg−1 in the LVAd and LVd, respectively) (Giori et al. 2014, in press), as well as those in the literature. Moreover, their use is advantageous once values are attained at proper soil moisture and allowances are made for aging effects. In general, weakly
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Table 3 Effects of organic waste amendments (straw, compost, and ash) and aging (0, 7, and 28 days) on the apparent distribution coefficients (Kd,app, L kg−1) of alachlor and diuron in two soils (LVAd and LVd soils) Kd,app values (L kg−1) Treatments
Alachlor
Diuron
0 day 7 days 28 days 0 day 7 days 28 days LVAd (control)
1.2
2.4
2.6
5.5
8.4
13.2
LVAd+straw
1.6
2.6
3.3
5.6
8.0
10.4
LVAd+compost 1.5
2.7
2.7
7.5
11.0
14.7
LVAd+ash
8.9
10.3
61.6
86.6
86.3 84.7
5.4
LVd (control)
3.8
6.0
6.5
50.4
73.5
LVd+straw
4.0
5.9
7.0
41.0
45.1
53.0
LVd+compost
3.9
6.0
6.2
49.4
75.8
87.4
LVd+ash
7.6
9.8
11.8
94.6
137.4
147.5
sorbed or easily desorbed pesticides are readily available for leaching and transformation in soils (Alexander 2000; Regitano et al. 2006). It is well known that diuron has greater sorption potential than alachlor, in part due to its lower solubility in water (40 versus 200 mg L−1) that favors hydrophobic partitioning. In addition, the retention of both herbicides in the LVd was higher than in the LVAd (Table 3), a result that had been expected, based on the greater organic carbon and clay contents and CEC of the former soil (Table 1). Both herbicides are sorbed to soils primarily by hydrophobic partitioning, which implies that sorption is mostly related to the organic fraction of the soil (Liu et al. 2010). Clay content may play an important role in their sorption, but only when organic carbon content in the soil is very low (<1 %). Tropical soils showed Koc values of 74–150 and 741–1,080 L kg−1 for alachlor and diuron, respectively (Inoue et al. 2006; Oliveira et al. 2001). The Kd,app values increased considerably with residence time (aging) of the molecule in the soils (Table 3). In certain cases, alachlor even showed high sorption potential (Kd,app >5 L kg−1). Increases in Kd,app values were more abrupt in the first 7 days of incubation, except for diuron in the LVd due to its high initial sorption (>94.8 % of applied amount) (Fig. 1). It is well known that pesticide sorption–desorption takes place in two distinct phases: a rapid initial phase followed by a slower secondary phase (Hatzinger and Alexander 1995). In general, the initial phase is due to rapid sorption at easily available (external) sites, while the slower
phase is associated with diffusion/immobilization towards the interior of the soil matrix (Alexander 2000), reducing pesticide desorption and availability to soil microorganisms. Within our incubation period (28 days), Kd,app values increased 1.6–2.2 times for alachlor and 1.3–2.3 times for diuron, and were more striking in the LVAd soil due to its lower buffering capacity (CEC). The mechanisms by which molecules and especially hydrophobic molecules become more retained with aging remain poorly understood. The solid phase of the organic matter (Brusseau et al. 1991), its nanopores (Chung and Alexander 1998), and the slow diffusion to the interior of soil aggregates (Brusseau et al. 1991) play important roles in this process, which may result in a fraction that is resistant to desorption and lead to the formation of non-extractable (bound) residues (Mamy and Barriuso 2007). It is likely that these mechanisms act jointly to some degree, enhancing pesticide sorption. Thus, it is important that aging effects are examined so that transport models can be adequately parameterized. Straw and compost had little effect on the sorption of alachlor and diuron (Table 3). Conversely, ash markedly increased their sorption ratifying its trapping efficiency for organic molecules. In the LVAd, ash increased sorption of alachlor by ~4 times and diuron by ~10 times. In the LVd, for both herbicides, ash increased sorption by ~2 times. Adding wheat ash to soils (1 %) enhanced sorption of diuron by up to 80 times (Yang et al. 2006). As a matter of fact, the sorption potential of the ash may exceed that of the soil’s humic substances (Amonette and Joseph 2009), suggesting that it should affect pesticide bioavailability and, subsequently, its environmental fate and agronomic efficacy. The high sorption potential of various classes of herbicides to ash may be explained by its high organic carbon content and by the heterogeneity and reactivity of its surface, which may include hydrophilic, hydrophobic, acidic, and basic groups (Amonette and Joseph 2009). The reactive nature of its surface is also reflected by its high CEC (Table 2). The Kd,app values at t0 were positively correlated with the organic carbon content of the soils before and after residue amendments (r=0.86 and 0.90 for the alachlor and diuron, respectively, p<0.01), except for the straw (Fig. 2). The organic carbon content of the amended soils was calculated by adding the total amounts in the soil and in the residue and then dividing by the total mass. The same trend was observed for the aged Kd,app (r>0.75 and p<0.01 for both t7 and t28, excluding the
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Water Air Soil Pollut (2014) 225:1935
14C-alachlor
Sorption (%)
90
LVAd
LVd
LVAd
LVd
75
60
45
30
14C-diuron
Sorption (%)
100
90
Ash Compost
80
Straw Soil (control) 70 0
8
16
24
Incubation (days)
32
0
8
16
24
32
Incubation (days)
Fig. 1 Sorbed percentages of alachlor and diuron as affected by residue amendments (straw, compost, and ash) and aging (0, 7, and 28 days)
straw), reinforcing that hydrophobic partition may increase with time due to either intra-particle diffusion into organic matter (Brusseau et al. 1991) or interparticle diffusion within organomineral aggregates in the soils (Nan and Alexander 1998). These results suggest that deriving organic matter content for amended soils may be meaningful even when they are not mixed, but layered on the surface, except by organic residues having a low degree of humification (highlighted by high C:N ratio) and low reactive surface, such as the straw that has epidermis covered with cuticle and hairs. 3.2 Soil Amendments on Leaching of Alachlor The addition of sugarcane residues affected the leaching potential of alachlor. Straw and ash reduced its leaching (<1.0 % of the applied alachlor was leached) whereas the used compost did not (~6 % of the applied alachlor was leached, similar to the control soil) (Fig. 3). The concentration in the leachate corresponded to 82.2 μg i.a. L−1 for the non-amended and to 9.9, 6.0, and 83.0 μg i.a. L−1 for the straw, ash, and compost amended soil, respectively. Although previous studies have shown limited movement in the soil profile (Clay et al. 1991), alachlor has been reported in surface and ground waters
(Vryzas et al. 2012a) and in deeper soil layers (up to 160 cm deep at concentrations of 211 or >0.1 μg L−1 at 18 h or 40 months after application, respectively) (Vryzas et al. 2012b), which raises concerns about the acceptable limits adopted for water resources in Brazil. Brazil has adopted 20 μg L−1 as standard for drinking water and the maximum limit for groundwater (BRASIL 2004, 2008), while the European Community (Directiva 98/83/CE 1998) and the USA (US-EPA 2012) have adopted values of 0.1 and 0.2 μg L−1, respectively. The US-EPA has classified alachlor as having a “high” leaching potential due to its volume of use, low sorption, and high persistence (US-EPA 1998). Alachlor residues remained mostly in the surface layer of the soil (0–5 cm+residues), with the ash showing greater retention (105 % of the amount applied) than the straw (87 %), which in turn was greater than the compost (68 %) and the control (45 %) (Fig. 4). Overall, >74 % of the applied alachlor was retained in the top 10cm layer. The literature showed that >60 % of the applied alachlor was in the soil surface layer (0–5 cm) for both non-till and conventional till areas (Clay et al. 1991). Furthermore, soil profile redistribution of alachlor was greater in the non-amended soil, but still relevant when compost was amended (Fig. 4) (Dorado
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8
Alachlor
Redistribution of Alachlor (% of applied) 0 15 30 45 60 75 90 105
Straw Other treatments
-1
Kd,app values (L kg )
6
a b
0-5 Soil depth (cm)
4
2
0 100
c c
5-10
10-15
Diuron
c d
b a
Straw c c a
-1
Kd,app values (L kg )
Compost
b
80
60
15-20
Ash
Soil (control)
a a a a
40
Fig. 4 Soil profile redistribution of alachlor as affected by straw, compost, and ash amendments. Bars represent the standard deviations. Different letters mean that treatments differ statistically within the same time period (Tukey’s test, p<0.05)
20
0 0
10
20
30
40
50
60
-1
Organic carbon (g kg )
Fig. 2 Apparent sorption distribution of alachlor and diuron (Kd,app) as affected by total organic carbon contents
Soil (control) Ash
Cumulative % alachlor leached
8
Straw Compost
a
6
a a
4
a
2 a a
b b
b
b
0 12
24
36
48
Time (hours) Fig. 3 Leaching of alachlor as affected by straw, compost, and ash amendments. Bars represent the standard deviations. Different letters mean that treatments differ statistically within the same time period (Tukey’s test, p<0.05)
et al. 2005). Composting materials with C:N ratios similar to or lower than ours (12:1, Table 2) usually enhance herbicides sorption due to their advanced stage of humification (Huang et al. 2006), but this enhancement was not abrupt in our case. Besides organic carbon content and C:N ratio, the quality of the added residue also dictates sorption strength and amount. For example, animal manure, another residue product rich in organic matter, also reduced atrazine (Langenbach et al. 2008) and metribuzin (Majumdar and Singh 2007) leaching. The high retention of alachlor on the soil surface amended with ash (Fig. 4) appears to be directly related to sorption, since ash markedly enhanced alachlor sorption (Kd,app =1.2 versus 5.4 L kg−1 at t0) (Table 3). Ashes were also highly efficient in retaining other organic compounds (Hiller et al. 2008; Singh 2009). However, the same reasoning could not be used for the straw since it had little effect on sorption (Kd,app =1.2 versus 1.6 L kg−1 at t0) (Table 3). The straw appears to work as a physical barrier to the transport, holding about 80 % of the applied alachlor even after heavy rainfall was simulated (170 mm in 48 h, starting soon after herbicide application). It is important to clear that this percentage (80 %) may be overestimated since herbicide application was not even (it was performed in small
1935, Page 8 of 9
drops) and rainfall was simulated at just one point (as proposed in the guideline for soils), but differently from soils, the straw has huge amounts of macropores that may have favored preferential flow of water only, masking part of the results. However, the presence of grass straw also reduced leaching of atrazine in soil columns (Langenbach et al. 2008), whereas sugarcane straw reduced surface runoff of atrazine and pendimethalin by at least 50 % (Selim et al. 2003). For sugarcane, the amount of sorbed herbicide is usually proportional to the amount of straw left on the soil and more soluble molecules usually run through the straw easily (and earlier) (Rossi et al. 2013). However, it will depend to a large degree on the period between pesticide application and the first rain, besides its quantity, intensity, and distribution (Rodrigues 1993). Therefore, straw management, herbicide properties, and rainfall patterns are important parameters dictating sorption and leaching of pesticides (Correia et al. 2013; Rossi et al. 2013). Based on screening models criteria (Kd <3–5 L kg−1 and t1/2 >14–21 d), our Kd,app suggests that alachlor should always leach from the studied soils, except when amended with ash. Therefore, this parameter may not provide adequate description of leaching in soils amended with non-humified organic residues, such as the straw.
4 Conclusions Sugarcane straw and compost had little effect whereas ash significantly increased alachlor and diuron sorption. Leaching corresponded to ~6.0 % in the control soil, but ash and straw amendments reduced it to less than 1 %. The compost had little effect on the leaching of alachlor. Unexpectedly, the straw was capable of holding 80 % of the applied alachlor in the leaching experiment. Despite this value may be overestimated, alachlor retention in the straw could not be predicted by the use of Kd,app values. Therefore, the transport potential of these herbicides may be overestimated if aging and sugarcane straw management practices are not taken into account by the models. Acknowledgments The authors thank FAPESP for providing financial support for the research (Grant 2012/15843-0) and CNPq for granting a scholarship to the first author. We are grateful to the Usina Estiva and the agronomy engineers Marcelo Rocha and Júlio Araújo for making the soils available and to our colleagues
Water Air Soil Pollut (2014) 225:1935 Carlos Alberto Dorelli and Rodrigo Pimpinato for their technical assistance in carrying out the experiments.
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