Journal of Environmental Quality
TECHNICAL REPORTS ORGANIC COMPOUNDS IN THE ENVIRONMENT
Degradation of Plant Cuticles in Soils: Impact on Formation and Sorptive Ability of Humin–Mineral Matrices Yaniv Olshansky, Tamara Polubesova, and Benny Chefetz*
O
ne of the important precursors for soil organic matter (SOM) is plant cuticle (Clemente et al., 2013; Kögel-Knabner, 2002; Nierop, 1998), a thin, predominantly lipid layer that covers all primary aerial surfaces of vascular plants (Kolattukudy, 2001). The main constituents of the cuticle are polymeric lipids, which are divided into two major classes: (i) cutin, a high-molecular-weight, polar, cross-linked polymer constructed of interesterified hydroxy-fatty acids and hydroxyepoxy-fatty acids with chain lengths of C16 and C18, and (ii) cutan, a nonsaponifiable polymer made up of long-chain fatty acids attached to an aromatic core via ester and ether linkages (Kögel-Knabner et al., 1994; Kolattukudy, 2001). In soil, plant cuticles undergo degradation and transformation (Chefetz, 2007; Clemente et al., 2013), and their degradation products are incorporated into SOM and in particular into humin, which makes up more than 50% of the SOM (Rice, 2001; Song et al., 2008). Operationally, humin is defined as the fraction of humus that is insoluble in aqueous solution at any pH and remains in the soil after removal of humic and fulvic acids (Stevenson, 1994). Humin is the residual SOM that is tightly associated with soil mineral surfaces (Song et al., 2011). According to this definition, nonhumic substances that are associated with mineral aggregates or surfaces may be also included in the humin fraction. Therefore, humin is better described as a humic-containing material rather than as a humic substance (Maccarthy, 2001). Humin is the most aliphatic fraction of SOM, and it has been suggested to contain residues of plant cuticles (Simpson et al., 2007). Humin is considered an efficient sorbent for organic pollutants and in some cases shows even higher affinity for them than whole soils (Olshansky et al., 2011; Simpson and Johnson, 2006). The high affinity of organic pollutants to humin has been attributed to the high content of polyethylene-like domains associated with plant-cuticle residues (Simpson and Johnson, 2006). Cuticular matter alone (cutin and cutan) has been reported to be a highly efficient sorbent for polar and nonpolar organic compounds (Chen et al., 2008, 2005; Li et al., 2010; Shechter
Abstract Plant cuticles are important precursors for soil organic matter, in particular for soil humin, which is considered an efficient sorbent for organic pollutants. In this study, we examined degradation and transformation of cuticles isolated from Lycopersicon esculentum fruit and Agave americana leaves in loamy sand and sandy clay loessial arid brown soils. We then studied sorption of phenanthrene and carbamazepine to humin–mineral matrices isolated from the incubated soils. Low degradation (22%) was observed for agave cuticle in a sandy clay soil system, whereas high degradation (68–78%) was obtained for agave cuticle in a loamy sand soil system and for loamy sand and sandy clay soils amended with tomato cuticle. During incubation, most of the residual organic matter was accumulated in the humin fraction. Sorption of phenanthrene was significantly higher for humin–mineral matrices obtained from soils incubated with plant cuticles as compared with soils without cuticle application. Sorption of carbamazepine to humin–mineral matrices was not affected by cuticle residues. Cooperative sorption of carbamazepine on humin–mineral matrices isolated from sandy clay soil is suggested. Sorption–desorption hysteresis of both phenanthrene and carbamazepine was lower for humin–mineral matrices obtained from soils incubated with plant cuticles as compared with nonamended soils. Our results show that cuticle composition significantly affects the rate and extent of cuticle degradation in soils and that plant cuticle application influences sorption and desorption of polar and nonpolar pollutants by humin–mineral matrices.
Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
Dep. of Soil and Water Sciences, Faculty of Agriculture, Food and Environment, The Hebrew Univ. of Jerusalem, P.O. Box 12, Rehovot 76100, Israel. Assigned to Associate Editor Joseph Pignatello.
J. Environ. Qual. 44:849–858 (2015) doi:10.2134/jeq2014.10.0452 Supplemental material is available online for this article. Received 30 Oct. 2014. Accepted 22 Mar. 2015. *Corresponding author ([email protected]).
Abbreviations: HPLC, high-performance liquid chromatography; OC, organic carbon; SOM, soil organic matter.
849
and Chefetz, 2008). However, only scarce information exists on the impact of plant cuticle degradation and transformation on SOM sorption ability. Chefetz (2007) reported a 30% decrease in sorption affinity of phenanthrene to sandy soils during incubation with tomato and pepper cuticles. Similarly, Shechter et al. (2010) showed decreases of 60 and 10% in the organic C normalized soil–water partition coefficient for organic compounds (KOC) values for phenanthrene sorption on purified cutin (obtained from tomato fruit) and cutan (obtained from agave leaves), which were incubated with sandy soils for 20 mo. Another study (Stimler et al., 2006) showed little or no effect of decomposition of tomato and pomelo cuticles incubated for 9 mo with sandy soil on the sorption of phenanthrene to that soil. The decrease in sorption efficiency of phenanthrene was related to decomposition of the cutin biopolymer. Thus, cuticles that contained sufficient amounts of cutan exhibited a much lower decrease in KOC values due to the high efficiency of cutan as a natural sorbent and its resistance to biodegradation (Chefetz, 2007; Shechter et al., 2010). Although the importance of humin and plant cuticles as natural sorbents for organic pollutants has been demonstrated, the impact of cuticle degradation on sorption of organic pollutants by the humin fraction of soils of different textures has not been investigated. Therefore, the objective of this study was to estimate the impact of cuticle degradation on humin formation and on the sorptive ability of humin–mineral matrices toward polar (carbamazepine) and nonpolar (phenanthrene) pollutants. The sorption–desorption behavior of carbamazepine and phenanthrene was studied with a “near-natural” humin– mineral matrix. To avoid destruction of the humin–mineral matrix, drastic procedures, such as treatments with hydrofluoric acid or extraction with organic solvents, were not applied. An alternative, milder method of humin isolation was used to determine the effect of cuticles on its formation and to obtain sorption data.
Materials and Methods Soils Samples of loessial arid brown soils (sandy clay and loamy sand) were obtained from two sites in Israel (Sa’ad and NirOz, respectively) located in areas that are traditionally used for intensive agriculture. The top soil organic layer (0–3 cm) was removed, and samples were collected from 3- to 30-cm depths. Soil samples were air dried and sieved through a 2-mm sieve. Soil properties were measured using standard soil testing methods (Sparks, 1996). The following soil properties are listed for sandy clay and loamy sand, respectively: clay content, 38.5 and 12.6%; silt content, 12.5 and 0%; sand content, 47.5 and 87.5%; specific surface area values, 168 ± 2 and 43 ± 0.4 m2 g-1; cation exchange capacities, 23.2 and 8.4 mmolc per100 g; organic C (OC) content, 0.37 ± 0.26% and 0.35 ± 0.25%; and pH, 8.1 ± 0.2 and 8.0 ± 0.4.
Cuticles Cuticles were isolated from fruits of tomato (Lycopersicon esculentum Mill.) and leaves of the succulent plant Agave americana L. Freshly picked leaves and fruits were washed several times with deionized water and boiled in water for 60 min. 850
Cuticle sheets were then manually peeled. The isolated cuticles were washed in deionized water and allowed to dry at room temperature. Dried cuticles were slightly ground before use in the incubation experiment.
Incubation of Soils with Cuticles The cuticles (12.5 g) were mixed with 250 g of soil and placed in 400-mL plastic containers. To ensure sufficient nutrient supply, N, P, and K were added to the soils at a level of 60 mg kg-1 each. After blending, water was added uniformly to the top of the soils. The volume of the water was adjusted to a moisture level of 80% of the water-holding capacity of each soil placed into container. The containers were incubated at 25 ± 1°C for 24 mo. During the incubation period, moisture levels were maintained constant by wetting the soils on a weekly basis according to weight loss. Control soils (soils without the addition of cuticles) were incubated and treated in the same way. Sufficient sets of samples were prepared to allow three replicates from each type of cuticle and each type of soil for four sampling periods. At every sampling point (i.e., 1, 6, 13, and 24 mo), the whole containers were sampled, and the soils were freeze-dried and kept in desiccators for further experiments.
Isolation of Humin–Mineral Matrices Humic and fulvic acids were extracted from the soil samples according to the procedure described by Swift (1996). Briefly, soils were treated with 1 mol L-1 HCl to remove CaCO3 and agitated with 0.1 mol L-1 NaOH under a N2 atmosphere for 24 h to extract humic and fulvic acids. The slurry was centrifuged at 23,000 g for 15 min, and the supernatants containing humic and fulvic acids were decanted. The extraction procedure was repeated a few times until no additional OC was detected in the supernatants. The extracts from the repeated procedure for each soil were combined and acidified to pH 2 using 6 mol L-1 HCl to precipitate humic acid. After further centrifugation, the supernatants containing fulvic acid were decanted. The OC contents of humic and fulvic acids were measured with a total OC analyzer (VCPH model, Shimadzu Scientific Instruments). The residual humin–mineral matrices were washed once with 0.05 mol L-1 CaCl2 and then several times with distilled water until the electrical conductivities of the supernatants were reduced to
TECHNICAL REPORTS ORGANIC COMPOUNDS IN THE ENVIRONMENT
Degradation of Plant Cuticles in Soils: Impact on Formation and Sorptive Ability of Humin–Mineral Matrices Yaniv Olshansky, Tamara Polubesova, and Benny Chefetz*
O
ne of the important precursors for soil organic matter (SOM) is plant cuticle (Clemente et al., 2013; Kögel-Knabner, 2002; Nierop, 1998), a thin, predominantly lipid layer that covers all primary aerial surfaces of vascular plants (Kolattukudy, 2001). The main constituents of the cuticle are polymeric lipids, which are divided into two major classes: (i) cutin, a high-molecular-weight, polar, cross-linked polymer constructed of interesterified hydroxy-fatty acids and hydroxyepoxy-fatty acids with chain lengths of C16 and C18, and (ii) cutan, a nonsaponifiable polymer made up of long-chain fatty acids attached to an aromatic core via ester and ether linkages (Kögel-Knabner et al., 1994; Kolattukudy, 2001). In soil, plant cuticles undergo degradation and transformation (Chefetz, 2007; Clemente et al., 2013), and their degradation products are incorporated into SOM and in particular into humin, which makes up more than 50% of the SOM (Rice, 2001; Song et al., 2008). Operationally, humin is defined as the fraction of humus that is insoluble in aqueous solution at any pH and remains in the soil after removal of humic and fulvic acids (Stevenson, 1994). Humin is the residual SOM that is tightly associated with soil mineral surfaces (Song et al., 2011). According to this definition, nonhumic substances that are associated with mineral aggregates or surfaces may be also included in the humin fraction. Therefore, humin is better described as a humic-containing material rather than as a humic substance (Maccarthy, 2001). Humin is the most aliphatic fraction of SOM, and it has been suggested to contain residues of plant cuticles (Simpson et al., 2007). Humin is considered an efficient sorbent for organic pollutants and in some cases shows even higher affinity for them than whole soils (Olshansky et al., 2011; Simpson and Johnson, 2006). The high affinity of organic pollutants to humin has been attributed to the high content of polyethylene-like domains associated with plant-cuticle residues (Simpson and Johnson, 2006). Cuticular matter alone (cutin and cutan) has been reported to be a highly efficient sorbent for polar and nonpolar organic compounds (Chen et al., 2008, 2005; Li et al., 2010; Shechter
Abstract Plant cuticles are important precursors for soil organic matter, in particular for soil humin, which is considered an efficient sorbent for organic pollutants. In this study, we examined degradation and transformation of cuticles isolated from Lycopersicon esculentum fruit and Agave americana leaves in loamy sand and sandy clay loessial arid brown soils. We then studied sorption of phenanthrene and carbamazepine to humin–mineral matrices isolated from the incubated soils. Low degradation (22%) was observed for agave cuticle in a sandy clay soil system, whereas high degradation (68–78%) was obtained for agave cuticle in a loamy sand soil system and for loamy sand and sandy clay soils amended with tomato cuticle. During incubation, most of the residual organic matter was accumulated in the humin fraction. Sorption of phenanthrene was significantly higher for humin–mineral matrices obtained from soils incubated with plant cuticles as compared with soils without cuticle application. Sorption of carbamazepine to humin–mineral matrices was not affected by cuticle residues. Cooperative sorption of carbamazepine on humin–mineral matrices isolated from sandy clay soil is suggested. Sorption–desorption hysteresis of both phenanthrene and carbamazepine was lower for humin–mineral matrices obtained from soils incubated with plant cuticles as compared with nonamended soils. Our results show that cuticle composition significantly affects the rate and extent of cuticle degradation in soils and that plant cuticle application influences sorption and desorption of polar and nonpolar pollutants by humin–mineral matrices.
Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
Dep. of Soil and Water Sciences, Faculty of Agriculture, Food and Environment, The Hebrew Univ. of Jerusalem, P.O. Box 12, Rehovot 76100, Israel. Assigned to Associate Editor Joseph Pignatello.
J. Environ. Qual. 44:849–858 (2015) doi:10.2134/jeq2014.10.0452 Supplemental material is available online for this article. Received 30 Oct. 2014. Accepted 22 Mar. 2015. *Corresponding author ([email protected]).
Abbreviations: HPLC, high-performance liquid chromatography; OC, organic carbon; SOM, soil organic matter.
849
and Chefetz, 2008). However, only scarce information exists on the impact of plant cuticle degradation and transformation on SOM sorption ability. Chefetz (2007) reported a 30% decrease in sorption affinity of phenanthrene to sandy soils during incubation with tomato and pepper cuticles. Similarly, Shechter et al. (2010) showed decreases of 60 and 10% in the organic C normalized soil–water partition coefficient for organic compounds (KOC) values for phenanthrene sorption on purified cutin (obtained from tomato fruit) and cutan (obtained from agave leaves), which were incubated with sandy soils for 20 mo. Another study (Stimler et al., 2006) showed little or no effect of decomposition of tomato and pomelo cuticles incubated for 9 mo with sandy soil on the sorption of phenanthrene to that soil. The decrease in sorption efficiency of phenanthrene was related to decomposition of the cutin biopolymer. Thus, cuticles that contained sufficient amounts of cutan exhibited a much lower decrease in KOC values due to the high efficiency of cutan as a natural sorbent and its resistance to biodegradation (Chefetz, 2007; Shechter et al., 2010). Although the importance of humin and plant cuticles as natural sorbents for organic pollutants has been demonstrated, the impact of cuticle degradation on sorption of organic pollutants by the humin fraction of soils of different textures has not been investigated. Therefore, the objective of this study was to estimate the impact of cuticle degradation on humin formation and on the sorptive ability of humin–mineral matrices toward polar (carbamazepine) and nonpolar (phenanthrene) pollutants. The sorption–desorption behavior of carbamazepine and phenanthrene was studied with a “near-natural” humin– mineral matrix. To avoid destruction of the humin–mineral matrix, drastic procedures, such as treatments with hydrofluoric acid or extraction with organic solvents, were not applied. An alternative, milder method of humin isolation was used to determine the effect of cuticles on its formation and to obtain sorption data.
Materials and Methods Soils Samples of loessial arid brown soils (sandy clay and loamy sand) were obtained from two sites in Israel (Sa’ad and NirOz, respectively) located in areas that are traditionally used for intensive agriculture. The top soil organic layer (0–3 cm) was removed, and samples were collected from 3- to 30-cm depths. Soil samples were air dried and sieved through a 2-mm sieve. Soil properties were measured using standard soil testing methods (Sparks, 1996). The following soil properties are listed for sandy clay and loamy sand, respectively: clay content, 38.5 and 12.6%; silt content, 12.5 and 0%; sand content, 47.5 and 87.5%; specific surface area values, 168 ± 2 and 43 ± 0.4 m2 g-1; cation exchange capacities, 23.2 and 8.4 mmolc per100 g; organic C (OC) content, 0.37 ± 0.26% and 0.35 ± 0.25%; and pH, 8.1 ± 0.2 and 8.0 ± 0.4.
Cuticles Cuticles were isolated from fruits of tomato (Lycopersicon esculentum Mill.) and leaves of the succulent plant Agave americana L. Freshly picked leaves and fruits were washed several times with deionized water and boiled in water for 60 min. 850
Cuticle sheets were then manually peeled. The isolated cuticles were washed in deionized water and allowed to dry at room temperature. Dried cuticles were slightly ground before use in the incubation experiment.
Incubation of Soils with Cuticles The cuticles (12.5 g) were mixed with 250 g of soil and placed in 400-mL plastic containers. To ensure sufficient nutrient supply, N, P, and K were added to the soils at a level of 60 mg kg-1 each. After blending, water was added uniformly to the top of the soils. The volume of the water was adjusted to a moisture level of 80% of the water-holding capacity of each soil placed into container. The containers were incubated at 25 ± 1°C for 24 mo. During the incubation period, moisture levels were maintained constant by wetting the soils on a weekly basis according to weight loss. Control soils (soils without the addition of cuticles) were incubated and treated in the same way. Sufficient sets of samples were prepared to allow three replicates from each type of cuticle and each type of soil for four sampling periods. At every sampling point (i.e., 1, 6, 13, and 24 mo), the whole containers were sampled, and the soils were freeze-dried and kept in desiccators for further experiments.
Isolation of Humin–Mineral Matrices Humic and fulvic acids were extracted from the soil samples according to the procedure described by Swift (1996). Briefly, soils were treated with 1 mol L-1 HCl to remove CaCO3 and agitated with 0.1 mol L-1 NaOH under a N2 atmosphere for 24 h to extract humic and fulvic acids. The slurry was centrifuged at 23,000 g for 15 min, and the supernatants containing humic and fulvic acids were decanted. The extraction procedure was repeated a few times until no additional OC was detected in the supernatants. The extracts from the repeated procedure for each soil were combined and acidified to pH 2 using 6 mol L-1 HCl to precipitate humic acid. After further centrifugation, the supernatants containing fulvic acid were decanted. The OC contents of humic and fulvic acids were measured with a total OC analyzer (VCPH model, Shimadzu Scientific Instruments). The residual humin–mineral matrices were washed once with 0.05 mol L-1 CaCl2 and then several times with distilled water until the electrical conductivities of the supernatants were reduced to
