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The role of biochar in modifying the environmental fate, bioavailability, and efficacy of pesticides in soils: a review.

Introduction

The potential role of biochar in carbon sequestration, reduction in the emission of greenhouse gases, and improving the soil fertility has led to the establishment of the International Biochar Initafive (www.biochar-intemational.org) and increased interest in this topic. Biochar as a soil amendment is increasingly attracting the attention of policy makers in USA, Australia, and elsewhere (e.g. Bracmort 2009). Biochar as an amendment to enhance soil fertility has only recently been recognised based on research on terra preta soils discovered in Amazonia, associated with Native American settlements. These are kilometer-sized fertile patches of black soils, containing black carbon or charcoal, among less intensely coloured and relatively infertile Oxisols (Glaser et al. 2001), However, it is well recognised that charcoal, the solid residue of partially combusted biomass, is a major constituent of the non-living organic matter of many soils and sediments (Schmidt and Noack 2000).

Charcoal and biochar are part of the combustion continuum which produces a range of partially combusted materials (e.g. soot, charcoal, etc.) arising from an incomplete combustion of vegetation or fossil fuel under different levels of oxygen environment during pyrolysis. This continuum is commonly termed as black carbon (BC). BC is ubiquitous in terrestrial and aquatic environments (Goldberg 1985; Schmidt and Noack 2000), and for the purposes of this paper, biochar can be considered as a part of the BC pool. In addition to dispersal through biomass and fossil fuel combustion in the environment, some agricultural practices have also contributed to the increasing amount of BC in agricultural soils. For example, it is an old practice in eastern and southern parts of China and in northern India to mix firewood-ashes with soils and livestock dung followed by heating and/or ageing for several months, before adding directly into the field as a organic manure fertiliser. Direct burning of plant residues in the field after harvest for land clearing, although discouraged, is still common in many parts of the world. Such agricultural practices provide direct input of BC into agricultural soils. Terrestrial BC, being erosion-prone, is readily transported by wind and water and is often deposited into aquatic ecosystems. It is therefore not surprising that BC has been found to make a significant proportion of sediments (Schmidt and Noack 2000). There is considerable research available on BC and its role in environmental fate and effect of organic contaminants (e.g. Accardi-Dey and Gschwend 2003). Such research is highly relevant in providing a basis for understanding the potential effect of biochar as a soil amendment on the fate and behaviour of pesticides in the environment.

It is increasingly being recognised that the presence of charcoal in soil could not only enhance the sorption of organic contaminants such as pesticides, but also influence the nature of the sorption mechanism and the availability of residues to organisms, including their effectiveness for weed or pest control. Research in recent years (e.g. Yang and Sheng 2003; Yang et al. 2006; Yu et al. 2006; Xu et al. 2008) has shown that the biochars produced from burning wood, wheat and rice residues were some 400-2500 times more effective than soil in sorbing pesticides. Despite the fact that the environmental fate and impact of contaminants (bioaccessibility, bioavailability, and toxicological impact) are strongly influenced by their sorption--desorption behaviour, the effect of BC or biochars on desorption behaviour of compounds has so far received limited attention.

The objective of this review therefore was to assess the potential effect of biochar addition to the environmental fate and efficacy of pesticides in soil, based on a review of recent work on sorption, desorption, persistence, and bioavailability of pesticides from soils amended with biochars or soils/sediment containing BC. Although the published research is limited, some of the observations emerging from these studies allow the building of new hypotheses and help identify future research needs from the standpoint of agronomic efficacy as well as fate and effects of pesticides in environments as affected by biochar addition to soils. The assessment raises some important questions, such as:

* Would biochar addition to soil lead to reduced efficacy of pesticide inputs requiring higher application rates?

* How would the properties of biochars and their ability to interact with pesticides vary with time and ageing?

* What are the implications of biochar addition to soils for bioavailability and persistence of pesticides and how would such practice impact the long-term fate of pesticide residues in environment?

* Should there be any concern due to accumulation of pesticide residues in soil and on food quality from produce grown in char-amended soils?

Obviously these questions need to be adequately addressed before biochar amendments to soils as an agronomic practice is recommended for a wide spread adoption for both environmental and economic reasons.

Key processes determining the efficacy and the environmental fate of pesticides

The environmental fate (persistence and movement), uptake by plants and biota, and efficacy of pesticides in soil for control of target organisms is governed by a variety of physical, chemical, and biological processes that are often complex and dynamic. These include sorption--lesorption, volatilisation, and chemical and biological degradation. The transport of organic contaminant from soils into water, air, or food is directly controlled by these processes. Sorption-desorption and degradation (biotic and abiotic) are perhaps the 2 most important processes, as the bulk of the chemicals are sorbed by organic and inorganic soil constituents within hours of contact with soil/sediment, and then chemically or microbially transformed/degraded with time. Sorption-desorption is a major process regulating the concentration of pesticide molecules in solutions and consequently affecting other processes such as bioavailability, degradation, leaching, and volatilisation. Organic matter and clay minerals in soil are the principal sorbents; however, different sorptive mechanisms operate depending upon the surface properties of the sorbent and the charge characteristics of the sorbate (e.g. a pesticide). Sorption of a range of pesticides in Australian soils has been reviewed by Kookana et al. (1998). While clays mainly govern sorption of cationic herbicides (e.g. paraquat, glyphosate), the sorption of non-ionic pesticides (e.g. diuron, chlorpyrifos) is often noted to be related to the organic matter content (Kookana et al. 1998). However, both the organic matter content and its chemistry are important for sorption-desorption of pesticides (Ahrnad et al. 2001).

The bulk of sorption or uptake by the solid phase from the solution phase occurs within a few hours of application of pesticide to soil, albeit that the slower phase of the process continues for a long time (ageing). Therefore, for a pesticide to be available for plant uptake or degradation/transformation by microorganisms it needs to be released back into pore water solution in soil/sediments. With reversible sorption, a pesticide is readily released back in response to a concentration decrease in solution. However, this is not always the case. Indeed, hysteresis (indicated by the differences between sorption and desorption isotherms) has frequently been observed for several pesticides (reviewed by Brusseau and Rao 1989), often caused by slow release kinetics or mass transfer limitations and/or due to strong sorption. The extent of sorption and desorption depends on not only the physico-chemical properties of the sorbate (e.g. hydrophobicity, charge characteristics of a pesticide) but also that of the sorbent such as soil and sediment. Both the physical properties (e.g. surface area, pore size distribution) and chemical nature of organic matter and clay minerals determine the release behaviour of pesticides in soils. The nature and properties of BC are therefore particularly relevant to sorption and more importantly to desorption behaviour of pesticides, which in turn affects the bioavailability and efficacy of pesticides. Let us now look at the properties of BC that are important for pesticide dynamics in soils.

Properties of black carbon including blochars

Black carbon appears in numerous forms in our environment. It is therefore difficult to describe through a generally accepted definition (Kuhlbusch et al. 1996). Little is known about formation, movement, and oxidation of charcoal, but there are indications that frequent vegetation fires and alluvial transport play a major role (Skjemstad et al. 1997). Charcoal withstands biological and chemical degradation to a considerable degree (Goldberg 1985) and, due to its accumulation, plays an important role in the global carbon cycle and carbon sequestration (Jenkinson 1990). The properties of charcoals from the biomass are strongly dependent on the conditions during the combustion process (Shafizadeh 1984). According to Schmidt and Noack (2000), charcoals produced by vegetation wildfires consist mainly of several randomly orientated stacks of graphitic sheets. However, the structure is strongly influenced by several combustion parameters, such as the fuel type, fuel load, fuel condition, weather conditions, substrate heterogeneity, fire intensity, and duration (Patterson et al. 1987). Consequently, chars commonly found in the environment and produced from different parent materials employing different pyrolysis techniques are expected to be highly heterogeneous in nature.

Surface area and porosity of different biochars

As mentioned earlier, the surface area, pore size distribution, and total porosity of a material can influence the release behaviour and bioavailability of pesticides. Yu et al. (2006) produced biochars from air-dried (Eucalyptus camaldulensis) wood chips in closed porcelain crucibles by heating in a muffle furnace to make 2 types of biochars (BC450 and BC850). Charcoal BC450 was produced by heating the furnace to 450[degrees]C in 1 h, holding at this temperature for 2h, then allowing the crucibles to cool to room temperature. Charcoal BC850 was produced by heating the furnace to 850[degrees]C in 1 h, holding at this temperature for 1 h, then allowing the crucibles to cool to room temperature. The C contents of charcoal BC450 and BC850 were 65.1% and 86.1%, respectively. The prepared biochar materials were hand-ground to a fine powder in a grinder and roller and passed through a 200-gm sieve. The specific surface area (SSA) and pore size distribution of the 2 biochars were evaluated using BET nitrogen adsorption techniques at 77K (Yu et al. 2006).

It was found that the shape of the BET nitrogen adsorption isotherm for the charcoal BC450 was similar to that for nonporous materials, suggesting a negligibly small micropore volume. In contrast, the BC850 was a microporous material with a high SSA. The SSAs of BC850 and BC450 were 566 and 27 [m.sup.2]/g, respectively. BC850 was a microporous material with all pores essentially <1 nm in pore width and the maximum peak occurring at pore widths of ~0.49 nm, whereas BC450 had a lower level of microporosity with peak maxima occurring at a pore width of ~1.1 nm (Yu et al. 2006). This may indicate the beginning of micropore formation at 450[degrees]C. The SSA for BC850 was also higher than that of chars derived from wheat residue (Chun et al. 2004), where the highest SSA was reported to be 438 [m.sup.2]/g (charred at 600[degrees]C). Chun et al. (2004) also found char surface area to increase with increasing charring temperatures (300[degrees]C to 600[degrees]C); however, SSA measurement of char produced at 7000C was lower than that of char produced at 600[degrees]C, which they speculated to be due to microporous structures which may have been destroyed at 700[degrees]C.

The chemistry of carbon in natural and synthetic biochars

Using [sup.13]C solid state nuclear magnetic resonance (NMR), Smernik et al. (2006) characterised the above 2 laboratoryprepared charcoals (BC450 and BC850) together with 2 charcoals collected from the Tanami desert, Australia, within 2 days of a wildfire in November 1998. Charcoal F-W (fieldwood) was sampled from a small burnt log (probably from a species of Eucalyptus). Charcoal F-G (field-grass) was sampled from under a burnt clump of spinifex grass. The C contents of charcoal F-W and F-G were 77.3% and 67.7%, respectively. [sup.13]C cross polarisation (CP) and direct polarisation (DP) NMR spectra of the 4 charcoals (Fig. 1) were found to be dominated by a signal in the region 110-140ppm, which can be assigned to aromatic carbon (Smernik et al. 2006). For each charcoal, corresponding CP and DP spectra exhibit a similar distribution of signal. Some small differences between the charcoal spectra were apparent, especially for the laboratory-prepared charcoal produced at 450[degrees]C (BC450), which contained a shoulder at 150-160 ppm that can be assigned to O-substituted aromatic C, and small signals in the O-alkyl and alkyl regions (0-100ppm). The presence of these signals indicated that the charring process for BC450 was incomplete.

From the above it is clear that BC have several properties that are expected to have important beating on pesticide dynamics in soils. Biochars, depending on the temperatures during formation, have high SSA, high microporosity, and highly aromatic carbon. All of these properties have strong influence on sorption-desorption, persistence, and bioavailability of pesticides. In the next section, let us examine how these properties affect both the efficacy and environmental fate of pesticides.

Effect of biochar and black carbon on environmental fate processes of pesticides

Sorption--desorption

Charcoals have been shown to be a strong sorbent for environmental contaminants, thereby playing a crucial role in governing their environmental fate and risks to human and ecosystem health (e.g. Karapanagioti et al. 2000; AccardiDey and Gschwend 2003; Lohmann et al. 2005). Biochars produced from burning of wheat and rice residues were reported to be up to 2500 times more effective than soil in sorbing diuron herbicide (Yang and Sheng 2003). These extraordinary sorption properties of biochars arise from their unique properties, especially their highly carbonaceous and aromatic nature and high SSA. Here, the research on pesticide sorption in soils with naturally differing chemistry of soil organic carbon (SOC) is instructive and helps explain the high sorption potential of biochars. SOC consists of a heterogeneous mix of organic materials, often classified as an amorphous, gel-like 'soft or rubbery' matrix or domain and a condensed, 'hard or glassy' matrix or domain (Chiou et al. 1983; Weber et al. 1992), including altered and relatively unaltered aliphatic polymers, polysaccharides (e.g. cellulose), lignin and lignin degradation products, fats, proteins, and pectines (Schaumann 2006). It is generally thought that sorption of organic compounds to SOC is influenced by its 2 major domains: a soft or rubbery domain and a hard or glassy domain. These 2 domains have very different sorptivity, and thus the amount and relative proportion of these materials in SOC influence the capacity of soil to sorb or sequester organic compounds. The extent of sorption of pesticides in soils has been found to depend on the aromaticity of SOC (Ahmad et al. 2001, Abelmann et al. 2005). The strong relationship between partition coefficient of a pesticide and the aromaticity of SOC, as determined by a solid state [sup.13]C NMR, is shown in Fig. 2. Biochars, being highly aromatic in nature, are therefore expected to have much stronger sorption of pesticides and other compounds in soils than SOC.

[FIGURE 1 OMITTED]

The bioaccessibility, bioavailability, efficacy, and toxicological impact are directly linked to the desorption behaviour of pesticides, as the compound needs to be released back into solution to be effective or have any impact on target or nontarget organisms. However, the effect of biochars on desorption behaviour of compounds is so far poorly understood. To assess the reversibility of sorbed compounds, or lack of it, generally the sorption and desorption flanks of isotherms are compared, and when these do not superimpose on each other (which is quite common), the phenomenon (termed as sorption-desorption hysteresis) is taken as a sign of partial irreversibility. Braida et al. (2003) studied sorption and desorption of benzene in water to pure-form maple-wood charcoal prepared by oxygen-limited pyrolysis and they found that the sorption of benzene on wood charcoal was highly irreversible. They noted swelling of a sorbent during benzene sorption and suggested that pore deformation during desorption caused the desorption hysteresis. It has also been suggested in other studies that surface-specific adsorption, entrapment into micropores, and partitioning into condensed structures of soil organic matter are among the main causes of chemical sequestration (e.g. Weberetal. 1992; Huangetal. 2003). Studies comparing sorption and desorption isotherms for diuron herbicide and of pyrimethanil fungicide in soils amended with the 2 different charcoals mentioned above (BC450 and BC850) have also been published (Yu et al. 2006, 2010). Highly non-linear sorption isotherm and relatively flat desorption isotherms were observed (e.g. Fig. 3 for diuron), showing an apparent sorption-desorption hysteresis for these pesticides in charcoal-amended soils. Based on a hysteresis index (H) to quantify the degree of the irreversibility, they showed that as the content of charcoal in the soil increased, the value of H also increased. For the soils amended with charcoal BC850, the H values increased rapidly from 1.32 for the soil with 0.1% of charcoal to 14.92 for 1.0% of charcoal. For the soils amended with charcoal BC450, the H value also progressively increased but at a smaller rate. This is consistent with the properties of the biochars, e.g. higher SSA and greater volume of nanopores in BC850, which may have provided greater sorption and entrapment of herbicide molecules. The comparison between the 2 types of biochars by Yu et al. (2006) also highlighted the fact that while the sorption capacity of a soil could be enhanced to the same level by adding differential amounts of biochars (e.g. 0.8% BC850 v. 5% BC450), the degree of reversibility may, however, be very different from different biochars.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

So far, little attention has been directed to the heterogeneity within charcoals in terms of their impact on sorption--desorption of pesticides. Ground temperatures during wildfires may vary between 200[degrees]C and 500[degrees]C for many fires, with highest temperatures for scrubland wildfires ranging up to 700-1000[degrees]C (e.g. Raison 1979; Scott 1989). However, artificial charcoals as used in most sorption studies consisted of chars produced from either one fixed temperature, or only a small range of temperatures (Sander and Pignatello 2005; Zhu and Pignatello 2005; Zhu et al. 2005). A study by James et al. (2005), however, employed a range of combustion temperatures for 3 species of softwood. They evaluated sorption of phenanthrene on various wood chars produced at varying heating temperatures and concluded that in addition to surface area, heterogeneous surface properties contribute to sorptive ability of biochars. More recently, Spokas et al. (2009), working with a biochar produced from sawdust at 5000C, noted that on a mass basis biochar was a less effective organic sorbent for 2 herbicides than other forms of SOC. Clearly, not all biochars have the same sorption properties. The implications of heterogeneity of biochars for sorption and desorption processes and, in turn, for other environmental fate processes such as volatilisation, degradation, leaching, and transport need to understood to seek the most appropriate balance between carbon sequestration and pesticide efficacy.

Biochar effect on bioavailability, biodegradation, and efficacy of pesticides

Bioavailability and bioaccumulation

In addition to the effect on sorption and accumulation of pesticides, soil organic matter provides a substrate to support microbial activity and thereby plays a direct role in their persistence and degradation. However, for a pesticide to be degraded by microorganisms or to be taken up by organisms and be effective in the control of target organisms, it needs to be bioavailable. Researchers have generally described bioavailability of organic compounds by one of the following definitions (Hunter et al. 2010):

(1) the fraction of organic compounds that is weakly/reversibly sorbed and can undergo rapid desorption from solids to the aqueous phase, or

(2) the potential for partitioning of the compound into organisms at equilibrium.

Regardless of the definition used, bioavailability of organic compounds depends on several factors including the soil/ sediment properties (e.g. SOC content, BC content, and their physical and chemical composition), contaminant properties (e.g. water solubility, Kow), and the contact time between the contaminant and soil or sediments. The physico-chemical characteristics of the sorbent matrix may have profound effects on the bioavailability of organic compounds, as different classes of SOC have shown different degrees of binding strength (Luthy et al. 1997). As mentioned above, the 2 major domains in SOC, a soft or rubbery domain and a hard or glassy domain, have different affinity for compounds. In addition, carbonaceous materials such as BC, biochars, and kerogen have been found in studies to strongly inhibit bioavailability.

Both extractability and bioavailability of organic compounds in soils have been found to depend on content and chemistry of SOC. For example, Tao et al. (2006) tested the hypotheses by assessing the extractability of polycyclic aromatic hydrocarbons (PAHs) by wheat roots (Triticum aestivum L.) from artificially contaminated soils in pot experiments. They added 4 PAHs (naphthalene, acenaphthylene, fluorene, phenanthrene) to 20 Chinese surface soils containing varying amounts of SOC. The apparent accumulation of PAHs by wheat roots was found to be positively and negatively correlated with dissolved organic matter and total organic matter, respectively. These results were explained on the basis that while total SOC reduced the extractability through sorption, dissolved organic matter mobilised the sorbed PAHs and facilitated their availability to wheat roots, thus showing the contrasting role of the soil organic matter based on its chemistry.

Many studies have reported reduced bioaccumulation and toxicity of organic contaminants due to the presence of BC (reviewed by Hunter et al. 2010). For instance, significantly reduced bioaccumulation in clams (Macoma balthica) was observed for benzo[a]pyrene and penta chlorobenzenes (PCBs) when contaminants were associated with activated carbon (a type of BC) compared with association with wood or diatoms (McLeod et al. 2004, 2007). Similarly, Millward et al. (2005) found that amendment of marine sediment with 3.4% activated carbon (dry weight basis) resulted in a 70-87% reduction in PCB uptake by a polychaete and amphipod. However, opposite effects on bioaccumulation of some compounds from sediments amended with coal and charcoal by oligochaete worms (Tubificidae) have also been reported (e.g. Jonker et al. 2004; Hunter et al. 2010), highlighting the complexity of interactions of organic compounds in the presence of BC.

Biodegradation and efficacy

Being highly microporous, carbonaceous, and reactive (from sorption standpoint), biochars and BC are particularly interesting materials from the point of view of their impact on ageing of residue, microbial activity, biodegradation of compounds with time, and ultimately the efficacy and build-up of residues in soils. Indeed, the incorporation of a small amount of fresh biochars in a soil has been shown to inhibit the microbial degradation of organic compounds including pesticides, as well as reducing their availability to plants and their efficacy (Zhang et al. 2005, Yang et al. 2006, Yu et al. 2009). The effect of biochar amendment to soils on biodegradation, persistence, and plant uptake of pesticide residue from pesticide-spiked soils was recently demonstrated by Yu et al. (2009). They amended soils with different amounts of 2 types of biochars produced at different temperatures and representing different physico-chemical properties (BC450 and BC 850 as described above) to a level of 0, 0.1, 0.5, and 1% by soil weight. Biodegradation as well as plant uptake of 2 pesticides (carbofuran and chlorpyrifos with differing hydrophobicities) by spring onions (Allium cepa) were studied. The study demonstrated that the bioavailability of the pesticides to microorganisms (for degradation) as well as for plant uptake (for efficacy) decreased as the content of the biochars in soil increased. Increased persistence of carbofuran insecticide with increased biochar contents in soil is shown in Fig. 4, indicating reduced bioavailability to soil microorganisms in the presence of even small amounts of biochars. Over 35 days, 86-88% of the pesticides dissipated from the unamended soil, whereas only 51% of carbofuran and 44% of chlorpyrifos was lost from the soil amended with 1.0% BC850. However, despite greater persistence of the pesticide residues in biochar-amended soils, the plant uptake of pesticides decreased markedly with increasing biochar content of the soil. Data on the plant uptake of 2 pesticides in the presence of different amounts of both biochars are shown in Fig. 5. With 1% of BC850 soil amendment, the total plant residues for chlorpyrifos and carbofuran decreased to 10% and 25% of that taken up by plants from the unamended soil, respectively. The BC850 char was particularly effective in reducing phytoavailability of both pesticides from soil, which is consistent with its high volume of nanopores and high SSA.

Similar results showing the reduced biodegradation of diuron and benzonitrile by selected microorganisms in the presence of wheat char have been reported by other workers (Zhang et al. 2005; Yang et al. 2006). The observation of decreased bioavailability and plant uptake has direct implications for the effectiveness of pesticides in controlling target organisms as well as the label rates required for pest control.

In Australia some 35 years ago, Toth and Milham (1975) observed the effect of carbon produced by burning paspalum (Paspalum dilatatum L.) under a wide range of conditions (ash-C) and noted that it adsorbed appreciable quantities of diuron from aqueous solution. They noted that 1 of the 2 ash-C products tested caused a significant reduction in the phytotoxicity of [greater than or equal to]16kg diuron/ha. They subsequently reported markedly reduced efficacy (by 60%) of 2 pre-emergent herbicides, thiobencarb (S-eithyl hexahydro-1,4-azepine-l-thiolcarbamate) and molinate (S-(4-chlorobenzyl)-N.N-diethylithiolcarbamate) when the herbicides were applied over rice stubble ash (Toth et al. 1981), and a decreased phytotoxicity of diuron herbicide when applied over ash of recently burned kangaroo grass (Themeda australis (R.Br.) Stapf) (Toth et al. 1999). Some recent studies have also noted similar reduction in efficacy of pesticides in the presence of biochars in soil. For example, Yang et al. (2006) reported reduced herbicidal efficacy of diuron and Xu et al. (2008) of clomazone to control barnyard grass in soils amended with wheat and rice straw biochars, respectively. The data (Table 1) and photograph (Fig. 6), taken from the study by Yang et al. (2006), effectively demonstrate that with increasing biochar content in soil, much higher rates of application of herbicides were needed to get the same weed control as in unamended soil. For example, even doubling the application rate of diuron failed to control weed growth in the presence of 0.5% of wheat biochar in soil (Table 1). Their measurement indicated that even 0.1% of biochar in soil would appreciably reduce the bioavailabily of diuron. It is worth noting that they used grass chars, and the wood chars are likely to be more effective in reducing herbicide efficacy due to their higher affinity for herbicides than the grass chars (Yu et al. 2006; Bomemann et al. 2007). Whether biochars in soil would lose or generate the capacity to inactivate herbicides with time due to organo-mineral and biological interactions remains to be seen. This is explored further in the following section.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The 'ageing effect' on biochar and pesticide residues

It is not clear whether the effectiveness of biochars in sorbing pesticides would increase or decrease with time and what implications it would have on the accumulation of pesticide residues or efficacy in char-amended soils. Given that biochars are highly microporous as well as offering reactive sites for minerals, nutrients, carbon, and other compounds, these materials may in fact become active substrates and sites for many biogeochemical interactions in soil with time. Many studies have shown that organic compounds become less bioavailable (lower bioaccumulation, biodegradation, or toxicity) with increased residence time in soils or sediments (see review by Alexander 1995; Semple et al. 2003; Hunter et al. 2010). Studies involving hydrophobic chlorinated pesticides (e.g. DDT, aldrin, dieldrin, heptachlor, chlordane, kepone, nonylphenol, and alkylbenzene sulfonate) have shown 'hockey-stick' shaped loss curves involving an initial stage of relatively rapid loss followed by little or no loss (Alexander 1995). Such ageing effects are not restricted to hydrophobic compounds only. In fact, hydrophilic compounds with relatively weak affinity for soil/sediments have also been shown to be sequestered in soil and sediments with time (Ahmad et al. 2004).

[FIGURE 6 OMITTED]

The configuration of SOC as a 3-dimensional network of polymer chains and its dynamic nature in terms of biogeochemical interactions is probably one the contributing factors to this phenomenon. The pore size distributions of biochars are particularly important in this relationship. Tests by Nam and Alexander (1998) with model sorbents have shown that with non-porous glass and polystyrene beads phenanthrene was rapidly mineralised, whereas with porous polystyrene beads (5- or 300-400-nm pores), little of the compound was desorbed and only <7% of sorbed phenanthrene was mineralised during the test period (240 h).

Evidence of microbially mediated processes enhancing sequestration of organic contaminants in aged sediment has also been observed (Macleod and Semple 2003). It is conceivable that certain pore spaces in biochars (if of right sizes) may provide a habitat for microorganisms from at least 2 standpoints. One, the biochar pore structure may offer protection to microorganisms (bacteria, fungi) from predator organisms (e.g. protozoa, nematodes) of larger size. Second, being attractive sorption surfaces for organic compounds (carbohydrates, amino acids, etc.) and nutrients, biochars may become sites of substrate accumulation for microorganisms (Pietikainen et al. 2000). However, the preferential interactions of carbon substrate with biochar may also influence the substrate availability to microorganisms in surrounding zones in soil and adversely affect microbial activity (Pietikainen et al. 2000). From the point of view of mineral-biochar interactions, it is possible that with time mineral surfaces may cover the reactive surfaces of biochar and thereby mask the true sorption capacity of biochars for organic compounds such as pesticides, thus losing their capacity to reduce the efficacy of pesticides discussed above. The studies based on sorption of pesticides before and after removal of paramagnetics (e.g. sesquioxides) from carbon surfaces by hydrofluoric acid treatment needed for NMR studies have shown that natural charcoal in soils does show increased sorption per unit mass after such a treatment (e.g. Ahangar et al. 2008; Singh and Kookana 2009). However, biochars produced from different parent materials and at different temperatures are likely to be highly heterogeneous in nature, and contact time in soil may have differential effect on their properties. The effect of contact time on nature and properties of biochars through a myriad of potential biogeochemical interactions in soils and the consequences for efficacy of pesticides or accumulation of residue certainly deserves urgent attention of researchers.

Conclusions and future research directions

Based on limited research cited in the above sections, biochar amendment to soils can conceivably reduce the bioavailability and efficacy of pesticides on one hand but also can influence the potential accumulation and ecotoxicological impact of pesticides and other organic contaminants on the other. The implication of biochar amendment to soil and potential implications for environmental accumulation, distribution, and food safety of pesticides need to be understood. Biochars produced with different temperatures and technologies and from different parent materials are likely to be highly heterogeneous in their chemical nature and properties. Consequently, their interactions with other soil constituents and applied agrochemical inputs are expected to be highly variable. Most studies in the literature have been performed on freshly prepared biochars, sometime even without hydration of biochars. Their relevance for field-applied and somewhat 'aged biochars' is not clear. How with time these biochars are going to be colonised with microorganisms and other materials, transformed, and functionalised in soil environments remains to be established. The long-term effect of biochar on the fate and behaviour of pesticides deserves urgent attention.

Some key future research needs are listed below.

1. The characteristics of biochars sourced from different parent materials and produced at different temperatures, utilising different technologies, need to be fully understood. Limited studies on woody and grass biochars prepared at different temperatures have shown marked differences in their properties and ability to interact with pesticides. 2. It is not clear if biochars contain toxic organic contaminants, generally produced during combustion, such as polynuclear aromatic hydrocarbons (PAHs) or even dioxin-like compounds. Recently, Singh et al. (2010) analysed 11 different biochars and found negligible levels of PAHs (<0.5 mg/kg) in these samples. The levels of contaminants, if any, in the variety of biochars likely to be used on soil and especially those sourced from waste materials need to be analysed.

3. Biochars, being highly reactive, are likely to undergo biogeochemical transformation leading to changes in their properties with contact time in the soil environment (ageing). The understanding of organo-mineral interactions of biochar in soils and consequences of aged biochars for pesticide, nutrient, and microbiological interactions needs to be developed.

4. Studies in the literature strongly suggest that efficacy of soil-applied pesticides (herbicides, insecticides) is likely to be compromised in the presence of biochars in soils; the extent of such effects of different biochars in various soils is yet to be studied. In particular, it is important to establish how these effects are going to be moderated with ageing of biochars in soils.

5. Strong sorption and its partial reversibility, essential sequestration of pesticide residues in microporus biochars, and reduced degradation have been noted. These processes can potentially lead to accumulation of pesticides/ contaminants residues in biochar-amended soils. However, several questions are pertinent here. What would be the long-term fate and effects of these contaminants? Do micropores in biochars provide a favourable or unfavourable environment for microorganisms leading to breakdown or accumulation of such residues? Would this residue be of any ecological consequence, especially if the residues are rendered biologically unavailable?

6. Particles of biochars in soils, being relatively lighter materials than other soil solids, are likely to be prone to preferential erosion and off-site transport in surface runoff. The consequences of movement of biochar colloids in off-site migration of pesticides and other contaminants need to be understood. The enrichment of such colloids in pesticide residue during transport is also conceivable. The impact of such processes on the receiving environment needs to be evaluated.

7. In terms of food quality, it is not clear whether the organo-mineral interactions of biochars in soils would influence (compromise or enhance) the food quality, especially of produce destined for certain value-added products and expected to have a certain special food quality attributes.

10.1071/SR10007

Acknowledgments

I would like to acknowledge the contributions of several of my colleagues who conducted research and co-authored articles with me on this topic over the years. In particular, I would like to acknowledge contributions by G. G. Ying, X. Y. Yu, Ludger Bomemann, Ron Smernik, Riaz Ahmad, Jan Skjemstad, Neera Singh, and Evelyn Krull.

Manuscript received 5 January 2010, accepted 28 April 2010

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Rai S. Kookana

CSIRO Sustainable Agriculture Research Flagship, PMB 2, Glen Osmond, NSW 5064, Australia. Email: Rai.Kookana@csiro.au
Table 1. Effect of different rates of diuron herbicide applications on
fresh weights of barnyard grass and remaining residue in soils amended
with different levels of biochars (source: Yang et al. 2006)

Soil + biochar +                       Biochar       Diuron
herbicide treatment                    content     application
                                         (%)      rate (kg/ha)

Control soil                               0             0
Control soil + diuron                      0           1.5
Soil + biochar (0.05) + diuron (1.5)    0.05           1.5
Soil + biochar (0.5) + diuron (3.0)      0.5             3
Soil + biochar (1.0) + diuron (3.0)        1             3
Soil + biochar (1.0) + diuron (6.0)        1             6

Soil + biochar +                     Fresh weight
herbicide treatment                   of barnyard
                                       grass (g)

Control soil                            0.0242
Control soil + diuron                   0.0007
Soil + biochar (0.05) + diuron (1.5)    0.0030
Soil + biochar (0.5) + diuron (3.0)     0.0224
Soil + biochar (1.0) + diuron (3.0)     0.0308
Soil + biochar (1.0) + diuron (6.0)     0.0378
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Author:Kookana, Rai S.
Publication:Australian Journal of Soil Research
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Geographic Code:8AUST
Date:Sep 1, 2010
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