The effects and mode of action of biochar on the degradation of methyl isothiocyanate in soil
Graphical abstract
Introduction
Biochar (BC) is a carbonaceous material made from a variety of organic feedstocks, such as crop straw, farmland weeds or forest trees, at low temperature (< 700 °C) and under limited oxygen conditions (Lehmann and Joseph, 2009). As a soil conditioner, biochar with high carbon content, rich pore structure, and stable physical and chemical properties (Chen and Meng, 2013) is widely used as a new type of fertilizer in agriculture. Many studies have shown that the application of biochar in agricultural soil can change the soil's physicochemical and biological properties, improve soil fertility, immobilize nutrient elements, and increase crop yield (Novak et al., 2009). The high stability and resistance of biochar to degradation, allow this material to mitigate climate change through direct carbon sequestration (Lehmann, 2007, Lehmann et al., 2011), as well as potentially reducing soil greenhouse gas emissions (i.e. N2O) following amendment (Ippolito et al., 2012, Spokas et al., 2009). In addition, biochar can adsorb organic contaminants and heavy metals (Karami et al., 2011, Minori et al., 2010), affecting their fate in soil. Zhang et al. suggested that biochar can suppress the degradation of diuron in soil through adsorption (Yang et al., 2006, Zhang et al., 2004), and Nicolai David found that biochar accelerated the degradation of atrazine (Nicolai David et al., 2013). Therefore, through adsorption or degradation, biochar has the potential to increase or reduce the concentration of organic pollutants in soil, such as pesticides in soil. However, when biochar is applied to the soil as a fertilizer it may also affect the efficacy of pesticides in soil (Graber et al., 2011, Kookana, 2010), especially fumigants.
Methyl isothiocyanate (MITC), the principal breakdown product of metham-sodium and dazomet, is a viable alternative to methyl bromide which is being phased out because it depletes stratospheric ozone (Ruzo, 2006). MITC is generally used prior to seeding or transplanting to control soil insects, fungi, nematodes, and weed seeds (Frick et al., 1998). The recommended application dose of MITC is 250 kg/ha, which is two orders of magnitude greater than the dose of most conventional pesticides (Kiely et al., 2004, Saeed et al., 1997). Therefore, the effect of biochar on the fate of MITC in soil is expected to be greater than on other pesticides. Moreover, MITC has a relatively high vapor pressure (20.7 mm Hg at 20 °C) and is easily emitted to the air, and this loss of MITC gas may cause environmental and health problems due to its irritant, lachrymatory and toxic qualities (Kiely et al., 2004). Hence, it is necessary to develop strategies to minimize emissions. The application of organic amendments (e.g. manure) on the soil surface has been demonstrated to substantially reduce volatilization losses (Dungan et al., 2003, Gan et al., 1998), and may increase the soil's capacity to degrade MITC by abiotic and biotic mechanisms. Wang et al. demonstrated that biochar can reduce 1.3-dichloropropene emission by > 92% by adsorption (Wang et al., 2014), and reduce chloropicrin losses by 85.7–97.7% by accelerated degradation (Wang et al., 2015). This clearly indicates that biochar has great potential in terms of reducing MITC emission.
When used as an organic amendment, biochar's strong adsorption capacity may decrease MITC degradation and increase the fumigant's residence time in soil, however, in contrast, the vast amount of free radicals (Graber et al., 2011) in biochar may potentially accelerate MITC degradation via radical reaction (Lu et al., 2014). To date, the influence of biochar on MITC degradation in soil has not been well understood. Moreover, different types of biochars have different physical and chemical characteristics depending on the specific feedstock and production parameters (Lua et al., 2004), and this may also influence the fate of MITC in soil. The primary objectives of this study were to investigate the effects of biochars produced from different feedstocks on MITC degradation in soil, including the effects of biochar characteristics, amendment rate, moisture, temperature, soil sterilization and soil type, and to identify the mechanisms of biochar's interaction with MITC. Information obtained from this study will be useful for evaluating the effects of biochars on the bioavailability and efficacy of MITC, as well as identifying the potential for biochar to reduce MITC emissions in agriculture.
Section snippets
Soil, biochar and chemicals
Three soil samples (soil-1, soil-2 and soil-3) used in this experiment were collected in Beijing, Hunan province, and Shanxi province of China, respectively, at the depth of 0–20 cm. The physical and chemical characteristics of the soil are shown in Table 1. All soil samples were passed through a 2 mm sieve before use. The study tested six different types of biochar (BC-1 to BC-6), produced from oak, mixed hardwoods, macadamia, wood pellets, pine bark and wood chip feedstocks. Table 2 lists the
Biochar characteristics
Table 3 lists the degradation parameters of MITC in soil amended with six different types of biochar (B-1 to B-6). The r2 values lay between 0.73 and 0.98, indicating that the first-order kinetics model closely simulated the degradation of MITC. Different types of biochar exhibited different effects on MITC degradation in soil. The degradation rate (k) in soil amended with BC-1 has reduced 73.9% than in unamended soil. There was no significant difference between the k values of soil amended
Conclusion
The study found that some types of biochars can delay the residence time and other types can decrease the persistence of MITC in soil, while some did not affect MITC fate in soil. The effects could be inferred from the H/C value, SSA, C content and pH of biochars. Biochars with higher H/C values and lower pH values (e.g. BC-4) have stronger MITC degradation capacity, while biochar with high SSA and low H/C values (e.g. BC-1) can slow down MITC degradation in soil. Biochar amendments with lower
Acknowledgements
This study was supported by the National Natural Science Foundation project of China (31572035) and Beijing Nova program (No. 2010B064). Thanks to Melanie M for the help in language and to Kurt Spokas for providing biochar used in experiments.
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