Degradation of methyl isothiocyanate and chloropicrin in forest nursery soils

Microbial degradation of plant toxins

Plants produce a variety of secondary metabolites in response to biotic and abiotic stresses. Although they have many functions, a subclass of toxic secondary metabolites mainly serve plants as deterring agents against herbivores, insects, or pathogens. Microorganisms present in divergent ecological niches, such as soil, water, or insect and rumen gut systems have been found capable of detoxifying these metabolites. As a result of detoxification, microbes gain growth nutrients and benefit their herbivory host via detoxifying symbiosis. Here, we review current knowledge on microbial degradation of toxic alkaloids, glucosinolates, terpenes, and polyphenols with an emphasis on the genes and enzymes involved in breakdown pathways. We highlight that the insect-associated microbes might find application in biotechnology and become targets for an alternative microbial pest control strategy.

Non-target impacts of pesticides on soil N transformations, abundances of nitrifying and denitrifying genes, and nitrous oxide emissions

Agriculture is the leading contributor to global nitrous oxide (N₂O) emissions, mostly from soils. We examined the non-target impacts of four pesticides on N transformations, N cycling genes and N₂O emissions from sugarcane-cropped soil. The pesticides, including a herbicide glyphosate (GLY), an insecticide imidacloprid (IMI), a fungicide methoxy ethyl mercuric chloride (MEMC) and a fumigant methyl isothiocyanate (MITC), were added to the soil and incubated in laboratory at 25 °C. The soil microcosms were maintained at two water contents, 55 % and 90 % water holding capacity (WHC), to simulate aerobic and partly anaerobic conditions, respectively. Half of the soil samples received an initial application of KNO₃ and were then maintained at 90 % WHC for 38 d, whilst the other half received (NH₄)₂SO₄ and were maintained at 55 % WHC for 28 d followed by 10 d at 90 % WHC to favour denitrification. Responses of individual functional genes involved in nitrification and denitrification to the pesticides and their relationships to N₂O emissions varied with time and soil water. Overall, MITC had pronounced repressive effects on AOA and AOB amoA gene abundances and gross nitrification. Under 55 % WHC during the initial 28 d, N₂O emissions were low for all treatments (≤62 μg N kg^-1 soil). However, under 90 % WHC (either during the first 28 d or the increase in water content from 55 to 90 % WHC after 28 d) the cumulative N₂O emissions increased markedly. Overall, under 90 % WHC the cumulative N₂O emissions were 19 (control) to 79-fold (MITC) higher than under 55% WHC; with the highest emissions observed in the MITC treatment (3140 μg N kg^-1 soil). This was associated with increases in gross nitrate consumption rates and abundances of denitrifying genes (nirK, nirS and qnorB). Therefore, to minimise N₂O emissions, MITC should not be applied to field under wet conditions favouring denitrification.

Exploring the destiny and distribution of thiocyanate in the water-soil-plant system and the potential impacts on human health

Endocrine disruptors like thiocyanate are some of the principal causes of chronic disorders worldwide. Prenatal and postnatal exposure to thiocyanate can interfere with normal neurological development in both fetuses and newborns. Currently, little information regarding thiocyanate levels and potential sources of exposure is available. In this study, we evaluated thiocyanate uptake and accumulation in chard and spinach grown under greenhouse conditions. Both chard and spinach are commonly used to produce baby foods. Three thiocyanate concentrations were compared: Control, T1 (30 ng mL^-1), and T2 (70 ng mL^-1). Thiocyanate accumulation depended on the concentration and exposure time. Chard was found to accumulate more thiocyanate than spinach, with leaf accumulation > stem accumulation (p < 0.0194) and maximum concentrations of 76 ng g^-1 (control), 112 ng g^-1, (T1), and 134 ng g^-1 (T2). The estimated daily intake (EDI) of thiocyanate for chard and spinach (fresh) exceeded the subchronic reference dose of 200 ng^-1 kg^-1 day^-1 and the chronic reference dose of 600 ng^-1 kg^-1 day^-1. In addition, the EDI of thiocyanate for spinach in baby food exceeded twice the chronic reference dose in the vulnerable newborn-1 year age group. However, all EDIs were lower than the lowest observed adverse effect level (LOAEL) of 1.9 × 10⁵ ng kg^-1 day^-1. Further studies are needed that increase our knowledge of thiocyanate levels and potential environmental sources to reduce opportunities for exposure, especially in vulnerable groups.

Chemical fumigation and biofumigation alter soil bacterial community diversity and composition

Chemical fumigation and biofumigation are used to reduce soil-borne diseases in agricultural production systems; however, non-targeted soil microorganisms may also be affected. This study compared the effects of chemical fumigation, either used alone or combined with an organic amendment, and biofumigation on soil bacterial community diversity and composition under controlled conditions over 160 days. Treatments included: fumigation with chloropicrin (CP), fumigation with metam sodium used alone (MS) or combined with barley plant residues (MSBR); biofumigation with mustard plant residues; addition of barley plant residues; and untreated control. Biofumigation had a greater impact on bacterial diversity at early time points, transiently decreasing species evenness and yielding the most dissimilar β-diversity after 3 days. MS fumigation did not affect bacterial diversity indices; however, MSBR transiently decreased species evenness after 8 days. CP-treated soil had decreased species evenness that did not recover over time and had the most dissimilar β-diversity at the end of the incubation compared to all other treatments. This study demonstrated that CP fumigation had the greatest and most persistent impact on bacterial diversity, whereas MS fumigation and biofumigation led to transient decreases in bacterial diversity.

Effect of application rate on chloropicrin half-life and simulated emissions across a range of soil conditions

The volatile release of agricultural fumigants from soil to air is a critical concern in terms of human and environmental health. A major control on the release of fumigants from soil to air is their degradation rate within the soil; however, this is a function of human/soil/environmental conditions and their inter-relationships. For the common fumigant chloropicrin (CP), it is known that application rate has a marked effect on degradation rate, with a potential further influence on CP emissions. We conducted batch degradation studies to better understand how CP degradation rate changes in response to application rate (56, 224, 392kgha^-1) under gradients of soil temperature (10, 25, and 40°C), soil moisture content (1, 8, and 15%), and organic matter content (1, 2, and 3%). A general trend of degradation rate decreasing with increasing application rate was observed across almost all such gradients, which is likely attributable to decreased microbial numbers and activity (i.e., degradation) at high (toxic) application rates. The effects of these ranges in degradation rate on emissions from soil to air were predicted using an analytical solution model, indicating that between the low and high application rates, total emissions percentage increased markedly (increases ranging from 69 to 99.8 percentage points, depending on prevalent conditions). The work will be useful to state and federal regulators in assessing the likely impact of CP use on air quality and human health.

Application rate affects the degradation rate and hence emissions of chloropicrin in soil

Increasingly stringent regulations to control soil-air emissions of soil fumigants has led to much research effort aimed at reducing emission potential. Using laboratory soil columns, we aimed to investigate the relationship between chloropicrin (CP) application rate and its emissions from soil across a wide range of CP applications (equivalent to 56-392kgha^-1). In contrast to the known behavior of other fumigants, total emission percentages were strongly and positively related to application rate (i.e., initial mass), ranging from 4 to 34% across the application rate range. When combined, data from a previous study and the present study showed good overall comparability in terms of CP application rate vs. emission percentage, yielding a second-order polynomial relationship with an R² value of 0.93 (n=12). The study revealed that mass losses of CP were strongly disproportional to application rate, also showing a polynomial relationship. Based on degradation studies, we consider that a shorter half-life (faster degradation) at lower application rates limited the amount of CP available for emission. The non-linear relationship between CP application rate and CP emissions (both as % of that applied and as total mass) suggests that low application rates likely lead to disproportionally low emission losses compared with higher application rates; such a relationship could be taken into account when assessing/mitigating risk, e.g., in the setting of buffer zone distances.

Analysis of the inhibitory effects of chloropicrin fumigation on nitrification in various soil types

Chloropicrin retards the conversion of ammonia to nitrite during the nitrification process in soil. In our study, the dynamic effect of chloropicrin fumigation on soil nitrification was evaluated in five different soil types to identify relationships between soil properties and the effect of fumigation on nitrification. Chloropicrin significantly inhibited nitrification in all soils; however, the recovery of nitrification varied greatly between the soils. Following chloropicrin fumigation, nitrification recovered to the control level in all soils, except in the acidic Guangxi soil. Nitrification recovered faster in fumigated sandy loam Beijing soil than in the other four fumigated soils. Soil texture and pH were two important factors that influenced chloropicrin's inhibitory effect on nitrification. An S-shaped function was fitted to soil NO₃^--N content to assess the nitrification recovery tendency in different soils. The time taken to reach maximum nitrification (t_max) ranged from 2.4 to 3.0 weeks in all unfumigated soils. Results demonstrated that t_max was greater in all fumigated soils than in untreated soils. Correlation calculations showed that t_max was strongly correlated to soil texture. The correlation analysis results indicated that the recovery rate of nitrification after chloropicrin fumigation is much faster in sandy loam soil than silty loam soil.

The effects and mode of action of biochar on the degradation of methyl isothiocyanate in soil

Biochar is used as a new type of fertilizer in agriculture; however, its effect on the fate of fumigants in soil is not fully understood. The objective of this study was to investigate the effects of biochar on methyl isothiocyanate (MITC) degradation in soil in laboratory incubation experiments, including the effects of biochar composition, amendment rate, moisture, temperature, soil sterilization and soil type. The dissipation pathways of MITC in biochars included adsorption and chemical degradation. The adsorption of MITC by biochars was positively correlated with the specific surface area (SSA) of the biochar. Biochar with a high SSA and low H/C value (such as biochar type BC-1) reduced MITC degradation in soil substantially; following BC-1 amendment, the degradation rate was 73.9% slower than in unamended soil. The degradation of MITC was positively correlated with the H/C value of biochar, and MITC degradation in soil increased 2.2-31.1 times following amendment with biochars with higher H/C values (e.g. biochar types BC-3-6). The biochar with the lowest organic matter and low H/C value did not affect the fate of MITC in soil. Biochars affect abiotic degradation processes more than biodegradation. When soil samples had a higher water content (>10%), higher temperature (40°C), and lower organic matter, the addition of BC-1 biochar reduced MITC degradation substantially; and this did not change significantly when the amendment rate increased. However, BC-4 biochar accelerated MITC degradation with increasing amendment rate, increasing temperature, and decreasing soil water content. The differences in degradation rates due to soil type were minimized by amendment with BC-4, but significant differences in BC-1. The results showed that the rational use of biochar has the potential to reduce MITC emission by accelerated degradation and adsorption.

Effect of application rate on fumigant degradation in five agricultural soils

Soil fumigation is an important pest management tool for many high value crops. To address the knowledge gap of how fumigant concentration in soil impacts dissipation, and thereby efficacy, this research determined the degradation characteristics of four fumigants as affected by application rate. Laboratory incubation experiments were conducted to determine degradation rates of 1,3-dichloropropene (both cis- and trans isomers), chloropicrin (CP), dimethyl disulfide (DMDS), and methyl iodide (MeI) in five agricultural soils. Fitted to pseudo first-order kinetics, the degradation rate constant (k) of CP, DMDS, and MeI decreased significantly as application rate increased while the 1,3-D isomers were the least affected by rate. Half-lives increased 12, 17, and 6-fold for CP, DMDS, and MeI, respectively, from the lowest to the highest application rate. At low application rates, the degradation rate of all fumigants in the Hueneme sandy loam soil was reduced by 50-95% in sterilized soil compared to the biologically active controls. However, this difference became much smaller or disappeared at high application rates indicating that biodegradation dominates at low concentrations but chemical degradation is more important at high concentrations. When co-applied, CP degradation was enhanced with biodegradation remained above 50%, while 1,3-D degradation was either reduced or not changed. Among the fumigants tested, the relative importance of biodegradation was DMDS>CP>MeI>1,3-D. These results are useful for determining effective fumigation rates and for informing regulatory decisions on emission controls under different fumigation scenarios.

Effect of co-formulation of 1,3-dichloropropene and chloropicrin on evaporative emissions from soil

Co-formulations of 1,3-dichloropropene (1,3-D) and chloropicrin (CP) are commonly used for preplant fumigation in the production of high-value crops. Various ratios of 1,3-D to CP are available in these co-formulations. Collation of previous field data suggested that when the two fumigants were co-applied, the emissions of CP were significantly lower than when CP was applied singly. However, none of these previous studies had a control treatment with CP applied alone, alongside a treatment where CP was co-applied with 1,3-D under the same climatic and edaphic conditions. This work aimed to address this issue by measuring emission fluxes from soil columns maintained under controlled conditions in which 1,3-D and CP were applied alone and as four commercial co-formulations with various 1,3-D:CP ratios. A strong positive relationship between CP emissions and CP percentage in the formulation was observed. Furthermore, strong positive relationships between CP degradation half-life and CP percentage in the formulation and between CP degradation half-life and total column emissions suggested that the lower emissions were due to faster CP degradation when the CP percentage (and hence initial application mass) in the formulation was low. The presence of 1,3-D did not significantly affect the degradation rate of CP, and, therefore, it is hypothesized that co-application was, in itself, not a significant factor in emission losses from the columns. The findings have implications for the accurate modeling of CP because the effect of initial mass applied on CP degradation rate is not usually considered.

Chloropicrin emissions after shank injection: two-dimensional analytical and numerical model simulations of different source methods and field measurements

Understanding the control mechanisms of fumigant movement in soil is a fundamental step for developing management strategies to reduce atmospheric emissions. Most soil fumigants including chloropicrin (CP) are applied by shank injection, and the application process often leaves vertical soil fractures that would potentially cause preferential fumigant movement and increased emissions. This potential transport pathway was evaluated by comparing cumulative emissions and soil air concentrations of CP from direct field measurements with those predicted using analytical and numerical models after assuming either point or rectangle sources for the injected CP. Results clearly showed that shank-injected CP, when treated as vertical rectangle sources, produced cumulative emission losses similar to the field measurements. Treating the shanked CP as point sources caused approximately 50% underprediction than the field measurements. The study also demonstrated that fumigant cumulative emissions can be predicted, with reasonable accuracy, using either analytical or numerical simulations.

Microbial aspects of accelerated degradation of metam sodium in soil

Preplant soil fumigation with metam sodium is used worldwide to control soilborne diseases. The development of accelerated degradation of pesticides in soil, including metam sodium, results in reduced pesticide efficacy. Therefore, we studied microbial involvement in accelerated degradation of methyl isothiocyanate (MITC) following repeated soil applications of the parent compound, metam sodium. MITC degradation was reduced in soil with a history of metam sodium applications following sterilization, indicating the key role of microorganisms in accelerated degradation. Accelerated degradation of MITC was induced by inoculation of soil with no previous application of metam sodium with soil with a history of metam sodium applications. We developed a method to extract the active microbial fraction responsible for MITC degradation from soil with a history of metam sodium applications. This concentrated soil extract induced accelerated degradation of MITC when added to two different soils with no previous application of metam sodium. An extensive shift in total bacterial community composition in concentrated soil extracts occurred after a single metam sodium application. Two Oxalobacteraceae strains, MDB3 and MDB10, isolated from Rehovot soil following triple application of metam sodium rapidly degraded MITC in soil with no previous application of metam sodium. Polymerase chain reaction-denaturing gradient gel electrophoresis analysis of bacterial community composition showed relative enrichment of MDB3 following metam sodium application, suggesting its potential in situ involvement in accelerated degradation development in Rehovot soil. Responses of resident Oxalobacteraceae community members to metam sodium applications differed between Rehovot and En Tamar soils. Isolate MDB10 did not induce accelerated degradation of MITC in En Tamar soil and, with the slow dissipation of MITC, soil suppressiveness of accelerated degradation is suggested. The isolation and identification of MITC-degrading bacteria might be helpful in developing tools for managing accelerated degradation.

Emission, distribution and leaching of methyl isothiocyanate and chloropicrin under different surface containments

The environmental fate of fumigants methyl isothiocyanate (MITC) and chloropicrin (CP) is of great concern for potential air and groundwater contamination while retaining sufficient concentrations for pest control efficacy. The emission, gas phase distribution, leaching, and persistence of MITC and CP were examined in repacked columns filled with sandy soils under three surface conditions: tarp without irrigation, tarp with limited irrigation, and 5-d irrigation without tarp cover. For MITC, cumulative emission constituted 62%, 36%, and 0.3% of the amount applied under tarp without irrigation, tarp with limited irrigation, and 5-d irrigation without tarp surface conditions, respectively. The corresponding cumulative emission losses were 45%, 30%, and 5.4% for CP. During the first 24h after injection, soil air concentrations of the two fumigants were much higher in the 15-25cm depth range than other depths in the soil profile. Small amounts of leaching occurred for both fumigants, indicating potential for groundwater contamination should heavy rain fall or irrigation occurs immediately after soil fumigation. Very small amounts of residual MITC and CP (<2%) were found in the soil 24 days after the experiment. The study clearly showed that atmospheric emission and degradation were the two primary pathways of MITC and CP dissipation during soil fumigation. Emission could be effectively reduced with 5-d irrigation if small leaching is acceptable or be prevented.