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.

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.