Predicting methyl iodide emission, soil concentration, and pest control in a two-dimensional chamber system

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.

Modeling variation in 1,3-dichloropropene emissions due to soil conditions and applicator practices

The fumigant 1,3-dichloropropene (1,3-D) is widely used for control of soil-borne pests and pathogens, but post-application emissions may lead to off-site transport and possible human exposure. The fraction of applied material emitted into the atmosphere and the magnitude of peak emissions are two quantities used by regulators to protect public health and are typically based on field estimates. However, the current body of field studies covers only a narrow subset of the broad range of application practices and soil conditions under which applications are performed and is subject to an unknown level of estimation error. Here we use the HYDRUS model to estimate cumulative and peak emissions of 1,3-D for 17 application methods used in California. The simulations are parameterized with soils data from 16 fields sampled immediately prior to fumigation in order to establish a representative distribution of initial soil conditions. The results demonstrate a wide range in cumulative emissions, with mean losses of initial applied mass between 10 and 58% over two weeks depending on application method. Emissions are highly variable in response to soil conditions, with coefficients of variation ranging from 16 to 54% for cumulative flux and 26 to 67% for peak three-hour flux depending on application method. The simulated distributions show similarities to the available field study estimates in terms of the mean and spread of distributions, particularly in the case of cumulative emissions, indicating that the modeling approach could be a useful tool to support regulatory decision-making in cases where field data is limited.

Introducing the 2-DROPS model for two-dimensional simulation of crop roots and pesticide within the soil-root zone

Mathematical models of pesticide fate and behaviour in soils have been developed over the last 30years. Most models simulate fate of pesticides in a 1-dimensional system successfully, supporting a range of applications where the prediction target is either bulk residues in soil or receiving compartments outside of the soil zone. Nevertheless, it has been argued that the 1-dimensional approach is limiting the application of knowledge on pesticide fate under specific pesticide placement strategies, such as seed, furrow and band applications to control pests and weeds. We report a new model (2-DROPS; 2-Dimensional ROots and Pesticide Simulation) parameterised for maize and we present simulations investigating the impact of pesticide properties (thiamethoxam, chlorpyrifos, clothianidin and tefluthrin), pesticide placement strategies (seed treatment, furrow, band and broadcast applications), and soil properties (two silty clay loam and two loam top soils with either silty clay loam, silt loam, sandy loam or unconsolidated bedrock in the lower horizons) on microscale pesticide distribution in the soil profile. 2-DROPS is to our knowledge the first model that simulates temporally- and spatially-explicit water and pesticide transport in the soil profile under the influence of explicit and stochastic development of root segments. This allows the model to describe microscale movement of pesticide in relation to root segments, and constitutes an important addition relative to existing models. The example runs demonstrate that the pesticide moves locally towards root segments due to water extraction for plant transpiration, that the water holding capacity of the top soil determines pesticide transport towards the soil surface in response to soil evaporation, and that the soil type influences the pesticide distribution zone in all directions. 2-DROPS offers more detailed information on microscale root and pesticide appearance compared to existing models and provides the possibility to investigate strategies targeting control of pests at the root/soil interface.

Phase partitioning, retention kinetics, and leaching of fumigant methyl iodide in agricultural soils

Although it is not currently being sold in the USA, the recent US registration of the fumigant methyl iodide has led to an increased interest in its environmental fate and transport. Although some work has now considered its volatile emissions from soil, there remains a lack of experimental data regarding its ability to be retained in soil and ultimately become transported with irrigation/rain waters. Using laboratory batch and soil column experiments, we aimed to better understand the phase partitioning of MeI, the ability of soils to retain MeI on the solid phase, and the potential for leaching of MeI and its primary degradation product, iodide, down a soil profile. Results indicated that MeI was retained by the solid phase of soil, being protected from volatilization and degradation, particularly in the presence of elevated organic matter. Retention was greater at lower moisture content, and maximum retention occurred after 56 days of incubation. At higher moisture content, the liquid phase also became important in retaining MeI within soil. Together with low observed K(D) values (0.10 to 0.57 mL g(-1)), these data suggest that MeI may be prone to leaching. Indeed, in a steady-state soil column study, initially retained MeI was transported with interstitial water. The MeI degradation product, iodide, was also readily transported in this manner. The data highlight a potentially significant process by which MeI fate and transport within the environment may be impacted.