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

Authors: ASHWORTH, DANIEL - University Of California; YATES, SCOTT - Retired ARS Employee

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/2019
Publication Date: 5/16/2019
Citation: Ashworth, D.J., Yates, S.R. 2019. Effect of application rate on chloropicrin half-life and simulated emissions across a range of soil conditions. Science of the Total Environment. 682:457-463. https://doi.org/10.1016/j.scitotenv.2019.05.203.
DOI: https://doi.org/10.1016/j.scitotenv.2019.05.203

Interpretive Summary: Owing to their volatile nature, agricultural fumigants can be emitted from soil into the air where they are a human respiratory risk and can contribute to the formation of near-surface smog. An important factor determining how much of an applied fumigant is released onto the air is its degradation rate within the soil; fumigants that degrade to a greater extent within the soil are less of a concern in terms of air quality. In this research, we studied the degradation of chloropicrin, a commonly used soil fumigant. The percentage loss of chloropicrin to air is thought to be related to its initial application rate owing to greater degradation at low application rates, which leads to very low emissions to air. However, little is known about how various soil/environmental factor affect this application rate/degradation rate relationship; therefore, we studied this relationship under varying temperatures, soil moisture contents, and organic matter contents. Across almost all treatment, the degradation of chloropicrin was lowest at high application rates, which we attribute to the toxic effect of the chloropicrin on microbes capable of degrading the fumigant. We then used a mathematical model to predict the effects of the differing degradation rates observed across the treatments on emissions from soil to air. We found that the impact of the various treatments on degradation rate had a significant impact on emissions. The work will be useful to state and federal regulators in assessing the likely impact of chloropicrin use on air quality and human health.

Technical Abstract: 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, 392 kg ha-1) under gradients of soil temperature (10–40°C), soil moisture content (1–15%), and soil organic matter content (1–3%). 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.