Defining the biosecurity risk posed by transported soil: Effects of storage time and environmental exposure on survival of soil biota

Soil frequently occurs as a contaminant on numerous sea, land and air transport pathways. It can carry unwanted invasive species, is widely recognized as a biosecurity risk, and is usually strictly regulated by biosecurity authorities. However, little is known about relative risk levels between pathways, thus authorities have limited capability to identify and target the riskiest soil pathways for management. We conducted a replicated one-year experimental study to test the hypotheses that biosecurity risks from soil organisms will increase both with declining transport duration and with increasing protection from environmental extremes. Soil was collected from two sites, a native forest remnant and, an orchard, and was situated on, in and under sea containers, and in cupboards, and assayed after 0, 3, 6 and 12 months for bacteria, fungi, nematodes and seeds. Results showed that for Pseudomonas sp., bacteria, nematodes and plants, viability declined over 12 months, irrespective of origin. Bacterial and fungal numbers showed higher survival in exposed environments, compared to enclosed environments. Overall, there was a rapid decline in the viability of some biota under exposed environments compared to biota protected from exposure to sunlight, wetting and desiccation. The results were consistent with observations of organisms in soil intercepted from several real-world pathways, and indicated there is potential to rank risks from transported soils based partly on transport duration and environmental exposure. This would help authorities to optimally allocate management resources according to pathway-specific risks.