Soil sterilisation methods for use in OECD 106: How effective are they?

Abstract

Under many circumstances chemical risk assessments for pharmaceuticals and other substances are required to differentiate between 'loss' of a chemical from the aqueous phase as a result of abiotic (sorption or precipitation reactions) or biotic (biodegradation) processes. To distinguish only abiotic processes, it is necessary to work under sterile conditions. Reported methods include poisoning the soil with sodium azide, irradiation and autoclaving. However, a key aspect of any testing is the representativeness of the matrix and so any sterilisation procedure needs to ensure that the integrity of the sample is maintained, in particular particle size distribution, pH and organic carbon partitioning potential. A number of controlled laboratory experiments were performed on 3 different types of soil. Results indicated that none of the methods successfully sterilised the soils and some physico-chemical changes in soils were identified post-treatment. Autoclaving destroyed the soil structure, therefore potentially affecting its sorption behaviour and sodium azide changed the pH of the loam soil solution by 0.53 pH units. Gamma irradiation exhibited least disruption to the tested soils physico-chemical properties. It was therefore concluded that gamma irradiation was the best available method for sterilising soils in preparation for sorption-desorption experiments; however care needs to be taken with this method to ensure that microbial activity is absent, or quantified if present. The changes to soils after sterilisation varied depending on the individual soil properties, indicating that soils should be studied on a case-by-case basis.

Abiotic conditions outweigh microbial origin during bacterial assembly in soils

“…Autoclaving consisted of 121°C for 60 min followed by a 1‐day room temperature incubation to encourage spore germination, followed by another round of autoclaving at 121°C for 60 min. Although gamma irrigation is often proposed as the preferred alternative to autoclaving for sterilizing soil, it is cost‐prohibitive at scale and we have found it less reliable in sterilizing soils, which is supported by other studies (McNamara et al ., 2003; Lees et al ., 2018). Additionally, both autoclaving and gamma irradiation alter soil chemical properties, including by releasing nutrients (McNamara et al ., 2003), but the same is true of in‐field soil sterilization techniques (Rovira, 1976).…”

Native soil microorganisms hinder the soil enrichment with antibiotic resistance genes following manure applications

“…All these studies also used γ-radiation to evaluate the role of indigenous soil bacteria in controlling the establishment of exogenous bacteria in soil. γ-radiation is considered as a reliable method that produces low soil disruption compared to other methodologies, being the most used and prospective technique for selective elimination of target organisms in soil ecological studies 12,13,27,28,35,36 . Indeed, we did not detect any artefact caused by γ-radiation in our study.…”

Gamma Irradiation with 50 kGy Has a Limited Effect on Agronomic Properties of Air-Dry Soil

“…Moist heat treatments are considered to be more detrimental to soil properties than dry heat treatments [3]. Gamma irradiation is considered to be the least disruptive to soil samples intended for physical and chemical analysis [4,9,10].…”

“…For instance, it has been reported that 20-100 kGy gamma irradiation may increase [5,9], decrease [12,13] or not affect [8,10] soil pH. Likewise, micronutrients may increase [5,13,14], decrease [15] or remain unchanged [5,15,16] after gamma irradiation.…”