Journal of Environmental Radioactivity 76 (2004) 265–272 www.elsevier.com/locate/jenvrad Short communication Seasonal changes of redox potential and microbial activity in two agricultural soils of tropical Australia: some implications for soil-to-plant transfer of radionuclides J.R. Twining  , M. Zaw, R. Russell, K. Wilde RadionuclideEnvironmentalPathwaysProject,AustralianNuclearScienceandTechnologyOrganisation, PMB1,Menai,NewSouthWales2234,Australia Received 11 February 2003; received in revised form 6 June 2003; accepted 10 June 2003 Abstract Very little is known of the factors controlling soil-to-plant transfer of radionuclides in tropical environments. As part of an IAEA/FAO coordinated research project (CRP) designed to elucidate some of those factors, near-surface samples of two agricultural red- earth soils (Blain and Tippera) were collected from a study site in the Northern Territory. The climate is tropical monsoonal with crops being grown over the wet season from Decem- ber to March/April. It is important to understand soil variables that may be related to this dramatic seasonality. In this investigation, soil redox state and microbial populations were assessed before and after the growing season with a view to generating hypotheses for future evaluation. The X-ray absorption near edge structure (XANES) technique was used to determine overall changes in the solid-state redox speciation of Fe and Mn in soils across the growing period. Fe speciation did not change but approximately 10% of the total Mn was oxidised from Mn(II) to Mn(III) and Mn(IV) in both soils between October 1999 and April 2000. An apparent disconnect between Fe and Mn was not unexpected given the >10 times higher concentration of Fe in the soils compared with Mn. These results have implications for the bioavailability of redox sensitive radionuclides such as Tc and Pu. Similarly, microbial population estimates were derived before and after the growing per- iod. Total bacterial populations did not vary from 10 6 to 10 7 colonies per gram. Fungal populations increased over the growing season from 3–6  10 5 to 1–4  10 6 colonies per gram of soil. Fungi have the potential to decrease soil pH and hence increase the bioavail-  Corresponding author. Tel.: +61-2-9717-3060; fax: +61-2-9717-9260. E-mailaddress: jrt@ansto.gov.au (J.R. Twining). 0265-931X/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2004.03.031