Short communication A sterile environment for growing, and monitoring, micro-organisms under a range of soil matric potentials H.J. McGovern, L.K. Deeks, P.D. Hallett, K. Ritz, I.M. Young * Soil-Plant Dynamics Unit, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK Received 19 October 1999; received in revised form 13 June 2000; accepted 14 July 2000 Abstract The energy status of soil water, as de®ned by the soil matric potential, plays an important role in regulating the dynamics of all soil organisms. In particular the relations between soil matric potential and soil microbes have been well documented. We have built a simple, reproducible, and inexpensive tension table designed to equilibrate soil samples in a sterile environment, or in an environment that ensures speci®c populations of soil micro-organisms are protected from competing populations. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Matric potential; Soil microbes; Sterility The soil water regime plays a pivotal role in soil biota dynamics (Young and Ritz, 2000; Linn and Doran, 1984). Speci®cally, the energy status of the water held within soil, as de®ned by the soil matric potential, has been shown to have large and signi®cant effects on the survival, reproduc- tion, and movement of soil-borne bacteria, fungi and nema- todes, both pathogenic and non-pathogenic (Grose et al., 1984; Glenn et al., 1987; Wallace, 1968; Li et al., 1996; Toyota et al., 1996b). As competition between micro-organisms can affect the growth and performance of any single species (Toyota et al., 1996a) it is important to maintain an uncontaminated envir- onment when studying selected organisms. Maintaining this sterile state can be a problem. For instance, when attempting to isolate the interaction of one species of bacteria or fungi on soil structure. Also, where a sterile environment is required, it has proved dif®cult to maintain sterility over time, within a soil where water status is changing. The equipment described here allows the matric potential of a sample to be maintained without the need to seal the sample in an airtight container, which could cause suffoca- tion to the target organism or an accumulation of by- products. This equipment therefore has the additional bene- ®t of allowing experiments to be run for a longer period of time than a sealed container would allow. Sand tables are commonly used in experiments to test the physical properties of soils at various matric potentials but, for the reasons mentioned previously, have limited use in biological experiments. We have designed and tested a ster- ile growing environment for micro-organisms that allows manipulation of matric potential without contamination from water- or air-borne bacteria or fungal spores. The basis of the design is an autoclavable desiccator (Fig. 1), with an internal diameter of 15 cm. This size allows for up to nine (60 mm diameter £ 57 mm height) cores to be incubated simultaneously. Two 0.3 mm Whatman Hepa- Vente ®lters were added to outlets on the lid to permit gas exchange. These ®lters are moderately hydrophobic and thus minimise moisture loss. A 9 mm (i.d.) autoclavable tube was passed through a hole in the lower part of the desiccator and coiled in the base. Fine holes were made in the coiled part of the tube and these were then covered with a ®ne netting to prevent sand entering the tube. Acid- washed, dry sand (250±500 mm) was then poured into the desiccator until it covered the tubing. Fine dry sand (125± 250 mm) was then added until it almost ®lled the base of the desiccator. The size range of the top layer of sand can be varied, depending on the desired matric potential range. However, initial trials indicated that air bubbles and uneven wetting were more likely to occur when very coarse sand or ®ne gravel sand was used. The portion of tubing outside the desiccator was connected to a bi-directional in-line ®lter unit (Nalgene), containing a 47 mm diameter, 0.2 mm cellu- lose nitrate membrane (Whatman). The connections at the ports for ®lters and tubing were sealed with silicon sealant Soil Biology & Biochemistry 33 (2001) 689±691 0038-0717/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0038-0717(00)00185-1 www.elsevier.com/locate/soilbio * Corresponding author. Present address: SIMBIOS, University of Aber- tay Dundee, Bell Street, Dundee DD1 1HG, UK. Tel.: 144-1382-308646; fax: 144-1382-808877. E-mail address: imy@tay.ac.uk (I.M. Young).