Time in a Bottle: Finding New Life for an Old (Yet Reliable) Test Method

The simple soil bottle presents an extremely useful tool for predicting performance of preservative treated, modified or naturally durable woods. The methodology was developed in the 1940s exclusively for evaluating wood preservatives against wood decay fungi. It has been adapted over several decades to include naturally durable woods, wood plastic composites, and engineered wood products, and we use it constantly here at the Forest Products Laboratory (FPL).

The basic premise of the soil bottle is a material is presented to an actively growing fungus in an otherwise sterile environment. The resistance of the material to fungal degradation is determined by comparison to reference materials (non-durable species or treated reference material). The soil bottle also presents an excellent tool for studying basic fungal biology whereby cellular changes in wood during the decomposition process can be analyzed.  The soil presents a refuge for the decay fungus as well as a source for moisture and transported ions relevant to the decay process.

Past, present and future research at FPL is looking at ways of modifying the standard soil bottle setup to be even more useful for the evaluation of wood and wood protectants. Here are just a few examples of where FPL researchers are pushing the boundaries of the standard soil bottle:

The western red cedar blocks on the left had extractives removed and did not stand up well against decay fungus, while the un-extracted blocks on the right exhibited tolerance.

Naturally durable woods – While evaluating North American wood species for covered bridge repair, researchers wanted to look at the effects of extractives on durability. We removed the extractives using and compared durability to un-extracted blocks. When the extractives from a naturally durable wood species are removed, much and in most cases all of its durability to decay fungi are also removed. More results from that study can be found here.

Fungal synergy – FPL researchers have included multiple fungi in a single bottle to examine the effects of fungal competition on decay rates. Referred to as “fungus fights,” these studies provided insight into what would likely occur in the natural environment. With only two exceptions, a single fungus was apparent in each soil bottle at the end of the test, indicating one fungus won the fight. The winning fungus was not always the most efficient, and the rot type, white or brown, did not affect the outcome on the preferred wood type. The zone lines on the samples to the right are indicative of interaction between two competing fungi. More information on those studies can be found here.

X-ray studies – FPL researchers are studying ion transport in decaying wood. For these tests, materials are subjected to a standard soil bottle test (8-12 weeks depending on wood type) and then the decayed wood is imaged using X-ray fluorescence microscopy at the Advanced Photon Source at Argonne Labs. X-ray fluorescence spectroscopy enables researchers to measure ion concentration in a spatially resolved way to pinpoint where in the wood decay fungi are releasing and moving cations, which are involved in nutrient transport and also wood decay. Researchers can also examine changes in ion concentration within the wood as a result of fungal colonization. A recent paper in Nature Scientific Reports details those studies.  We’ve also conducted modified soil bottles where we exposed very small slivers of wood so that we can image ions in and around the fungal hyphae as the fungus interacts with the small wood slivers.The X-ray image to the right shows a wood block colonized by Serpula lacrymans, a brown rot fungus common in buildings. (Photo credit: Advanced Photon Source at Argonne Labs)

Different stacking configurations used in the copper proximity study.

Copper tolerance – Copper tolerant fungi are a common nuisance in preservative treated wood and can lead to early failure of products in service. These fungi are adapted to tolerate the levels of copper preservatives that usually control non-tolerant wood decay fungi. Recent studies conducted at FPL used a modified soil bottle setup to examine the effects of proximity to copper treated wood on decay of untreated wood as well as expression of genes believed to be involved in copper tolerance. The results of this study can be found here.

Modified woods – In contrast with naturally durable woods, modified woods have been altered to resist decay and are often difficult to assess in the standard soil bottle.  Chemically modified woods are a good example as they resist decay either by moisture exclusion or blocking the action of decay enzymes. To better understand the mechanisms of modified woods, we’re currently looking at how to modify parameters in the soil bottle decay test to better suit these materials.

FPL has a rich history of soil bottle tests over the past 70 years and continues to find new and innovative ways to adapt this test to suit our investigative needs.  Stay tuned!

Blog post by Grant Kirker, FPL research forest products technologist