Turning a large area of huge leafy richness into smoking remains covering an ash filled wasteland has many far-reaching effects on animals and plants alike, but the effect on bacteria in the soil is less immediately obvious. Recent research (in a rather underresearched field) shows that as well as the lack of vegetation, and corresponding signals from plants and fungi, the sheer presence of ash in the soil can drastically change bacterial populations.
It's been fairly common knowledge for a while that ash tends to increase the amount of nitrogen in the soil, one of the reasons you get fertile farming areas under volcanoes (and ensuing unfortunate disasters). One of the things the researchers discovered was that a potential reason for this is the huge abundance of nitrifying bacteria found in forest soils recently exposed to fire. Ammonia oxidizing bacteria (that release nitrogen into the soil) were found in far higher concentrations in fire-ravaged soil than in normal soil, as shown below:
Bar chart showing numbers of Ammonia-oxidizing bacteria in different layers of the soil. Numbers were generated using real-time PCR
This increase in nitrogen production and nitrogen-producing bacteria was found to be independent of any pH changes in soil caused by the ash. One suggestion as to why the ash might be promoting nitrifying bacteria is that the ash helps to remove phenols or terpenes found in soil, which inhibit nitrification. This is supported the fact that soils containing large numbers of phenol-compounds tend to show a reduction in the number of nitrifying bacteria (and consequentially contain less nitrogen).
For those who are more scientifically inclined an interesting thing about this study was that PCR was used to quantify the number of AOB within the soil (see the bar chart). This is because AOB are apparently very difficult to grow on conventional media, so colony counting assays would have been tricky and less reliable.
This increased nitrification has many positive benefits for the surrounding ecosystem. By releasing more nitrogen into the soil the bacteria make it easier for new plants to grow, and colonize the empty space left behind. Several forests (especially in parts of Australia) rely on fires to help spread their seeds and clear way for new growth, and it looks as though bacteria play an important part in creating a nutrient rich environment in which the new seeds and proliferate.
Ball, P., MacKenzie, M., DeLuca, T., & Montana, W. (2010). Wildfire and Charcoal Enhance Nitrification and Ammonium-Oxidizing Bacterial Abundance in Dry Montane Forest Soils Journal of Environment Quality, 39 (4) DOI: 10.2134/jeq2009.0082
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