(Left: The fungal superhighway: soil fungi colonise different areas in the soil, often depending on the associations they have with plants growing aboveground. Image credit: http://flic.kr/p/dpRSsP.)
Soil animals and microbes play important roles in ecosystems: they control the rates of greenhouse gas emissions, mediate the nutrients available for plant growth, and many more functions besides. The more we know about the types of soil communities that inhabit a patch of soil we're interested in, the more we can do to predict how that patch will behave.
The work of Franciska de Vries and colleagues, for example, has shown that soils with more fungi in them could be more resilient to drought. However, the biogeography of soil microbes and animals remains elusive. Mapping what lies beneath has the potential to satisfy much more than our curiosity: knowing which microbial neighbourhood you're in could really help with growing your food.
We know that soil communities tend to be strongly linked to environmental factors, like pH. Because it's much easier to measure pH across large scales, we can use it to help us predict the biogeography of soil communities. Robert Griffiths and co-workers investigated the biogeography of soil bacteria in British soils in 2011, producing a map of the similarities between bacterial communities.
(Right: A map of the similarities between bacterial communities in the British Isles, produced by Griffiths et al. (2011).)
The map reveals broad geographic patterns in soil bacteria, suggesting that land use is a useful way of predicting soil communities: for example, bacteria found in peaty soils in the Scottish Highlands and the English Pennines are similar, despite being many hundred miles apart. Further investigation revealed that much of the variation in soil bacterial communities was explained by pH, plant communities and land use types.
Above ground, plants have also been shown to help us predict the sorts of soil organisms that exist beneath them. Ruth Mitchell and colleagues have used a statistical technique known as Co-Correspondence Analysis to study how well one community (in this case, the plants) can predict another (the microbes). Ruth and co-workers found that the plant community could indeed be used to predict the soil microbial community. Plants are useful in this respect because they are easier to survey than soil microbes and, because they grow over longer periods, provide a 'summary' of the environmental conditions driving the microbial communities.
In some of my own work, I've identified differences in the microbial communities that occur beneath distinctive landforms in a peat bog, by carrying out an intensive survey of both plant and microbial communities. This approach could be useful because the landforms can be easily identified from aerial photographs, allowing us to potentially map predicted patterns of their associated soil microbial communities over larger areas. Doing this could help us to improve our predictions of how different elements of the ecosystem might be functioning, using computer models.
(Left: A map of peatland landforms from Whitfield's PhD. Each landform, easily identifiable from its plant cover, has a distinct microbial community.)
Our knowledge of the what lies beneath is improving all the time, and there is a growing library of resources to get into if you're interested. A great example is the European Atlas of Soil Biodiversity [information found here], which was launched in 2010. The Global Soil Biodiversity Initiative is working to publish a Global Soil Biodiversity Atlas, so watch this space!
Mike Whitfield is also on the editorial board for Beneath Our Feet. Check out his bio on our site here.