New directions in managing soil biology
From the February 2009 edition of Agriculture Today.
The more we know about soil biology the more we realise we don’t know.
We know, for instance, that soil microbes are a huge part of the earth’s genetic diversity.
We know that one gram of soil contains around 10 billion bacteria of up to 50,000 different species, and up to 200 metres of fungal hyphae.
We know these microbes play a vital role in breaking down organic matter to produce plant nutrients.
But we still don’t really understand how they affect plants.
An interesting review paper published last year summarises what we know about microbes’ positive and negative effects on plants.
The paper, by van der Heijden et al, is titled The unseen majority: soil microbes as drivers of plant diversity and productivity in terrestrial ecosystems and was published in Ecology Letters (www.wiley.com) in March 2008.
If you are interested in soil biology this paper provides an up to date summary of current soil biological knowledge relevant to agriculture.
I found the authors’ suggestions for future research particularly interesting.
For instance, we need to know how changes in microbial diversity and composition influence vegetation, but this requires experimental systems which can manipulate microbial diversity without influencing other factors, a hard ask.
We need to understand how altered land use and climate change affect microbial communities and, by association, plants.
We need to know much more about how different functional groups such as free living nitrogen fixers, root associated bacteria, and fungi affect plants.
And we need to find experimental systems that allow us to culture microbes.
Many microbes cannot be cultured, so we don’t know how they respond to or modify their environment.
New techniques developed in the past few years have helped scientists make some very interesting discoveries.
We now realise that Archae, a group of organisms similar to but separate from bacteria, have a big role in transforming ammonium into nitrate.
Bacteria other than Rhizobia can nodulate legumes.
There are many species of Arbuscular mychorrizal fungi that help plants obtain soil phosphorus.
Those that can be cultured in the laboratory are more resistant to soil perturbation; we know very little about the role of many other AM species because they decline in disturbed soils and most still can’t be cultured.
DNA analysis has found that microbes with the same gene sequences often have quite different functions in the soil.
Knowing which genes are associated with processes such as nitrogen fixation, denitrification, decomposition and phosphorus acquisition may help us manage our soils in future.
