Founded on more than 50 years’ worth of data from a unique field experiment, the researchers considered almost 9,000 genes and used X-ray imaging to look at soil pores smaller than the width of a human hair, and in concert with previous work, have started forming what they envisage will be a universal ‘Theory of Soil.’
In healthy soils, relatively low nitrogen levels limit microbes’ ability to utilise carbon compounds, so they excrete them as polymers which act as a kind of ‘glue’ - creating a porous, interconnected structure in the soil which allows water, air and nutrients to circulate.
Writing in the journal Scientific Reports, the researchers reveal that the Victorian-era switch from manure to ammonia and phosphorous based fertilisers has caused microbes to metabolise more carbon, excrete less polymers and fundamentally alter the properties of farmland soils when compared to their original grassland state.
Lead researcher Professor Andrew Neal said, “We noticed that as carbon is lost from soil, the pores within it become smaller and less connected. This results in fundamental changes in the flow of water, nutrients and oxygen through soil and forces several significant changes to microbial behaviour and metabolism. Low carbon, poorly connected soils are much less efficient at supporting growth and recycling nutrients.”
A lack of oxygen in soil results in microbes having to turn to nitrogen and sulphur compounds for their energy – inefficient processes, he said, which result in increased emissions of the greenhouse gas nitrous oxide among other issues.
The closed soil structure also means microbes need to expend more energy on activities such as searching out and degrading less easily accessible organic matter for nutrients.
Conversely, in carbon-rich soil there is an extensive network of pores which allow for greater circulation of air, nutrients and retention of water.
While soil carbon was already known to drive climate and water cycles the world over, it took a chance discussion between experts working at very different scales to discover the reason why.
The idea to look at this link between the living and non-living components of soil came about through a discussion between an expert in microbial genetics Professor Andrew Neal, and Professor John Crawford – now at the University of Glasgow - who studies the way complex systems behave.
“Despite carbon’s critical role, the mechanisms underlying carbon dynamics and the link to soil water were poorly understood,” said Neal. “Society struggles with the concept of what soil is and how it can be managed effectively because it is such a complex combination of biological, chemical and physical processes.