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A radical new way of thinking has solved the mystery of why adding manure to soil improves crop yields – and offers other benefits... Scientists finally unearth why soil carbon is so valuable

A radical new way of thinking has solved the mystery of why adding manure to soil improves crop yields – and offers other benefits too.

Scientists at Rothamsted Research found that relatively low nitrogen levels in healthy soils limit the ability of microbes to utilise carbon compounds,  excrete them as glue-like polymers to create a porous, interconnected structure 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 fertiliser caused microbes to metabolise more carbon, excrete less polymers and fundamentally alter the properties of farmland soils.

Lost carbon

As carbon is lost from soil, the scientists found that the pores within it become smaller and less connected. They found that low carbon, poorly connected soils, are much less efficient at supporting growth and recycling nutrients – limiting yields.

Lead researcher Professor Andrew Neal said: “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.”

A lack of oxygen in soil results in microbes having to turn to nitrogen and sulphur compounds for their energy – inefficient processes, says Professor Neal. Among other issues, this results in increased emissions of the greenhouse gas nitrous oxide

The closed soil structure also means microbes need to expend more energy on 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.

Nutritious manure

“Manure is high in carbon and nitrogen, whereas ammonia-based fertilisers are devoid of carbon. Decades of such inputs – and soil processes typically act over decades – have changed the way soil microbes get their energy and nutrients, and how they respire.”

While soil carbon was already known to drive climate and water cycles the world over, Prof Neal said 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 Prof Andrew Neal and Prof John Crawford – now at Glasgow  University – who studies the way complex systems behave.

Prof Neal says: “Carbon dynamics and the link to soil water were poorly understood. 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.”

Dynamic interaction

The two scientists took inspiration from a theory proposed by Richard Dawkins in the 1980s that many structures we encounter are in fact products of organisms’ genes. This helped them understand soil as a product of microbial genes, incorporating organic materials.

“We have shown for the first time a dynamic interaction between soil structure and microbial activity – fuelled by carbon – which regulates water storage and gaseous flow rates in soil with real consequences for how microbes respire.”

The group, which also involved scientists from Nottingham University, is the first to seriously study the details of this intimate two-way relationship between the microscopic life in soil and its structure at scales relevant to microbial processes.

The results also have implications for farmers, where the addition of nitrogen and phosphorous fertilisers – and not carbon – may be leading to a degradation of natural soil fertility that could be detrimental to long term productivity.