The Victorian-era switch from manure to artificial fertiliser has resulted in damage to the microbial communities that maintain soil structure and soil health, according to new work from Rothamsted Research. Andrew Neal, lead researcher of the study, says it’s all to do with carbon in the soil.
According to Professor Neal, microbial communities in the soil use carbon to create a glue-like polymer which maintains soil structure. As the soil loses carbon, the microbe community changes, and in turn the soil pores become smaller and less connected. Over decades, the result is drastically reduced availability of water, nutrients and oxygen.
X-ray imaging was used to analyse soil structure, allowing a microscopic journey through the pore network…
Why is carbon being lost from the soil?
“Manure is high in carbon and nitrogen, whereas ammonia based fertilisers are devoid of carbon,” says Neal. Results suggest that decades of artificial fertiliser inputs on UK soil have changed the balance of carbon and nitrogen in the soil, changing the way microbes get their energy, which has in turn altered the soil structure.
In the experiment, long-term pasture land fertilised with manure was found to have an excellent soil structure, home to a diverse community of microbial life. By comparison, soil with high nitrogen content, but lower carbon content, had an altered microbial community which was associated with less pores and connectivity in the soil.
As the pores close up, and oxygen in the soil is lost, microbes are forced to turn to nitrogen and sulphur compounds for their energy. This inefficient process drains nutrients from the soil and results in increased emissions of the greenhouse gas nitrous oxide.
The authors say that addition of nitrogen and phosphorous fertilizers – without a source of carbon – is likely to be degrading the natural fertility of the soil, causing long-term reduction in yield potential, as well as increasing risk of flooding and drought damage.
A chance encounter…
Whilst 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 the 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.
“The question of how soil health can be managed effectively is difficult because it is such a complex combination of biological, chemical and physical processes,” says Neal. The latest study is the first to dig into the intimate two-way relationship between the microscopic life in soil and its structure at a sufficiently small scale.
The report adds to a growing body of evidence suggesting that the recycling of carbon into the soil, through manure or through more innovative solutions (link to UEA, paper crumble), is crucial to restoring the fertility of UK soils.
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