Chemical fertilisers reduce the level of bacteria in soil

Chemical fertilisers reduce the number of nutrient solubilising bacteria associated with the roots of wheat, according to new research.

A team headed up by scientists from Rothamsted Research has found that the addition of fertiliser decreased the proportion of bacteria that help make nutrients such as nitrogen (N), potassium (K), phosphorous (P), iron, and zinc more readily available from soil.

The results point to the idea that the addition of fertiliser means that plants no longer need to interact with these beneficial bacteria to access the nutrients required to grow.

The authors say this knowledge will benefit the development of more targeted biofertilisation strategies.

Lead author Tessa Reid said:

Current high-yielding dwarf crop varieties rely on unsustainable levels of inorganic chemical fertilisers. Understanding the effect of the fertilisation regime on growth promoting bacteria is essential to optimise microbiome function in the sustainable intensification of agriculture.

Results of study

In the study, the beneficial plant growth-promoting bacteria were found to have decreased, both on and around the roots of fertilised plants.

Strikingly, the amount of growth-promoting bacteria living on the roots fell from 91% of total bacteria for unfertilised plants, to just 19% for those that received the fertiliser dose.

Although the mechanism behind the decline is unknown, it does back up other Rothamsted findings that fertilisers are essentially ‘short-circuiting’ the natural cycling of nutrients by soil microbes.

Study lead, Dr. Tim Mauchline said:

“This study gives evidence that wheat plants can select growth-promoting bacteria in their root environment which could establish mutually beneficial associations with plants. However, high levels of inorganic chemical fertiliser application reduces the abundance of these bacteria in soil around plant roots.”

Whereas most other studies on the soil microbiome have focused solely on describing the microbial community, here the team complemented this approach with microbial functional screens.

They isolated bacterial species from the soil samples, grew them up in the laboratory and subjected them to a series of tests to monitor their ability to solubilise key macro and micro-nutrients.

“We hope that this work contributes to a shift from simple studies looking at which species are present to a conceptual framework which attempts to identify and explain patterns in the soil microbiome in both farmed and natural systems,” Reid concluded.