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Fertilisers Make Plants Weaker

As well as causing soil damage and environmental problems from run-off, research published in Proceedings of the Royal Society of London B (http://tinyurl.com/hzwkj58) has found that nitrogen-based fertilisers also reduce a plant community’s resistance to fungal diseases. The researchers warn that prolonged use of artificial fertilisers can lead to the extinction of the most resistant plant species in a community, making the remaining species more susceptible to diseases.

In experimental field trials, researchers from Flinders University in Adelaide and Fudan University in Shanghai tested the biodiversity resilience of an isolated plant community in a native alpine meadow on the Qinghai–Tibetan Plateau. The 20-hectare experimental meadow, which has been spared from yak grazing for the past 20 years, contains a rich and diverse plant community.

“In this diverse and pristine ecosystem, we have established that extended fertilisation of soils not only alters the structure of natural plant communities, it also exacerbates pathogen emergence and transmission,” the researchers concluded.

Prof Corey Bradshaw of Flinders University says that this is one of the ways in which crop monocultures succumb to sudden outbreaks of severe disease. “Having more species in an ecosystem provides a sort of insurance policy against disease for any given species because of what’s known as the ‘dilution effect’,” he said. “This means that when there are a lot of species in any given area, the chance of passing a disease pathogen from an infected individual to a neighbour of the same species is lower, so the entire community benefits from an overall lower prevalence and severity of diseases.”

The project’s leader, Prof Shurong Zhou of Fudan University, described one of the main mechanisms of this disease-dilution effect: “Adding fertilisers makes certain species outcompete others, leaving the overall biodiversity of a system lower and more susceptible to disease,” Zhou said. “In other words, while some species benefit from adding nitrogen, the overall effect at the community level could be worse because the surviving species end up being more diseased.”

Bradshaw added: “These experiments provide powerful information about how species diversity maintains ecosystem function, and how agriculture and other human interventions can accelerate ecosystem degradation”.