Biodiversity challenged by excessive nutrient use in agriculture

A German study has found excessive use of nutrients in farming are having a negative impact on biodiversity

One of the reasons for the global threat to biodiversity is that we humans introduce more nutrients into our environment than would naturally be present there, for example, when fertilising agricultural land.

In addition, precipitation re-distributes excess nutrients to other areas, and nutrients can also enter our soils through air pollution.

Natural grasslands are a habitat for many different plant species including grasses, herbs, wildflowers and orchids, many of which can be threatened by human activities and impacts.

Plants need three things to grow: carbon dioxide (CO₂) from the air, water and nutrients from the soil. The latter are usually scarce in semi-natural European meadows.

Although this limits the growth of individual plants, it favours the possibility of many different species growing side by side.

Excessive amounts of nutrients, however, create the image that is ubiquitous in our landscape today: lush green meadows but without the colourful flowers of former times.

nutrients reduce biodiversity

It has long been known that excess nutrients reduce biodiversity, but until now the process behind it has been unclear.

Researchers from the German Centre for Integrative Biodiversity Research (iDiv), the Helmholtz Centre for Environmental Research (UFZ) and Martin Luther University Halle-Wittenberg (MLU) have now investigated the causes of this phenomenon in collaboration with a large international team.

To do this, they recorded the plant species community on fertilised and unfertilised grassland in experiments at 59 sites on six continents over a period of 13 years.

Dr Emma Ladouceur is lead author of the study and a scientist at iDiv and UFZ. She said: “The composition of plant species in grasslands always varies slightly; that is quite normal. For some species, the conditions in one year are not optimal, and they can only produce a few seeds, or no new plants emerge from them.

“On the other hand, other species whose seeds were already in the soil or were carried in by wind or animals can germinate. Through these processes, different species occupy new spaces in natural areas.

“The higher the species diversity, the higher the probability that species adapted to the living conditions are present and are providing ecosystem services humans depend on.”

more species loss

As expected, this experiment also consistently showed a decline in species on the nutrient-enriched plots over time.

In order to be able to attribute the observations to changes in plant species, the scientists divided the plant communities into three categories: species that were permanently present at a site over the entire study period, species that disappeared, and those that newly arrived.

The analyses showed that, on the one hand, more species got lost in the fertilised areas than in the unfertilised grasslands during the study period; on the other hand, fewer new species arrived during this time. The species composition also changed in the unfertilised areas.

However, losses and gains were balanced out here – the number of species remained constant in total.

In addition, the researchers recorded the aboveground biomass produced. As expected, the biomass of the harvested plants was higher on the fertilised trial plots than on the unfertilised ones, observable in the first year of experiments.

About 60% of this increase was due to plant species that were permanently present at a site.

The remaining 40% of the biomass was produced by newly added species, although fewer new species were added each year than on the unfertilised experimental plots.

practical nature conservation

Ladouceur added: “Our results show that when unnaturally large amounts of nutrients are present, a few species benefit particularly.

“Many other not-so-competitive species fall by the wayside - we lose them, and new species have a hard time establishing and taking hold.”

Senior author Professor Stanley Harpole, head of the Physiological Diversity Research Group at UFZ, iDiv and MLU said: “Our study makes a significant contribution to better understanding the effects of unnatural nutrient inputs on our biodiversity.”

With their study, the researchers are also providing valuable information for practical nature conservation.

“For nature conservation areas next to an agricultural area, it is important to know how the run-off of nutrients affects the semi-natural ecosystem so that targeted measures can then be taken to protect it,” Harpole concluded.

The study is published in Ecology Letters.