The 35-year-old mystery of why fertilisers decrease biodiversity has finally been solved. The secret? They increase competition for sunlight.

Adding fertilisers to grasslands increases the productivity of plants, but leads to a major drop in biodiversity.

The amount of nitrogen and phosphorus available to plants has doubled in the last 50 years, but the reason why this has harmed diversity has not been easy to answer.

The debate has centred on whether fertilisers increase competition above or below ground for sunlight or soil resources?

To resolve the argument, Yann Hautier and Andy Hector from the University of Zurich, Switzerland built their own experimental plant community from scratch.

Hautier's team grew 32 plant communities for four years, before transferring them to a glass house.

Each community comprised four different sets of six species. Half were fertilised, the others were left unfertilised. Half of each of these sets had light added, using a system of three fluorescent tubes that were raised as the canopy grew, while the other half were left to grow in normal light conditions

After two years, the sets that were fertilised in normal light conditions showed a significant increase in productivity and biomass, but lost around one-third of their species diversity compared to the unfertilised groups. Those that were fertilised and given additional light showed no significant loss of diversity.

To uncover whether underground competition for root space had any influence on biodiversity, the team added two new species of plant at the beginning of the second year

The roots of half these plants were contained in plastic tubes, which prevented any below-ground competition: the other half were left exposed.

Removing below-ground competition from fertilised plots had no detectable impact on the mortality of the seedlings, compared to those that were exposed to full root competition, says Hautier

In the fertilised groups without additional light, there was no difference with or without the root-tube they died both ways. Even if we remove competition below ground, these plants are unable to grow.

This is a rare example of a simple experiment providing an unambiguous answer to an important ecological question. If these results are general to temperate grasslands which seems likely then we can start to develop more targeted policies to offset one of the most important sources of diversity loss in grasslands.