Nitrogen-Based Fertilizers Increase Yield But Diminish Species Diversity in Grasslands

A new study highlights the double-edged-sword nature of nitrogen-based fertilizers, reporting that the increased yield in grasslands regions leads to an imbalance of species.

The worldwide study, published in the journal Nature, found that, on average, additional nitrogen imparted in soil from fertilizer will increase the amount of grass than can be grown, but that the benefit goes to a smaller number of plant species. The species that are crowded out are generally better adapted at surviving in harsher environments, which could lead to a greater swings in the amount of available forage at any given time, according to the study.

"More nitrogen means more production, but it's less stable," said University of Nebraska-Lincoln biologist Johannes M.H. Knops, a study co-author. "There are more good years and more bad years. Not all years are going to be good and the bad years are going to be worse."

To reach their conclusions, Knops and his international team of colleagues monitored grasslands across 41 locations on five continents for three years.

The sites monitored in the study included grasslands in China, tall grass prairies in the US, savanna in Tanzania, pastures in Switzerland and old fields in Germany, the researchers said in a statement.

The worldwide grasslands study reveled a number of common trends. In unfertilized grasslands there was greater stability of all grass species, which the researchers attribute to asynchrony - which refers to different species thriving at different times, thus resulting in more consistency in the grassland over time.

The number of species present in fertilized grasslands was lower than in unfertilized control plots. Unfertilized plots averaged 32 species per square meter, while fertilized plots had just 4.4 species per square meter.

Compared to control plots, the researchers found that fertilization decreased asynchrony and increased variation in production levels over time.

Knops and his collaborators' research is part of the larger Nutrient Network experiment, which is a collection of several studies detailing the relationships between grassland diversity, productivity and stability. Knops called the project an unprecedented experiment.

"In the past you didn't see a collaborative effort at a really large scale like this in biology or in ecology," he said.