How much nitrogen is available worldwide in grasslands

05.11.2019  | Beate Kittl | News WSL 

Buffalo herds in North American prairies, gnus and zebras on the African savannahs or deer herds in the Swiss National Park: grasslands not only provide habitat for numerous large animals but also are very important for global nutrient cycles. Grasslands are naturally found on roughly one third of the global land area and store approximately one quarter of the total sequestered carbon, most of it in the soil.

Nitrogen is one of the most important nutrients for plants. It directly controls growth and thereby the productivity of grassland ecosystems – and in turn, how much grass grows and how many animals can live there. Carbon sequestration, by which the greenhouse gas carbon dioxide is removed from the atmosphere, also depends on nitrogen. It is well known how microorganisms extract nitrogen from dead biomass and convert it into a form that is available again for plants (nitrogen mineralisation). However, until now the available quantities of nitrogen in grasslands have never been determined worldwide using a coordinated approach under real conditions. A team headed by Anita Risch of the Swiss Federal Institute for Forest, Snow and Landscape Research WSL has now accomplished this for the first time.

It is expensive to directly measure the soil nitrogen available for plants. That is why researchers are often limited to taking soil samples into the laboratory to measure mineralisation under standardised conditions. Anita Risch and her colleagues from the Nutrient Network Project (www.nutnet.org) have now measured nitrogen conversion in 30 natural grassland ecosystems worldwide directly in the soil. All researchers used the same equipment and methods and sent all the collected samples to WSL in Birmensdorf for analysis.

Better understanding of nutrient cycles

The results show for the first time the global pattern of nitrogen availability measured in grasslands.  In particular, it turned out that the field measurements significantly differed from measurements in the laboratory. «We now better understand what happens in the nitrogen cycle under natural conditions worldwide,» Anita Risch explains. «This is important if we want to understand the impact of man-made global changes such as over-fertilisation of ecosystems.» Natural grassland ecosystems generally contain significantly less nitrogen that those on which humans increase biomass production through fertilisation. In another part of the study, the team investigated these impacts by using fertilisation experiments on the same grasslands; the results should become available during the coming year.

Furthermore, the results will help other scientists to gain more information on the availability of nitrogen in grassland ecosystems under natural conditions from their laboratory values , as Anita Risch's team has linked the laboratory and field data to different factors such as temperature, clay content of the soil and microbial mass. In this combination, the estimates of available nitrogen is more meaningful.

Nutrient Network

Nutrient Network (NutNet.org) is an international research cooperative. More than 100 research groups from all over the world work together in the Network. Grassland ecosystems are studied using the same experimental set-up and the same methods for collecting data. The key question is how fertilisers and herbivores affect ecosystem functions such as productivity, diversity or nutrient cycles.

Natural grasslands cover roughly one third of the global land area. Many of them are or were the habitat of vast herds of large herbivores. Some examples:

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