In the Department of Botany, there are two very sweet days every year. On those days, researchers and students drive to a meadow near Ahja River, bringing 250 kilograms of sugar with them, and spread it to the soil following a specific methodology. They have been doing it for 15 years already as part of one of the longest running plant experiments in Estonia, to study the effect of soil fertility on plant growth and biodiversity.
15 Years and 8 Tons of Sugar
Just imagine – during the last 15 years nearly 8 tonnes of sugar have been added to the meadow. It might seem wasteful but the scientists of the Department of Botany have a good reason for using sugar in a plant experiment. Since sugar is a carbon compound it can be used to change soil fertility, and study its impact on plant diversity.
When plant diversity decreases – something that is happening right now worldwide – it damages the whole ecosystem: biochemical cycles, other living organisms and humans as well. In order to conserve or restore plant diversity, scientists have developed ways to study how environmental factors affect plant diversity. One of those research methods involves changing soil fertility by adding a source of carbon or fertilizer to the soil.
Plants On a Sugar Diet
The experiment at the Ahja meadow involves fertilization of one part of the 2-hectare area every spring. The effects of adding fertilizer are probably familiar to most people, as it is often used in gardens and potato fields: soil fertility increases, plants grow bigger and give more harvest.
Natural plant communities usually do not need fertilizers. Unfortunately, humans fertilize natural ecosystems even unintentionally. A great amount of nitrogen finds its way into the nature through air pollution, and it acts like a fertilizer. As a result, plant diversity decreases because plants grow larger and a given area can fit less large plants than small ones. There are many reasons for this – a simple lack of space, or competition for soil nutrients and light.
The other part of the Ahja meadow gets sprinkled with sugar every spring and autumn, because sugar has exactly the opposite effect of fertilizing. How come? As plants get their nutrients from the soil, it is difficult to restrict their food intake in the nature. Instead, microorganisms living in the soil can achieve this job. As microorganisms consume the sugar, they become hyperactive and need more other nutrients from the soil as well. So, there will be less food left for plants. Thus, adding sugar to the soil is like putting plants on a diet.
The 15th Anniversary of the Experiment
The Department of Botany started the experiment in 2002 on the initiative of Martin Zobel, a professor of plant ecology. This year the experiment celebrates its 15th birthday, and during the years, many studies have been conducted at the experimental site.
When the experiment had been going on for 10 years, the results were reported in an extensive scientific article, which showed that fertilizing increased plant growth and decreased plant diversity. On the other hand, sugar addition made plants smaller and, though its impact to diversity was small, the areas influenced by sugar had a different set of species.
How much does soil fertility need to be decreased in order to achieve high plant diversity, and how much time will this process take – these are two questions that will probably be answered in the coming years of further research. Let’s hope that for the experiment’s 20th anniversary more significant results can be reported!
Similar experiments have been conducted in other places in the world, for example in Sweden and in the United States, but the Estonian experiment is known to be running for the longest time. In some plant communities elsewhere, soil fertility has been reduced in other ways. For example, in Czech Republic, scientists have added sawdust (also a carbon source), and in Germany, soil has been excavated to make the soil layer thinner.
Adding sugar can also have other benefits for plants. Some studies have found that it can keep out invasive alien species that often prefer nutrient-rich soils. Additionally, it has been demonstrated that adding sugar is beneficial for restoring species-rich meadows. These topics are to be further studied by Estonian students and scientists since this exciting plant experiment will be continued for a long time.
Riin Tamme and Lena Neuenkamp are ecologists at the Department of Botany in the University of Tartu.