More Plant Species = Better Soil
The loss of biodiversity, meaning the loss of plant and animal species, is currently one of the biggest threats to Life on Earth. This was recently confirmed by the latest UN report on biodiversity and ecosystem services. In this context, I present my own research project on biodiversity from my Master studies. I analyzed if a higher number of plant species is beneficial for the health of the soil – and I found out, yes it is!
Soils are amazing constructs, formed out of rocks by the united forces of nature: The air is oxidizing minerals, the wind is giving and taking away particles, the rain is spending live but is also taking parts of the surface away. Roots are cracking and loosening everything. Microbes are the most numerous and versatile guests. Worms and bugs are its builders and inhabitants. Together, nature’s forces and time make soils to wise, old and incredibly important spaces.
The life of soils under humans
Soils cover most of the land on Earth and are one of humans most important resources. Soils are essential for many plant species and therefore for us humans. We need them to spend water and nutrients for our crops, as the ground so we can build our houses upon them, and for many more things.
Unfortunately, we use the soils so much that they are very exhausted. They get contaminated, parts of them get washed away, their nutrient content declines, and many physical properties deteriorate. Apart from sealed soils by cities and streets, mining, and big industries, a big problem are extensive areas with monocultures or removed vegetation. This decreases plant biodiversity, which is defined as the variety of plant species. And for instance soils with monocultures in agriculture are known to decrease much in quality.
Biodiversity and soil properties
In a project during my Master studies, I wanted to find out what effects plant biodiversity has on soil properties. In particular, I looked at soil porosity and soil density called bulk density. When a soil has high porosity, it means it has many pores that are spaces filled with air or water. High porosity is generally good because it benefits the growth of plants and many soil organisms as well as soil stability, which protects the soil from erosion.
Soils with many pores have automatically less solid components per defined volume and have therefore a lower bulk density. Hence, soil porosity and bulk density are opposing properties.
The Jena Experiment
As a student in the group of Ecological Modelling at the University in Jena, I studied the effects of plant biodiversity on a big experimental field site – the Jena Experiment in Jena, Germany. It is a huge project with many collaborating scientists, where the effects of plant biodiversity on the whole ecosystem are investigated. Is it different or better when there are 60 plants of one species or 60 plants all from different species? To answer this question, the experimental field has many smaller squares with different numbers of plant species: 0, 1, 2, 4, 8, 16, or 60. The plants include grasses, legumes, small herbs, and tall herbs.
The Jena Experiment: How does the number of plant species affect the soil and the whole ecosystem?
For those who are interested in the details of the experiment, in this paragraph I describe the method I used: I took soil samples from the different squares with the varying numbers of plant species. For that, I used small metal cylinders (4.05 cm high, diameter = 5.7 cm) and pushed them into the soil. I took the cylinders with the soil out of the ground and brought them back to the lab (a trailer on the Jena Experiment site). I put them in a box with a sand bed and saturated the soil with water for two days and weighed the soil samples to get the wet weight. In the end, I dried the soil completely in the oven at 105°C and weighed the samples again for the dry weight. With the weights, I could calculate the soil density and porosity as you can see in the following graphic.
Method for the determination of bulk density and soil porosity: Take soil sample, saturate with water, weigh, dry, weigh, and calculate 🙂
Complex mechanisms improve soil properties
Your gut feeling may tell you, the more different species the better, just because it is more natural. And that is exactly right! The result of the experiment was that soil with higher plant biodiversity was significantly less dense and more porous, hence good for the soil.
But why that is, is actually quite complicated and can be answered by other biodiversity studies. One relevant point is that more plant species cause the growth of more microorganisms in the soil. This myriad of tiny organisms enhances the formation and stability of soil aggregates, which are groups of soil particles binding together. These aggregates make the soil more porous. And apparently, more plant species stimulate the growth of the microbes and hence the formation of aggregates that make the soil more porous and less dense.
Plant biodiversity improves soil properties: A high number of plant species leads to high porosity and low bulk density, which is good for the soil.
Not only my study but also hundreds of other studies from the Jena Experiment show that a higher number of plant species is beneficial for the soil and the whole ecosystem. Amongst others, the group of my project showed that higher plant numbers increase the capacity of the soil to take up water, which is connected to the increased porosity. This is important for the water household of the soil. It is of advantage that rainwater can infiltrate and be stored in the soil so it is available for plant growth. Contrary, when water cannot infiltrate, it leads to soil erosion and flooding in the worst case.
We can use this knowledge to decide what we do with our soils. For instance, when soils are to be renatured because a street or building is removed, it is highly recommendable to plant many different species. In agriculture, it is a good idea to avoid monocultures when it is possible. Or if you have your own garden, try to plant many different flowers and herbs!
Literature and further information:
My project was supervised by Christine Fischer and Anke Hildebrandt and got published as part of this paper:
Fischer, C., Tischer, J., Roscher, C., Eisenhauer, N., Ravenek, J., Gleixner, G., Attinger, S., Jensen, B., De Kroon, H., Mommer, L., Hildebrandt, A. (2015). Plant species diversity affects soil hydraulic properties in an experimental grassland by mediating soil properties. Plant and Soil 397(1).
Learn more about the Jena Experiment here: