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Low-land soils

Forget about biodiversity for the first 10-20 years

If we stop cultivating low-land fields and let nature take over, we will get more biodiversity, we often hear. Correct, says expert: but the best thing we can do for biodiversity is to harvest everything that comes up for the first 10-20 years.

By Birgitte Svennevig, , 4/4/2022

It sounds both romantic and simple: Stop cultivating farmer's low-land fields and leave them to themselves, so that nature can return and create beautiful flowering meadows, buzzing with life.

Romantic scenery is not a priority for Henning S. Jensen, associate professor emeritus at the Department of Biology, University of Southern Denmark; He is more interested in discussing if it really is so straightforward to restore biodiversity in formerly cultivated low-lands.

- Low-land fields are an important tool for increasing biodiversity in Danish nature. But the most effective solution is not to just let nature take over, he says.

More meadows, bogs, and lakes

There was a time, before Danish farmers in the 19th century began to cultivate low-lands and wet soils, when these soils were meadows, ponds, lakes, swamp forests, bogs and other types of wetlands, homes of a myriad of different animal and plant species.

- There is a desire to recreate these types of landscapes. And it is tempting to think that if you just stop cultivating, a piece of land will by itself return to its original biologically diversified, says Henning S. Jensen.

But as with so much else, restoration of nature does not just happen by itself, he points out.

Stinging nettles and thistles

- Yes, you can "just" leave such a piece of land to itself. Lots of new plants will grow there, but that is not a success in itself, he says.

Plants like stinging nettles or thistles may become so dominant that no other species get a chance - not even the species that traditionally belong to wetlands in Denmark.

Several reasons to stop cultivating lowland soils

  1. Lowland soils emit a lot of CO2 when cultivated. Approx. 40% of agricultural CO2 pollution comes from the cultivation of lowland soils.
  2. Lowland soils can be used to remove nitrogen from the aquatic environment.
  3. They can absorb a lot of rain when streams are flooded - and thus they protect against floods in cities further down the stream.

The reason is that a lot of nutrients remain in the soil after cultivation. Often, you find enough phosphorus in the soil to keep nutrient-loving plants like stinging nettles and thistles thriving for 40-50 years. Only when the phosphorus has been used up, will the original species return – and with them the biodiversity, that we all are longing for.

- With this in mind, it is relevant to consider whether you can help nature get through all the phosphorus in the soil faster – and thus reach the biodiversity goal faster, says Henning S. Jensen.

Faster use of nutrients

One method could be to let plants like reed or cat tails grow on the land and harvest them. In the harvest process, absorbed phosphorus will be removed.

- If you do this for 10-20 years, you can remove enough phosphorus from the soil that different types of plants will get a chance to grow there - not just the phosphorus-loving stinging nettles and thistles. This paves the way for the biodiversity that we all want, says Henning S. Jensen.

Reed and cattails

Incidentally, the harvested mass of plants like reed and cat tail can be useful, he points out; it can be converted to biogas.

And the by-product from the gas production can also be used in the form of sludge as fertilizer and soil improvement in agriculture.

- Then it will be recycled, and that is the right direction to go in, he says.

 

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Meet the researcher

Henning S. Jensen is an expert in freshwater ecology and lake restoration and an associate professor emeritus at the Department of Biology.

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Editing was completed: 04.04.2022