Less nitrogen fixing nodules as a result of a soil legacy from vetch

When clover grows on a place with soil organisms from vetch, the plant will produce less nitrogen fixing nodules at that place, a study from Sigri Saar and colleagues shows. With this discovery the researchers understand better how plants grow together in one community.

White clover (Trifolium repens) is an important feed crop for cows. It is able to generate its own nitrogen source, as the plant grows small nodules, around the roots. In this nodules bacteria catch nitrogen from the air which they subsequently transform to a form that can be taken up by clover. Almost all Fabeacea, such as clover but also vetch are able to do so. This in contrast to most other plants.

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However, the developments of these nodules is determined by many different factors. It has been shown for instance that clover creates less nodules when there is a lot of nitrogen in the soil already. Also the occurrence of other plants in the neighborhood can influence the development of these nodules. Researchers of the research group Soil Biology & Biological Soil Quality, a group connected to the Centre for Soil Ecology wanted to understand better which factors determine the formation of new nodules. More specifically they wanted to know what the existence of old plant roots means for a new generation of plants. They also wanted to know whether this effect was only local (only this area in the soil) or whether the plant responds in a systemic way.

The study
To do so they grew plants of white clover in pots that were divided into two departments. The one half was filled with normal soil. The other half was filled with either soil with soil organisms from various other plant species or with the roots of these plant species. They found that clover roots that were growing on soil that contained soil organisms from tufted vetch (Vicia cracca) produced less root nodules. This only applied to the soil with soil organisms; in the other half of the pot with normal soil there was no difference.

"Maybe vetch has different bacteria, resulting in a different collaboration", Janna Barel, PhD student at the research group Soil Biology & Biological Soil Quality says. However, another explanation might be possible as well. "We did not measure this in detail", Barel explains.

The researchers moreover discovered that the existence of dead roots of vetch not resulted into less nodules. In other words: the effect of adding roots is broader than just adding certain new soil organisms. The existence of vetch roots, moreover, did result into smaller clover roots. "Most likely this is the result of nutrients that become available after decomposing the root litter"

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Green manure
The study from Saar, Barel and their coworkers may eventually helps to better understand how different plant species can grow together in one community. In this study they specifically showed the effect of the occurrence of a previous generation.

The knowledge they obtain can eventually help to better understand which types of green manure deliver the best results and when these crops can be best ploughed under.

In accordance to Barel this study is a warming up for research that is going to answer these bigger questions. This work is already in preparation. "We expect to publish them soon", says Barel. At the end of this month she hopes to defend her PhD on this topic.