Boosting plant diversity in agriculture can increase plant biomass and improve plant–microbe interactions, both of which promote the storage of carbon in the soil, demonstrates a study carried out at the University of Helsinki.

The study investigated whether increasing plant diversity through use of undersown species in field ecosystems can affect the structure and functioning of microbial communities to promote soil health and carbon sequestration. Specifically, the study examined how the growth, community structure, and carbon use efficiency of root microbes respond to plant species.

“Our findings indicate that promoting plant diversity in agriculture could be an effective strategy for improving carbon sequestration of agricultural soils. This finding provides much needed evidence-based support for developing sustainable agricultural policy in the European Union,” says project lead and Professor Anna-Liisa Laine from the Faculty of Biological and Environmental Sciences of the University of Helsinki.
According to the researchers, the expansion of agricultural land has caused a reduction in soil carbon, a problem whose resolution is central to curbing climate change.

“In practice, even a small improvement in the carbon retention capacity of fields can be significant, as a large share of land area globally has been harnessed for food production,” Laine muses.

The study was carried out in an exceptionally extensive biodiversity experiment with barley as the experimental cereal. The researchers investigated how companion species to barley, such as red clover (Trifolium pratense), alfalfa (Medicago sativa) and chicory (Cichorium intybus), affect barley yields, as the crop must not be compromised at the expense of sustainability in agroecosystems. Barley was grown in test plots either on its own or with one, two, four or eight undersown species.

In ecology, a positive relationship has already been demonstrated experimentally between the number of plant species and ecosystem functions, such as soil carbon retention. However, field ecosystems differ considerably from experiments emulating wild plant communities, as the former contain a single dominant species. The researchers were surprised by how quickly soil microbes responded positively to plant diversity, even though barley was the dominant species in the test plots.

How can farmers cultivate diversity in their fields?

While clovers have long been used as undersown species, the experiment carried out here provides new information on how soil processes react when the diversity of such species is increased to as many as eight species. In practice, adding eight species to agroecosystems can be challenging, but the addition of four already produced desired results.

Laine points out that undersown species compete for space, light and nutrients with crops, making it necessary to keep their density low enough to ensure that this competition is not reflected in growth and yield.