Clover fixes atmospheric nitrogen, and plants growing nearby benefit. But does clover gain anything from its neighbours in return? Recent research published in PLOS ONE this week reveals that, in mixed cropping, both nitrogen-fixing plants and their neighbours improve in weight and quality.
The research -- performed by a team from Wageningen University, part of Wageningen UR, together with UK colleagues -- revealed that levels of both carbon and especially nitrogen, a measure of food value, were higher in plant mixtures.
The Dutch-English team comprises researchers from Wageningen University, Lancaster University and the Centre for Ecology and Hydrology in Lancaster. They concluded that clover plants and grasses transport carbon into the ground more quickly and produce more biomass of a higher quality if both plant species grow close to each other instead of being surrounded by plants of the same species.
Clover species live together with root inhabiting bacteria that remove nitrogen from the air and make it available to the plants. Non-nitrogen-fixing neighbouring plants benefit as well because nitrogen in clover is released into the soil due to leakage from the roots and breakdown of dead roots. This 'clover love' has been known about for a long time but the question whether it was mutual love remained unanswered.
The researchers from Wageningen University and Lancaster have shown that there is true love between plants that fix nitrogen and those that do not. As a result, these plant species contribute jointly to a higher yield in mixed crops as compared to monocultures.
This research showed that white clover, in particular, was able to rapidly transport the carbon it had absorbed during the day to underground plant parts- but only if it grew in the company of other plant species. Transport was then three times faster. Sweet vernal grass in a mixed culture also incorporated and transported carbon more rapidly.
In addition, the plant communities lost less carbon through plant and soil respiration if they were composed of plant species mixtures both compared to when the plant species were cultivated in a monoculture. The same plant species that were successful in speeding up their carbon transport on the short term also produced more biomass in mixed cropping on the longer term over a period of one year.
Fixing extra carbon of plant individuals growing in plant species mixtures did not occur at the expense of plant quality. The nitrogen content turned out to be significantly higher in all non-nitrogen-fixing plant species, while it remained steady in the clover plants. This was expressed in the lower carbon/nitrogen proportion that decreased from 28 to 22 in the non-nitrogen-fixing plant species. This resulted in more nitrogen per unit of carbon in the harvestable material originating from mixed crops.
Further research is necessary to understand how the underlying processes exactly operate, but there are already strong indications that mycorrhizal fungi play an important role. These fungi live in and around plant roots and indirectly help to stimulate carbon transport and nutrient metabolism in their host plants.
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