Highlighted article: Gerlinde B. De Deyn, Helen Quirk, Simon Oakley, Nick Ostle, Richard D. Bardgett (2012) Increased Plant Carbon Translocation Linked to Overyielding in Grassland Species Mixtures. PLoS ONE 7(9): e45926. doi:10.1371/journal.pone.0045926
Plant biomass yield is often greater in areas where species richness is high than it is in monocultures. This has implications for agriculture, and also the use of non-farmed land as a carbon sink, as more biomass means more carbon assimilation. However, the relationship between growth and species richness on a plot of land is not constant or clear, so a group in Lancaster investigated it. I think their research threw up more questions than it answered, but the authors found intriguing links between lifetime biomass yield and speed of carbon transport from the leaves to other parts of the plants and found that non-legumes and legumes alike benefit from growing alongside one another.
Deyn et al. planted seedlings in monocultures or in a mixture and sampled soil and plant matter at 2, 24, and 48 hours, and finally at 8 days, after labeling carbon in the system using a 13CO2 pulse (Ostle et al., 2003). They assessed carbon assimilation and carbon/nitrogen ratio. Two years later, all the above-ground vegetation was harvested and weighed to obtain ‘yield’ data. The species used were common grassland species Trifolium repens and Lotus cornicalatus (both leguminous species), Plantgo lanceolata, Anthoxanthum odoratum, Achillea millefolium, and Lolium perenne.
On the whole, yield and nitrogen content were both higher in plants grown on a mixed plot. This was to be expected for the non-leguminous plants, which benefited from the nitrogen-fixing legumes. Higher yield was correlated with the fast transport of carbon from the leaves to other parts of the plants early in the carbon pulse analysis. The exception to this rule was L. perenne, which despite sitting almost squarely in the middle of the other tests, had a higher yield when grown in a monoculture. It is possible that the other plants changed the soil microbiota, which was not tested in this research, to a less favorable dynamic.
A surprising finding was that the leguminous species benefited as much as the non-legumes from growing in the mixed plot. What can they gain from the presence of other plants? Deyn et al. noted that the soil pH was low in the leguminous monoculture, which they suggest may inhibit nitrogen fixation and therefore growth. It is clear that more research needs to be done to find the link between species diversity and over-yielding, particularly on the soil pH theory, and also on the finding that being able to assimilate carbon quickly has a big positive impact on yield.
Teaching resources: A simple version of this research could be tried with children. Grow cress or, for older students, a plant like Arabidopsis, close together or individually and see what happens. You will not be able to discuss carbon or nitrogen content, but note the height of the seedling, and the length and branching in the roots. You can discuss competition and adaptation.
Image credit: Lotus corniculatus from Fredrik Lähnn via Wikimedia.