Plant diversity enhanced yield and mitigated drought impacts in intensively managed grassland communities

There is a global requirement to improve the environmental sustainability
of intensively managed grassland monocultures that rely on high rates of
nitrogen fertiliser, which is associated with negative environmental
impacts. Multi-species grass-legume mixtures are a promising tool for
stimulating both productivity and sustainability in intensively managed
grasslands, but questions remain about the benefit of increasing the
diversity of plant functional groups. We established a plot-scale
experiment that manipulated the diversity of plant communities from a
six-species pool comprising three functional groups: grasses, legumes and
herbs (two species each). Communities were grown as monocultures, or
mixtures of two to six species, following a simplex design. This allowed
us to quantify species’ identity effects and functional group interaction
effects. To investigate the impact of severe weather events, main plots
were split and two levels of water supply, ‘rainfed’ and a two-month
experimental drought, were applied. Mineral nitrogen fertiliser was
applied at 150 kg ha-1 yr-1, and a Lolium perenne monoculture received 300
kg ha-1 yr-1 (300N) as a highly fertilised comparison. Annual aboveground
biomass was measured for two years. We found a strong positive effect on
yield due to functional group interactions. Multi-species communities with
three functional groups yielded more than the best-performing monoculture
over a large range of different relative abundances of the three
functional groups. The highest diversity community outperformed the 300N
L. perenne under rainfed conditions. Although species’ monoculture yields
were generally reduced by the experimental drought, the interaction
effects were not affected by drought and remained sufficiently large for
the six-species mixture under drought to have similar yields to both the
best-performing monoculture and the 300N L. perenne under rainfed
conditions. Synthesis and applications. Plant diversity can result in
higher yields, mitigate the yield risks associated with extreme weather
events, and displace fertiliser inputs and their embedded greenhouse gas
emissions. Multi-species swards offer a practical, farm-scale management
action to enhance the sustainability of intensive grassland production.