Nitrogen availability and plant functional composition modify biodiversity-multifunctionality relationships

The ability of an ecosystem to deliver multiple functions at high levels
(multifunctionality) typically increases with biodiversity but there is
substantial variation in the strength and direction of biodiversity
effects, suggesting context-dependency. A better understanding of the
drivers of this context dependency is essential to predict effects of
global change on ecosystems. To determine how different factors modulate
the effect of diversity on multifunctionality, we established a large
grassland experiment with 216 communities, crossing a manipulation of
plant species richness (1, 4, 8, 20 species) with manipulations of
resources (nitrogen enrichment), plant functional composition (gradient in
mean specific leaf area [SLA] to manipulate abundances of exploitative,
fast-growing vs. conservative, slow-growing species), plant functional
diversity (variance in SLA) and enemy abundance (foliar fungal pathogen
removal). We measured ten above- and belowground functions, related to
productivity, nutrient cycling and energy transfer between trophic levels,
and calculated ecosystem multifunctionality. Plant species richness and
functional diversity both increased multifunctionality, but their effects
were context dependent. Species richness increased multifunctionality only
when communities were assembled with fast growing (high SLA) species. This
was because slow species were more redundant in their functional effects,
whereas different fast species tended to promote different functions.
Functional diversity also increased multifunctionality but this effect was
dampened by nitrogen enrichment. However, unfertilised, functionally
diverse communities still delivered more functions than low diversity,
fertilised communities. Our study suggests that a shift towards
fast-growing exploitative communities will not only alter ecosystem
functioning but also the strength of biodiversity-functioning
relationships, which highlights the potentially complex effects of global
change on multifunctionality.