Disentangling the roles of plant functional diversity and plaint traits in regulating plant nitrogen accumulation and denitrification in freshwaters

1. There is a growing recognition that functional measures of diversity,
based on quantification of functionally important species traits, are
useful for explaining variation in ecosystem processes. However, the
mechanisms linking functional diversity to different processes remain
poorly understood, hindering development of a predictive framework for
ecosystem functioning based on species traits. 2. The current
understanding of how the functional traits of aquatic plants (macrophytes)
affect nitrogen (N) cycling by regulating microbial communities and their
activity in freshwater habitats is particularly limited. Denitrifying
bacteria are typically associated with the roots of both aquatic and
terrestrial plants and denitrification is the main cause of loss of N from
ecosystems. Disentangling the interplay between plants and microbial
denitrifiers is key to understanding variation in rates of denitrification
from local to landscape scales. 3. In a mesocosm experiment, we varied the
species richness (monocultures or two- species mixtures) and composition
of macrophytes. We quantified effects of both macrophyte functional
diversity, quantified as functional trait dissimilarity, and functional
trait composition, quantified as community weighted mean trait values, on
N removal in wetlands. We used structural equation modelling to
disentangle the direct and indirect influences of traits on N accumulation
in plant biomass, denitrification activity and abundance of key bacterial
denitrification genes (nirSand nirK). 4. Both functional
diversity and functional trait composition regulated N removal, explaining
70 – 94% variation in the underlying ecosystem processes. Increased
macrophyte functional diversity increased plant N accumulation, and
indirectly enhanced denitrification by increasing denitrification gene
abundance. Among traits, greater plant relative growth rates, specific
leaf area and aboveground biomass increased plant N accumulation.
Denitrification activity increased with increasing belowground biomass but
decreased with increasing root diameter. 5. These findings improve our
understanding of N removal in freshwater wetlands dominated by
macrophytes, and have broad ecological implications for wetland management
targeting enhanced ecosystem services. Our results highlight the potential
for optimising denitrification and plant N accumulation in wetlands and
thereby improving water purification by increasing macrophyte functional
diversity and ensuring the presence of key traits in macrophyte
assemblages.