What plant functional traits can reduce nitrous oxide emissions from intensively managed grasslands?

Plant species exert a dominant control over the nitrogen (N) cycle of natural and managed grasslands. Although in intensively managed systems that receive large external N inputs the emission of the potent greenhouse gas nitrous oxide (N2O) is a crucial component of this cycle, a mechanistic relationship between plant species and N2O emissions has not yet been established. Here we use a plant functional trait approach to study the relation between plant species strategies and N2O emissions from soils. Compared to species with conservative strategies, species with acquisitive strategies have higher N uptake when there is ample N in the soil, but also trigger N mineralization when soil N is limiting. Therefore, we hypothesized that (1) compared to conservative species, species with acquisitive traits reduce N2O emissions after a high N addition; and (2) species with conservative traits have lower N2O emissions than acquisitive plants if there is no high N addition. This was tested in a greenhouse experiment using monocultures of six grass species with differing above-and below-ground traits, growing across a gradient of soil N availability. We found that acquisitive species reduced N2O emissions at all levels of N availability, produced higher biomass and showed larger N uptake. As such, acquisitive species had 87% lower N2O emissions per unit of N uptake than conservative species (p<.05). Structural equation modelling revealed that specific leaf area and root length density were key traits regulating the effects of plants on N2O emission and biomass productivity. These results provide the first framework to understand the mechanisms through which plants modulate N2O emissions, pointing the way to develop productive grasslands that contribute optimally to climate change mitigation.