Can flooding-induced greenhouse gas emissions be mitigated by trait-based plant species choice?

Intensively managed grasslands are large sources of the potent greenhouse gas nitrous oxide (N2O) and important regulators of methane (CH4) consumption and production. The predicted increase in flooding frequency and severity due to climate change could increase N2O emissions and shift grasslands from a net CH4 sink to a source. Therefore, effective management strategies are critical for mitigating greenhouse gas emissions from flood-prone grasslands. We tested how repeated flooding affected the N2O and CH4 emissions from 11 different plant communities (Festuca arundinacea, Lolium perenne, Poa trivialis, and Trifolium repens in monoculture, 2- and 4-species mixtures), using intact soil cores from an 18-month old grassland field experiment in a 4-month greenhouse experiment. To elucidate potential underlying mechanisms, we related plant functional traits to cumulative N2O and CH4 emissions. We hypothesized that traits related with fast nitrogen uptake and growth would lower N2O and CH4 emissions in ambient (non-flooded) conditions, and that traits related to tissue toughness would lower N2O and CH4 emissions in flooded conditions. We found that flooding increased cumulative N2O emissions by 97 fold and cumulative CH4 emissions by 1.6 fold on average. Plant community composition mediated the flood-induced increase in N2O emissions. In flooded conditions, increasing abundance of the grass F. arundinacea was related with lower N2O emissions; whereas increases in abundance of the legume T. repens resulted in higher N2O emissions. In non-flooded conditions, N2O emissions were not clearly mediated by plant traits related with nitrogen uptake or biomass production. In flooded conditions, plant communities with high root carbon to nitrogen ratio were related with lower cumulative N2O emissions, and a lower global warming potential (CO2 equivalent of N2O and CH4). We conclude that plant functional traits related to slower decomposition and nitrogen mineralization could play a