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 (NO) and important regulators of methane (CH) consumption and production. The predicted increase in flooding frequency and severity due to climate change could increase NO emissions and shift grasslands from a net CH 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 NO and CH 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 NO and CH emissions. We hypothesized that traits related with fast nitrogen uptake and growth would lower NO and CH emissions in ambient (non-flooded) conditions, and that traits related to tissue toughness would lower NO and CH emissions in flooded conditions. We found that flooding increased cumulative NO emissions by 97 fold and cumulative CH emissions by 1.6 fold on average. Plant community composition mediated the flood-induced increase in NO emissions. In flooded conditions, increasing abundance of the grass F. arundinacea was related with lower NO emissions; whereas increases in abundance of the legume T. repens resulted in higher NO emissions. In non-flooded conditions, NO 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 NO emissions, and a lower global warming potential (CO equivalent of NO and CH). We conclude that plant functional traits related to slower decomposition and nitrogen mineralization could play a significant role in mitigating NO emissions in flooded grasslands.