De-intensification Effects on Methane-Cycling Microorganisms and the Methane Sink Function of Grasslands (MetGrass)

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Methane (CH4) is the second most relevant greenhouse gas and contributes substantially to global warming. In 2021, global political efforts have been started to mitigate its emission. The soil microbiome is the most important terrestrial methane sink, but this is highly dependent on land use management. A major challenge for today´s agriculture is to adjust land-use intensity to ensure productive and at the same time sustainable grassland production systems to balance greenhouse gas emissions and increasing food demands of a growing world population. Two fundamentally different groups of microbes are crucial for CH4-cycling. Methanotrophic bacteria act as biological sink by oxidizing atmospheric CH4, while methanogens produce methane in anoxic zones, aggregates and deeper soil layers. In the previous project, the partners have revealed (1) the negative impact of high land-use intensity on methanotrophic bacteria in grassland soils, and (2) the high seasonal microbiome dynamics that determine if grasslands are sources or sinks of CH4. The results raise several questions regarding the magnitude and dynamics of land-use intensity effects on CH4-cycling microbiomes and the resulting net surface fluxes. In the new proposal MetGrass, we will thus study the effects of grassland land-use de-intensification on methanotrophs and methanogens, by contributing to the joint multi-site grassland experiments REX and LUX. How a reduction of fertilization, mowing, and grazing will impact biodiversity, activity, abundance and the distribution over the soil profile of methanotrophs and methanogens hast not been investigated in any ecosystem-scale experiment to date. We will test four complementary hypotheses with the overall aim to assess (1) how de-intensification leads to changes of the species composition and abundance of methanogenic and methanotrophic microbes, (2) how this is associated with changed CH4 net surface fluxes and (3) how rapid these changes are. In work package (WP)1, we will study the long-term recovery effects of the CH4 sink function and methanotrophs in grassland soils after de-intensification. WP2 assesses short-term responses of methanotrophs and methanogens to single management events (i.e. fertilization and mowing). In WP3, we aim to develop microbiome-based predictive model of grassland soil CH4 fluxes. WP4 will assess the tipping point of the CH4 sink function of a grassland soil upon increasing rates of fertilization and will thereby provide a mechanistic understanding of underlying microbiome dynamics. We will employ a unique combination of state-of-the-art approaches, reflecting the interdisciplinary nature of the MetGrass team. MetGrass will be able (a) to address pressing questions on the effect of land-use de-intensification on the functional diversity and activity of these key soil microorganisms in grasslands and (b) will deliver the basis for improvement of grassland management towards sustainable land use.