Plant productivity in pristine ecosystems like boreal and tropical cloud forests is limited by soil nutrients, primarily nitrogen (N). Mosses are major contributors to ecosystem productivity in these habitats, and most of them are colonized by N2-fixing cyanobacteria, thereby providing N to the ecosystem. Despite this key role, critical knowledge gaps exist. In particular, the climatic controls of moss-associated N2 fixation remain unclear, limiting our ability to quantify and project climate change effects on this fundamental ecosystem function. Further, it is unknown whether mosses and associated cyanobacteria share a mutualistic (both partners benefit) or parasitic (one partner benefits at the expense of the other) relationship. Yet, the balance of this association is crucial for maintaining ecosystem productivity. I will combine field, laboratory and modelling approaches to fill these knowledge gaps by addressing 4 objectives. I will (1) identify the climatic controls of N2 fixation in mosses from contrasting ecosystems: boreal forests and tropical cloud forests, (2) ascertain the degree of mutualism or parasitism between moss and cyanobacteria using transcriptomics, (3) determine nutrient exchange rates between moss and cyanobacteria using nanoSIMS. The ultimate goal is (4) to model ecosystem N input via N2 fixation in boreal and tropical ecosystems. In SYMBIONIX, I will uniquely combine biogeochemistry with ecology across scales, offering a break-through in ecosystem research. Application of cutting-edge methods will enable progress previously unachievable in ecosystem ecology. I will interlink a vital ecosystem function to its molecular and ecological foundation. A synthesis will be achieved via the parameterization and validation of global circulation models, resulting in improved predictions of ecosystem nutrient cycling in a future climate. Targeted combination of cross-scale approaches will make this initiative field-leading in ecosystem ecology.
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