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Priming and soil organic matter decomposition

The objective of this project is to determine if priming (see below) is a universal property of soil organic matter degrading microorganisms, or if it is limited to specific microbial functional groups such as mycorrhizal fungi, saprotrophic fungi and bacteria. We also aim to tease out the underlying molecular mechanism(s), and determine degree to which priming is influenced by variations in carbon and nutrient availability, as well as other environmental factors. The knowledge gained will improve our understanding of the importance of priming for soil carbon decomposition and sequestration in a changing environment.

Plants allocate 20–30 % of recently photosynthesized carbon below ground, supporting the growth and activity of roots, mycorrhizal fungi, and rhizosphere microbial communities. A part of this carbon is exuded by roots as sugars and other labile compounds. Recent evidence suggests that these exudates can cause a massive stimulation of soil organic matter decomposition, a phenomenon referred to as priming. Attempts at quantifying priming effects have produced results varying from negative all the way up to a 380 % increase in soil organic matter decomposition. Our understanding of what causes such erratic responses is limited by a lack of knowledge about the underlying mechanisms. Neither the microorganisms nor the enzymes involved in priming have been identified, and it is not known what kind of modifications of the soil organic matter that occur at the molecular level. 

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