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The role of lignin for saprotrophic decomposition of plant litter and the genesis of soil organic matter in boreal soils
PI: Mats Öquist
A striking feature of soil organic matter (SOM) in the moor layer of boreal forests is that it contains 40-50% carbohydrate polymers, even in SOM that has been decomposed for decades to centuries. This is in contrast to all common conceptual decomposition models assuming aromatic and aliphatic polymers to constitute the fraction of recalcitrant SOM. We will elucidate if this accumulation of polymeric carbohydrates stems from protection by recalcitrant lignin or is a result of cellulose crystallinity. As model substrates we will use plant material of Populus genotypes differing both in lignin content and composition of the phenylpropanoid sub-units. We will also use 13C labelled cellulose with varying degree of crystallinity. The role of labile C substrates, according to the priming hypotheses, and N on decomposition will be explored. The decomposition of the substrates will be followed using 2D HSQC NMR identifying what molecular moieties in the lignocellulose complexes that are decomposed over time. 16S rRNA tracking and in depth metagenome characterization will give inferences about the emerging metabolic capabilities of the community and to trace the expression of the relevant hydrolytic genes over time and across treatments. The information derived from this detailed analysis will finally allow to systematically test the multi decadal-old hypothesis that currently shape our view of saprotrophic decomposition of plant litter in moor layers at early stages of SOM genesis.