Breaking it down - complex organic compounds

What is it about?

Soils in coniferous woodlands is made up of layers, with dead leaves and plant litter comprising the uppermost layer and the deeper, humus layer containing more decomposed organic matter. The latter, however, has relatively high levels of compounds that are very difficult to break down. Nitrogen is needed for plant growth and in the humus layer, nitrogen primarily exists in complex organic materials. Release of nitrogen from these organic sources drives plant productivity, but requires special enzymes to break up complex constituents. Wood decomposing fungi – particularly white-rot fungi – are renowned for producing potent enzymes called peroxidases that break down complex organic molecules to produce accessible nitrogen and carbon. SYMBIOTIC FUNGI Wood decomposers are not present in the humus layer, however, and ectomycorrhizal fungi instead dominate. These fungi live in a symbiotic relationship with plant roots, where they exchange nitrogen for sugars directly from plant photosynthesis. Ectomycorrhizal fungi have generally been considered poor at digesting organic matter, but links have been found between peroxidase activity and these fungi. This study investigates one common group of ectomycorrhizal fungi, the Cortinarius species (i.e. webcaps), to examine whether they produce enzymes to digest organic matter for nitrogen release. TESTING NORTHERN FORESTS The researchers took humus samples from northern woodlands and found a correlation between peroxidase activity and the amount of Cortinarius DNA in the samples. In a more closely investigated mycorrhizal Cortinarius species, they discovered similar genetic potential to produce peroxidase enzymes more commonly found in the most potent wood decomposers. Adding inorganic nitrogen to the forest soil prior to sampling led to reductions in peroxidase activity, suggesting that when nitrogen is readily available, fungi react and produce less of the enzymes needed to release nutrients from organic materials. Contrary to previous ideas, this research demonstrates that symbiotic ectomycorrhizal fungi – not only free-living decomposer fungi – play an important role in degrading organic matter in coniferous forests.

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