Project Summary: Plant and ecosystem productivity are strongly influenced by nutrient availability, largely determined by rates of decomposition of plant litter. Decomposition rates of above-ground plant tissues (leaves and stems) and associated rates of nutrient mineralization are well characterized by indices of 'litter quality', such as nutrient content, carbon:nutrient ratios, or lignin:nutrient ratios. However, for roots, for reasons that we do not yet understand, relationships between decomposition rate and such indices of litter quality are less consistent. Mycorrhizae are ubiquitous and strongly influence root chemistry, in ways that traditional indices of litter quality are likely to miss. For these reasons, including the mycorrhizal status of decomposing roots could substantially increase our ability to predict root decomposition rates in terrestrial ecosystems.
Most plant species are associated with soil fungi, forming root-fungus associations called mycorrhizae. This association facilitates nutrient and water uptake by plants and provides carbon through photosynthesis to fungi. Much is known about how this association affects living roots and whole plants, but the consequences of mycorrhizal infection for root litter decomposition have never been investigated. Differences in the degree of mycorrhizal infection are likely to strongly influence root decomposition rates, by altering both the carbon and nutrient quality of the roots for decomposer microorganisms. While mycorrhizae usually increase nutrient concentrations in all plant tissues by enhancing nutrient uptake, they also affect root nutrient concentrations because of the high-nutrient content fungal structures that are built inside the root as the mycorrhizal association develops. While high in nutrient content, these structures often contain a recalcitrant carbon skeleton (e.g., chitin), so that rates of decomposition are likely to deviate from predictions based on indices of litter quality.
This project would use greenhouse experiments and natural gradients in the mycorrhizal status of plants to generate root litter material from a broad range of species in which the nutrient content and mycorrhizal status of the root litter vary independently. Following the characterization of litter for nutrient and carbon 'quality' (including fractions not typically examined, such as chitin), mycorrhizal status would be assessed, and decomposition experiments would be conducted under laboratory and field conditions. Comparing decomposition rates of litter material from different treatments would allow a quantitative assessment of the influence of mycorrhizae and associated changes in root chemistry on root decomposition rates, and thus has the potential to substantially advance understanding of the controls over litter decomposition and nutrient availability in terrestrial ecosystems.