Allen 9616536 Plants devote considerable energy to the production and maintenance of small-diameter roots and their associated mycorrhizal fungi. How this below-ground allocation of carbon responds to altered availability of resources such as nitrogen is fundamental to our understanding of ecosystem function. There are currently two competing theories: (1) increased N availability decreases C allocation to roots; and (2) increased N availability increases C allocation to roots. Understanding fine root production and mortality is at the heart of this conflict. The overall hypothesis of the PIs is that the type of root-mycorrhizal association is a dominant factor controlling carbon allocation to roots, root longevity and the response to root systems to changes in N availability. The PIs propose to directly and independently quantify root production and mortality, mycorrhizal community composition and abundance, root respiration, soil CO2 flux, and above-ground litterfall at four long-term study sites that represent a range of ecosystems typical of North America. The study sites capitalize on contrasts between ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) conifers and hardwoods and represent a wide range of environmental conditions. Experimental additions of N are designed to address the issue of C allocation to roots at a scale that will resolve the controversy about how root growth and turnover at the ecosystem level responds to increasing N availability. By comparing and contrasting EM and AM dominated ecosystems under a variety of climatic and edaphic regimes, we will be able to make more universal statements about the factors controlling fine root turnover.
