Defining nutritional constraints on carbon cycling in boreal forests – towards a less `phytocentric' perspective

Growing interest in possible global climate change has underlined the need for better information concerning the way in which carbon partitioning between ecosystem components is influenced by constraints on nutrient availability. Micro-organisms play a fundamental role in the cycling of carbon and nutrients in all ecosystems but the role of fungi in particular is pivotal in boreal forest ecosystems. Traditional models of nutrient cycling are based on methods and concepts developed in agricultural systems where microorganisms are considered primarily as nutrient processors providing plants with inorganic nutrients. The filamentous nature of fungi, their ability to translocate carbon and nutrients between different substrates and the capacity of ectomycorrhizal fungi to utilise organic nutrients have all been largely ignored. In this article, a new model is suggested which emphasises competition for organic nutrients between decomposer organisms and plants, with the plants depending on their associated mycorrhizal fungi for nutrient acquisition. Antagonistic interactions involving nutrient transfer between decomposer and mycorrhizal fungi are proposed as important pathways in nutrient cycling. Due to the nutrient conservative features of decomposer fungi, inorganic nutrients are considered less important for plant nutrition. The implications of the new nutrient cycling model on the carbon balance of boreal forests are discussed.