In terrestrial ecosystems, global warming has the potential to cause a positive feedback by increasing respiration rates of soil microorganisms, contributing to atmospheric CO©ü associated warming. Although some studies have shown that the fungal to bacterial ratio decreases with warming, little is known about the physiological responses of soil fungi to long term warming. If soil warming leads to an increase in metabolic rates of fungi then respiration rates will increase with temperature, but if fungal metabolism acclimates to warmer soil temperatures over time or encounters other limitations then respiration rates will remain unchanged or decline. Soil warming may favor the production of more fungal storage compounds if more carbon resources are available to fungi under warmed conditions or if fungi shift allocation of carbon resources from growth towards storage at higher temperatures. Alternatively, higher fungal respiration rates could deplete storage pools. Two long term soil warming studies at Harvard Forest were used to examine fungal respiration rates and production of storage compounds under warmed conditions. Hyphal in-growth bags were used to estimate fungal respiration rates and the neutral lipid fatty acid (NLFA) content of soils and roots were analyzed as an indicator of fungal storage compounds.
Respiration rates per unit volume of substrate from the hyphal in-growth bags were not significantly different between the heated and control plots. Results of the NLFA analysis show no difference in the amount of fungal storage compounds present in soils from heated or control plots, but there was a significant increase of fungal storage compounds in roots from heated versus control plots. This may indicate that mycorrhizal fungi are producing more or consuming less storage compounds, or are allocating less carbon to extraradical hyphal growth under warmed conditions. Increased fungal neutral lipid production may occur as a stress response to increased temperature via a shift in fungal community structure towards taxa that store more carbon in root vesicles or in response to differential C allocation by the host plant due to warming induced changes in nutrient availability.