As climate is altered, belowground responses will play a critical role in determining carbon balance in ecosystems. We focused on soil fungi, which are central to both decomposition of plant litter (saprophytic fungi) and the direct transfer of photosynthetically fixed carbon from plants to soils (mycorrhizal fungi). Specifically, we asked how changes in the magnitude and timing of precipitation affected soil fungi and fungal mediation of soil carbon pools. Annual precipitation in a northern California grassland was increased beginning in 2001 to reflect the predictions of HadCM2 and CCM1, with approximately 20% more rain added either during the current winter rainy season or as an extension of the rainy season into spring.
We found that the size of hyphal networks and fungal community composition responded directly to both rain additions rather than to changes in the plant community. Because treatments were imposed on ambient rainfall, we also observed that the response of fungi to rain additions varied both seasonally and annually, with greater increases in soil hyphae when the system was more drought stressed. Altered fungal communities in the rainfall addition treatments led to as much as 300% increases in rates of litter decomposition that were independent of soil moisture. Total and fungal carbon pools (total carbon, glomalin, ergosterol) also increased, but represented less than 2% of the potential carbon losses to the atmosphere through respiration. The responses we observed in the soil fungal community appear to be decoupled from those of other soil microbes and from the plant community, suggesting that a greater understanding of soil fungi may help to reduce uncertainty about ecosystem responses to future climate change.