Effects of elevated CO2 and nitrogen on ponderosa pine fine roots and associated fungal components

The effects of CO2 and nitrogen treatments on ponderosa pine (Pinus ponderosa Dougl. ex P. Laws. & C. Laws.) fine roots and associated fungal structures were monitored for a year (October 1992 to October 1993) using a minirhizotron camera system. The trees were grown in native soil in open-top field-exposure chambers at PlacerviUe, CA and exposed to ambient (similar to 350 mu mol mol(-1)) air or ambient air plus either 175 or 350 mu mol mol(-1) CO2 and three levels of nitrogen addition (0, 100 and 200 kg ha(-1)); however, the 100 kg ha(-1) N treatment at ambient plus 175 mu mol mol(-1) CO2 treatment was omitted from the experimental design. Roots were classified as new, white, brown, decaying or missing and their lengths and diameters measured. The occurrence of mycorrhizae and fungal hyphae was also recorded. The majority (>90%) of roots observed were smaller than 2 mm and the mean diameter decreased during the study. None of the root parameters measured showed a significant response to elevated CO2. The elevated CO2 treatments consistently showed an increase in root area density averaging 50% larger compared to ambient CO2, but this response was not statistically significant due to the high spatial variability of root distribution. Only new root area density showed a significant nitrogen response. The most new roots were initiated between April and June and the highest level of root loss occurred between June and August. The occurrence of mycorrhizae and fungal hyphae increased in response to CO2 treatment but not the nitrogen. Their highest levels of occurrence were during August and October 93.