LIFETIME AND TEMPORAL OCCURRENCE OF ECTOMYCORRHIZAE ON PONDEROSA PINE(PINUS-PONDEROSA LAWS) SEEDLINGS GROWN UNDER VARIED ATMOSPHERIC CO2 AND NITROGEN LEVELS

Climate change (elevated atmospheric CO2, and altered air temperatures , precipitation amounts and seasonal patterns) may affect ecosystem pr ocesses by altering carbon allocation in plants, and carbon flux from plants to soil. Mycorrhizal fungi, as carbon sinks, are among the firs t soil biota to receive carbon from plants, and thereby influence carb on release from plants to soil. One step in this carbon release is via fine root and mycorrhizal turnover. It is necessary to know the lifet ime and temporal occurrence of roots and mycorrhizae to determine the capacity of the soil ecosystem to sequester carbon assimilated abovegr ound. In this study, ponderosa pine (Pinus ponderosa Laws) seedlings w ere grown under three levels of atmospheric CO2 (ambient, 525 and 700 mu mol CO2 mol(-1)) and three levels of annual nitrogen additions (0,1 00 and 200 kg N ha(-1)) in open-top chambers. At a two-month frequency during 18 months, we observed ectomycorrhizal root tips observed usin g minirhizotron tubes and camera. The numbers of new mycorrhizal root tips, the numbers of tips that disappeared between two consecutive rec ording events, and the standing crop of tips at each event were determ ined. There were more mycorrhizal tips of all three types seen during the summer compared with other times of the year. When only the standi ng crop of mycorrhizal tips was considered, effects of the CO2 and N a ddition treatments on carbon allocation to mycorrhizal tips was weakly evident. However, when the three types of tips were considered collec tively, tips numbers flux of carbon through mycorrhizae was greatest i n the: (1) high CO2 treatment compared with the other CO2 treatments, and (2) intermediate N addition treatment compared with the other N ad dition treatments. A survival analysis on the entire 18 month cohort o f tips was done to calculate the median lifetime of the mycorrhizal ro ot tips. Average median lifetime of the mycorrhizal tips was 139 days and was not affected by nitrogen and CO2 treatments.