Effects of elevated CO2, N-fertilization, and season on survival of ponderosa pine fine roots

We used minirhizotrons to assess the effects of elevated CO2, N, and season on the life-span of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) fine roots. CO2 levels were ambient air (A), ambient air + 175 mu mol.mol(-1) (A + 175) and ambient air + 350 mu mol.mol(-1) (A + 350). N treatments were 0, 100, and 200 kg N.ha(-1) per year (N0, N100, and N200, respectively). Fine root survival was strongly influenced by season and seemed to be most strongly linked to soil temperature. Roots born in the fall and winter had longer median root life-span (MRLs) than those born during the spring and summer. Elevated CO2 increased root life-span, but N fertilization decreased it. Under A, MRL was 74 +/- 12 days (mean +/- SE) and was significantly different from the MRL for the A + 350 treatment (102 +/- 14 days). MRL under A + 175 averaged 92 +/- 10 days. MRL was 116 +/- 13 days for the N0 treatment and was significantly greater than MRL for the N100 (70 +/- 10 days) and N200 (62 +/- 14 days) treatments. Assuming that longer lived fine roots continue their resource acquisition functions, then elevated CO2 may have the effect of extending the resource acquisition period. In contrast, fine roots in N-rich environments have shorter life-spans than fine roots in N-poor environments.