Effects of CO2 and N fertilization on decomposition and N immobilization in ponderosa pine litter

Naturally senesced needles from ponderosa pine (Pinus ponderosa Dougl.), grown from seed in open-top chambers under three levels of CO2 (350, 525 and 700 mu l l(-1)) and three levels of N fertilization (0, 10 and 20 g N m(-2) yr(-1)), were used in a field litterbag decomposition study and in a laboratory study on potential microbial and nonmicrobial N immobilization. The litterbag studies revealed no statistically significant effects of either CO2 or N treatment on mass loss, N concentration, or N content over a 26-month period. The laboratory study of potential N-15 immobilization revealed no statistically significant effects of CO2 or N treatment on either total or microbial immobilization. Elevated (CO2) did have a significant negative effect on nonmicrobial immobilization, however. Natural abundance of N-15 was significantly greater with elevated (CO2) in both live and naturally senesced needles under all N treatments. This pattern combined with N-15 natural abundance in soils suggests that saplings grown under elevated (CO2) were either taking up more N from surface horizons or from a more recalcitrant soil N pool in either horizon.