Ponderosa Pine Responses to Elevated CO2and Nitrogen Fertilization

Abstract

The effects of elevated CO2 (ambient, +175, and +350 μl l−1) and nitrogen fertilization (0, 100, and 200 kg N ha−1 yr−1 as ammonium sulfate) on C and N accumulations in biomass and soils planted with ponderosa pine (Pinus ponderosa Laws) over a 6-year study period are reported. Both nitrogen fertilization and elevated CO2 caused increases in C and N contents of vegetation over the study period. The pattern of responses varied over time. Responses to CO2 decreased in the +175 μl l−1 and increased in the +350 μl l−1 after the first year, whereas responses to N decreased after the first year and became non-significant by year six. Foliar N concentrations were lower and tree C:N ratios were higher with elevated CO2 in the early years, but this was offset by the increases in biomass, resulting in substantial increases in N uptake with elevated CO2. Nitrogen budget estimates showed that the major source of the N for unfertilized trees, with or without elevated CO2, was likely the soil organic N pool. There were no effects of elevated CO2 on soil C, but a significant decrease in soil N and an increase in soil C:N ratio in year six. Nitrogen fertilization had no significant effect on tree C:N ratios, foliar N concentrations, soil C content, soil N content, or soil C:N ratios. There were no significant interactions between CO2 and N treatments, indicating that N fertilization had no effect on responses to CO2 and that CO2 treatments had no effect on responses to N fertilization. These results illustrate the importance of long-term studies involving more than one level of treatment to assess the effects of elevated CO2.