c 1997 Kluwer Academic Publishers. Printed in the Netherlands. Effects of CO2 and nitrogen fertilization on vegetation and soil nutrient content in juvenile ponderosa pine

This paper summarizes the data on nutrient uptake and soil responses in opentop chambers planted with ponderosa pine (Pinus ponderosa Laws.) treated with both N and CO2. Based upon the literature, we hypothesized that 1) elevated CO2 would cause increased growth and yield of biomass per unit uptake of N even if N is limiting, and 2) elevated CO2 would cause increased biomass yield per unit uptake of other nutrients only by growth dilution and only if they are non-limiting. Hypothesis 1 was supported only in part: there were greater yields of biomass per unit N uptake in the first two years of growth but not in the third year. Hypothesis 2 was supported in many cases: elevated CO2 caused growth dilution (decreased concentrations but not decreased uptake) of P, S, and Mg. Effects of elevated CO2 on K, Ca, and B concentrations were smaller and mostly non-significant. There was no evidence that N responded in a unique manner to elevated CO2, despite its unique role in rubisco. Simple growth dilution seemed to explain nutrient responses in almost all cases. There were significant declines in soil exchangeable K+, Ca2+, Mg2+ and extractable P over time which were attributed to disturbance effects associated with plowing. The only statistically significant treatment effects on soils were negative effects of elevated CO2 on mineralizeable N and extractable P, and positive effects of both N fertilization and CO2 on exchangeable Al3+. Soil exchangeable K+, Ca2+, and Mg2+ pools remained much higher than vegetation pools, but extractable P pools were lower than vegetation pools in the third year of growth. There were also large losses of both native soil N and fertilizer N over time. These soil N losses could account for the observed losses in exchangeable K+, Ca2+, Mg2+ if N was nitrified and leached as NO3