Effects of elevated CO₂ on nutrient cycling in a sweetgum plantation

D. W. Johnson

Environmental and Resource Sciences, University of Nevada-Reno, Reno, Nevada 89557 USA(e-mail: dwj@cabnr.unr.edu; phone: +1-775-784-4511; fax: +1-775-784-4789))

W. Cheng

Department of Environmental Studies, University of California-Santa Cruz, Santa Cruz, California 95064 USA

J. D. Joslin

Belowground Forest Research Oak Ridge, Tennessee 37830 USA

R. J. Norby

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 USA

N. T. Edwards

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 USA

D. E. Todd

Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 USA

Abstract

The effects of elevated CO₂ on nutrient cycling and selected belowground processes in the closed-canopy sweetgum plantation were assessed as part of a free-air CO₂ enrichment (FACE) experiment at Oak Ridge, Tennessee. We hypothesized that nitrogen (N) constraints to growth response to elevated CO₂ would be mitigated primarily by reduced tissue concentrations (resulting in increased biomass production per unit uptake) rather than increased uptake. Conversely, we hypothesized that the constraints of other nutrients to growth response to elevated CO₂ would be mitigated primarily by increased uptake because of adequate soil supplies. The first hypothesis was not supported: although elevated CO₂ caused reduced foliar N concentrations, it also resulted in increased uptake and requirement of N, primarily because of greater root turnover. The additional N uptake with elevated CO₂ constituted between 10 and 40% of the estimated soil mineralizeable N pool. The second hypothesis was largely supported: elevated CO₂ had no significant effects on tissue concentrations of P, K, Ca, or Mg and caused significantly increased uptake and requirement of K, Ca, and Mg. Soil exchangeable pools of these nutrients are large and should pose no constraint to continued growth responses. Elevated CO₂ also caused increased microbial biomass, reduced N leaching and increased P leaching from O horizons (measured by resin lysimeters), reduced soil solution NH4+, SO42-, and Ca²⁺ concentrations, and increased soil solution pH. There were no statistically significant treatment effects on soil nutrient availability as measured by resin capsules, resin stakes, or in situ incubations. Despite significantly lower litterfall N concentrations in the elevated CO₂ treatment, there were no significant treatment effects on translocation or forest floor biomass or nutrient contents. There were also no significant treatment effects on the rate of decomposition of fine roots. In general, the effects of elevated CO₂ on nutrient cycling in this study were not large; future constraints on growth responses imposed by N limitations will depend on changes in N demand, atmospheric N deposition, and soil mineralization rates.

Keywords

Carbon dioxide, Forest, Nutrients, Uptake, Nutrient cycling

Article ID: 5150932

Effects of elevated CO2 on nutrient cycling in a sweetgum plantation

Effects of elevated CO₂ on nutrient cycling in a sweetgum plantation