Effects of elevated CO₂on nutrient cycling in a sweetgum plantation

Abstract

Abstract. The effects of elevated CO 2 on nutrient cycling and selected belowground processes in the closed-canopy sweetgum plantation were assessed as part of a free-air CO 2 enrichment (FACE) experiment at Oak Ridge, Tennessee. We hypothesized that nitrogen (N) constraints to growth response to elevated CO 2 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 CO 2 would be mitigated primarily by increased uptake because of adequate soil supplies. The first hypothesis was not supported: although elevated CO 2 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 CO 2 constituted between 10 and 40% of the estimated soil mineralizeable N pool. The second hypothesis was largely supported: elevated CO 2 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 CO 2 also caused increased microbial biomass, reduced N leaching and increased P leaching from O horizons (measured by resin lysimeters), reduced soil solution NH þ 4 , SO 2À 4 , and Ca 2þ 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 CO 2 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 CO 2 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.

Fine-root production dominates response of a deciduous forest to atmospheric CO 2 enrichment

“…However, increased fine-root production may represent a futile mechanism for improving N nutrition in this forest stand. The increased C supply in CO 2 -enriched plots was used to support more root production, but the resulting increase in N uptake has so far supported only increased root production and not whole-tree N nutrition or aboveground growth (31).…”

“…Relationship between N uptake and root-length duration. N uptake per plot for each year (1998 -2002) was calculated as the N content (dry matter production times N concentration) of woody increment, leaf litter, and annual fine-root production plus net foliar leaching (31). Root-length duration is the area under the plots of fine-root standing crop (Fig.…”

CO 2 enhancement of forest productivity constrained by limited nitrogen availability

“…Beginning in April 1998, two plots were exposed continuously during daylight hours throughout the growing season to an elevated concentration of CO 2 (∼550 ppm), using the FACE apparatus (39). No N fertilizer was added to the FACE plots; annual N deposition is 12-15 kg·ha −1 (40 (19). There were two fertilized and two control plots (12 × 16 m) in each of three blocks.…”

Overstory Community Composition and Elevated Atmospheric CO2 and O3 Modify Understory Biomass Production and Nitrogen Acquisition

“…The total amount of N acquired by the understory vegetation, as measured by N content and 15N recovery, was not significantly affected by the treatments. Other elevated CO 2 experiments have also found that overstory and understory vegetation are able to obtain approximately equal amounts of N in control and elevated CO 2 treatments (Finzi et al, 2002;Johnson et al, 2004). …”

“…In order for trees to maintain increased productivity under elevated CO 2 , they must increase N use efficiency or N uptake (Pataki et al, 2003). A study of nutrient cycling at the Oak Ridge National Laboratory FACE experiment (Johnson et al, 2004) provided evidence that trees are able to increase their N uptake, likely through greater fine root production, although the source of this additional N is unclear.…”