Fine-root biomass and fluxes of soil carbon in young stands of paper birch and trembling aspen as affected by elevated atmospheric CO2 and tropospheric O3

Abstract

Rising atmospheric CO may stimulate future forest productivity, possibly increasing carbon storage in terrestrial ecosystems, but how tropospheric ozone will modify this response is unknown. Because of the importance of fine roots to the belowground C cycle, we monitored fine-root biomass and associated C fluxes in regenerating stands of trembling aspen, and mixed stands of trembling aspen and paper birch at FACTS-II, the Aspen FACE project in Rhinelander, Wisconsin. Free-air CO enrichment (FACE) was used to elevate concentrations of CO (average enrichment concentration 535 µl l) and O (53 nl l) in developing forest stands in 1998 and 1999. Soil respiration, soil pCO, and dissolved organic carbon in soil solution (DOC) were monitored biweekly. Soil respiration was measured with a portable infrared gas analyzer. Soil pCO and DOC samples were collected from soil gas wells and tension lysimeters, respectively, at depths of 15, 30, and 125 cm. Fine-root biomass averaged 263 g m in control plots and increased 96% under elevated CO. The increased root biomass was accompanied by a 39% increase in soil respiration and a 27% increase in soil pCO. Both soil respiration and pCO exhibited a strong seasonal signal, which was positively correlated with soil temperature. DOC concentrations in soil solution averaged ~12 mg l in surface horizons, declined with depth, and were little affected by the treatments. A simplified belowground C budget for the site indicated that native soil organic matter still dominated the system, and that soil respiration was by far the largest flux. Ozone decreased the above responses to elevated CO, but effects were rarely statistically significant. We conclude that regenerating stands of northern hardwoods have the potential for substantially greater C input to soil due to greater fine-root production under elevated CO. Greater fine-root biomass will be accompanied by greater soil C efflux as soil respiration, but leaching losses of C will probably be unaffected.

Impacts of elevated atmospheric CO2 on forest trees and forest ecosystems: knowledge gaps

“…Increased root growth of forest trees under elevated atmospheric CO 2 has been reported by several researchers (Matamala and Schlesinger, 2000;Pregitzer et al, 2000;King et al, 2001;Pritchard et al, 2001). Consistent findings show that the production and mortality of fine roots produced by trees growing under CO 2 enrichment are significantly increased (Matamala and Schlesinger, 2000;Pregitzer et al, 2000;King et al, 2001;Pritchard et al, 2001).…”

“…Consistent findings show that the production and mortality of fine roots produced by trees growing under CO 2 enrichment are significantly increased (Matamala and Schlesinger, 2000;Pregitzer et al, 2000;King et al, 2001;Pritchard et al, 2001). Species differ in the responsiveness of their root systems to increased atmospheric CO 2 , suggesting that differences in the ability of certain species to compete against others could be dramatically changed under elevated CO 2 (Pritchard et al, 2001).…”

“…Few forest tree studies have as yet estimated impacts of elevated atmospheric CO 2 on carbon sequestration. However, observations at two FACE studies suggest that soil respiration rates are higher under elevated CO 2 (King et al, 2001;Schlesinger and Richter, 2001). Schlesinger and Richter (2001) suggest that a large portion of the additional C added to soils is likely returned to the atmosphere.…”

A new approach to trenching experiments for measuring root–rhizosphere respiration in a lowland tropical forest

“…The heterotrophic component of soil respiration is strongly influenced by substrate availability (Raich and Tufekcioglu, 2000;Vasconcelos et al, 2004), which is closely related to aboveground litterfall on a global scale (Raich and Nadelhoffer, 1989;Davidson et al, 2002), and is thus also ultimately driven by aboveground growth and production (Rey et al, 2002). Recently, a microcosm study showed that increased litter inputs under elevated CO 2 would greatly increase microbial respiration in the soil (Liu et al, 2008) and FACE experiments have shown large increases in root respiration, which may be related to increased root biomass or higher specific root respiration rates (Andrews et al, 1999;King et al, 2001;Pregitzer et al, 2008).…”

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

“…Statistical analysis of understory biomass, N concentration, N content, atom percent excess 15N, and 15N recovery was performed in SAS (version 8.2, SAS Institute Inc., Cary, NC) using PROC GLM for a split-plot randomized complete block design as detailed in King et al (2001). An alpha level of 0.05 was used and significant differences were further analyzed using least squared means.…”