Interactive effects of atmospheric CO2 and soil-N availability on fine roots of Populus tremuloides

The objective of this experiment was to understand how atmospheric carbon d ioxide (CO2) and soil-nitrogen (N) availability influence Populus tremuloid es fine-root growth and morphology. Soil-N availability may limit the growt h response of forests to elevated CO2 and interact with atmospheric CO2 to alter litter quality and ecosystem carbon (C) and N cycling. We established a CO2 x N factorial field experiment and grew six genotypes of P. tremuloi des for 2.5 growing seasons in 20 large open-top chamber/root-box experimen tal units at the University of Michigan Biological Station in northern lowe r Michigan (USA). Ln this paper we describe an integrated examination of ho w atmospheric CO2 and soil-N availability influence fine-root morphology, g rowth, mortality, and biomass. We also studied the relationship between roo t biomass and total soil respiration. Over 80% of the absorbing root length of P. tremuloides was accounted fur b y roots <0.4 mm in diameter, and specific root length(100-250 m/g) was much greater than reports for other temperate and boreal deciduous trees. Eleva ted atmospheric CO2 increased the diameter and length of individual roots. In contrast, soil N had no effect on root morphology. Fine-root length prod uction and mortality, measured with minirhizotrons, was controlled by the i nteraction between atmospheric CO2 and soil N. Rates of root production and mortality were significantly greater at elevated CO2 when trees grew in hi gh-N soil, but there were no CO2 effects at low soil N. Fine-root biomass i ncreased 137-194% in high-N compared to low-N soil, and elevated atmospheri c CO2 increased fine-root biomass (52%) in high soil N, but differences in low soil N were not significant. Across all treatments, dynamic estimates o f net fine-root production were highly correlated with fine-root biomass (s oil cores; r = 0.975). Mean rates of soil respiration were more than double in high-N compared to low-N soil. and elevated atmospheric CO2, when compa red to ambient atmospheric CO2, increased mean rates of soil respiration 19 % in 1995 and 25% in 1996. Across all treatments, total root biomass was li nearly related to mean rates of soil respiration (r(2) = 0.96). Our results indicate that atmospheric CO2 and soil-N availability strongly interact to influence P. tremuloides fine-root morphology, growth, and C tu rnover. Aspen-dominated ecosystems of the future are likely to have greater productivity fueled by greater nutrient uptake due to greater root length production. Further, it appears that elevated atmospheric CO2 will result i n greater C inputs to soil through greater rates of fine-root production an d turnover, especially in high-fertility soils. Increased C inputs to soil result in greater rates of soil respiration. At this rime, it is not clear what effects increased rates of root turnover will have on C storage in the soil.