Abstract. The objective of this experiment was to understand how atmospheric carbon dioxide (CO 2 ) and soil-nitrogen (N) availability influence Populus tremuloides fine-root growth and morphology. Soil-N availability may limit the growth response of forests to elevated CO 2 and interact with atmospheric CO 2 to alter litter quality and ecosystem carbon (C) and N cycling. We established a CO 2 ϫ N factorial field experiment and grew six genotypes of P. tremuloides for 2.5 growing seasons in 20 large open-top chamber/rootbox experimental units at the University of Michigan Biological Station in northern lower Michigan (USA). In this paper we describe an integrated examination of how atmospheric CO 2 and soil-N availability influence fine-root morphology, growth, mortality, and biomass. We also studied the relationship between root biomass and total soil respiration.Over 80% of the absorbing root length of P. tremuloides was accounted for by roots Ͻ0.4 mm in diameter, and specific root length (100-250 m/g) was much greater than reports for other temperate and boreal deciduous trees. Elevated atmospheric CO 2 increased the diameter and length of individual roots. In contrast, soil N had no effect on root morphology. Fine-root length production and mortality, measured with minirhizotrons, was controlled by the interaction between atmospheric CO 2 and soil N. Rates of root production and mortality were significantly greater at elevated CO 2 when trees grew in high-N soil, but there were no CO 2 effects at low soil N. Fine-root biomass increased 137-194% in high-N compared to low-N soil, and elevated atmospheric CO 2 increased fine-root biomass (52%) in high soil N, but differences in low soil N were not significant. Across all treatments, dynamic estimates of net fine-root production were highly correlated with fine-root biomass (soil cores; r ϭ 0.975). Mean rates of soil respiration were more than double in high-N compared to low-N soil, and elevated atmospheric CO 2 , when compared to ambient atmospheric CO 2 , increased mean rates of soil respiration 19% in 1995 and 25% in 1996. Across all treatments, total root biomass was linearly related to mean rates of soil respiration (r 2 ϭ 0.96).Our results indicate that atmospheric CO 2 and soil-N availability strongly interact to influence P. tremuloides fine-root morphology, growth, and C turnover. 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 CO 2 will result in greater C inputs to soil through greater rates of fine-root production and turnover, especially in high-fertility soils. Increased C inputs to soil result in greater rates of soil respiration. At this time, it is not clear what effects increased rates of root turnover will have on C storage in the soil.
We tested the hypothesis that elevated CO would stimulate proportionally higher photosynthesis in the lower crown of Populus trees due to less N retranslocation, compared to tree crowns in ambient CO. Such a response could increase belowground C allocation, particularly in trees with an indeterminate growth pattern such as Populus tremuloides. Rooted cuttings of P. tremuloides were grown in ambient and twice ambient (elevated) CO and in low and high soil N availability (89 ± 7 and 333 ± 16 ng N g day net mineralization, respectively) for 95 days using open-top chambers and open-bottom root boxes. Elevated CO resulted in significantly higher maximum leaf photosynthesis (A ) at both soil N levels. A was higher at high N than at low N soil in elevated, but not ambient CO. Photosynthetic N use efficiency was higher at elevated than ambient CO in both soil types. Elevated CO resulted in proportionally higher whole leaf A in the lower three-quarters to one-half of the crown for both soil types. At elevated CO and high N availability, lower crown leaves had significantly lower ratios of carboxylation capacity to electron transport capacity (V /J) than at ambient CO and/or low N availability. From the top to the bottom of the tree crowns, V /J increased in ambient CO, but it decreased in elevated CO indicating a greater relative investment of N into light harvesting for the lower crown. Only the mid-crown leaves at both N levels exhibited photosynthetic down regulation to elevated CO. Stem biomass segments (consisting of three nodes and internodes) were compared to the total A for each segment. This analysis indicated that increased A at elevated CO did not result in a proportional increase in local stem segment mass, suggesting that C allocation to sinks other than the local stem segment increased disproportionally. Since C allocated to roots in young Populus trees is primarily assimilated by leaves in the lower crown, the results of this study suggest a mechanism by which C allocation to roots in young trees may increase in elevated CO.
The objective of this experiment was to understand how atmospheric carbon dioxide (CO 2) and soil-nitrogen (N) availability influence Populus tremuloides fine-root growth and morphology. Soil-N availability may limit the growth response of forests to elevated CO 2 and interact with atmospheric CO 2 to alter litter quality and ecosystem carbon (C) and N cycling. We established a CO 2 N factorial field experiment and grew six genotypes of P. tremuloides for 2.5 growing seasons in 20 large open-top chamber/root-box experimental units at the University of Michigan Biological Station in northern lower Michigan (USA). In this paper we describe an integrated examination of how atmospheric CO 2 and soil-N availability influence fine-root morphology, growth, mortality, and biomass. We also studied the relationship between root biomass and total soil respiration. Over 80% of the absorbing root length of P. tremuloides was accounted for by 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. Elevated atmospheric CO 2 increased the diameter and length of individual roots. In contrast, soil N had no effect on root morphology. Fine-root length production and mortality, measured with minirhizotrons, was controlled by the interaction between atmospheric CO 2 and soil N. Rates of root production and mortality were significantly greater at elevated CO 2 when trees grew in high-N soil, but there were no CO 2 effects at low soil N. Fine-root biomass increased 137-194% in high-N compared to low-N soil, and elevated atmospheric CO 2 increased fine-root biomass (52%) in high soil N, but differences in low soil N were not significant. Across all treatments, dynamic estimates of net fine-root production were highly correlated with fine-root biomass (soil cores; r 0.975). Mean rates of soil respiration were more than double in high-N compared to low-N soil, and elevated atmospheric CO 2 , when compared to ambient atmospheric CO 2 , increased mean rates of soil respiration 19% in 1995 and 25% in 1996. Across all treatments, total root biomass was linearly related to mean rates of soil respiration (r 2 0.96). Our results indicate that atmospheric CO 2 and soil-N availability strongly interact to influence P. tremuloides fine-root morphology, growth, and C turnover. 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 CO 2 will result in greater C inputs to soil through greater rates of fine-root production and turnover, especially in high-fertility soils. Increased C inputs to soil result in greater rates of soil respiration. At this time, it is not clear what effects increased rates of root turnover will have on C storage in the soil.
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