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.

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Abstract. Our ability to predict whether elevated atmospheric CO 2 will alter the cycling of C and N in terrestrial ecosystems requires understanding a complex set of feedback mechanisms initiated by changes in C and N acquisition by plants and the degree to which changes in resource acquisition (C and N) alter plant growth and allocation. To gain further insight into these dynamics, we grew six genotypes of Populus tremuloides Michx. that differ in autumnal senescence (early vs. late) under experimental atmospheric CO 2 (35.7 and 70.7 Pa) and soil-N availability (low and high) treatments. Atmospheric CO 2 concentrations were manipulated with open-top chambers, and soil-N availability was modified in open-bottom root boxes by mixing different proportions of native A and C horizon soil. Net N mineralization rates averaged 61 ng N·g Ϫ1 ·d Ϫ1 in low-N soil and 319 ng N·g Ϫ1 ·d Ϫ1in high-N soil. After 2.5 growing seasons, we harvested above-and belowground plant components in each chamber and determined total biomass, N concentration, N content, and the relative allocation of biomass and N to leaves, stems, and roots. Elevated CO 2 increased total plant biomass 16% in low-N soil and 38% in high-N soil, indicating that the growth response of P. tremuloides to elevated CO 2 was constrained by soil-N availability. Greater growth under elevated CO 2 did not substantially alter the allocation of biomass to above-or belowground plant components. At both levels of soil-N availability, elevated CO 2 decreased the N concentration of all plant tissues. Despite declines in tissue N concentration, elevated CO 2 significantly increased whole-plant N content in high-N soil (ambient ϭ 137 g N/chamber; elevated ϭ 155 g N/chamber), but it did not influence whole-plant N content in low-N soil (36 g N/chamber). Our results indicate that plants in high-N soil obtained greater amounts of soil N under elevated CO 2 by producing a proportionately larger fine-root system that more thoroughly exploited the soil. The significant positive relationship between fine-root biomass and total-plant N content we observed in high-N soil further supports this contention. In low-N soil, elevated CO 2 did not increase fine-root biomass or production, and plants under ambient and elevated CO 2 obtained equivalent amounts of N from soil. In high-N soil, it appears that greater acquisition of soil N under elevated CO 2 fed forward within the plant to increase rates of C acquisition, which further enhanced plant growth response to elevated CO 2 .

Nutrients such as nitrogen (N) and phosphorus (P) often limit plant growth rate and production in natural and agricultural ecosystems. Limited availability of these nutrients is also a major factor influencing long-term plant and ecosystem responses to rising atmospheric CO levels, i.e., the commonly observed short-term increase in plant biomass may not be sustained over the long-term. Therefore, it is critical to obtain a mechanistic understanding of whether elevated CO can elicit compensatory adjustments such that acquisition capacity for minerals increases in concert with carbon (C) uptake. Compensatory adjustments such as increases in (a) root mycorrhizal infection, (b) root-to-shoot ratio and changes in root morphology and architecture, (c) root nutrient absorption capacity, and (d) nutrient-use efficiency can enable plants to meet an increased nutrient demand under high CO. Here we examine the literature to assess the extent to which these mechanisms have been shown to respond to high CO. The literature survey reveals no consistent pattern either in direction or magnitude of responses of these mechanisms to high CO. This apparent lack of a pattern may represent variations in experimental protocol and/or interspecific differences. We found that in addressing nutrient uptake responses to high CO most investigators have examined these mechanisms in isolation. Because such mechanisms can potentially counterbalance one another, a more reliable prediction of elevated CO responses requires experimental designs that integrate all mechanisms simultaneously. Finally, we present a functional balance (FB) model as an example of how root system adjustments and nitrogen-use efficiency can be integrated to assess growth responses to high CO. The FB model suggests that the mechanisms of increased N uptake highlighted here have different weights in determining overall plant responses to high CO. For example, while changes in root-to-shoot biomass allocation, r, have a small effect on growth, adjustments in uptake rate per unit root mass, [Formula: see text], and photosynthetic N use efficiency, p*, have a significantly greater leverage on growth responses to elevated CO except when relative growth rate (RGR) reaches its developmental limit, maximum RGR (RGR).

Abstract. Predicting forest responses to rising atmospheric CO 2 will require an understanding of key feedbacks in the cycling of carbon and nitrogen between plants and soil microorganisms. We conducted a study for 2.5 growing seasons with Populus tremuloides grown under experimental atmospheric CO 2 and soil-N-availability treatments. Our objective was to integrate the combined influence of atmospheric CO 2 and soil-N availability on the flow of C and N in the plant-soil system and to relate these processes to the performance of this widespread and economically important tree species. Here we consider treatment effects on photosynthesis and canopy development and the efficiency with which this productive capacity is translated into aboveground, harvestable yield.We grew six P. tremuloides genotypes at ambient (35 Pa) or elevated (70 Pa) CO 2 and in soil of low or high N mineralization rate at the University of Michigan Biological Station, Pellston, Michigan, USA (45Њ35Ј N, 84Њ42Ј W). In the second year of growth, net CO 2 assimilation rate was significantly higher in elevated-CO 2 compared to ambient-CO 2 plants in both soil-N treatments, and we found little evidence for photosynthetic acclimation to high CO 2 . In the third year, however, elevated-CO 2 plants in low-N soil had reduced photosynthetic capacity compared to ambient-CO 2 , low-N plants. Plants in high-N soil showed the opposite response, with elevated-CO 2 plants having higher photosynthetic capacity than ambient-CO 2 plants. Net CO 2 assimilation rate was linearly related to leaf N concentration (log : log scale), with identical slopes but different intercepts in the two CO 2 treatments, indicating differences in photosynthetic N-use efficiency. Elevated CO 2 increased tissue dark respiration in high-N soil (ϩ22%) but had no significant effect in low-N soil (ϩ9%). There were no CO 2 effects on stomatal conductance. At the final harvest, stem biomass and total leaf area increased significantly due to CO 2 enrichment in high-N but not in low-N soil. Treatment effects on wood production were largely attributable to changes in leaf area, with no significant effects on growth efficiency. We conclude that harvest intervals for P. tremuloides on fertile sites will shorten with rising atmospheric CO 2 , but that tree size at canopy closure may be unaffected.