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.

Summary• How nitrogen (N) deposition impacts ectomycorrhizal (EM) fungal communities has been little studied in deciduous forests or across spatial scales. Here, it was tested whether N addition decreases species richness and shifts species composition across spatial scales in temperate deciduous oak forests.• Combined molecular (terminal restriction fragment length polymorphism (T-RFLP), sequencing) and morphological approaches were used to measure EM fungal operational taxon unit (OTU) richness, community structure and composition at the spatial scale of the root, soil core and forest during a 3-yr N fertilization experiment in Quercus-dominated forests near Chicago, IL, USA.• In N treatments, significantly lower OTU richness at the largest but not smaller spatial scales and a different community structure were detected. The effects of N appeared to be immediate, not cumulative. Ordination indicated the composition of EM fungal communities was determined by forest site and N fertilization.• The EM fungi responded to a N increase that was low compared with other fertilization studies, suggesting that moderate increases in N deposition can affect EM fungal communities at larger spatial scales in temperate deciduous ecosystems. While responses at large spatial scales indicate that environmental factors can drive changes in these communities, untangling the impacts of abiotic from biotic factors remain limited by detection issues.

Leaf 15N signature is a powerful tool that can provide an integrated assessment of the nitrogen (N) cycle and whether it is influenced by rising atmospheric CO2 concentration. We tested the hypothesis that elevated CO2 significantly changes foliage δ15N in a wide range of plant species and ecosystem types. This objective was achieved by determining the δ15N of foliage of 27 field‐grown plant species from six free‐air CO2 enrichment (FACE) experiments representing desert, temperate forest, Mediterranean‐type, grassland prairie, and agricultural ecosystems. We found that within species, the δ15N of foliage produced under elevated CO2 was significantly lower (P<0.038) compared with that of foliage grown under ambient conditions. Further analysis of foliage δ15N by life form and growth habit revealed that the CO2 effect was consistent across all functional groups tested. The examination of two chaparral shrubs grown for 6 years under a wide range of CO2 concentrations (25–75 Pa) also showed a significant and negative correlation between growth CO2 and leaf δ15N. In a select number of species, we measured bulk soil δ15N at a depth of 10 cm, and found that the observed depletion of foliage δ15N in response to elevated CO2 was unrelated to changes in the soil δ15N. While the data suggest a strong influence of elevated CO2 on the N cycle in diverse ecosystems, the exact site(s) at which elevated CO2 alters fractionating processes of the N cycle remains unclear. We cannot rule out the fact that the pattern of foliage δ15N responses to elevated CO2 reported here resulted from a general drop in δ15N of the source N, caused by soil‐driven processes. There is a stronger possibility, however, that the general depletion of foliage δ15N under high CO2 may have resulted from changes in the fractionating processes within the plant/mycorrhizal system.

We tested the hypotheses that increased belowground allocation of carbon by hybrid poplar saplings grown under elevated atmospheric CO would increase mass or turnover of soil biota in bulk but not in rhizosphere soil. Hybrid poplar saplings (Populus×euramericana cv. Eugenei) were grown for 5 months in open-bottom root boxes at the University of Michigan Biological Station in northern, lower Michigan. The experimental design was a randomized-block design with factorial combinations of high or low soil N and ambient (34 Pa) or elevated (69 Pa) CO in five blocks. Rhizosphere microbial biomass carbon was 1.7 times greater in high-than in low-N soil, and did not respond to elevated CO. The density of protozoa did not respond to soil N but increased marginally (P < 0.06) under elevated CO. Only in high-N soil did arbuscular mycorrhizal fungi and microarthropods respond to CO. In high-N soil, arbuscular mycorrhizal root mass was twice as great, and extramatrical hyphae were 11% longer in elevated than in ambient CO treatments. Microarthropod density and activity were determined in situ using minirhizotrons. Microarthropod density did not change in response to elevated CO, but in high-N soil, microarthropods were more strongly associated with fine roots under elevated than ambient treatments. Overall, in contrast to the hypotheses, the strongest response to elevated atmospheric CO was in the rhizosphere where (1) unchanged microbial biomass and greater numbers of protozoa (P < 0.06) suggested faster bacterial turnover, (2) arbuscular mycorrhizal root length increased, and (3) the number of microarthropods observed on fine roots rose.