John Lussenhop

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 dioxide (CO2) 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 CO2 and interact with atmospheric CO2 to alter litter quality and ecosystem carbon (C) and N cycling. We established a CO2 × 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 CO2 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 great...

GAS EXCHANGE, LEAF NITROGEN, AND GROWTH EFFICIENCY OF POPULUS TREMULOIDES IN A CO2-ENRICHED ATMOSPHERE

Predicting forest responses to rising atmospheric CO2 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 CO2 and soil-N-availability treatments. Our objective was to integrate the combined influence of atmospheric CO2 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) CO2 and in soil of low or high N mineralization rate at the University of Michigan Biological Station, Pellston, Michigan, USA (45°35′ N, 84°42′ W). In the second year of growth, net...

Atmospheric CO2, soil-N availability, and allocation of biomass and nitrogen by Populus tremuloides.

Our ability to predict whether elevated atmospheric CO2 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 CO2 (35.7 and 70.7 Pa) and soil-N availability (low and high) treatments. Atmospheric CO2 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−1·d−1 in low-N soil and 319 ng N·g−1·d−1 in 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 CO2 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 CO2 was constrained by soil-N availability. Greater growth under elevated CO2 did not substantially alter the allocation of biomass to above- or belowground plant components. At both levels of soil-N availability, elevated CO2 decreased the N concentration of all plant tissues. Despite declines in tissue N concentration, elevated CO2 significantly increased whole-plant N content in high-N soil (ambient = 137 g N/chamber; elevated = 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 CO2 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 CO2 did not increase fine-root biomass or production, and plants under ambient and elevated CO2 obtained equivalent amounts of N from soil. In high-N soil, it appears that greater acquisition of soil N under elevated CO2 fed forward within the plant to increase rates of C acquisition, which further enhanced plant growth response to elevated CO2.

Root system adjustments: regulation of plant nutrient uptake and growth responses to elevated CO2

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 CO2 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 CO2 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 CO2. Here we examine the literature to assess the extent to which these mechanisms have been shown to respond to high CO2. The literature survey reveals no consistent pattern either in direction or magnitude of responses of these mechanisms to high CO2. 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 CO2 most investigators have examined these mechanisms in isolation. Because such mechanisms can potentially counterbalance one another, a more reliable prediction of elevated CO2 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 CO2. The FB model suggests that the mechanisms of increased N uptake highlighted here have different weights in determining overall plant responses to high CO2. 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,

Collembola as mediators of microbial symbiont effects upon soybean

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Soil nematodes indicate food web responses to elevated atmospheric CO2

, and photosynthetic N use efficiency, p*, have a significantly greater leverage on growth responses to elevated CO2 except when relative growth rate (RGR) reaches its developmental limit, maximum RGR (RGRmax).

Changes in spatial distribution of fungal propagules associated with invertebrate activity in soil

Abstract We tested the hypotheses that increased belowground allocation of carbon by hybrid poplar saplings grown under elevated atmospheric CO2 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) CO2 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 CO2. The density of protozoa did not respond to soil N but increased marginally (P < 0.06) under elevated CO2. Only in high-N soil did arbuscular mycorrhizal fungi and microarthropods respond to CO2. In high-N soil, arbuscular mycorrhizal root mass was twice as great, and extramatrical hyphae were 11% longer in elevated than in ambient CO2 treatments. Microarthropod density and activity were determined in situ using minirhizotrons. Microarthropod density did not change in response to elevated CO2, 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 CO2 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.

A new dawn for soil biology: video analysis of root-soil-microbial-faunal interactions

Abstract The hypothesis that collembola affect rhizobia and mycorrhizas of soybean ( Glycine max ) and thus indirectly change leaf tissue nutrient concentration was studied in pot and field experiments. When a high density of the collembolan species Folsomia candida , was added to pots, the number of nodules per plant increased 52%. When moderate densities of two collembolan species, Folsomia candida and Tullbergia granulata , were added in factorial combinations to cylinders sunk in the soil around soybean in fields, the following responses were observed: 40% greater mycorrhizal root length, and 5% higher leaf tissue N, but no changes in leaf P, nodule number or root mass. Collembola density in the field was too low to increase nodule number per plant as observed in pot experiments: there was no mechanism to explain the 5% increase in leaf tissue N associated with collembola in the field. In the field, intermediate densities of collembola were associated with greater mycorrhizal root length, but since available soil P concentrations were high, longer mycorrhizal root length was not associated with higher leaf tissue P. A path model showed that if mycorrhizas had been positively associated with higher leaf P, the indirect effect of collembola would have been significantly higher leaf tissue P. This study showed that both available soil P and collembola density determine mycorrhizal benefits. In natural habitats, intermediate collembola density and low soil P are expected to maximize benefits of mycorrhizas to plants.