Search

Contents Summary 68 I. Introduction 68 II. Have ECM fungi retained genes with lignocellulolytic potential from saprotrophic ancestors? 69 III. Are genes with saprotrophic function expressed by ECM fungi when in symbiosis? 71 IV. Do transcribed enzymes operate to obtain N from SOM? 71 V. Is the organic N derived from SOM transferred to the plant host? 71 VI. Concluding remarks 72 Acknowledgements 72 References 72 Summary: The view that ectomycorrhizal (ECM) fungi commonly participate in the enzymatic liberation of nitrogen (N) from soil organic matter (SOM) has recently been invoked as a key mechanism governing the biogeochemical cycles of forest ecosystems. Here, we provide evidence that not all evolutionary lineages of ECM have retained the genetic potential to produce extracellular enzymes that degrade SOM, calling into question the ubiquity of the proposed mechanism. Further, we discuss several untested conditions that must be empirically validated before it is certain that any lineage of ECM fungi actively expresses extracellular enzymes in order to degrade SOM and transfer N contained therein to its host plant. [ABSTRACT FROM AUTHOR]

• The Rhinelander free-air CO2 enrichment (FACE) experiment is designed to understand ecosystem response to elevated atmospheric carbon dioxide (+CO2) and elevated tropospheric ozone (+O3). The objectives of this study were: to understand how soil respiration responded to the experimental treatments; to determine whether fine-root biomass was correlated to rates of soil respiration; and to measure rates of fine-root turnover in aspen ( Populus tremuloides) forests and determine whether root turnover might be driving patterns in soil respiration. • Soil respiration was measured, root biomass was determined, and estimates of root production, mortality and biomass turnover were made. • Soil respiration was greatest in the +CO2 and +CO2 +O3 treatments across all three plant communities. Soil respiration was correlated with increases in fine-root biomass. In the aspen community, annual fine-root production and mortality (g m−2) were positively affected by +O3. • After 10 yr of exposure, +CO2 +O3-induced increases in belowground carbon allocation suggest that the positive effects of elevated CO2 on belowground net primary productivity (NPP) may not be offset by negative effects of O3. For the aspen community, fine-root biomass is actually stimulated by +O3, and especially +CO2 +O3. [ABSTRACT FROM AUTHOR]

There is considerable uncertainty about how rates of soil carbon (C) and nitrogen (N) cycling will change as CO2 accumulates in the Earth's atmosphere. We summarized data from 47 published reports on soil C and N cycling under elevated CO2 in an attempt to generalize whether rates will increase, decrease, or not change. Our synthesis centres on changes in soil respiration, microbial respiration, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization, because these pools and processes represent important control points for the below-ground flow of C and N. To determine whether differences in C allocation between plant life forms influence soil C and N cycling in a predictable manner, we summarized responses beneath graminoid, herbaceous and woody plants grown under ambient and elevated atmospheric CO2. The below-ground pools and processes that we summarized are characterized by a high degree of variability (coefficient of variation 80-800%), making generalizations within and between plant life forms difficult. With few exceptions, rates of soil and microbial respiration were more rapid under elevated CO2, indicating that (1) greater plant growth under elevated CO2 enhanced the amount of C entering the soil, and (2) additional substrate was being metabolized by soil microorganisms. However, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization are characterized by large increases and declines under elevated CO2, contributing to a high degree of variability within and between plant life forms. From this analysis we conclude that there are insufficient data to predict how microbial activity and rates of soil C and N cycling will change as the atmospheric CO2 concentration continues to rise. We argue that current gaps in our understanding of fine-root biology limit our ability to predict the response of soil microorganisms to rising atmospheric CO2, and that understanding differences in fine-root longevity and biochemistry between plant species are necessary for developing a predictive model of soil C and N cycling under elevated CO2. [ABSTRACT FROM AUTHOR]

In most natural ecosystems a significant portion of carbon fixed through photosynthesis is allocated to the production and maintenance of fine roots, the ephemeral portion of the root system that absorbs growth-limiting moisture and nutrients. In turn, senescence of fine roots can be the greatest source of C input to forest soils. Consequently, important questions in ecology entail the extent to which increasing atmospheric CO2 may alter the allocation of carbon to, and demography of, fine roots. Using microvideo and image analysis technology, we demonstrate that elevated atmospheric CO2 increases the rates of both fine root production and mortality. Rates of root mortality also increased substantially as soil nitrogen availability increased, regardless of CO2concentration. Nitrogen greatly influenced the proportional allocation of carbon to leaves vs. fine roots. The amount of available nitrogen in the soil appears to be the most important factor regulating fine root demography in Populus trees. [ABSTRACT FROM AUTHOR]

We grew cuttings of two early (mid Oct.) and two late (early Nov.) leaf-fall Populus tremuloides Michx. genotypes (referred to as genotype pairs) for c. 150 din open-top chambers to understand how twice-ambient (elevated) CO2, and soil N availability would affect growth and C allocation. For the study, we selected genotypes differing in leaf area duration to find out if late-season photosynthesis influenced C allocation to roots. Both elevated CO2 and high soil N availability significantly increased estimated whole-tree photosynthesis, but they did so in different ways. Elevated CO2 stimulated leaf-level photosynthesis rates, whereas high soil N availability resulted in greater total plant leaf area. The early leaf-fall genotype pair had significantly higher photosynthesis rates per unit leaf area than the late leaf-fall genotype pair and elevated CO2 enhanced this difference. The early leaf-fall genotype pair had less leaf area than the late leaf-fall genotype pair, and their rate of leaf area development decreased earlier in the season. Across both genotype pairs, high soil N availability significantly increased fine root length production and mortality by increasing both the amount of root length present, and by decreasing the life span of individual roots. Elevated CO2 resulted in significantly increased fine root production and mortality in high N but not low N soil and did not affect fine root life span. The early leaf-fall genotype pair had significantly greater fine root length production than the late leaf-fall genotype pair across all CO2 and N treatments. These differences in belowground C allocations are consistent with the hypothesis that belowground C and N cycling is strongly influenced by soil N availability and will increase under elevated atmospheric CO2. In addition, this study reinforces the need for better understanding of the variation in tree responses to elevated CO2, within and among species. [ABSTRACT FROM AUTHOR]

Two important processes which may limit productivity gains in forest ecosystems with atmospheric CO2 are reduction in photosynthetic capacity following prolonged exposure to high CO2 and diminution of positive growth responses when soil nutrients, particularly N, are limiting. To examine the interacting effects of soil fertility and CO2 enrichment on photosynthesis and growth in trees we grew hybrid poplar (Populus x euramericana) for 158 d in the field at ambient and twice ambient CO2 and in soil with low or high N availability. We measured the timing and rate of canopy development, and final above- and belowground dry weight. Single leaf net CO2 assimilation (A) increased at elevated CO2 over the majority of the growing season in both fertility treatments. At high fertility, the maximum size of individual leaves, total leaf number, and seasonal leaf area duration (LAD) also increased at elevated CO2, leading to a 49% increase in total dry weight. In contrast, at low fertility leaf area growth was unaffected by CO2 treatment. Total dry weight nonetheless increased 25% due to CO2 effects on A. Photosynthetic capacity (A at constant internal p(CO2), (C1)) was reduced in high CO2 plants after 100 d growth at low fertility and 135 d growth at high fertility. Analysis of A responses to changing C1 indicated that this negative adjustment of photosynthesis was due to a reduction in the maximum rate of CO2 fixation by Rubisco. Maximum rate of electron transport and phosphate regeneration capacity were either unaffected or declined at elevated CO2, Carbon dioxide effects on leaf respiration were most pronounced at high fertility, with increased respiration mid-season and no change (area basis) or reduced (mass basis) respiration late-season in elevated compared to ambient CO2 plants. This temporal variation correlated with changes in leaf N concentration and leaf mass per area. Our results demonstrate the importance of considering both structural and physiological pathways of net C gain in predicting tree responses to rising CO2 under conditions of suboptimal soil fertility. [ABSTRACT FROM AUTHOR]

• The Rhinelander free-air CO2 enrichment (FACE) experiment is designed to understand ecosystem response to elevated atmospheric carbon dioxide (+CO2) and elevated tropospheric ozone (+O3). The objectives of this study were: to understand how soil respiration responded to the experimental treatments; to determine whether fine-root biomass was correlated to rates of soil respiration; and to measure rates of fine-root turnover in aspen ( Populus tremuloides) forests and determine whether root turnover might be driving patterns in soil respiration. • Soil respiration was measured, root biomass was determined, and estimates of root production, mortality and biomass turnover were made. • Soil respiration was greatest in the +CO2 and +CO2 +O3 treatments across all three plant communities. Soil respiration was correlated with increases in fine-root biomass. In the aspen community, annual fine-root production and mortality (g m−2) were positively affected by +O3. • After 10 yr of exposure, +CO2 +O3-induced increases in belowground carbon allocation suggest that the positive effects of elevated CO2 on belowground net primary productivity (NPP) may not be offset by negative effects of O3. For the aspen community, fine-root biomass is actually stimulated by +O3, and especially +CO2 +O3. [ABSTRACT FROM AUTHOR]

We grew cuttings of two early (mid Oct.) and two late (early Nov.) leaf-fall Populus tremuloides Michx. genotypes (referred to as genotype pairs) for c. 150 d in open-top chambers to understand how twice-ambient (elevated) CO 2 and soil N availability would affect growth and C allocation. For the study, we selected genotypes differing in leaf area duration to find out if late-season photosynthesis influenced C allocation to roots. Both elevated CO 2 and high soil N availability significantly increased estimated whole-tree photosynthesis, but they did so in different ways. Elevated CO 2 stimulated leaf-level photosynthesis rates, whereas high soil N availability resulted in greater total plant leaf area. The early leaf-fall genotype pair had significantly higher photosynthesis rates per unit leaf area than the late leaf-fall genotype pair and elevated CO 2 enhanced this difference. The early leaf-fall genotype pair had less leaf area than the late leaf-fall genotype pair, and their rate of leaf area development decreased earlier in the season. Across both genotype pairs, high soil N availability significantly increased fine root length production and mortality by increasing both the amount of root length present, and by decreasing the life span of individual roots. Elevated CO 2 resulted in significantly increased fine root production and mortality in high N but not low N soil and did not affect fine root life span. The early leaf-fall genotype pair had significantly greater fine root length production than the late leaf-fall genotype pair across all CO 2 and N treatments. These differences in belowground C allocations are consistent with the hypothesis that belowground C and N cycling is strongly influenced by soil N availability and will increase under elevated atmospheric CO 2 . In addition, this study reinforces the need for better understanding of the variation in tree responses to elevated CO 2 , within and among species.

Two important processes which may limit productivity gains in forest ecosystems with rising atmospheric CO 2 are reduction in photosynthetic capacity following prolonged exposure to high CO 2 and diminution of positive growth responses when soil nutrients, particularly N, are limiting. To examine the interacting effects of soil fertility and CO 2 enrichment on photosynthesis and growth in trees we grew hybrid poplar (Populus × euramericana) for 158 d in the field at ambient and twice ambient CO 2 and in soil with low or high N availability. We measured the timing and rate of canopy development, the seasonal dynamics of leaf level photosynthetic capacity, respiration, and N and carbohydrate concentration, and final above- and belowground dry weight. Single leaf net CO 2 assimilation (A) increased at elevated CO 2 over the majority of the growing season in both fertility treatments. At high fertility, the maximum size of individual leaves, total leaf number, and seasonal leaf area duration (LAD) also increased at elevated CO 2 , leading to a 49% increase in total dry weight. In contrast, at low fertility leaf area growth was unaffected by CO 2 treatment. Total dry weight nonetheless increased 25% due to CO 2 effects on A. Photosynthetic capacity (A at constant internal p(CO 2 ), ((C 1 )) was reduced in high CO 2 plants after 100 d growth at low fertility and 135 d growth at high fertility. Analysis of A responses to changing C 1 indicated that this negative adjustment of photosynthesis was due to a reduction in the maximum rate of CO 2 fixation by Rubisco. Maximum rate of electron transport and phosphate regeneration capacity were either unaffected or declined at elevated CO 2 . Carbon dioxide effects on leaf respiration were most pronounced at high fertility, with increased respiration mid-season and no change (area basis) or reduced (mass basis) respiration late-season in elevated compared to ambient CO 2 plants. This temporal variation correlated with changes in leaf N concentration and leaf mass per area. Our results demonstrate the importance of considering both structural and physiological pathways of net C gain in predicting tree responses to rising CO 2 under conditions of suboptimal soil fertility.