Litterfall 15N abundance indicates declining soil nitrogen availability in a free-air CO2 enrichment experiment

Forest productivity increases in response to carbon dioxide (CO2) enrichment of the atmosphere. However, in nitrogen-limited ecosystems, increased productivity may cause a decline in soil nitrogen (N) availability and induce a negative feedback on further enhancement of forest production. In a free-air CO2 enrichment (FACE) experiment, the response of sweetgum (Liquidambar styraciflua L.) productivity to elevated CO2 concentrations [CO2] has declined over time, but documenting an associated change in soil N availability has been difficult. Here we assess the time history of soil N availability through analysis of natural 15N abundance in archived samples of freshly fallen leaf litterfall. Litterfall delta15N declined from 1998 to 2005, and the rate of decline was significantly faster in elevated [CO2]. Declining leaf litterfall delta15N is indicative of a tighter ecosystem N cycle and more limited soil N availability. By integrating N availability over time and throughout the soil profile, temporal dynamics in leaf litterfall delta15N provide a powerful tool for documenting changes in N availability and the critical feedbacks between C and N cycles that will control forest response to elevated atmospheric CO2 concentrations.     

Ecosystem Impacts of Geoengineering: A Review for Developing a Science Plan

Geoengineering methods are intended to reduce climate change, which is already having demonstrable effects on ecosystem structure and functioning in some regions. Two types of geoengineering activities that have been proposed are: carbon dioxide (CO(2)) removal (CDR), which removes CO(2) from the atmosphere, and solar radiation management (SRM, or sunlight reflection methods), which reflects a small percentage of sunlight back into space to offset warming from greenhouse gases (GHGs). Current research suggests that SRM or CDR might diminish the impacts of climate change on ecosystems by reducing changes in temperature and precipitation. However, sudden cessation of SRM would exacerbate the climate effects on ecosystems, and some CDR might interfere with oceanic and terrestrial ecosystem processes. The many risks and uncertainties associated with these new kinds of purposeful perturbations to the Earth system are not well understood and require cautious and comprehensive research.     

Hierarchy theory as a guide to mycorrhizal research on large-scale problems

Mycorrhizal researchers are faced with the dilemma of designing experiments on small-scale processes that contribute to the solution of large-scale problems, such as global change. Hierarchy Theory provides a paradigm that indicates the need for mycorrhizal research and suggests criteria that help to prioritize research objectives. In general, mycorrhizal research is justified whenever direct or indirect effects on mycorrhizal physiology or ecology form a critical constraint to large-scale system function. The paper attempts to extract the relevant concepts from the theory and provides a series of illustrative examples from mycorrhizal research.     

Nitrogen uptake, distribution, turnover, and efficiency of use in a CO2-enriched sweetgum forest

The Progressive Nitrogen Limitation (PNL) hypothesis suggests that ecosystems in a CO2-enriched atmosphere will sequester C and N in long-lived biomass and soil organic pools, thereby limiting available N and constraining the continued response of net primary productivity to elevated [CO2]. Here, we present a six-year record of N dynamics of a sweetgum (Liquidambar styraciflua) stand exposed to elevated [CO2] in the free-air CO2 enrichment (FACE) experiment at Oak Ridge, Tennessee, USA. We also evaluate the concept of PNL for this ecosystem from the perspective of N uptake, content, distribution, and turnover, and N-use efficiency. Leaf N content was 11% lower on a leaf mass basis (NM) and 7% lower on a leaf area basis (NA) in CO2-enriched trees. However, there was no effect of [CO2] on total canopy N content. Resorption of N during senescence was not altered by [CO2], so NM of litter, but not total N content, was reduced. The NM of fine roots was not affected, but the total amount of N required for fine-root production increased significantly, reflecting the large stimulation of fine-root production in this stand. Hence, total N requirement of the trees was higher in elevated [CO2], and the increased requirement was met through an increase in N uptake rather than increased retranslocation of stored reserves. Increased N uptake was correlated with increased net primary productivity (NPP). N-use efficiency, however, did not change with CO2 enrichment because increased N productivity was offset by lower mean residence time of N in the trees. None of the measured responses of plant N dynamics in this ecosystem indicated the occurrence of PNL, and the stimulation of NPP by elevated [CO2] was sustained for the first six years of the experiment. Although there are some indications of developing changes in the N economy, the N supply in the soil at this site may be sufficient to meet an increasing demand for available N, especially as the roots of CO2-enriched trees explore deeper in the soil profile.     

Differential response of CO2 uptake parameters of soil- and sand-grown phaseolus vulgaris (L.) plants to absorbed ozone flux

Shoots of a soil- or sand-grown dwarf bean variety were exposed to O(3) concentrations in the range of 500 to 900 ppb for up to 5 h. The measured exchange rates of water vapor and CO(2) during exposures were used to calculate stomatal and mesophyll conductances averaged across all leaves. Changes in conductances were related to exposure duration and absorbed O(3) totals (AOT). Both conductances were more sensitive to AOT in sand-grown plants, which also had more visible injury under comparable AOT values. Measurements of the relationship between CO(2) exchange and internal CO(2) concentration of single leaflets of treated plants also showed greater sensitivity of CO(2)-saturated photosynthesis in sand-grown plants. Diffusional processes were not likely to have been the cause of dissimilar responses because the O(3) absorption rate was lower in sand-grown plants. A difference in the scaveninng capacities in cells is suggested to be the cause of the differences in sensitivity to acute O(3) exposure.