Publications

In controlled N-nutrition experiments, differences in <i>δ</i> ¹⁵N composition of leaves and roots are regularly found. In this paper we report results from a survey of nitrogen stable isotope signatures of leaves and roots of 16 plant species growing under natural conditions in a meadow and a forest understorey, which differed in nitrate and ammonium availability. Significant differences between leaf and root were observed. The range of <i>Δ</i> ¹⁵N [leaf-root] values was m 0.97 to +0.86‰, small compared to published values from controlled N-nutrition experiments, but almost as large as the range of leaf <i>δ</i> ¹⁵N values (m 1.04 to +1.08‰). Forbs showed the largest differences between leaves and roots and showed a significant difference with respect to habitat. Grasses and legumes did not show significant differences in <i>Δ</i> ¹⁵N [leaf-root] between the two habitats. Care must be taken when using leaf <i>δ</i> ¹⁵N values as representative for whole-plant ¹⁵N composition in these two habitats.

Models project that land ecosystems may be able take up a considerable proportion of the carbon dioxide released by human activities, thereby counteracting the anthropogenic emissions. In their Perspective, <a href="http://www.sciencemag.org/cgi/content/full/302/5650/1512"> Hungate <em>et al</em>.</a> argue that these carbon uptake estimates are too high because the models do not take other nutrients such as nitrogen into account appropriately. The authors estimate that there will not be enough nitrogen available to sustain the high carbon uptake scenarios. Nutrients other than nitrogen may also affect carbon uptake in ways not captured by most models.

This paper discusses the disturbances within and between ecosystems that result in element redistribution involving element transport in the biosphere, hydrosphere, lithosphere and atmosphere. It also discusses the disturbances (e.g. floods/landslides and vulcanism/dust storms) that redistribute and mobilize element in ratios that frequently differ from ecosystem stoichiometries, influence biogeochemical interactions on large spatial and temporal scales, and create long-lasting biological and biogeochemical legacies (e.g. widespread occurrence of N limitation of net primary production in terrestrial ecosystems). Tabulated data are given showing the effects of fire disturbance on element redistribution for a tropical forest under plantation management, for Mediterranean scrub forests, and for pine and <i>Eucalyptus</i> forests. The consequences (negative and positive feedback within ecosystem) of element redistribution are also presented. In addition, the limitations of disturbance assessment, learned views and challenges for the future are given.

Elevated carbon dioxide (CO₂) caused greater accumulation of carbon (C) and nutrients in both vegetation and O horizons over a 5-yr sampling period in a scrub oak ecosystem in Florida. Elevated CO₂ had no effect on any measured soil property except extractable phosphorus (P), which was lower with elevated CO₂ after five years. Anion and cation exchange membranes showed lower available nitrogen (N) and zinc (Zn) with elevated CO₂. Soils in both elevated and ambient CO₂ showed decreases in total C, N, sulfur (S), and cation exchange capacity, and increases in base saturation, exchangeable Ca²⁺, and Mg²⁺ over the 5-yr sampling period. We hypothesize that these soil changes were a delayed response to prescribed fire, which was applied to the site just before the initiation of the experiment. In the ambient CO₂ treatment, the increases in vegetation and O horizon C, N, and S were offset by the losses of soil total C, N, and S, resulting in no statistically significant net changes in ecosystem C, N, or S over time. In the elevated CO₂ treatment, the increases in vegetation and O horizon C content outweighed the losses in soil C, resulting in a statistically significant net increase in ecosystem C content. Nitrogen and S contents showed no statistically significant change over time in the elevated CO₂ treatment, however. Comparisons of vegetation contents and soil pools of potassium (K), calcium (Ca), and magnesium (Mg) suggest that a substantial proportion of these nutrients were taken up from either groundwater or deep soil horizons. This study demonstrates that changes in ecosystem C sequestration due elevated CO₂ or any other factor cannot be accurately assessed in the absence of data on changes in soils.

The effects of CO₂ elevation on the dynamics of fine root (FR) mass and ectomycorrhizal (EM) mass and colonization were studied in situ in a Florida scrub oak system over four years of postfire regeneration. Soil cores were taken at five dates and sorted to assess the standing crop of ectomycorrhizal and fine roots. We used ingrowth bags to estimate the effects of elevated CO₂ on production of EM roots and fine roots. Elevated CO₂ tended to increase EM colonization frequency but did not affect EM mass nor FR mass in soil cores (standing mass). However, elevated CO₂ strongly increased EM mass and FR mass in ingrowth bags (production), but it did not affect the EM colonization frequency therein. An increase in belowground production with unchanged biomass indicates that elevated CO₂ may stimulate root turnover. The CO₂-stimulated increase of belowground production was initially larger than that of aboveground production. The oaks may allocate a larger portion of resources to root/mycorrhizal production in this system in elevated rather than ambient CO₂.

Plant productivity and ecosystem productivity are strongly influenced by nutrient availability, which is largely determined by the decomposition rate of plant litter. Belowground litter inputs (dead roots, mycorrhizae, and exudates) are larger than aboveground litterfall in many systems. Chemical quality and diameter primarily control decomposition for coarse roots, but these patterns do not hold for finer classes of roots, which are frequently colonized by mycorrhizae. Though mycorrhizal status is known to drastically alter root chemistry, morphology, life span, and exudation, it has never been explicitly considered as a factor affecting root decomposition. We hypothesize that mycorrhizal status substantially influences fine root decomposition rates. Both ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) fungi can change root properties but do so in different ways. Dominant tree species of most cold and temperate forests rely heavily on EM associations. EM fungi form massive structures that envelop fine roots. Roots infected by ectomycorrhizae have higher nitrogen concentrations than nonmycorrhizal roots, which would be expected to increase decomposition rates, but much of this nitrogen is bound in recalcitrant forms, such as chitin, so the net effect on decomposition is difficult to predict. AM fungi lack elaborate, macroscopic structures and may not alter root chemistry as profoundly. In addition to mycorrhizal roots, external fungal hyphae can contribute significantly to ecosystem carbon budgets and thereby influence rates of soil carbon turnover. Hyphae have commonly been considered a rapidly decomposing carbon pool, though this has never been demonstrated experimentally. If hyphae are produced at the expense of rapidly decomposing root exudates, then the net effect of hyphal litter production might be to reduce soil microbial activity and overall carbon cycling rates. Based on known differences in morphology and chemistry, EM hyphae may be more recalcitrant than AM hyphae. In summary, we submit that mycorrhizal status could substantially influence fine root decomposition and soil carbon processing rates, potentially explaining some of the variation observed within and among individual plant species and ecosystems.

Elevated atmospheric carbon dioxide (C_a) usually reduces stomatal conductance, but the effects on plant transpiration in the field are not well understood. Using constant-power sap flow gauges, we measured transpiration from <em>Quercus myrtifolia</em> Willd., the dominant species of the Florida scrub-oak ecosystem, which had been exposed <em>in situ</em> to elevated C_a (350 µmol mol^<b>−</b>1 above ambient) in open-top chambers since May 1996. Elevated C_a reduced average transpiration per unit leaf area by 37%, 48% and 49% in March, May and October 2000, respectively. Temporarily reversing the C_a treatments showed that at least part of the reduction in transpiration was an immediate, reversible response to elevated C_a. However, there was also an apparent indirect effect of C_a on transpiration: when transpiration in all plants was measured under common C_a, transpiration in elevated C_a-grown plants was lower than that in plants grown in normal ambient C_a. Results from measurements of stomatal conductance (g_s), leaf area index (LAI), canopy light interception and correlation between light and g_s indicated that the direct, reversible C_a effect on transpiration was due to changes in g_s caused by C_a, and the indirect effect was caused mainly by greater self-shading resulting from enhanced LAI, not from stomatal acclimation. By reducing light penetration through the canopy, the enhanced self-shading at elevated C_a decreased stomatal conductance and transpiration of leaves at the middle and bottom of canopy. This self-shading mechanism is likely to be important in ecosystems where LAI increases in response to elevated C_a.

Methane consumption by temperate forest soils is a major sink for this important greenhouse gas, but little is known about how tree species influence CH₄ uptake by soils. Here, we show that six common tree species in Siberian boreal and temperate forests significantly affect potential CH₄ consumption in laboratory microcosms. Overall, soils under hardwood species (aspen and birch) consumed CH₄ at higher rates than soils under coniferous species and grassland. While NH₄⁺ addition often reduces CH₄ uptake, we found no effect of NH₄⁺ addition, possibly because of the relatively high ratio of CH₄-to-NH₄⁺ in our incubations. The effects of soil moisture strongly depended on plant species. An increase in soil moisture enhanced CH₄ consumption in soils under spruce but had the opposite effect under Scots pine and larch. Under other species, soil moisture did not affect CH₄ consumption. These results could be explained by specific responses of different groups of CH₄-oxidizing bacteria to elevated moisture.

The use of stable isotopes of N and O in N₂O has been proposed as a way to better constrain the global budget of atmospheric N₂O and to better understand the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N₂O formation in soil. This study compared the isotopic composition of N₂O emitted from soils under different tree species in the Brazilian Amazon. We also compared the effect of tree species with that of soil moisture, as we expected the latter to be the main factor regulating the proportion of nitrifier- and denitrifier-derived N₂O and, consequently, isotopic signatures of N₂O. Tree species significantly affected <i>δ</i> ¹⁵N in nitrous oxide. However, there was no evidence that the observed variation in <i>δ</i> ¹⁵N in N₂O was determined by varying proportions of nitrifier- vs. denitrifier-derived N₂O. We submit that the large variation in <i>δ</i> ¹⁵N-N₂O is the result of competition between denitrifying and immobilizing microorganisms for NO 3 m . In addition to altering <i>δ</i> ¹⁵N-N₂O, tree species affected net rates of N₂O emission from soil in laboratory incubations. These results suggest that tree species contribute to the large isotopic variation in N₂O observed in a range tropical forest soils. We found that soil water affects both ¹⁵N and ¹⁸O in N₂O, with wetter soils leading to more depleted N₂O in both ¹⁵N and ¹⁸O. This is likely caused by a shift in biological processes for ¹⁵N and possible direct exchange of ¹⁸O between H₂O and N₂O.