Shrub establishment under experimental global changes in a California grassland

Accelerating invasion of grasslands by woody species is a widespread global phenomenon. The native shrub Baccharis pilularis has recently increased in abundance in some California grasslands, with large local community and ecosystem effects. I investigated potential contributions of (1) future global climate and atmospheric changes and (2) variation in moisture and nutrient availability to increased Baccharis germination and early establishment rates. I examined responses of Baccharis seeds and seedlings to simulated warming (+ 1−2 °C) and elevated CO₂ (+ 300 ppm) in a 2-year field experiment. Warming and CO₂ treatments were applied at ambient and increased water and nitrogen levels chosen to simulate future increases in precipitation (+ 50%) and N deposition (+ 7 gN m⁻² y⁻¹). Elevated CO₂ and water addition each increased or accelerated germination. Herbivory strongly reduced seedling populations during the winter wet season; drought further reduced seedling survival in the spring. Overall Baccharis survivorship was extremely low (<0.1%) across all treatments, complicating the interpretation of global change effects.     

Transition from P‐ to N‐limited phytoplankton growth in an artificial lake on flooded cutaway peatland in Ireland

Question: Which nutrient limits primary production in a lake created by flooding industrial cutaway peatland? Location: Clongawny Lake (53°10’N, 07°53’W), County Offaly, Ireland Methods: Nutrient concentrations in lake water and the dynamics of phytoplankton populations were monitored over a 38‐month period. The ratio of dissolved inorganic nitrogen to total phosphorus (DIN:TP) and nutrient enrichment bio‐assays were used to investigate temporal changes in nutrient limitation. Results: Primary production in the new lake was phytoplankton‐driven due to the scarcity of recolonizing macrophytes. Phytoplankton growth was initially phosphorus‐limited. The runoff of phosphate fertilizer from an adjacent coniferous forestry plantation raised the TP concentration of lake water 5.5‐fold. Consequently, the biovolume of phytoplankton increased 30‐fold, and chlorophyll‐a concentrations increased eightfold, reaching hyper‐eutrophic levels. A concurrent depletion of nitrogen in lake water reduced the DIN:TP ratio from 17.8 to 0.6, and phytoplankton growth rapidly became nitrogen‐limited. Phytoplankton composition shifted from dinoflagellates to minute, unicellular chlorophytes, with a coincident decline in species diversity. Cyanobacteria did not proliferate, most likely due to the acidic nature of the lake. Conclusions: Results illustrated the vulnerability of newly created cutaway peatland lakes to developing severe phytoplankton blooms and coincident secondary nitrogen limitation in the presence of moderate external phosphorus inputs.     

Biomass and photosynthetic productivity of water hyacinth (Eichhornia crassipes) as affected by nutrient supply and mirid (Eccritotarus catarinensis) biocontrol

The biological control of water hyacinth is affected by water nitrogen and phosphorus content and this was investigated experimentally at five levels of nutrient supply by measuring plant photosynthetic and growth responses, and mirid reproduction and herbivory of nutrient treated plants. Low nitrogen (2–0.2 mg L⁻¹) and phosphorus (0.2–0.01 mg L⁻¹) supply decreased hyacinth photosynthesis, growth and biomass accumulation relative to plants supplied 200 mg L⁻¹ N and 20 mg L⁻¹ P. This effect depended more on nitrogen supply than phosphorus supply. Chlorophyll fluorescence showed that the photosynthetic light reactions of low nutrient plants were affected and leaves had decreased chlorophyll content, density of functional photosystems II and dissipated a greater proportion of absorbed energy as heat. Gas exchange parameters showed reduced carboxylation efficiency, rates of RuBP regeneration and light saturated photosynthetic rates, but not quantum yields. Effects on photosynthesis translated into lower plant dry biomass. Mirid herbivory exacerbated the effects of low nutrients noted for chlorophyll fluorescence, gas exchange parameters and biomass accumulation, however, these effects were not always significant and there was no obvious correlation between the level of nutrients supplied and the effect of mirid herbivory. Low nutrient supply did, however, affect mirid performance reducing the number of adult insects, nymphs and herbivory intensity suggesting that in the long-term mirid populations would be significantly affected by water nutrient status.     

Growth and nitrogen fixation ofAeschynomene under water stressed conditions

Summary Studies on the tolerance ofAeschynomene americana L. to periods of flooding or soil moisture deficit were conducted in an attempt to elucidate nitrogen fixation as affected by soil moisture. Nitrogenase activity was not reduced significantly in pot-grown Aeschynomene plants subjected to flooding in greenhouse conditions. After 20 days of withholding water from the soil, nitrogenase activities of the drought-stressed plants were much lower than those of either the well-watered or flooded plants. Leaf water potentials were similar in flooded and control plants; however, the droughted plants had leaf water potentials that were 4 bars lower than those of the control plants. Aeschynomene plants were tolerant to long-term periods of flooding, but exhibited a reduction in nitrogenase activity and leaf water status when subjected to soil moisture deficits.     

Exercise-induced hypoxaemia in highly trained cyclists at 40% peak oxygen uptake

A group of 15 competitive male cyclists [mean peak oxygen uptake, VO2peak 68.5 (SEM 1.5 ml x kg(-1) x min(-1))] exercised on a cycle ergometer in a protocol which began at an intensity of 150 W and was increased by 25 W every 2 min until the subject was exhausted. Blood samples were taken from the radial artery at the end of each exercise intensity to determine the partial pressures of blood gases and oxyhaemoglobin saturation (SaO2), with all values corrected for rectal temperature. The SaO2 was also monitored continuously by ear oximetry. A significant decrease in the partial pressure of oxygen in arterial blood (PaO2) was seen at the first exercise intensity (150 W, about 40% VO2peak). A further significant decrease in PaO2 occurred at 200 W, whereafter it remained stable but still significantly below the values at rest, with the lowest value being measured at 350 W [87.0 (SEM 1.9) mmHg]. The partial pressure of carbon dioxide in arterial blood (PaCO2) was unchanged up to an exercise intensity of 250 W whereafter it exhibited a significant downward trend to reach its lowest value at an exercise intensity of 375 W [34.5 (SEM 0.5) mmHg]. During both the first (150 W) and final exercise intensities (VO2peak) PaO2 was correlated significantly with both partial pressure of oxygen in alveolar gas (P(A)O2, r = 0.81 and r = 0.70, respectively) and alveolar-arterial difference in oxygen partial pressure (P(A-a)O2, r = 0.63 and r = 0.86, respectively) but not with PaCO2. At VO2peak PaO2 was significantly correlated with the ventilatory equivalents for both oxygen uptake and carbon dioxide output (r = 0.58 and r = 0.53, respectively). When both P(A)O2 and P(A-a)O2 were combined in a multiple linear regression model, at least 95% of the variance in PaO2 could be explained at both 150 W and VO2peak. A significant downward trend in SaO2 was seen with increasing exercise intensity with the lowest value at 375 W [94.6 (SEM 0.3)%]. Oximetry estimates of SaO2 were significantly higher than blood measurements at all times throughout exercise and no significant decrease from rest was seen until 350 W. The significant correlations between PaO2 and P(A)O2 with the first exercise intensity and at VO2peak led to the conclusion that inadequate hyperventilation is a major contributor to exercise-induced hypoxaemia.     

Initial decay of woody fragments in soil is influenced by size, vertical position, nitrogen availability and soil origin

Fast-growing bacteria and fungi are expected to cause the initial stage of decomposition of woody fragments in and on soils, i.e. the respiration of sugars, organic acids, pectin and easily accessible cellulose and hemi-cellulose. However, little is known about the factors regulating initial wood decomposition. We examined the effect of wood fragment size, vertical position, nitrogen addition and soil origin on initial wood decay and on the relative importance of fungi and bacteria therein. Two fractions of birch wood were used in microcosm experiments, namely wood blocks (dimensions: 3 × 0.5 × 0.5 cm) and sawdust (dimensions: 0.5–2 mm). The woody fragments were enclosed in nylon bags and placed on top of- or buried in an abandoned arable soil and in a heathland soil. After 15, 25 and 40 weeks of incubation, fungal biomass was quantified (as ergosterol and chitin content) and bacterial numbers were determined. The results indicated that initial wood decay was mostly caused by fungi; bacteria were only contributing in sawdust in/on abandoned arable soil. Larger fragment size, burial of fragments and nitrogen addition positively influenced fungal biomass and activity. Fungal biomass and decay activities were much lower in woody fragments incubated in/on heathland soil than in those incubated in/on abandoned arable soil, indicating that soil origin is also an important factor determining initial wood decay.     

Conversion of the nitrogen content in liquid manure into biomass and polyglutamic acid by a newly isolated strain of Bacillus licheniformis

Extensive spreading of liquid manure onto agricultural fields causes eutrophication of ground and surface water and also pollution of the atmosphere due to the high ammonium nitrogen content. A poly(gamma-glutamic acid) (PGA)-producing strain of Bacillus licheniformis was isolated in this study and investigated for its ability to reduce the ammonium nitrogen by converting ammonium into biomass and PGA as depot forms of nitrogen. In batch cultivations swine manure and an optimized mineral salts medium were used for PGA production. For example the cultivation of B. licheniformis strain S2 in liquid manure, which was modified by adding of 18 g citrate and 80 g glycerol l(-1) and exhibited a carbon to nitrogen ratio of 15.5:1, led to severe reduction of the ammonium content from 2.83 to 0.1 g x l(-1) and to the production of 0.16 g PGA and 7.5 g cell dry mass l(-1) within 410 h. Approximately 28% (w/w) of the total nitrogen was converted into cellular biomass, whereas 0.1% (w/w) was used for the production of PGA. In addition, approximately 33% (w/v) of the original ammonium was lost by stripping.     

Reproductive allocation of an annual,<Emphasis Type="Italic"> Xanthium canadense</Emphasis>, at an elevated carbon dioxide concentration

Stimulation of vegetative growth by an elevated CO₂ concentration does not always lead to an increase in reproductive yield. This is because reproductive yield is determined by the fraction of biomass allocated to the reproductive part as well as biomass production. We grew Xanthium canadense at low N (LN) and high N levels (HN) under an ambient (360 mol mol^-1) and elevated (700 mol mol^-1) CO₂ concentration ([CO₂]) in open-top chambers. Reproductive yield was analysed as the product of: (1) the duration of the reproductive period, (2) the rate of dry mass acquisition in the reproductive period, and (3) the fraction of acquired biomass allocated to the reproductive part. Elevated [CO₂] increased the total amount of biomass that was allocated to reproductive structures, but this increase was caused by increased capsule mass without a significant increase in seed production. The increase in total reproductive mass was due mainly to an increase in the rate of dry mass acquisition in the reproductive period with a delay in leaf senescence. This positive effect was partly offset by a reduction in biomass allocation to the reproductive part at elevated [CO₂] and HN. The duration of the reproductive period was not affected by elevated [CO₂] but increased by HN. Seed production was strongly constrained by the availability of N for seed growth. The seed [N] was very high in X. canadense and did not decrease significantly at elevated [CO₂]. HN increased seed [N] without a significant increase in seed biomass production. Limited seed growth caused a reduction in biomass allocation to the reproductive part even though dry mass production was increased due to increased [CO₂] and N availability.     

Functional correlates of leaf demographic response to gap release in saplings of a shade-tolerant tree,<Emphasis Type="Italic"> Elateriospermum tapos</Emphasis>

For a shade-tolerant SE Asian tropical tree, Elateriospermum tapos (Euphorbiaceae), we studied field-established saplings in gaps and the shaded understory to test the hypothesis that differences in leaf demography and leaf life span under contrasting light regimes should be functionally correlated with architecture, self-shading and nitrogen distribution within the sapling crown. Rates of leaf production and net leaf gain were greater for saplings in gaps than those in the understory. Median leaf life span was approximately 26 months in the gap saplings, while it was estimated to be greater than 38 months in the understory saplings. Consequently, gap saplings had a greater standing leaf number and experienced greater degrees of self-shading than understory saplings. Light availability at individual leaves, estimated by a combination of canopy photos and a three-dimensional architecture model, were negatively correlated with leaf age in gap saplings but not so in understory saplings. Leaf nitrogen content per unit area (N_area) was influenced more by light availability than by leaf age in the gap saplings. In contrast, in understory saplings, N_area was neither correlated with light availability nor with leaf age, and did not decrease significantly before 38 months in leaf age. We conclude that saplings of this shade-tolerant species apparently prolong their leaf life span in the shaded understory through slower rates of leaf production, lower standing number of leaves and lower degrees of self shading than in gap, and that the rate of decline of N_area with leaf age depends on architecture and self-shading regimes that respond to changes in light regimes.     

Stimulation of soil nitrification and denitrification by grazing in grasslands: do changes in plant species composition matter?

Stimulation of nitrification and denitrification by long term (from years to decades) grazing has commonly been reported in different grassland ecosystems. However, grazing generally induces important changes in plant species composition, and whether changes in nitrification and denitrification are primarily due to changes in vegetation composition has never been tested. We compared soil nitrification- and denitrification-enzyme activities (NEA and DEA, respectively) between semi-natural grassland sites experiencing intensive (IG) and light (LG) grazing/mowing regimes for 13 years. Mean NEA and DEA (i.e. observed from random soil sampling) were higher in IG than LG sites. The NEA/DEA ratio was higher in IG than LG sites, indicating a higher stimulation of nitrification. Marked changes in plant species composition were observed in response to the grazing/mowing regime. In particular, the specific phytomass volume of Elymus repens was lower in IG than LG sites, whereas the specific volume of Lolium perenne was higher in IG than LG sites. In contrast, the specific volume of Holcus lanatus, Poa trivialis and Arrhenatherum elatius were not significantly different between treatments. Soils sampled beneath grass tussocks of the last three species exhibited higher DEA, NEA and NEA/DEA ratio in IG than LG sites. For a given grazing regime, plant species did not affect significantly soil DEA, NEA and NEA/DEA ratio. The modification of plant species composition is thus not the primary factor driving changes in nitrification and denitrification in semi-natural grassland ecosystems experiencing long term intensive grazing. Factors such as trampling, N returned in animal excreta, and/or modification of N uptake and C exudation by frequently defoliated plants could be responsible for the enhanced microbial activities.