Production and nutrient dynamics of plant communities on a sub-antarctic island

Summary Investigations of the seasonal changes in vegetation standing crop have enabled an assessment of annual net primary production (ANP) at a fjaeldmark, open fernbrake and closed fernbrake at Marion Island (46°54S, 37°45E). These communities represent a successional sequence on relatively dry ridges and slopes on the island. Together, they are representative of a large proportion of the island's lowland (c. 300 m above sea level) vegetation. Aboveground ANP's were 728 g m^-2 y^-1 at closed fernbrake, 502 g m^-2 y^-1 at open fernbrake and 226 g m^-2 y^-1 at fjaeldmark. Total (above-plus below) ANP's were 1958 g m^-2 y^-1, 1578 g m^-2 y^-1 and 685 g m^-2 y^-1, respectively. These values are greater than those found for most tundra and tundra-like shrub and dwarf shrub-dominated communities of the northern hemisphere. The island's oceanic climate ensures a long growing season (c. 300 days for vascular plants, 365 days for bryophytes) and aboveground productivities for the island communities (including two mire-grasslands reported on previously), based on the length of the growing season, were 0.9 to 2.9 g m^-2 d^-1, lower than for most comparable shrub and dwarf shrub sub-Arctic or alpine communities and more similar to low Artic and low alpine sedge-moss and grass-herb communities. Production efficiencies (0.7% to 2.1% of photosynthetically active radiation) were in the range reported for northern hemisphere subpolar vegetations.     

Production and nutrient dynamics of plant communities on a sub-Antarctic Island

Summary Studies of plant standing crop and nutrient concentrations have enabled an assessment of the seasonal changes in nutrient standing stocks (the mass of nutrients per m²) in a fjaeldmark and two fernbrake communities on Marion Island (46°54S, 37°45E). These communities are an important component of the island's vegetation on rocky plateaux and slopes. For most species the aboveground accumulations of N, P and K early in the season were more rapid than increases in the aerial biomass. Rates of Ca, Mg or Na accrual were either similar to, or lower than, rates of aboveground growth. Nutrient (N+P+K+Ca+Mg+Na) standing stocks at the three communities were high; 71 g m^-2 at fjaeldmark, 116 g m^-2 at open fernbrake and 154 g m^-2 at closed fernbrake. The aboveground component accounted for 47% to 65% of these values. N was the most abundant element in the vegetation, followed by K (closed fernbrake) or Ca (open fernbrake and fjaeldmark). Nutrient standing stocks at the two fernbrakes were mostly higher than for most sub-Arctic and alpine dwarf-shrub tundras. Nutrient pool sizes (i.e. the total quantities of nutrients contained in the soil/plant system to a depth of 25 cm) were lower than those reported for arctic tundra meadows but were similar to, and often greater than, those found at heath communities, sub-Arctic dry meadows and dwarf-shrub tundras and some boreal forests. Annual net primary productions of the fernbrake vegetations were high and substantial quantities of nutrients are aquired annually from the soils by the vegetations. Depending on plant species, either N or K was the element taken up in the largest quantity, whereas P was mostly taken up in the lowest amount. A large proportion (mostly all) of the Ca and Mg and a substantial proportion of the N taken up aboveground was lost in the litterfall but little of K taken up was lost in this way.     

Symbiotic N2-fixation in alpine tundra: ecosystem input and variation in fixation rates among communities

Annual inputs of symbiotic N₂-fixation associated with 3 species of alpine Trifolium were estimated in four alpine communities differing in resource supplies. We hypothesized that fixation rates would vary according to the degree of N, P, and water limitation of production, with the higher rates of fixation in N limited communities (dry meadow, moist meadow) and lower rates in P and water limited communities (wet meadow, fellfield). To estimate N₂-fixation rates, natural abundance of N isotopes (¹⁵N) were measured in field collected Trifolium and reference plants and in Trifolium plants grown in N-free medium in a growth chamber. All three Trifolium species relied on a large proportion of atmospherically-fixed N₂ to meet their N requirements, ranging from 70 to 100%. There were no apparent differences in the proportion of plant N derived from fixation among the communities, but differences in the contribution of the Trifolium species to community cover resulted in a wide range of annual N inputs from fixation, from 127 mg m^–2 year^–1 in wet meadows to 810 mg m^–2 year^–1 in fellfields. Annual spatially integrated input of symbiotic N₂-fixation to Niwot Ridge, Colorado was estimated at 490 mg m^–2 year^–1 (5 kg ha^–1 year^–1), which is relatively high in the context of estimates of net N mineralization and N deposition.     

Production and nutrient dynamics of plant communities on a sub-Antarctic island

Summary Studies of plant standing crop and nutrient concentrations have enabled an assessment of the seasonal changes in nutrient standing stocks (the mass of nutrients per m²) in two mire-grasslands at Marion Island (46°54S, 37°45E). Mire-grasslands are an important component of the island's vegetation, occurring on very wet peats and dominated by graminoids and bryophytes. Peak aboveground standing stocks of N, P and K in the vascular plant species of the mire-grasslands mostly occurred earlier in the season than did peak aboveground biomass, implying that aboveground accumulation rates of these nutrients were greater than the rate of biomass accumulation. Maximum Ca standing stocks coincided in the season with peak shoot biomass. Depending on the plant species, peak Mg and Na standing stocks occurred either before, or later than, peak shoot biomass. Total (above-plus belowground) standing stocks of nutrients (N+P+K+Ca+Mg+Na) at the time of peak aboveground biomass were 51 g m^-2 at study mire 1 and 44 g m^-2 at study mire 2. The most abundant element in the vegetation was N, followed by K. The net quantities of most nutrients translocated into the aboveground growth were mostly greater than the seasonal mean standing stocks in the aerial biomass. Except for Ca, nutrient standing stocks in the vegetation of the mire-grasslands are in the upper part of the range reported for sub-Arctic and Arctic graminoid communities. They are more similar to standing stocks at oceanic moorlands, montane grasslands and heath communities. Low Ca concentrations occur in the plants so that Ca standing stocks are lower than in most comparable northern hemisphere communities. Pool sizes (i.e. total quantities contained in the plant/soil system to a depth of 25 cm) of N, P, K and Ca are in the lower part of the range reported for wet, graminoid-dominated tundra and tundra-like communities of the northern hemisphere.     

Plant responses to nutrient addition and predictive ability of vegetation N:P ratio in an austral fen

1. Previous studies of the N:P ratio in wetland plants have been carried out in northern hemisphere wetlands where atmospheric nitrogen deposition is higher. There is little research on foliar N:P ratio as a potential indicator of nutrient limitation in vegetation communities in southern hemisphere wetlands. This study aimed to redress this knowledge gap and answer the following questions: how well does the plant tissue nitrogen to phosphorus (N:P) ratio predict wetland plant community nutrient limitation, as indicated by vegetation standing stocks and below-ground biomass, in southern hemisphere fens? Secondly, what are the impacts of realistic upper levels of farm nutrient run-off on natural montane fen vegetation?
2. Low (35 kg ha⁻¹ year⁻¹) and high (70 kg ha⁻¹ year⁻¹) levels of nitrate-N or ammonium-N with and without P (20 kg ha⁻¹ year⁻¹) were added to 81 vegetation plots over a period of 2.75 years. Species composition, plant nutrient status, and above-ground live vegetation standing stocks were assessed after 3 years, and below-ground biomass after 2 years.
3. Plant tissue analysis suggested the community was N limited or N and P co-limited; we found greater standing stocks of vegetation in plots treated with 70 kg ha⁻¹ year⁻¹ ammonium-N, indicating N limitation. No difference between other treatments was found in above-ground standing stocks or below-ground biomass. Plant species cover increased in both high N treatments, consistent with N limitation. These changes in plant species cover were accompanied by significant decreases in species richness in both high N treatments. Native species dominated the vegetation and this was unaffected by nutrient addition (90% cover).
4. This is one of the first studies to test and find support for the N:P ratio in southern hemisphere wetlands. Observed declines in species richness after N fertilisation in an N-limited fen suggests increased N may pose risks to austral wetlands. Responses by plant communities (changes in composition, biomass) to lower levels of nutrient addition may require longer periods of fertilisation to be apparent in slow growing ecosystems.

     

Interactive effects of lettuce (Lactuca sativa L.), irradiance, and ferulic acid in axenic, hydroponic culture

Ferulic acid (FA) is released by living roots and by decaying plant material and is involved in chemical interactions between plants. Effects of FA on plant growth and root development of lettuce (Lactuca sativa L. cv. Grand Rapids) cultivated in axenic nutrient solution were studied in two factorial experiments. Root and shoot growth was impeded when 200 M trans-FA was added to the nutrient solution and the light intensity was in the range of 250–380 mol m^-2 s^-1. Root growth showed a stronger response to FA than did shoot growth. At 200 M, FA strongly inhibited root hair formation and reduced mean lengths of primary, secondary and tertiary roots, but stimulated primary and secondary root branching. Both isomerization to the cis isomer and the presence of the plant reduced the concentration of trans-FA in the nutrient solution during the two weeks exposure period. A third experiment was conducted to assess the influence of irradiance on the phytotoxicity of FA. At a light intensity of 489 mol m^-2 s^-1, or in the presence of microorganisms, the concentration of FA in the nutrient solution was lowered and the phytotoxic effects were reduced.     

Soil microbial counts in relation to site characteristics at a subantarctic Island

Enumerations of colony-forming units of aerobic bacteria and fungi at Marion Island (46°54S, 37°45E) showed that the mire and bog peats investigated possessed greater numbers of microorganisms on a dry soil weight basis than did the slope fernbrake soils but that counts from both sites were similar per unit soil volume. Afjaeldmark soil of an exposed rocky ridge contained very low microbial populations. Manuring by sea-birds and seals caused an increase in levels of soil N and P and in the numbers of bacteria and fungi. Regression analyses indicated that inorganic N and soil pH explained more of the variation in bacterial and fungal counts respectively than all other soil parameters included in the regression. Variation in botanical composition between the sites was examined using multivariate analysis and the resulting pattern compared with the soil microorganism and nutrient levels. This analysis emphasized the importance of manuring influences in elevating NH₄-N and bacterial levels in the soil and allowing the occurrence of a vegetation cover dominated by copriphilous species. No correlation occurred between site botanical factors and soil fungi counts.     

Natural and constructed littoral zones as nutrient traps in eutrophicated shallow lakes

It is generally known that the water quality of shallow lakes can be influenced significantly by marginal wetlands. In order to study the efficacy of constructed littoral wetlands in the IJsselmeer area (The Netherlands) for water quality improvement, a field survey was carried out in 2003. Vegetation, soil, pore water and surface water characteristics were measured in spring and summer in two types of littoral zones: natural and constructed for 8–16 years. The study showed that constructed wetlands perform well and are suitable to enlarge the vegetated littoral zone in the IJsselmeer area. In both natural and constructed sites vegetation biomass varied between 2,200 g m⁻² for helophyte vegetation and 1,300 g m⁻² for low herbaceous vegetation. Nutrient concentrations in the pore water of constructed sites tended to be higher than in natural sites. and concentrations in pore water were much lower when vegetation was present, probably as a result of plant uptake. The N and P accumulation rate in the soil of constructed wetlands was 20 g N m⁻² y⁻¹ and 3 g P m⁻² y⁻¹ in vegetated plots; without vegetation the rate was much lower (8 g N m⁻² y⁻¹ and 1.8 g P m⁻² y⁻¹). We conclude that concerning their effect on water quality, constructed sites may replace natural sites, at least after 8–16 years. Principal component analysis showed a relationship between vegetation biomass and flooding, and nutrient concentrations in soil and pore water. Biomass was negatively correlated with extractable nutrients and positively with soil total N and P content. Flooding duration was negatively related to pore water salinity and positively to pore water nutrients. Due to their high biomass, helophyte stands retained significantly more nutrients than low pioneer vegetation and are therefore more suitable for improving water quality. Handling editor: S. Declerck     

Nitrogen and phosphorus addition differentially enhance seed production of dominant species in a temperate steppe

Previous studies have demonstrated changes in plant growth and reproduction in response to nutrient availability, but responses of plant growth and reproduction to multiple levels of nutrient enrichment remain unclear. In this study, a factorial field experiment was performed with manipulation of nitrogen (N) and phosphorus (P) availability to examine seed production of the dominant species, Stipa krylovii, in response to N and P addition in a temperate steppe. There were three levels of N and P addition in this experiment, including no N addition (0 g N m⁻² year⁻¹), low N addition (10 g N m⁻² year⁻¹), and high N addition (40 g N m⁻² year⁻¹) for N addition treatment, and no P addition (0 g P m⁻² year⁻¹), low P addition (5 g P m⁻² year⁻¹), and high P addition (10 g P m⁻² year⁻¹) for P addition treatment. Low N addition enhanced seed production by 814%, 1371%, and 1321% under ambient, low, and high P addition levels, respectively. High N addition increased seed production by 2136%, 3560%, and 3550% under ambient, low, and high P addition levels, respectively. However, P addition did not affect seed production in the absence of N addition, but enhanced it under N addition. N addition enhanced seed production mainly by increasing the tiller number and inflorescence abundance per plant, whereas P addition stimulated it by decreasing the plant density yet stimulating height of plants and their seed number per inflorescence. Our results indicate seed production is not limited by P availability but rather by N availability in the temperate steppe, whereas seed production will be increased by P addition when N availability is improved. These findings enable a better understanding of plant reproduction dynamics in the temperate steppe under intensified nutrient enrichment and can inform their improved management in the future.     

Transpiration of a 64-year-old maritime pine stand in Portugal

Alpine plant species have been shown to exhibit a more pronounced increase in leaf photosynthesis under elevated CO₂ than lowland plants. In order to test whether this higher carbon fixation efficiency will translate into increased biomass production under CO₂ enrichment we exposed plots of narrow alpine grassland (Swiss Central Alps, 2470 m) to ambient (355 l l^-1) and elevated (680 l l^-1) CO₂ concentration using open top chambers. Part of the plost received moderate mineral nutrient additions (40 kg ha^-1 year^-1 of nitrogen in a complete fertilizer mix). Under natural nutrient supply CO₂ enrichment had no effect on biomass production per unit land area during any of the three seasons studied so far. Correspondingly, the dominant species Carex curvula and Leontodon helveticus as well as Trifolium alpinum did not show a growth response either at the population level or at the shoot level. However, the subdominant generalistic species Poa alpina strongly increased shoot growth (+47%). Annual root production (in ingrowth cores) was significantly enhanced in C. curvula in the 2nd and 3rd year of investigation (+43%) but was not altered in the bulk samples for all species. Fertilizer addition generally stimulated above-ground (+48%) and below-ground (+26%) biomass production right from the beginning. Annual variations in weather conditions during summer also strongly influenced above-ground biomass production (19–27% more biomass in warm seasons compared to cool seasons). However, neither nutrient availability nor climate had a significant effect on the CO₂ response of the plants. Our results do not support the hypothesis that alpine plants, due to their higher carbon uptake efficiency, will increase biomass production under future atmospheric CO₂ enrichment, at least not in such late successional communities. However, as indicated by the response of P. alpina, species-specific responses occur which may lead to altered community structure and perhaps ecosystem functioning in the long-term. Our findings further suggest that possible climatic changes are likely to have a greater impact on plant growth in alpine environments than the direct stimulation of photosynthesis by CO₂. Counter-intuitively, our results suggest that even under moderate climate warming or enhanced atmospheric nitrogen deposition positive biomass responses to CO₂ enrichment of the currently dominating species are unlikely.     

Photosynthetic apparatus in primary leaves of barley seedlings grown under blue or red light of very low photon flux densities

Chlorophyll (Chl) a and Chl b contents, rate of CO₂ gas exchange, quenching coefficients of chlorophyll fluorescence, and endogenous phytohormones have been studied in primary leaves of barley seedlings cultivated under blue (BL) or red (RL) light. Photon flux densities (PFD) were between 0.3 and 12 mol m^-2 s^-1. Plants grown at PFD of 0.3 mol m^-2 s^-1 demonstrated in BL tenfold and in RL threefold decreased Chl content compared to plants grown at 12 mol m^-2 s^-1. Chl a/b ratio increased from 2.3–2.5 to 4.4–4.5 in BL, not in RL, following the decrease in PFD at plant cultivation from 12 to 0.3 mol m^-2 s^-1. Plants cultivated at weak BL demonstrated severalfold decreased rate of photosynthetic CO₂ uptake, whereas decrease in PFD of RL from 12 to 0.3 mol m^-2 s^-1 caused only 20% de cline in the rate of photosynthesis. Decrease in PFD during a plant cultivation reduced the maximum quantum yield of photosynthesis in BL, not in RL leaves. Light response curves of non-photochemical and photochemical quenching of chlorophyll fluorescence calculated on the basis of absorbed quanta were not affected by PFD of RL during plant cultivation. On the contrary, both non-photochemical quenching and accumulation of Q_A ^-, reduced primary acceptor of Photosystem II, occurred at lower amounts of absorbed quanta in leaves of BL plants grown at 0.3 than at 12 mol m^-2 s^-1. Two photoregulatory reactions were suggested to exert the light control of the development of photosynthetic apparatus in the range of low PFDs. The photoregulatory reaction saturating by very low PFDs of RL was supposed to be mediated by phytochrome. Phytochrome was proposed to enhance (as related to other pigment-protein complexes of thylakoids) the accu mulation of chlorophyll- b-binding light-harvesting complex of Photosystem II (LHC II). It acts independently of the pigment mediating the second photoregulatory reaction, as evidenced by the results of experiments with plant growth under mixed blue plus red light. The contents of cytokinins and indole-3-acetic acid in a leaf were not significantly affected by either light quality and PFD thus indicating those phytohormones not to be involved into photoregulatory processes.     

Long‐term nutrient addition increases respiration and nitrous oxide emissions in a New England salt marsh

Salt marshes may act either as greenhouse gas (GHG) sources or sinks depending on hydrological conditions, vegetation communities, and nutrient availability. In recent decades, eutrophication has emerged as a major driver of change in salt marsh ecosystems. An ongoing fertilization experiment at the Great Sippewissett Marsh (Cape Cod, USA) allows for observation of the results of over four decades of nutrient addition. Here, nutrient enrichment stimulated changes to vegetation communities that, over time, have resulted in increased elevation of the marsh platform. In this study, we measured fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in dominant vegetation zones along elevation gradients of chronically fertilized (1,572 kg N ha^?1 year^?1) and unfertilized (12 kg N ha^?1 year^?1) experimental plots at Great Sippewissett Marsh. Flux measurements were performed using darkened chambers to focus on community respiration and excluded photosynthetic CO₂ uptake. We hypothesized that N‐replete conditions in fertilized plots would result in larger N₂O emissions relative to control plots and that higher elevations caused by nutrient enrichment would support increased CO₂ and N₂O and decreased CH₄ emissions due to the potential for more oxygen diffusion into sediment. Patterns of GHG emission supported our hypotheses. Fertilized plots were substantially larger sources of N₂O and had higher community respiration rates relative to control plots, due to large emissions of these GHGs at higher elevations. While CH₄ emissions displayed a negative relationship with elevation, they were generally small across elevation gradients and nutrient enrichment treatments. Our results demonstrate that at decadal scales, vegetation community shifts and associated elevation changes driven by chronic eutrophication affect GHG emission from salt marshes. Results demonstrate the necessity of long‐term fertilization experiments to understand impacts of eutrophication on ecosystem function and have implications for how chronic eutrophication may impact the role that salt marshes play in sequestering C and N.     

Deep rooting and global change facilitate spread of invasive grass

Abiotic global change factors, such as rising atmospheric CO₂, and biotic factors, such as exotic plant invasion, interact to alter the function of terrestrial ecosystems. An invasive lineage of the common reed, Phragmites australis, was introduced to North America over a century ago, but the belowground mechanisms underlying Phragmites invasion and persistence in natural systems remain poorly studied. For instance, Phragmites has a nitrogen (N) demand higher than native plant communities in many of the ecosystems it invades, but the source of the additional N is not clear. We exposed introduced Phragmites and native plant assemblages, containing Spartina patens and Schoenoplectus americanus, to factorial treatments of CO₂ (ambient or +300 ppm), N (0 or 25 g m^?2 year^?1), and hydroperiod (4 levels), and focused our analysis on changes in root productivity as a function of depth and evaluated the effects of introduced Phragmites on soil organic matter mineralization. We report that non-native invasive Phragmites exhibited a deeper rooting profile than native marsh species under all experimental treatments, and also enhanced soil organic matter decomposition. Moreover, exposure to elevated atmospheric CO₂ induced a sharp increase in deep root production in the invasive plant. We propose that niche separation accomplished through deeper rooting profiles circumvents nutrient competition where native species have relatively shallow root depth distributions; deep roots provide access to nutrient-rich porewater; and deep roots further increase nutrient availability by enhancing soil organic matter decomposition. We expect that rising CO₂ will magnify these effects in deep-rooting invasive plants that compete using a tree-like strategy against native herbaceous plants, promoting establishment and invasion through niche separation.     

Enhancement of short-term nitrogen uptake by greenhouse roses under intermittent N-deprivation

A recirculating nutrient solution system was utilized to study the effect of intermittent N deprivation on N uptake by mature Royalty rose over the course of one flowering cycle.Plants received a nutrient solution lacking N for 4, 8 or 16 days, after which one containing NO₃–N (0.75 mM) was supplied for 4 days. N-deprivation resulted in a 2–3 fold increase in N uptake rate compared to control plants supplied continuously with N. The magnitude of this deprivation-enhanced N uptake was not affected by either the duration of N-deprivation or the plant developmental stage. Over the course of the flowering cycle, the total cumulative N uptake by the plants was 95, 66, and 44% of the control plants in the 4, 8 and 16-day deprivation treatments, respectively. A characteristic diurnal pattern of N uptake occurred in both N-starved and control plants. Uptake oscillated between minimum rates in the morning and maximum rates in the evening, the latter occurring 4–6 hr after the maximum rate of transpiration.     

Carbon-Degrading Enzyme Activities Stimulated by Increased Nutrient Availability in Arctic Tundra Soils

Climate-induced warming of the Arctic tundra is expected to increase nutrient availability to soil microbes, which in turn may accelerate soil organic matter (SOM) decomposition. We increased nutrient availability via fertilization to investigate the microbial response via soil enzyme activities. Specifically, we measured potential activities of seven enzymes at four temperatures in three soil profiles (organic, organic/mineral interface, and mineral) from untreated native soils and from soils which had been fertilized with nitrogen (N) and phosphorus (P) since 1989 (23 years) and 2006 (six years). Fertilized plots within the 1989 site received annual additions of 10 g N⋅m^-2⋅year^-1 and 5 g P⋅m^-2⋅year^-1. Within the 2006 site, two fertilizer regimes were established – one in which plots received 5 g N⋅m^-2⋅year^-1 and 2.5 g P⋅m^-2⋅year^-1 and one in which plots received 10 g N⋅m^-2⋅year^-1 and 5 g P⋅m^-2⋅year^-1. The fertilization treatments increased activities of enzymes hydrolyzing carbon (C)-rich compounds but decreased phosphatase activities, especially in the organic soils. Activities of two enzymes that degrade N-rich compounds were not affected by the fertilization treatments. The fertilization treatments increased ratios of enzyme activities degrading C-rich compounds to those for N-rich compounds or phosphate, which could lead to changes in SOM chemistry over the long term and to losses of soil C. Accelerated SOM decomposition caused by increased nutrient availability could significantly offset predicted increased C fixation via stimulated net primary productivity in Arctic tundra ecosystems.     

Herbicide management of invasive cattail (<Emphasis Type="Italic">Typha</Emphasis> × <Emphasis Type="Italic">glauca</Emphasis>) increases porewater nutrient concentrations

Invasive wetland plants are the primary targets of wetland management to promote native communities and wildlife habitat, but little is known about how commonly implemented restoration techniques influence nutrient cycling. We tested how experimental mowing, herbicide application, and biomass harvest (i.e., removal of aboveground biomass) treatments of Typha-invaded mesocosms altered porewater nutrient (NO₃ ^?, NH₄ ⁺, PO ₄ ^?3 ) concentration and supply rate, vegetation response, and light penetration to the soil surface. We found that while herbicide application eliminated the target species, it also reduced native plant density and biomass, as well as increased porewater nutrient concentration (PO ₄ ^?3 , NO₃ ^?) and supply rates (N, P, K) up to a year after treatments were implemented. Because herbicide application promotes nutrient enrichment, it may increase the likelihood of reinvasion by problematic wetland invaders, as well as cause eutrophication and deleterious algal blooms in adjacent aquatic systems. Our data suggest that biomass harvest should be considered by managers aiming to reduce Typha abundance without eradicating native diversity, avoid nutrient leaching, as well as possibly utilizing biomass for bioenergy.     

Nitrogen dynamics in alpine ecosystems of the northern Caucasus

Net N mineralization, nitrification, microbial biomass N and ¹⁵N natural abundance were studied in a toposequence of representative soils and plant communities in the alpine zone of the northern Caucasus. The toposequence was represented by (1) low-productive alpine lichen heath (ALH) of wind-exposed ridge and upper slope; (2) more productive Festuca varia grassland (FG) of middle slope; (3) most productive Geranium gymnocaulon/Hedusarum caucasicummeadow (GHM) of lower slope; (4) low-productive snowbed community (SBC) of the slope bottom. N availability, net N mineralization and nitrification were higher in soils of alpine grassland and meadow of the middle part of the toposequence compared with soils of lichen heath and snowbed community of extreme habitats in the alpine zone. There was no correlation between intensities of N transformation processes and favorable (low soil acidity, low C/N ratio, long vegetation period, relatively high temperature, absence of hydromorphic features) and unfavorable (opposite) factors, indicating that the intensity of N mineralization and nitrification in the alpine soils is controlled by a complex combination of these factors. Potential net N mineralization and nitrification in alpine soils determined in the short-term laboratory incubation were considerably higher than those determined in the long-term field incubation. The differences of potential nitrification between soils of various plant communities did not correspond to the field determined pattern indicating the importance of on-site climatic conditions for control of nitrification in high mountains. The result of comparison of N transformation potentials in incubated and native soils indicated that nitrification potential was significantly increased after long-term soil incubation. It means that net nitrification determined in the field was probably overestimated, especially in the meadow soils. A soil translocation experiment indicated that low temperature was an important factor limiting net N mineralization and nitrification in alpine soils: net N mineralization and especially nitrification increased when alpine soils were translocated into the subalpine zone and mean annual temperature increased by about 3°C. Additional N input increased N availability (NH₄ ⁺-N) and potential nitrification in soils of the lower part of the toposequense (GHM and SBC), and potential net N mineralization in two soils of extreme habitats (ALH and SBC). A positive correlation was found between soil ¹⁵N and net N mineralization and nitrification; the relative ¹⁵N enrichment was characteristic of grassland and meadow ecosystems. ¹⁵N of total soil N pool increased during the field mineralization experiment; there was a positive tendency between the change in ¹⁵N and net N mineralization and nitrification, however the relationship was not significant. Foliar ¹⁵N of dominant plant species varied widely within community, however, a tendency of higher foliar ¹⁵N for species growing on the soils with higher net N mineralization, nitrification and ¹⁵N was observed.     

Turbulent breakage of protein precipitates in mechanically stirred bioreactors

Experimental data relating to the breakage of isoelectric Soya protein precipitates in a mechanically agitated bioreactor are provided and examined in the light of a proposed mechanistic model which relates the size of the maximum attainable aggregate diameter to the energy dissipation rate in the vessel. The analysis suggests that protein precipitation results in the formation of scale-invariant fractal aggregates with a dimensionality of 2.2. Comparing the fractal dimensionality of the protein precipitates with reported values based on computer simulation studies suggests that the aggregates undergo considerable restructuring during agitation.List of Symbols A Hamaker constant (J) - D impeller diameter (m) - d _p primary particle diameter (m) - d _f maximum aggregate diameter (m) - G shear rate (s^–1) - H ₀ separation distance between two primary particles (m) - k constant in Eq. (5) - K constant in Eq. (6) - N impeller speed (rpm or rps) - r radial position in an aggregate, measured from the centre (m) - t time of exposure to shear (mins) - T _e eddy period (s^–1) - v _f aggregate volume (m³) Greek Symbols aggregate dimensionality constant - energy dissipation rate (W/kg) - dynamic viscosity of particle-free liquid (kg/ms) - kinematic viscosity of particle-free liquid (m²/s) - collision probability (–) - _p aggregate density (kg/m³) - _p continuous phase density (kg/m³) - aggregate mechanical strength (N/m²) - shear stress (N/m²) - particle concentration in an aggregate (m³/m³) - (r) porosity at radial position, r     

Nutrient and water dynamics of Amazonian <Emphasis Type="Italic">canga</Emphasis> vegetation differ among physiognomies and from those of other neotropical ecosystems

Ferriferous savannas, also known as cangas in Brazil, are nutrient-impoverished ecosystems adapted to seasonal droughts. These ecosystems support distinctive vegetation physiognomies and high plant diversity, although little is known about how nutrient and water availability shape these ecosystems. Our study was carried out in the cangas from Carajás, eastern Amazonia, Brazil. To investigate the N cycling and drought adaptations of different canga physiognomies and compare the findings with those from other ecosystems, we analyzed nutrient concentrations and isotope ratios (δ¹³C and δ¹⁵N) of plants, litter, and soils from 36 plots distributed in three physiognomies: typical scrubland (SB), Vellozia scrubland (VL), and woodland (WD). Foliar δ¹⁵N values in cangas were higher than those in savannas but lower than those in tropical forests, indicating more conservative N cycles in Amazonian cangas than in forests. The lower δ¹⁵N in savanna formations may be due to a higher importance of mycorrhizal species in savanna vegetation than in canga vegetation. Elevated δ¹³C values indicate higher water shortage in canga ecosystems than in forests. Foliar and litter nutrient concentrations vary among canga physiognomies, indicating differences in nutrient dynamics. Lower nutrient availability, higher C:N ratios, and lower δ¹⁵N values characterize VL, whereas WD is delineated by lower δ¹³C values and higher soil P. These results suggest lower water restriction and lower P limitation in WD, whereas VL shows more conserved N cycles due to lower nutrient availability. Differences in nutrient and water dynamics among physiognomies indicate different ecological processes; thus, the conservation of all physiognomies is required to ensure the maintenance of functional diversity in this unique ecosystem.     

Productivity and water relations of burnt and long-unburnt semi-arid shrubland in Western Australia

This study provides a comparison between vegetation of relatively recent and long-unburnt shrubland in terms of structural and functional groups, annual net primary productivity and water relations. Adjacent areas of vegetation long-unburnt or burnt 5 years previously were compared within a remnant block of Acacia–Allocasuarina–Melaleuca arid shrubland at Kalannie, south west Western Australia. Species were classified according to growth and life form, fire response, phenology and rooting morphology and densities, mean plant above-ground dry weights and shoot:root dry mass ratios of each assessed. Species compositions, seedling densities and absence of recruitment in the long-unburnt area suggested marked dependence on fire in maintenance of biodiversity. Comparisons of above-ground standing dry biomass and annual net primary productivity of total (above-ground plus below-ground) dry matter showed the 4.09 kg m^–2 biomass of long-unburnt vegetation to be increasing at 0.52 kg m^–2 year^–1 versus 0.45 and 0.18 kg m^–2 year^–1 for vegetation of the burnt area. Water relations of soils indicated consistently wetter profiles in burnt than long-unburnt areas and no deep drainage during the year of study. Lower water stress of key species in burnt than long-unburnt areas were indicated by less negative pre-dawn water potentials and higher stomatal conductance during the year of study and more negative carbon-isotope composition (¹³C) in wood laid down over the past 5 years. Budgets for water use were estimated for both sites and compared with annual net primary productivities. Data suggested much greater transpiration loss per unit dry matter gain by the rapidly growing plants at the burnt site (437 ml H₂O g^–1 DM) than by the plants of the long-unburnt community (92 ml H₂O g^–1 DM). Results are discussed in relation to composition and functioning of other Western Australian ecosystems. It is clear that time since fire affects productivity and water-use of vegetation of semi-arid shrublands and is therefore an important consideration for management and protection of remnant vegetation.