Response of roselle (Hibiscus sabdariffa) to heavy metals contamination in soils with different organic fertilisations

The response of green roselle (Hibiscus sabdariffa) to Cu/Pb contamination and manure application in soil was investigated using pot experiments. Subsamples of a mineral soil were treated with increasing doses (0–500 mg kg^?1) of Cu/Pb only and/or amended (at 10% w/w) with poultry or swine manure. Roselle plants were grown, monitored for changes in growth rate and post-harvest aboveground dry biomass and tissue Cu/Pb concentrations were determined. The plants were typically greenish with linear growth profiles at all metal doses, indicating some level of tolerance. Dry biomass yields decreased as metal dose increased. Poultry manure enhanced roselle biomass yields better than swine manure. Tissue Cu/Pb concentrations increased linearly as metal doses increased in unamended soils; whereas nonlinear responses were observed in manure-amended soils. Soil-to-plant transfer factors, T _f (%) indicated that Cu (13≤T _f (% )≤60) was more phytoavailable to roselle than Pb (11≤T _f (% )≤20). Tissue metal concentrations were modelled from soil pH, organic matter, plant available and pseudototal metal; but the models appeared more reliable with plant available metal as a covariate than with pseudototal metal content. These observations may become useful whenever phytoextraction is the remedial option for soils moderately contaminated by toxic metals.     

Above- and belowground responses of Arctic tundra ecosystems to altered soil nutrients and mammalian herbivory

Theory and observation indicate that changes in the rate of primary production can alter the balance between the bottom-up influences of plants and resources and the top-down regulation of herbivores and predators on ecosystem structure and function. The exploitation ecosystem hypothesis (EEH) posited that as aboveground net primary productivity (ANPP) increases, the additional biomass should support higher trophic levels. We developed an extension of EEH to include the impacts of increases in ANPP on belowground consumers in a similar manner as aboveground, but indirectly through changes in the allocation of photosynthate to roots. We tested our predictions for plants aboveground and for phytophagous nematodes and their predators belowground in two common arctic tundra plant communities subjected to 11 years of increased soil nutrient availability and/or exclusion of mammalian herbivores. The less productive dry heath (DH) community met the predictions of EEH aboveground, with the greatest ANPP and plant biomass in the fertilized plots protected from herbivory. A palatable grass increased in fertilized plots while dwarf evergreen shrubs and lichens declined. Belowground, phytophagous nematodes also responded as predicted, achieving greater biomass in the higher ANPP plots, whereas predator biomass tended to be lower in those same plots (although not significantly). In the higher productivity moist acidic tussock (MAT) community, aboveground responses were quite different. Herbivores stimulated ANPP and biomass in both ambient and enriched soil nutrient plots; maximum ANPP occurred in fertilized plots exposed to herbivory. Fertilized plots became dominated by dwarf birch (a deciduous shrub) and cloudberry (a perennial forb); under ambient conditions these two species coexist with sedges, evergreen dwarf shrubs, and Sphagnum mosses. Phytophagous nematodes did not respond significantly to changes in ANPP, although predator biomass was greatest in control plots. The contrasting results of these two arctic tundra plant communities suggest that the predictions of EEH may hold for very low ANPP communities, but that other factors, including competition and shifts in vegetation composition toward less palatable species, may confound predicted responses to changes in productivity in higher ANPP communities such as the MAT studied here.     

加拿大一枝黄花和小飞蓬叶浸提液对大豆幼苗生长的复合化感效应

加拿大一枝黄花(Solidago canadensis)和小飞蓬(Conyza canadensis)常共同入侵至同一农田生态系统。探究了两者对大豆幼苗生长的复合化感作用。加拿大一枝黄花叶浸提液显著抑制大豆幼苗地上生物量。而小飞蓬叶浸提液也明显抑制大豆幼苗地上生物量(未达到显著水平)。与对照处理相比,两者复合叶浸提液处理未显著影响大豆幼苗地上生物量。两者复合叶浸提液处理下大豆幼苗地上生物量显著大于加拿大一枝黄花叶浸提液单一处理,同时也大于小飞蓬叶浸提液单一处理(未达到显著水平)。两者叶浸提液单一处理均明显降低大豆幼苗株高(未达到显著水平)。与对照处理相比,两者复合叶浸提液处理未显著影响大豆幼苗株高。两者复合叶浸提液处理下大豆幼苗株高明显大于两者叶浸提液单一处理(未达到显著水平)。因此,两者叶浸提液对大豆幼苗的生长均具有一定程度的抑制效应,且加拿大一枝黄花叶浸提液对大豆幼苗产生的化感作用(尤其是地上生物量)明显大于小飞蓬。两者复合化感作用明显低于两者单一化感作用,尤其是两者对大豆幼苗地上生物量的复合化感作用显著低于加拿大一枝黄花的单一化感作用。所以,化感效应可能在两者共同入侵(即两者复合作用)进程中所起的贡献低于在两者单一入侵进程中所起的作用。     

Impact of canal development on intertidal microalgal productivity: Comparative assessment of Patterson Lakes and Ralphs Bay,South East Australia

We assessed the potential impact of a proposed canal development in an estuarine sandflat at Ralphs Bay, Tasmania on intertidal microalgal productivity and species composition, by comparing it over summer and winter seasons with a well- established (30 year old) canal estate at Patterson Lakes, Victoria. Pulse amplitude modulation (PAM) fluorometry was used to generate a relative measure of photosynthetic performance, which combined with microalgal chlorophyll biomass and irradiance provides an assessment of potential primary productivity. We present a sophisticated mathematical model for calculating benthic microalgal production and the contribution to total primary production, taking into account sediment light attenuation as estimated from sediment grain size. Ralphs Bay had a total productive microalgal biomass of 44 mg chlorophyll a m⁻² which was six times higher than Patterson Lakes, while the relative productivity of Ralphs Bay was four times greater compared to Patterson Lakes where productivity was virtually absent in the subtidal zone of the canal waterway. Ralphs Bay exhibited a more or less homogeneous spatial distribution of microphytobenthos biomass but this was subject to some seasonal variation in species composition, abundance and productivity. By contrast, at Patterson Lakes biomass distribution, diversity and productivity was highly spatially variable in the canal system in both seasons. Patterson Lakes exhibited 60% lower microphytobenthos species richness than Ralphs Bay but little variation in species composition occurred between seasons in the canal estate. This suggests that the dominant diatom species in Patterson Lakes, Pinnularia yarrensis, Gyrosigma balticum and Pleurosigma salinarum, are well adapted to the disturbance regime within the canal estate. The proposed canal development at Ralphs Bay is estimated to cause a decrease in microalgal productivity by both reducing available marine substrate (66% reduction) and replacing productive intertidal phytobenthic habitat with nonproductive canal substrate. These combine to cause a decline in productivity of 92% with significant flow-on effects predicted for higher trophic levels such as migratory wading birds.     

Photosynthetic and growth responses of Japanese sasabamo (Potamogeton wrightii Morong) under different photoperiods and nutrient conditions

Controlled laboratory experiments were conducted to examine how photosynthesis and growth occur in Potamogeton wrightii Morong under different photoperiods and nutrient conditions. The experiment was based on a 3×2 factorial design with three photoperiods (16, 12 and 8 h) of 200 μE · m^?2·s^?1 irradiance and two nutrient conditions, high (90 μmol N · L^?1·d^?1 and 9 μmol P · L^?1·d^?1) and low (30 μmol N L^?1·d^?1 and 3 μmol P · L^?1·d^?1). After 14, 28, 56 and 70 days of growth, plants were harvested to determine net photosynthesis rate and various growth parameters. Above- and below-ground biomass were investigated on days 56 and 70 only. Plants under low nutrient conditions had greater leaf area, more chlorophyll a, a higher rate of net photosynthesis and accumulated more above- and below-ground biomass than plants in the high nutrient condition. Plants with an 8 h photoperiod in the low nutrient condition had a significantly higher rate of net photosynthesis, whereas 8 h photoperiod plants in the high nutrient condition had a lower rate of net photosynthesis and their photosynthetic capacity collapsed on day 70. We conclude that P. wrightii has the photosynthetic plasticity to overcome the effects of a shorter photoperiod under a tolerable nutrient state.     

Photosynthetic studies of Chondrus crispus

The net photosynthesis of intertidal, subtidal, carposporic, tetrasporic, and winter versus summer acclimatized plants of Chondrus crispus Stackhouse were evaluated under different temperatures and quantities of light. The optimum temperature and light conditions for net photosynthesis of C. crispus are seasonally and spatially variable, and there is an adaptive shift in the photosynthetic capacity at different seasons and positions on the shore. Plants collected during the fall and winter had lower light optima (465 to 747 ft-c) for net photosynthesis than spring and summer specimens (about 1000 ft-c). Intertidal populations exhibited a higher rate of net photosynthesis between 250 and 2819 ft-c than subtidal plants. Summer materials have a greater tolerance to high temperatures and a higher temperature optimum than winter materials. Shallow subtidal populations (-6m) exhibited a higher temperature optimum than deep subtidal plants (-12m). Tetrasporic plants (diploid) showed a higher rate of net photosynthesis than carposporic plants (haploid). It is suggested that the diploid plants of C. crispus may extend deeper in the subtidal zone, because they have a higher rate of net photosynthesis than carposporic plants. The results of the present studies are compared with previous physiological studies of C. crispus.Published with the approval of the Director of the New Hampshire Agriculture Experiment Station as Scientific Contribution Number 742.     

Disturbance by ice and life-history adaptations of the seagrassZostera marina

The formation of winter ice, and its movement with the tides, has had a major influence on the life-history parameters of shallow-water populations of the rhizomatous marine angiosperm eelgrass (Zostera marina) in Nova Scotia, Canada. In this region, and annual form of eelgrass inhabits intertidal and shallow subtidal mudflats and a perennial form is common in subtidal areas. Where lowtide water depth was greater than winter ice thickness (25 cm), ice movement removed much of the aboveground biomass from perennial ramets, but did not influence the density of ramets. Measurements of primary production showed that perennial plants allocated a greater proportion of their total production to below-ground structures than measured in all previous studies on eelgrass. Thus perennials are well anchored in the sediment, are more able to withstand removal by ice, and have reserves available for production of new leaves when ice melts. In the spring, seed germination was greatest in areas where ice had removed whole plants (or their aboveground parts) than among mature ramets which had survived winter intact. Survival of seedlings (new genets) was not affected by shading from the adult canopy, but a shading experiment showed that competition for light with mature ramets had a significant negative effect on morphology, growth and the allocation of seedling biomass to below-ground parts, thus reducing the ability of new genets to survive ice disturbance in the next winter. The annual form ofZ. marina was restricted to areas where low tide water depth was much less than winter ice thickness. Annual plants did not survive winter, had small investment in below-ground parts and high reproductive effort, and overwintered as seeds. The genetic status of the two forms and the restriction of the annual to very shallow water are discussed in the light of previous work on eelgrass     

Phosphorus limitation ofCymodocea nodosa growth

Experimental nutrient (N, P, and N+P) additions to shallow (ca. 1 m)Cymodocea nodosa (Ucria) Aschers. stands growing in patches and in a continuous meadow in a Mediterranean Bay (Alfacs Bay, NE Spain) in 1988 demonstrated mid-summer growth to be strongly P-limited, as suggested by the high N:P ratios (>35) in unmanipulated plants. P additions resulted in increased leaf P content, reduced N:P ratios, and enhanced shoot growth and turnover in both populations. These effects promoted in turn a stand response, leading to increased biomass and, therefore, increased areal productivity in the plots receiving P. The stand response was largely attributable to a doubling of shoot density, indicating enhanced rhizome growth and branching.     

Uranium Accumulation of Crop Plants Enhanced by Citric Acid

Citric acid was applied to soil to enhance U accumulation in four crop plants. While the highest enhanced U accumulation of aboveground tissues (a.c. 2000 mg kg⁻¹ dry weight) occurred in the leaves of Indian mustard (Brassica juncea), the highest enhanced U accumulation of roots (a.c. 3500 mg kg⁻¹ dry weight) occurred in canola (Brassica napus var. napus). Uranium translocation among tissues of test plants is in the relation of roots>shoots ≅ leaves. The flowers of sunflower (Helianthus annuus) contained similar or higher U concentrations than those found in shoots, but concentrations in seeds are close to zero. In conclusion, Indian mustard is recommended as a potential species for phytoextraction for U-contaminated soil due to its high U accumulation of aboveground biomass (a.c. 2200 μg per plant). There is no evidence that two types of soils cause a significant difference of the enhanced U accumulation (p<0.05). Results, however, indicate that additional citric acid may result in downward U migration that may contaminate groundwater. Speciation of U that is taken up by plants is also discussed in the end.     

Carbon sequestration potential of Rhizophora mucronata and Avicennia marina as influenced by age, season, growth and sediment characteristics in southeast coast of India

This work analysed the carbon sequestration potential in two species of mangroves (Rhizophora mucronata and Avicennia marina) along with their growth, biomass, sediment characteristics for four seasons of the year 2009–2010, in planted stands of different age (1–17.5 years) in the Vellar-Coleroon estuarine complex, India. The mangroves were recorded to store significant amount of biomass. Avicennia marina performed better to display 75 % higher rate of carbon sequestration than that in Rhizophora mucronata. This could be attributed to growth efficiency and high biomass production. For instance, Avicennia marina exhibited 2.7 fold higher girth, 24 % higher net canopy photosynthesis, 2 fold aboveground biomass (AGB), 40 % more belowground biomass (BGB) and 77.3 % higher total biomass, than R. mucronata did. Seasonally the rate of carbon sequestration was 7.3 fold higher in post-monsoon, 3.4 fold in monsoon, 73 % more in summer than that in pre-monsoon. The rate of carbon sequestration was positively correlated with age of planted site, tree height, tree diameter, net canopy photosynthesis, AGB, BGB, total biomass, carbon stock, growth efficiency, AGB/tree height tree girth, leaf area index, silt content (p?<?0.01). The carbon sequestration was negatively corrected with soil temperature and clay content (p?<?0.05). Mangroves were found to be a productive system and important sink of carbon in the tropical coastal zone, but increasing soil temperature due to global warming would have a negative impact on carbon sequestration potential of the mangroves.     

Above- and belowground biomass and nutrient pools of Spartina alterniflora (smooth cordgrass) in a South American salt marsh

In order to examine the role of position in the tidal range on biomass production and nutrient pools in Spartina alterniflora in an Argentinian estuary, we estimated productivity, the concentration of C, N and P in tissues and pools (concentration×biomass) of these elements in low (LM) and high (HM) zones. Aboveground biomass of S. alterniflora was higher in HM than in LM. Aboveground primary productivity was 106 and 439 g dry wt m^?2 year^?1 in LM and HM, respectively. Belowground biomass was similar in LM and HM. Belowground primary productivity was 526 and 744 g dry wt m^?2 year^?1 for LM and HM, respectively. Nutrient pools were higher in HM than in LM. Biomass and productivity values were low, which makes nutrient pools low. The lower values of the parameters analysed in LM than in HM indicate that position in the tidal range is an important factor in this system, possibly due to the effect of flooding. Moreover, this pattern is opposite to the general one observed in the northern hemisphere, meaning that studying marshes from different environments is worth doing. Because pools were higher in HM, this zone would be more important for nutrient input to the estuary.     

Rates of primary productivity and growth in<Emphasis Type="Italic"> Ecklonia radiata</Emphasis> measured at different depths,over an annual cycle,at West Island,South Australia

The pattern of growth (biomass accumulation) in Ecklonia radiata throughout the year and across a depth profile was investigated using the traditional hole-punch method, and the information presented in context with concurrently measured in situ net productivity rates. The rate of net daily productivity showed a lack of consistent seasonal variability, remaining constant throughout the year at two of the four depths measured (3 m and 12 m), and becoming higher during winter at another (5 m). Throughout the year, rates of net daily productivity differed significantly across the depth profile. Net daily productivity rates averaged 0.017 g C g^–1 dwt day^–1 and 0.005 g C g^–1 dwt day^–1 at a depth of 3 m (1,394 mol O₂ g^–1 dwt day^–1) and 10 m (382 mol O₂ g^–1 dwt day^–1) respectively. In contrast, the biomass accumulation rate of E. radiata was highly seasonal, with low rates of growth occurring in autumn (0.002 g dwt g^–1 dwt day^–1 at both 3 and 10 m) and summer (0.007 and 0.004 g dwt g^–1 dwt day^–1 at 3 and 10 m respectively) and higher rates in spring (0.016 and 0.007 g dwt g^–1 dwt day^–1 at 3 and 10 m respectively) and winter (0.015 and 0.008 g dwt g^–1 dwt day^–1 at 3 and 10 m respectively). The proportion of assimilated carbon used for biomass accumulation varied throughout the year, between 5% and 41% at 3 m and between 28% and 128% at 10 m. The rates of biomass accumulation at all depths represented only a small proportion of the amount of carbon assimilated annually.Communicated by P.W. Sammarco, Chauvin     

Functional diversity of mesograzers in an eelgrass–epiphyte system

Historically, small invertebrate grazers in marine plant communities have been considered to be a relatively homogeneous group in their impact on ecosystem processes. However, recent studies propose that species composition is an important agent in determining grazer effects. We used four mesocosm experiments to test the biomass-specific and density-dependent effects of common mesograzers in temperate regions (Littorina littorea, Rissoa membranacea, Idotea baltica and Gammarus oceanicus) on epiphyte and eelgrass biomass and productivity. Mesograzer species identity strongly influenced epiphyte accumulation and eelgrass growth, where Rissoa was the most efficient mesograzer (per biomass) and Gammarus had the weakest impact. Density-dependent effects varied considerably among species. Both gastropod species reduced epiphyte accumulation in direct proportion to their density, and Littorina had the strongest negative effect on epiphyte biomass. The impact of Idotea seemed to level off to a threshold value and Gammarus had no density-dependent effect on epiphyte accumulation at all. Rissoa and Idotea increased eelgrass productivity in accordance with their effect on epiphyte accumulation, whereas Littorina showed a less positive effect than could be expected by its strong impact on epiphyte biomass. Gammarus had no significant impact on eelgrass growth. Our results show that the different functional traits of superficially similar mesograzers can have important consequences for ecosystem processes in macrophyte systems.     

Photosynthetic characteristics of giant kelp (Macrocystis pyrifera) determined in situ

Rates of net photosynthesis and nocturnal respiration by individual blades of the giant kelp Macrocystis pyrifera (L.) C. Agardh in southern California, were determined in situ by measuring oxygen production in polyethylene bags during spring/summer of 1983. Mature blades from different depths in the water column exhibited different photosynthetic characteristics. Blades from the surface canopy (0 to 1 m depth) exhibited higher photosynthetic capacity under saturating irradiance and higher photosynthetic efficiency at low irradiances than blades from 3 to 5 or 7 to 9 m depths. Saturating irradiance was lower for canopy blades than for deeper blades. Canopy blades showed no short-term photoinhibition, but photosynthetic rates of deeper blades were significantly reduced during 1 to 2 h incubations at high irradiances. Results of 1 to 2 wk acclimation experiments indicated that differences between photosynthetic characteristics of blades from different depths were primarily attributable to acclimation light conditions. Vertical displacement of blades within the kelp canopy occurred on a time-scale of 1 min to 1 h. Blades continually moved between the unshaded surface layer and deeper, shaded layers. Vertical movement did not maximize photosynthesis by individual blades; only a small proportion of blades making up a dense surface canopy maintained light-saturated photosynthetic rates during midday incubations. The relatively high photosynthetic rates exhibited by canopy blades over the entire range of light conditions probably resulted from acclimation to intermittent high and low irradiances, a consequence of vertical displacement. Vertical displacement also reduced the afternoon depression in photosynthesis of individual canopy blades. The overall effect of vertical displacement was optimization of total net photosynthesis by the kelp canopy and, therefore, optimization of whole-plant production.     

Production of tropical copepods in Kingston Harbour, Jamaica: the importance of small species

The copepod community observed during an 18-month period at the mouth of eutrophic Kingston Harbour, Jamaica, was dominated by small species of Parvocalanus, Temora, Oithona, and Corycaeus. Mean copepod biomass was 22.1 mg AFDW m⁻³ (331 mg m⁻²). Annual production was 1679 kJ m⁻², partitioned as 174 kJ m⁻² naupliar, 936 kJ m⁻² copepodite, 475 kJ m⁻² egg and 93 kJ m⁻² exuvial production. All nauplii, most copepodites and many adults, equivalent to half of the biomass and production, were missed by a standard 200-μm plankton net, emphasizing the importance of nauplii and small species in secondary production estimates. The evidence suggests that growth rates and production are generally not food limited, and we speculate that size-selective predation shapes the structure of the harbour community. Biomass and production are higher than previous estimates for tropical coastal waters, but comparable to other eutrophic tropical embayments and many productive temperate ecosystems. Far from being regions of low productivity, tropical zooplankton communities may have significant production and deserve greater research attention than they currently receive. Received: 19 September 1997 / Accepted: 21 October 1997     

Sustainability of agricultural and wild cereals to aerotechnogenic exposure

In recent decades, the problem of the constantly increasin level of anthropogenic load on the environment is becoming more and more acute. Some of the most dangerous pollutants entering the environment from industrial emissions are heavy metals. These pollutants are not susceptible to biodegradation over time, which leads to their accumulation in the environment in dangerous concentrations. The purpose of this work is to study the sustainability of cultivated and wild plants of the Poaceae family to aerotechnogenic pollution in the soil. The content of heavy metals in couch grass (Elytrigia repens (L.) Nevski), meadow bluegrass (Poa pratensis L.) and soft wheat (Triticum aestivum) plants grown in the impact zone of Novocherkassk Power Station has been analyzed. Contamination of cultivated and wild cereals with Pb, Zn, Ni and Cd has been established. It has been shown that the accumulation of heavy metals is individual for each plant species. An average and close correlation have been established between the total HM content and the content of their mobile forms in the soil and their content in plants. For the plants studied, the translocation factor (TF) and the distribution coefficient (DC) of HM have been calculated. The TF is formed by the ratio of the concentration of an element in the root plant dry weight to the content of its mobile compounds in the soil. The DC value makes it possible to estimate the capacity of the aboveground parts of plants to absorb and accumulate elements under soil pollution conditions and is determined as the ratio of the metal content in the aboveground biomass to its concentration in the roots. TF and DC values have shown a significant accumulation of elements by plants from the soil, as well as their translocation from the root system to the aboveground part. It has been revealed that even within the same Poaceae family, cultural species are more sensitive to man-made pollution than wild-growing ones.

     

Relationship between seasonal growth,photosynthetic production and apex mortality of Caulerpa paspaloides (Chlorophyceae)

Seasonal changes in rates of rhizome elongation, frond initiation, photosynthesis, respiration, starch content and mortality of rhizome apices were measured for a population of Caulerpa paspaloides (Bory) Greville located off Key Largo, Florida, USA. Maximal growth rates occur during spring and coincide with high photosynthetic production and low mortality of rhizome apices. A secondary period of rapid growth occurs during fall; however, rates are less than during spring. Minimal growth rates occur during winter and summer. Reduced growth during winter coincides with (1) low photosynthetic rates, (2) low photosynthesis: respiration ratios, and (3) reduced frond biomass, indicating that photosynthetic production is limiting. Reserve carbohydrate (starch) is apparently utilized to maintain vegetative growth during the winter. Increased mortality of rhizome apices is responsible for most of the reduction in growth during summer and fall.     

Grassland establishment under varying resource availability: a test of positive and negative feedback

The traditional logic of carbon (C) and nitrogen (N) interactions in ecosystems predicts further increases or decreases in productivity (positive feedback) in response to high and low fertility in the soil, respectively; but the potential for development of feedback in ecosystems recovering from disturbance is less well understood. Furthermore, this logic has been challenged in grassland ecosystems where frequent fires or grazing may reduce the contribution of aboveground litter inputs to soil organic matter pools and nutrient supply for plant growth, relative to forest ecosystems. Further, if increases in plant productivity increase soil C content more than soil N content, negative feedback may result from increased microbial demand for N making less available for plant growth. We used a field experiment to test for feedback in an establishing grassland by comparing aboveground net primary productivity (ANPP) and belowground pools and fluxes of C and N in soil with enriched, ambient, and reduced N availability. For eight years annual N enrichment increased ANPP, root N, and root tissue quality, but root C:N ratios remained well above the threshold for net mineralization of N. There was no evidence that N enrichment increased root biomass, soil C or N accrual rates, or storage of C in total, microbial, or mineralizable pools within this time frame. However, the net nitrogen mineralization potential (NMP) rate was greater following eight years of N enrichment, and we attributed this to N saturation of the microbial biomass. Grassland developing under experimentally imposed N limitation through C addition to the soil exhibited ANPP, root biomass and quality, and net NMP rate similar to the ambient soil. Similarity in productivity and roots in the reduced and ambient N treatments was attributed to the potentially high nitrogen-use efficiency (NUE) of the dominant C4 grasses, and increasing cover of legumes over time in the C-amended soil. Thus, in a developing ecosystem, positive feedback between soil N supply and plant productivity may promote enhanced long-term N availability and override progressive N limitation as C accrues in plant and soil pools. However, experimentally imposed reduction in N availability did not feed back to reduce ANPP, possibly due to shifts in NUE and functional group composition.     

Differences in productivities between the Great Australian Bight and the Gulf of Carpentaria,Australia, in summer

Phytoplankton standing crop (chlorophyll a) and primary productivity were recorded, and zooplankton biomass was estimated in the two large bays of Australia, the Great Australian Bight on the south coast (December, 1965) and the Gulf of Carpentaria on the north coast (December, 1968). In the Gulf of Carpentaria, the phytoplankton standing crop (average, 27.3 mg chlorophyll a m^-2) and primary productivity (average, 133.1 mg C m^-2 h^-1), as well as zooplankton biomass (average, 305.3 mg wet weight m^-3) are much higher than in the Great Australian Bight (12.1 mg chlorophyll a m^-2, 18.2 mg C m^-2 h^-1, 7.1 mg wet weight m^-3, respectively). The unexpectedly low productivity values in the Great Australian Bight are attributable to environmental conditions of this bay, which obtains neither replenishment of nutrients from the land nor receives upwelling of deep water.     

Additive effects of aboveground polyphagous herbivores and soil feedback in native and range-expanding exotic plants

Plant biomass and plant abundance can be controlled by aboveground and belowground natural enemies. However, little is known about how the aboveground and belowground enemy effects may add up. We exposed 15 plant species to aboveground polyphagous insect herbivores and feedback effects from the soil community alone, as well as in combination. We envisaged three possibilities: additive, synergistic, or antagonistic effects of the aboveground and belowground enemies on plant biomass. In our analysis, we included native and phylogenetically related range-expanding exotic plant species, because exotic plants on average are less sensitive to aboveground herbivores and soil feedback than related natives. Thus, we examined if lower sensitivity of exotic plant species to enemies also alters aboveground-belowground interactions. In a greenhouse experiment, we exposed six exotic and nine native plant species to feedback from their own soil communities, aboveground herbivory by polyphagous insects, or a combination of soil feedback and aboveground insects and compared shoot and root biomass to control plants without aboveground and belowground enemies. We observed that for both native and range-expanding exotic plant species effects of insect herbivory aboveground and soil feedback added up linearly, instead of enforcing or counteracting each other. However, there was no correlation between the strength of aboveground herbivory and soil feedback. We conclude that effects of polyphagous aboveground herbivorous insects and soil feedback add up both in the case of native and related range-expanding exotic plant species, but that aboveground herbivory effects may not necessarily predict the strengths of soil feedback effects.