A “toy model” analysis of causes of nitrogen limitation in terrestrial ecosystems

Nitrogen (N) limitation to net primary production is widespread and influences the responsiveness of ecosystems to many components of global environmental change. Logic and both simple simulation (Vitousek and Fieldin in Biogeochemistry 46: 179–202, 1999) and analytical models (Menge in Ecosystems 14:519–532, 2011) demonstrate that the co-occurrence of losses of N in forms that organisms within an ecosystem cannot control and barriers to biological N fixation (BNF) that keep this process from responding to N deficiency are necessary for the development and persistence of N limitation. Models have focused on the continuous process of leaching losses of dissolved organic N in biologically unavailable forms, but here we use a simple simulation model to show that discontinuous losses of ammonium and nitrate, normally forms of N whose losses organisms can control, can be uncontrollable by organisms and can contribute to N limitation under realistic conditions. These discontinuous losses can be caused by temporal variation in precipitation or by ecosystem-level disturbance like harvest, fire, and windthrow. Temporal variation in precipitation is likely to increase and to become increasingly important in causing N losses as anthropogenic climate change proceeds. We also demonstrate that under the conditions simulated here, differentially intense grazing on N- and P-rich symbiotic N fixers is the most important barrier to the responsiveness of BNF to N deficiency.

     

Does Phaeocystis spp. contribute significantly to vertical export of organic carbon?

Phaeocystis spp. cell and colony mass fluxes and their contribution to the vertical particulate organic carbon (POC) export from a wide range of stations were quantified by short-term sediment traps. The compilation of available data, ranging from polar to sub-arctic and boreal regions, revealed that Phaeocystis colonial and single cells frequently are observed in shallow sediment traps at 30–50 m depth (average of 7 ± 11% of POC export). A strong vertical export decline between 40 m and 100 m diminished the contribution of Phaeocystis spp. cell carbon to vertical export of POC to only 3 ± 2% at 100 m depth, with two exceptions (deeper mixed stations). Estimates of potential corresponding mucus contribution increased the average Phaeocystis spp. contribution to <5% of POC export. The vertical flux attenuation efficiency is higher for Phaeocystis spp. than for diatoms. The overall contribution of Phaeocystis spp. to vertical carbon export based on direct investigations of vertical organic carbon export is small.     

Can remotely sensed meteorological data significantly contribute to reduce costs of tsetse surveys?

A 0.125 degree raster or grid-based Geographic Information System with data on tsetse, trypanosomiasis animal production, agriculturerkina> and land use has recently been developed in Togo. This paper addresses the problem of generating tsetse distribution and abundance maps from remotely sensed data, using a restricted amount of field data. A discriminant analysis model is tested using contemporary tsetse data and remotely sensed, low resolution data acquired from the National Oceanographic and Atmospheric Administration and Meteosat platforms. A split sample technique is adopted where a randomly selected part of the field measured data (training set) serves to predict the other part (predicted set). The obtained results are then compared with field measured data per corresponding grid-square. Depending on the size of the training set the percentage of concording predictions varies from 80 to 95 for distribution figures and from 63 to 74 for abundance. These results confirm the potential of satellite data application and multivariate analysis for the prediction, not only of the tsetse distribution, but more importantly of their abundance. This opens up new avenues because satellite predictions and field data may be combined to strengthen or substitute one another and thus reduce costs of field surveys.     

Could TCTP contribute to Armin Braun's paradigm of tumor reversion in plants?

Comment on: Brioudes F, et al. Proc Natl Acad Sci USA 2010; 107:16384-9.     

How much carbon is stored in the terrestrial ecosystems of the Chilean Patagonia?

We estimated the amount of carbon (C) stored in terrestrial ecosystems of the Chilean Patagonia and the proportion within protected areas. We used existing public databases that provide information on C stocks in biomass and soils. Data were analysed by ecosystem and forest type in the case of native forests. Our results show that some ecosystems have been more extensively studied both for their stocks in biomass and soils (e.g. forests) compared with others (e.g. shrublands). Forests and peatlands store the largest amount of C because of their large stocks per hectare and the large area they cover. The total amount of C stored per unit area varies from 261.7 to 432.8 Mg C ha⁻¹, depending on the published value used for soil organic C stocks in peatlands, highlighting the need to have more precise estimates of the C stored in this and other ecosystems. The mean stock in national parks (508 Mg C ha⁻¹) is almost twice the amount stored in undisturbed forests in the Amazon. State and private protected areas contain 58.9% and 2.1% of the C stock, respectively, playing a key role in protecting ecosystems in this once pristine area.     

Does virus-induced lysis contribute significantly to bacterial mortality in the oxygenated sediment layer of shallow oxbow lakes?

Despite the recognition that viruses are ubiquitous components of aquatic ecosystems, the number of studies on viral abundance and the ecological role of viruses in sediments is scarce. In this investigation, the interactions between viruses and bacteria were studied in the oxygenated silty sediment layer of a mesotrophic oxbow lake. A long-term study (13 months) and a diel study revealed that viruses are a numerically important and dynamic component of the microbial community. The abundance and decay rates ranged from 4.3 x 10(9) to 7.2 x 10(9) particles ml of wet sediment(-1) and from undetectable to 22.2 x 10(7) particles ml(-1) h(-1), respectively, and on average the values were 2 orders of magnitude higher than the values for the overlying water. In contrast to our expectations, viruses did not contribute significantly to the bacterial mortality in the sediment, since on average only 6% (range, 0 to 25%) of the bacterial secondary production was controlled by viruses. The low impact of viruses on the bacterial community may be associated with the quantitatively low viral burden that benthic bacteria have to cope with compared to the viral burden with which bacterial assemblages in the water column are confronted. The virus-to-bacterium ratio of the sediment varied between 0.9 and 3.2, compared to a range of 5.0 to 12.4 obtained for the water column. We speculate that despite high numbers of potential hosts, the possibility of encountering a host cell is limited by the physical conditions in the sediment, which is therefore not a favorable environment for viral proliferation. Our data suggest that viruses do not play an important role in the processing and transfer of bacterial carbon in the oxygenated sediment layer of the environment investigated.     

Could the intrinsic rate of increase represent the fitness in terrestrial ectotherms?

The intrinsic rate of increase (r_m) has been considered as an important indicator of fitness in terrestrial ectotherms since long. It is actually an equivalent to the instantaneous growth rate of the exponential equation for describing the density-independent population growth. In terrestrial ectotherms, r_m has been demonstrated to be temperature-dependent. The temperature at which r_m was maximal, was considered to be the “optimal” temperature for fitness in Amarasekare and Savage (2012), but this definition needs further analysis. Only r_m cannot provide thorough representation of fitness. Because body size can affect the competitive abilities in many terrestrial ectotherms, both population size and body size should be considered in measuring the fitness of ectotherms. The rule of “bigger is better” requires relatively low temperature to increase in body size, whereas relatively high temperature is required for a rapid increase in population size. Thus, there is presumably a trade-off in temperature for adjusting individual body size and population size to achieve maximum fitness. We hypothesized that this temperature could be reflected by the intrinsic optimum temperature for developmental rate in the Sharpe–Schoolfield–Ikemoto model, and it led to a temperature estimate around 20 °C. However, the traditional viewpoint based on the temperature corresponding to the maximal intrinsic rate of increase provides a temperature estimate around 30 °C. This study suggests that a low temperature around 20 °C might authentically represent the optimal ambient temperature for fitness in terrestrial ectotherms. It implies that thermal biologists who are interested in the effect of temperature on the fitness in terrestrial ectotherms should pay more attention to their performance at low temperature rather than high temperature.     

Conversion of native terrestrial ecosystems in Hawai‘i to novel grazing systems: a review

The remote oceanic islands of Hawai‘i exemplify the transformative effects that non-native herbivorous mammals can bring to isolated terrestrial ecosystems. We reviewed published literature containing systematically collected, analyzed, and peer-reviewed original data specifically addressing direct effects of non-native hoofed mammals (ungulates) on terrestrial ecosystems, and indirect effects and interactions on ecosystem processes in Hawai‘i. The effects of ungulates on native vegetation and ecosystems were addressed in 58 original studies and mostly showed strong short-term regeneration of dominant native trees and understory ferns after ungulate removal, but unassisted recovery was dependent on the extent of previous degradation. Ungulates were associated with herbivory, bark-stripping, disturbance by hoof action, soil erosion, enhanced nutrient cycling from the interaction of herbivory and grasses, and increased pyrogenicity and competition between native plants and pasture grasses. No studies demonstrated that ungulates benefitted native ecosystems except in short-term fire-risk reduction. However, non-native plants became problematic and continued to proliferate after release from herbivory, including at least 11 species of non-native pasture grasses that had become established prior to ungulate removal. Competition from non-native grasses inhibited native species regeneration where degradation was extensive. These processes have created novel grazing systems which, in some cases, have irreversibly altered Hawaii’s terrestrial ecology. Non-native plant control and outplanting of rarer native species will be necessary for recovery where degradation has been extensive. Lack of unassisted recovery in some locations should not be construed as a reason to not attempt restoration of other ecosystems.     

Biological nitrogen fixation: An efficient source of nitrogen for sustainable agricultural production?

A fundamental shift has taken place in agricultural research and world food production. In the past, the principal driving force was to increase the yield potential of food crops and to maximize productivity. Today, the drive for productivity is increasingly combined with a desire for sustainability. For farming systems to remain productive, and to be sustainable in the long-term, it will be necessary to replenish the reserves of nutrients which are removed or lost from the soil. In the case of nitrogen (N), inputs into agricultural systems may be in the form of N-fertilizer, or be derived from atmospheric N₂ via biological N₂ fixation (BNF).Although BNF has long been a component of many farming systems throughout the world, its importance as a primary source of N for agriculture has diminished in recent decades as increasing amounts of fertilizer-N are used for the production of food and cash crops. However, international emphasis on environmentally sustainable development with the use of renewable resources is likely to focus attention on the potential role of BNF in supplying N for agriculture. This paper documents inputs of N via symbiotic N₂ fixation measured in experimental plots and in farmers' fields in tropical and temperate regions. It considers contributions of fixed N from legumes (crop, pasture, green manures and trees), Casuarina, and Azolla, and compares the relative utilization of N derived from these sources with fertilizer N.     

Carbon storage in terrestrial ecosystems: do browsing and grazing herbivores matter?

Large mammalian herbivores manifest a strong top‐down control on ecosystems that can transform entire landscapes, but their impacts have not been reviewed in the context of terrestrial carbon storage. Here, we evaluate the effects of plant biomass consumption by large mammalian herbivores (>10 kg adult biomass), and the responses of ecosystems to these herbivores, on carbon stocks in temperate and tropical regions, and the Arctic. We calculate the difference in carbon stocks resulting from herbivore exclusion using the results of 108 studies from 52 vegetation types. Our estimates suggest that herbivores can reduce terrestrial above‐ and below‐ground carbon stocks across vegetation types but reductions in carbon stocks may approach zero given sufficient periods of time for systems to respond to herbivory (i.e. decades). We estimate that if all large herbivores were removed from the vegetation types sampled in our review, increases in terrestrial carbon stocks would be up to three orders of magnitude less than many of the natural and human‐influenced sources of carbon emissions. However, we lack estimates for the effects of herbivores on below‐ground biomass and soil carbon levels in many regions, including those with high herbivore densities, and upwards revisions of our estimates may be necessary. Our results provide a starting point for a discussion on the magnitude of the effects of herbivory on the global carbon cycle, particularly given that large herbivores are common in many ecosystems. We suggest that herbivore removal might represent an important strategy towards increasing terrestrial carbon stocks at local and regional scales within specific vegetation types, since humans influence populations of most large mammals.     

Does genotypic variation in nitrogen remobilisation efficiency contribute to nitrogen efficiency of winter oilseed-rape cultivars (Brassica napus L.)?

Aims

Winter oilseed-rape production is characterized by a low N efficiency, due to low N uptake and insufficient N remobilisation to the seeds. In particular, a reduction of leaf N losses might be one way to improve N efficiency of this crop. It was tested if variations in leaf N losses and in stem residual N amounts at maturity exist between cultivars differing in N efficiency.

Methods

In a 3-year field experiment, four oilseed rape cultivars were cultivated at limiting, medium, and high N supply.

Results

N harvest indices in this study were comparatively high (around 0.79) and leaf N losses amounted to at most 13 kg N ha^?1. 86 % of the leaf N present at the beginning of flowering was remobilised, irrespective of N rate or cultivar. Nevertheless, genotypic variation in leaf N loss existed. They were mainly due to differences in leaf N accumulation until flowering. Residual N in stems (up to 33 kg N ha^?1) was higher than leaf N losses and varied more between treatments but was not related to genotypic variation in yield.

Conclusions

N uptake after flowering was more important than N remobilisation from vegetative biomass for genotypic variation in seed yield both at low and high N supply.     

Do dung beetle larvae need microbial symbionts from their parents to feed on dung?

1. Adult dung beetles eat small particles of their food using soft mouthparts, but their larvae have biting jaws with which they chew material in their natal brood ball.
2. It was assumed that microbes supplied by the parents help breakdown food material in the brood ball, which was thought to act as a fermentation chamber. It has been generally accepted that larval dung beetles gain access to nutrients in the dung via symbionts.
3. Various dung treatments used here, including heat sterilisation and anti‐microbials, suggest that the maternal contribution to the brood ball is a pre‐digested meal for the newly hatched larva, and that microbes from the parents are not involved in larval feeding within this group of dung beetles.
4. However, manipulation of the dung by the mother supplied the larva with smaller dung particles from the unsorted dung, and an additional ‘maternal gift’ secretion may provide partially digested dung, both of which affect survival of the larva.
5. This overturns the largely untested dogma that larval dung beetles depend on microbial symbionts from the adults for feeding, but confirms that adult ‘conditioning’ of the dung is important for the larva in selecting out larger dung particles from the brood ball.

     

Nitrogen fixation in the High Arctic: a source of ‘new’ nitrogen?

Biological nitrogen (N₂) fixation performed by diazotrophs (N₂ fixing bacteria) is thought to be one of the main sources of plant available N in pristine ecosystems like arctic tundra. However, direct evidence of a transfer of fixed N₂ to non-diazotroph associated plants is lacking to date. Here, we present results from an in situ ¹⁵N–N₂ labelling study in the High Arctic. Three dominant vegetation types (organic crust composed of free-living cyanobacteria, mosses, cotton grass) were subjected to acetylene reduction assays (ARA) performed regularly throughout the growing season, as well as ¹⁵N–N₂ incubations. The ¹⁵N-label was followed into the dominant N₂ fixer associations, soil, soil microbial biomass and non-diazotroph associated plants three days and three weeks after labelling. Mosses contributed most to habitat N₂ fixation throughout the measuring campaigns, and N₂ fixation activity was highest at the beginning of the growing season in all plots. Fixed ¹⁵N–N₂ became quickly (within 3 days) available to non-diazotroph associated plants in all investigated vegetation types, proving that N₂ fixation is an actual source of available N in pristine ecosystems.     

Does migration of hybrids contribute to post-zygotic isolation in flycatchers?

In the face of hybridization, species integrity can only be maintained through post-zygotic isolating barriers (PIBs). PIBs need not only be intrinsic (i.e. hybrid inviability and sterility caused by developmental incompatibilities), but also can be extrinsic due to the hybrid's intermediate phenotype falling between the parental niches. For example, in migratory species, hybrid fitness might be reduced as a result of intermediate migration pathways and reaching suboptimal wintering grounds. Here, we test this idea by comparing the juvenile to adult survival probabilities as well as the wintering grounds of pied flycatchers (Ficedula hypoleuca), collared flycatchers (Ficedula albicollis) and their hybrids using stable isotope ratios of carbon (delta13C) and nitrogen (delta15N) in feathers developed at the wintering site. Our result supports earlier observations of largely segregated wintering grounds of the two parental species. The isotope signature of hybrids clustered with that of pied flycatchers. We argue that this pattern can explain the high annual survival of hybrid flycatchers. Hence, dominant expression of the traits of one of the parental species in hybrids may substantially reduce the ecological costs of hybridization.     

Could EU herbal monographs contribute to Malta’s treatment armamentarium?

Ten years have passed since Directive 2004/24/EC regulating herbal medicinal products across the EU were published. The directive created the Committee on Herbal Medicinal Products within the European Medicines Agency whose remit includes the creation and publishing of official EU monographs on herbal medicinal products. These monographs include the official uses of the products and their evidence for efficacy and safety. To this effect, we are interested in analysing the potential impact herbal product EU monographs could have on the therapeutic treatment options available for prescribers in Malta. Therefore our aim was two-fold. First, to rationalise the spread of indications of the herbal substances listed in the community herbal monograph inventory and subsequently determine if these herbal substances could potentially contribute to the treatment options available in our local scenario (Malta). 128 EU monographs were analysed resulting in a total of 230 indications which subsequently codified into 42 unique ATC codes. The Malta Medicines List contains 1456 unique ATC codes. Comparative analysis of the Malta Medicines List revealed that the 21 therapeutic areas had 4 or less pharmaceutically used substances (5th level ATC codes) registered and therefore in our opinion are areas with limited therapeutic choice. The following 4 therapeutic areas, A05 bile and liver therapy, A13 tonics, A15 appetite stimulants and D03 preparations for treatment of wounds and ulcers, could potentially benefit from the registration of herbal medicinal products according to the EU herbal monographs. If such registration is effected the aforementioned areas would no longer be considered limited because more than 4 therapeutic choices would be available to prescribers. This study is the first study across the EU to analyse the potential impact of published EU herbal monographs on therapeutic coverage in an EU member state and confirms the notion that herbal products could potentially increase the treatment options available in areas where few medical products have been registered due to Malta's small market size.     

Could poor seed dispersal contribute to predation by introduced rodents in a Hawaiian dry forest?

Dry forests are among the most diverse, yet threatened, communities in Hawai’i. Dry forests throughout the archipelago suffer from a lack of natural regeneration of trees. Two factors that may limit tree recruitment include poor seed dispersal and seed predation by rodents. Poor or limited dispersal of fleshy-fruited species results in seeds and fruits falling directly under parents. Dispersed and non-dispersed seeds may differ in their vulnerability to predation. We tested effects of seed location (under/away from parent trees) and pulp (presence/absence) on predation of four native species that suffer from limited dispersal and one readily-dispersed alien species in Kanaio Natural Area Reserve, Maui. Three natives (Diospyros sandwicensis, Pleomele auwahiensis, Santalum ellipticum), had significantly more seeds removed under parent trees than in exposed sites away from trees. For the one alien (Bocconia frutescens) and two native trees (D. sandwicensis, P. auwahiensis) that were evaluated, significantly more intact fruits were removed than were cleaned seeds. Presence of teeth marks and gnawed seed husk fragments indicate introduced rodents are destroying many of the seeds they remove. These results suggest that seed predation is disproportionately concentrated among poorly-dispersed seeds and may contribute to recruitment failure.