Temperature response of denitrification rate and greenhouse gas production in agricultural river marginal wetland soils

Soils are predicted to exhibit significant feedback to global warming via the temperature response of greenhouse gas (GHG) production. However, the temperature response of hydromorphic wetland soils is complicated by confounding factors such as oxygen (O₂), nitrate () and soil carbon (C). We examined the effect of a temperature gradient (2–25 °C) on denitrification rates and net nitrous oxide (N₂O), methane (CH₄) production and heterotrophic respiration in mineral (Eutric cambisol and Fluvisol) and organic (Histosol) soil types in a river marginal landscape of the Tamar catchment, Devon, UK, under non‐flooded and flooded with enriched conditions. It was hypothesized that the temperature response is dependent on interactions with ‐enriched flooding, and the physicochemical conditions of these soil types. Denitrification rate (mean, 746 ± 97.3 μg m^?2 h^?1), net N₂O production (mean, 180 ± 26.6 μg m^?2 h^?1) and net CH₄ production (mean, 1065 ± 183 μg m^?2 h^?1) were highest in the organic Histosol, with higher organic matter, ammonium and moisture, and lower concentrations. Heterotrophic respiration (mean, 127 ± 4.6 mg m^?2 h^?1) was not significantly different between soil types and dominated total GHG (CO₂eq) production in all soil types. Generally, the temperature responses of denitrification rate and net N₂O production were exponential, whilst net CH₄ production was unresponsive, possibly due to substrate limitation, and heterotrophic respiration was exponential but limited in summer at higher temperatures. Flooding with increased denitrification rate, net N₂O production and heterotrophic respiration, but a reduction in net CH₄ production suggests inhibition of methanogenesis by or N₂O produced from denitrification. Implications for management and policy are that warming and flood events may promote microbial interactions in soil between distinct microbial communities and increase denitrification of excess with N₂O production contributing to no more than 50% of increases in total GHG production.     

Rates and pathways of CH4 oxidation in ferruginous Lake Matano,Indonesia

This study evaluates rates and pathways of methane (CH₄) oxidation and uptake using ¹⁴C‐based tracer experiments throughout the oxic and anoxic waters of ferruginous Lake Matano. Methane oxidation rates in Lake Matano are moderate (0.36 nmol L^?1 day^?1 to 117 μmol L^?1 day^?1) compared to other lakes, but are sufficiently high to preclude strong CH₄ fluxes to the atmosphere. In addition to aerobic CH₄ oxidation, which takes place in Lake Matano's oxic mixolimnion, we also detected CH₄ oxidation in Lake Matano's anoxic ferruginous waters. Here, CH₄ oxidation proceeds in the apparent absence of oxygen (O₂) and instead appears to be coupled to some as yet uncertain combination of nitrate (), nitrite (), iron (Fe) or manganese (Mn), or sulfate () reduction. Throughout the lake, the fraction of CH₄ carbon that is assimilated vs. oxidized to carbon dioxide (CO₂) is high (up to 93%), indicating extensive CH₄ conversion to biomass and underscoring the importance of CH₄ as a carbon and energy source in Lake Matano and potentially other ferruginous or low productivity environments.     

The effect of O2 and pressure on thiosulfate oxidation by Thiomicrospira thermophila

Microbial sulfur cycling in marine sediments often occurs in environments characterized by transient chemical gradients that affect both the availability of nutrients and the activity of microbes. High turnover rates of intermediate valence sulfur compounds and the intermittent availability of oxygen in these systems greatly impact the activity of sulfur‐oxidizing micro‐organisms in particular. In this study, the thiosulfate‐oxidizing hydrothermal vent bacterium Thiomicrospira thermophila strain EPR85 was grown in continuous culture at a range of dissolved oxygen concentrations (0.04–1.9 mM) and high pressure (5–10 MPa) in medium buffered at pH 8. Thiosulfate oxidation under these conditions produced tetrathionate, sulfate, and elemental sulfur, in contrast to previous closed‐system experiments at ambient pressure during which thiosulfate was quantitatively oxidized to sulfate. The maximum observed specific growth rate at 5 MPa pressure under unlimited O₂ was 0.25 hr^?1. This is comparable to the μ_max (0.28 hr^?1) observed at low pH (<6) at ambient pressure when T. thermophila produces the same mix of sulfur species. The half‐saturation constant for O₂ () estimated from this study was 0.2 mM (at a cell density of 10⁵ cells/ml) and was robust at all pressures tested (0.4–10 MPa), consistent with piezotolerant behavior of this strain. The cell‐specific was determined to be 1.5 pmol O₂/cell. The concentrations of products formed were correlated with oxygen availability, with tetrathionate production in excess of sulfate production at all pressure conditions tested. This study provides evidence for transient sulfur storage during times when substrate concentration exceeds cell‐specific and subsequent consumption when oxygen dropped below that threshold. These results may be common among sulfur oxidizers in a variety of environments (e.g., deep marine sediments to photosynthetic microbial mats).     

Ordinary stoichiometry of extraordinary microorganisms

All life on Earth seems to be made of the same chemical elements in relatively conserved proportions (stoichiometry). Whether this stoichiometry is conserved in settings that differ radically in physicochemical conditions (extreme environments) from those commonly encountered elsewhere on the planet provides insight into possible stoichiometries for putative life beyond Earth. Here, we report measurements of elemental stoichiometry for extremophile microbes from hot springs of Yellowstone National Park (YNP). Phototrophic and chemotrophic microbes were collected in locations spanning large ranges of temperature (24 °C to boiling), pH (1.6–9.6), redox (0.1–7.2 mg L^?1 dissolved oxygen), and nutrient concentrations (0.01–0.25 mg L^?1 , 0.7–12.9 mg L^?1 , 0.01–42 mg L^?1 NH₄⁺, 0.003–1.1 mg L^?1 P mostly as phosphate). Despite these extreme conditions, the microbial cells sampled had a major and trace element stoichiometry within the ranges commonly encountered for microbes living in the more moderate environments of lakes and surface oceans. The cells did have somewhat high C:P and N:P ratios that are consistent with phosphorus (P) limitation. Furthermore, chemotrophs and phototrophs had similar compositions with the exception of Mo content, which was enriched in cells derived from chemotrophic sites. Thus, despite the extraordinary physicochemical and biological diversity of YNP environments, life in these settings, in a stoichiometric sense, remains much the same as we know it elsewhere.     

Soil nitrogen dynamics in switchgrass seeded to a marginal cropland in South Dakota

The potential ecological impacts of switchgrass (Panicum virgatum L.), as a biofuel feedstock, have been assessed under different environmental conditions. However, limited information is available in understanding the integrated analysis of nitrogen (N) dynamics including soil nitrate (), nitrous oxide (N₂O) emissions, and leaching under switchgrass land management. The specific objective was to explore N dynamics for 2009 through 2015 in switchgrass seeded to a marginally yielding cropland based on treatments of N fertilization rate (N rate; low, 0; medium, 56; high, 112 kg N ha^?1) and landscape position (shoulder, backslope, and footslope). Our findings indicated that N rate impacted soil (0–5 cm depth) and surface N₂O fluxes but did not impact leaching during the observed years. Medium N (56 kg N ha^?1) was the optimal rate for increasing biomass yield with reduced environmental problems. Landscape position impacted the N dynamics. At the footslope position, soil , soil leaching, and N₂O fluxes were higher than the other landscape positions. Soil N₂O fluxes and leaching had downward trends over the observed years. Growing switchgrass on marginally yielding croplands can store soil N, reduce N losses via leaching, and mitigate N₂O emissions from soils to the atmosphere over the years. Switchgrass seeded on marginally yielding croplands can be beneficial in reducing N losses and can be grown as a sustainable bioenergy crop on these marginal lands.     

RISK OF POPULATION EXTINCTION FROM FIXATION OF NEW DELETERIOUS MUTATIONS

The fixation of new deleterious mutations is analyzed for a randomly mating population of constant size with no environmental or demographic stochasticity. Mildly deleterious mutations are far more important in causing loss of fitness and eventual extinction than are lethal and semilethal mutations in populations with effective sizes, N_e, larger than a few individuals. If all mildly deleterious mutations have the same selection coefficient, s against heterozygotes and 2s against homozygotes, the mean time to extinction, , is asymptotically proportional to for 4N_es > 1. Nearly neutral mutations pose the greatest risk of extinction for stable populations, because the magnitude of selection coefficient that minimizes is about ? = 0.4/N_e. The influence of variance in selection coefficients among mutations is analyzed assuming a gamma distribution of s, with mean and variance . The mean time to extinction increases with variance in selection coefficients if is near ?, but can decrease greatly if is much larger than ?. For a given coefficient of variation of , the mean time to extinction is asymptotically proportional to for . When s is exponentially distributed, (c = 1) is asymptotically proportional to . These results in conjunction with data on the rate and magnitude of mildly deleterious mutations in Drosophila melanogaster indicate that even moderately large populations, with effective sizes on the order of N_e = 10³, may incur a substantial risk of extinction from the fixation of new mutations.     

Biophysical analysis of the dynamics of calmodulin interactions with neurogranin and Ca2+/calmodulin‐dependent kinase II

Calmodulin (CaM) functions depend on interactions with CaM‐binding proteins, regulated by . Induced structural changes influence the affinity, kinetics, and specificities of the interactions. The dynamics of CaM interactions with neurogranin (Ng) and the CaM‐binding region of /calmodulin‐dependent kinase II (CaMKII_290−309) have been studied using biophysical methods. These proteins have opposite dependencies for CaM binding. Surface plasmon resonance biosensor analysis confirmed that and CaM interact very rapidly, and with moderate affinity ( ). Calmodulin‐CaMKII_290−309 interactions were only detected in the presence of , exhibiting fast kinetics and nanomolar affinity ( ). The CaM–Ng interaction had higher affinity under ‐depleted ( and k ₋₁ = 1.6 × 10⁻¹s⁻¹) than ‐saturated conditions ( ). The IQ motif of Ng (Ng₂₇₋₅₀) had similar affinity for CaM as Ng under ‐saturated conditions ( ), but no interaction was seen under ‐depleted conditions. Microscale thermophoresis using fluorescently labeled CaM confirmed the surface plasmon resonance results qualitatively, but estimated lower affinities for the Ng ( ) and CaMKII_290−309( ) interactions. Although CaMKII_290−309 showed expected interaction characteristics, they may be different for full‐length CaMKII. The data for full‐length Ng, but not Ng₂₇₋₅₀, agree with the current model on Ng regulation of /CaM signaling.     

Nitrate or ammonium: Influences of nitrogen source on the physiology of a green alga

In freshwaters, algal species are exposed to different inorganic nitrogen (N_i) sources whose incorporation varies in biochemical energy demand. We hypothesized that due to the lesser energy requirement of ammonium ()‐use, in contrast to nitrate ()‐use, more energy remains for other metabolic processes, especially under CO₂‐ and phosphorus (P_i) limiting conditions. Therefore, we tested differences in cell characteristics of the green alga Chlamydomonas acidophila grown on or under covariation of CO₂ and P_i‐supply in order to determine limitations, in a full‐factorial design. As expected, results revealed higher carbon fixation rates for ‐grown cells compared to growth with under low CO₂ conditions. ‐grown cells accumulated more of the nine analyzed amino acids, especially under P_i‐limited conditions, compared to cells provided with . This is probably due to a slower protein synthesis in cells provided with . In contrast to our expectations, compared to ‐grown cells ‐grown cells had higher photosynthetic efficiency under P_i‐limitation. In conclusion, growth on the N_i‐source did not result in a clearly enhanced C_i‐assimilation, as it was highly dependent on P_i and CO₂ conditions (replete or limited). Results are potentially connected to the fact that C. acidophila is able to use only CO₂ as its inorganic carbon (C_i) source.     

Environmental factors function as constraints on soil nitrous oxide fluxes in bioenergy feedstock cropping systems

Nitrous oxide (N₂O) is a potent greenhouse gas and major component of the net global warming potential of bioenergy feedstock cropping systems. Numerous environmental factors influence soil N₂O production, making direct correlation difficult to any one factor of N₂O fluxes under field conditions. We instead employed quantile regression to evaluate whether soil temperature, water‐filled pore space (WFPS), and concentrations of soil nitrate () and ammonium () determined upper bounds for soil N₂O flux magnitudes. We collected data over 6 years from a range of bioenergy feedstock cropping systems including no‐till grain crops, perennial warm‐season grasses, hybrid poplar, and polycultures of tallgrass prairie species each with and without nitrogen (N) addition grown at two sites. The upper bounds for soil N₂O fluxes had a significant and positive correlation with all four environmental factors, although relatively large fluxes were still possible at minimal values for nearly all factors. The correlation with was generally weaker, suggesting it is less important than in driving large fluxes. Quantile regression slopes were generally lower for unfertilized perennials than for other systems, but this may have resulted from a perpetual state of nitrogen limitation, which prevented other factors from being clear constraints. This framework suggests efforts to reduce concentrations of in the soil may be effective at reducing high‐intensity periods—”hot moments”—of N₂O production.     

Isotopically anomalous organic carbon in the aftermath of the Marinoan snowball Earth

Throughout most of the sedimentary record, the marine carbon cycle is interpreted as being in isotopic steady state. This is most commonly inferred via isotopic reconstructions, where two export fluxes (organic carbon and carbonate) are offset by a constant isotopic fractionation of ~25 (termed ). Sedimentary deposits immediately overlying the Marinoan snowball Earth diamictites, however, stray from this prediction. In stratigraphic sections from the Ol Formation (Mongolia) and Sheepbed Formation (Canada), we observe a temporary excursion where the organic matter has anomalously heavy C and is grossly decoupled from the carbonate C. This signal may reflect the unique biogeochemical conditions that persisted in the aftermath of snowball Earth. For example, physical oceanographic modeling suggests that a strong density gradient caused the ocean to remain stratified for about 50,000 years after termination of the Marinoan snowball event, during which time the surface ocean and continental weathering consumed the large atmospheric CO₂ reservoir. Further, we now better understand how C records of carbonate can be post‐depostionally altered and thus be misleading. In an attempt to explain the observed carbon isotope record, we developed a model that tracks the fluxes and isotopic values of carbon between the surface ocean, deep ocean, and atmosphere. By comparing the model output to the sedimentary data, stratification alone cannot generate the anomalous observed isotopic signal. Reproducing the heavy C in organic matter requires the progressively diminishing contribution of an additional anomalous source of organic matter. The exact source of this organic matter is unclear.     

Predicting soils and environmental impacts associated with switchgrass for bioenergy production: a DAYCENT modeling approach

Switchgrass (Panicum virgatum L.) production has the potential to improve soils and the environment. However, little is known about the long‐term future assessment of soil and environmental impacts associated with switchgrass production. In this study, soil organic carbon (SOC), soil nitrate (), water‐filled pore space (WFPS), carbon dioxide (CO₂) and nitrous oxide (N₂O) fluxes, and biomass yield from switchgrass field were predicted using DAYCENT models for 2016 through 2050. Measured data for model calibration and validation at this study site managed with nitrogen fertilization rates (N rates) (low, 0 kg N ha^?1; medium, 56 kg N ha^?1; and high, 112 kg N ha^?1) and landscape positions (shoulder and footslope) for switchgrass production were collected from the previously published studies. Modeling results showed that the N fertilization can enhance SOC and soil NO₃^?, but increase soil N₂O and CO₂ fluxes. In this study, medium N fertilization was the optimum rate for enhancing switchgrass yield and reducing negative impact on the environment. Footslope position can be beneficial for improving SOC, , and yield, but contribute higher greenhouse gas (GHG) emissions compared to those of the shoulder. An increase in temperature and decrease in precipitation (climate scenarios) may reduce soil , WFPS, and N₂O flux. Switchgrass production can improve and maintain SOC and , and reduce N₂O and CO₂ fluxes over the predicted years. These findings indicate that switchgrass could be a sustainable bioenergy crop on marginally yielding lands for improving soils without significant negative impacts on the environment in the long run.     

Variance component tests of multivariate mediation effects under composite null hypotheses

Mediation effects of multiple mediators are determined by two associations: one between an exposure and mediators (‐) and the other between the mediators and an outcome conditional on the exposure (‐). The test for mediation effects is conducted under a composite null hypothesis, that is, either one of the ‐ and ‐ associations is zero or both are zeros. Without accounting for the composite null, the type 1 error rate within a study containing a large number of multimediator tests may be much less than the expected. We propose a novel test to address the issue. For each mediation test , , we examine the ‐ and ‐ associations using two separate variance component tests. Assuming a zero‐mean working distribution with a common variance for the element‐wise ‐ (and ‐) associations, score tests for the variance components are constructed. We transform the test statistics into two normally distributed statistics under the null. Using a recently developed result, we conduct hypothesis tests accounting for the composite null hypothesis by adjusting for the variances of the normally distributed statistics for the ‐ and ‐ associations. Advantages of the proposed test over other methods are illustrated in simulation studies and a data application where we analyze lung cancer data from The Cancer Genome Atlas to investigate the smoking effect on gene expression through DNA methylation in 15 114 genes.     

Nanodiamond‐Embedded p‐Type Copper(I) Oxide Nanocrystals for Broad‐Spectrum Photocatalytic Hydrogen Evolution

Copper(I) oxide (Cu₂O) is an attractive photocatalyst because of its abundance, low toxicity, environmental compatibility, and narrow direct band gap, which allows efficient light harvesting. However, Cu₂O exhibits poor photocatalytic performance and photostability because of its short electron diffusion length and low hole mobility. Here, it is demonstrated that nanodiamond (ND) can greatly improve the photocatalytic hydrogen evolution reaction (HER) of the p‐type photocatalyst Cu₂O nanocrystals by nanocomposition. Compared with pure Cu₂O nanocrystals, this composite shows a tremendous improvement in HER performance and photostability. HER rates of 100.0 mg NDs‐Cu₂O nanocrystals are 1597 and 824 under the simulated solar light irradiation (AM 1.5, 100 mW cm^?2) and visible light irradiation (420–760 nm, 77.5 mW cm^?2), respectively. The solar‐to‐hydrogen conversion efficiency of this composite is 0.85%, which is nearly ten times higher than that of pure Cu₂O. The quantum efficiency of the composite is high, with values of 0.17% at and 0.23% at . The broad spectral response of ND provides numerous carriers for the subsequent reactions. The electron‐donating ability of ND and suitable band structures of the two components promote electron injection from ND to Cu₂O. These results suggest the broad applicability of ND to ameliorate the photoelectric properties of semiconductors.     

Microbial diversity and activity in seafloor brine lake sediments (Alaminos Canyon block 601, Gulf of Mexico)

The microbial communities thriving in deep‐sea brines are sustained largely by energy rich substrates supplied through active seepage. Geochemical, microbial activity, and microbial community composition data from different habitats at a Gulf of Mexico brine lake in Alaminos Canyon revealed habitat‐linked variability in geochemistry that in turn drove patterns in microbial community composition and activity. The bottom of the brine lake was the most geochemically extreme (highest salinity and nutrient concentrations) habitat and its microbial community exhibited the highest diversity and richness indices. The habitat at the upper halocline of the lake hosted the highest rates of sulfate reduction and methane oxidation, and the largest inventories of dissolved inorganic carbon, particulate organic carbon, and hydrogen sulfide. Statistical analyses indicated a significant positive correlation between the bacterial and archaeal diversity in the bottom brine sample and inventories. Other environmental factors with positive correlation with microbial diversity indices were DOC, H₂S, and DIC concentrations. The geochemical regime of different sites within this deep seafloor extreme environment exerts a clear selective force on microbial communities and on patterns of microbial activity.     

Explaining the doubling of N2O emissions under elevated CO2 in the Giessen FACE via in‐field 15N tracing

Rising atmospheric CO₂ concentrations are expected to increase nitrous oxide (N₂O) emissions from soils via changes in microbial nitrogen (N) transformations. Several studies have shown that N₂O emission increases under elevated atmospheric CO₂ (eCO₂), but the underlying processes are not yet fully understood. Here, we present results showing changes in soil N transformation dynamics from the Giessen Free Air CO₂ Enrichment (GiFACE): a permanent grassland that has been exposed to eCO₂, +20% relative to ambient concentrations (aCO₂), for 15 years. We applied in the field an ammonium‐nitrate fertilizer solution, in which either ammonium () or nitrate () was labelled with ¹⁵N. The simultaneous gross N transformation rates were analysed with a ¹⁵N tracing model and a solver method. The results confirmed that after 15 years of eCO₂ the N₂O emissions under eCO₂ were still more than twofold higher than under aCO₂. The tracing model results indicated that plant uptake of did not differ between treatments, but uptake of was significantly reduced under eCO₂. However, the and availability increased slightly under eCO₂. The N₂O isotopic signature indicated that under eCO₂ the sources of the additional emissions, 8,407 μg N₂O–N/m² during the first 58 days after labelling, were associated with reduction (+2.0%), oxidation (+11.1%) and organic N oxidation (+86.9%). We presume that increased plant growth and root exudation under eCO₂ provided an additional source of bioavailable supply of energy that triggered as a priming effect the stimulation of microbial soil organic matter (SOM) mineralization and fostered the activity of the bacterial nitrite reductase. The resulting increase in incomplete denitrification and therefore an increased N₂O:N₂ emission ratio, explains the doubling of N₂O emissions. If this occurs over a wide area of grasslands in the future, this positive feedback reaction may significantly accelerate climate change.     

The Arabidopsis root stele transporter NPF2.3 contributes to nitrate translocation to shoots under salt stress

In most plants, constitutes the major source of nitrogen, and its assimilation into amino acids is mainly achieved in shoots. Furthermore, recent reports have revealed that reduction of translocation from roots to shoots is involved in plant acclimation to abiotic stress. NPF2.3, a member of the NAXT (nitrate excretion transporter) sub‐group of the NRT1/PTR family (NPF) from Arabidopsis, is expressed in root pericycle cells, where it is targeted to the plasma membrane. Transport assays using NPF2.3‐enriched Lactococcus lactis membranes showed that this protein is endowed with transport activity, displaying a strong selectivity for against Cl^?. In response to salt stress, translocation to shoots is reduced, at least partly because expression of the root stele transporter gene NPF7.3 is decreased. In contrast, NPF2.3 expression was maintained under these conditions. A loss‐of‐function mutation in NPF2.3 resulted in decreased root‐to‐shoot translocation and reduced shoot content in plants grown under salt stress. Also, the mutant displayed impaired shoot biomass production when plants were grown under mild salt stress. These mutant phenotypes were dependent on the presence of Na⁺ in the external medium. Our data indicate that NPF2.3 is a constitutively expressed transporter whose contribution to translocation to the shoots is quantitatively and physiologically significant under salinity.     

MATERNAL INHERITANCE AND ITS EFFECT ON ADAPTIVE EVOLUTION: A QUANTITATIVE GENETIC ANALYSIS OF MATERNAL EFFECTS IN A NATURAL PLANT POPULATION

A mother can influence a trait in her offspring both by the genes she transmits (Mendelian inheritance) and by maternal attributes that directly affect that trait in her offspring (maternal inheritance). Maternal inheritance can alter the direction, rate, and duration of adaptive evolution from standard Mendelian models and its impact on adaptive evolution is virtually unexplored in natural populations. In a hierarchical quantitative genetic analysis to determine the magnitude and structure of maternal inheritance in the winter annual plant, Collinsia verna, I consider three potential models of inheritance. These range from a standard Mendelian model estimating only direct (i.e., Mendelian) additive and environmental variance components to a maternal inheritance model estimating six additive and environmental variance components: direct additive and environmental variances; maternal additive and environmental variances; and the direct-maternal additive () and environmental covariances. The structure of maternal inheritance differs among the 10 traits considered at four stages in the life cycle. Early in the life cycle, seed weight and embryo weight display substantial , a negative , and a positive . Subsequently, cotyledon diameter displays and of roughly the same magnitude and negative . For fall rosettes, leaf number and length are best described by a Mendelian model. In the spring, leaf length displays maternal inheritance with significant and and a negative . All maternally inherited traits show significant negative . Predicted response to selection under maternal inheritance depends on and as well as . Negative results in predicted responses in the opposite direction to selection for seed weight and embryo weight and predicted responses near zero for all subsequent maternally inherited traits. Maternal inheritance persists through the life cycle of this annual plant for a number of size-related traits and will alter the direction and rate of evolutionary response in this population.     

Differential responses of two wetland graminoids to high ammonium at different pH values

Enhanced soil ammonium () concentrations in wetlands often lead to graminoid dominance, but species composition is highly variable. Although is readily taken up as a nutrient, several wetland species are known to be sensitive to high concentrations or even suffer toxicity, particularly at low soil pH. More knowledge about differential graminoid responses to high availability in relation to soil pH can help to better understand vegetation changes. The responses of two wetland graminoids, Juncus acutiflorus and Carex disticha, to high (2 mmol·l^?1) versus control (20 μmol·l^?1) concentrations were tested in a controlled hydroponic set up, at two pH values (4 and 6). A high concentration did not change total biomass for these species at either pH, but increased C allocation to shoots and increased P uptake, leading to K and Ca limitation, depending on pH treatment. More than 50% of N taken up by C. disticha was invested in N‐rich amino acids with decreasing C:N ratio, but only 10% for J. acutiflorus. Although both species appeared to be well adapted to high loadings in the short term, C. disticha showed higher classic detoxifying responses that are early warning indicators for decreased tolerance in the long term. In general, the efficient aboveground biomass allocation, P uptake and N detoxification explain the competitive strength of wetland graminoids at the expense of overall biodiversity at high loading. In addition, differential responses to enhanced affect interspecific competition among graminoids and lead to a shift in vegetation composition.     

THE MAINTENANCE OF POLYGENIC VARIATION IN FINITE POPULATIONS

Models of the maintenance of genetic variance in a polygenic trait have usually assumed that population size is infinite and that selection is weak. Consequently, they will overestimate the amount of variation maintained in finite populations. I derive approximations for the equilibrium genetic variance, in finite populations under weak stabilizing selection for triallelic loci and for an infinite “rare alleles” model. These are compared to results for neutral characters, to the “Gaussian allelic” model, and to Wright's approximation for a biallelic locus under arbitrary selection pressures. For a variety of parameter values, the three-allele, Gaussian, and Wrightian approximations all converge on the neutral model when population size is small. As expected, far less equilibrium genetic variance can be maintained if effective population size, N, is on the order of a few hundred than if N is infinite. All of the models predict that comparisons among populations with N less than about 10⁴ should show substantial differences in . While it is easier to maintain absolute when alleles interact to yield dominance or overdominance for fitness, less additivity also makes more susceptible to differences in N. I argue that experimental data do not seem to reflect the predicted degree of relationship between N and . This calls into question the ability of mutation-selection balance or simple balancing selection to explain observed . The dependence of on N could be used to test the adequacy of mutation-selection balance models.     

A new proposal for measurement of the adjusted mean crowding through consideration of size variability in habitat units

The mean crowding has previously been measured under the assumption that all quadrats or habitat units have the same size, even though the actual habitat units such as seeds or leaves are generally variable in size. A new index, ‘adjusted mean crowding’, which is adjusted for this variability can be given as where Q is the total number of habitat units in the whole area, x_j the number of individuals in the jth habitat unit, and a_j is defined as the ‘relative size’ of the jth habitat unit, i.e. a_y=y_y/(∑y_j/Q) where y_j is the actually measured size of the jth habitat unit. It is expected that and for the uniform distribution and the random distribution ‘per unit size’, respectively. The comparison between and regressions ( analysis) for the egg distribution pattern of Callosobruchus chinensis or C. maculatus proved that the regression is biased by a positive correlation between the egg number per seed and seed size rather than by a density-dependent change in the ovipositional behavior.