The impact of sampling frequency and sampling times on chamber-based measurements of N2O emissions from fertilized soils

An automated closed‐chamber system was developed to measure N₂O fluxes in the field. It was deployed at two N‐fertilized grassland sites in two successive years, together with replicated manual chambers, to investigate the spatial and temporal variability in fluxes, and the likely impact of sampling frequency on cumulative flux values. The automated system provided flux data at 8‐h intervals, while manual sampling was conducted at intervals of 3–7 days. The autochambers showed fluctuations in emissions not detected by manual sampling. However, integrated flux values based on the more intensive measurements were on average no more than 14% greater than those based on data from the autochambers that were obtained at the same time as manual sampling. This difference was not significant and well within the spatial variability determined with manual chambers. If daily sampling intervals were used immediately after fertilization, the agreement was closer still, increasing the confidence that can be placed in manual procedures. Diurnal variations in temperature and flux were small, and results from sampling at mid‐day were not significantly different from those based on early morning or evening sampling. Where diurnal fluctuations in temperature and flux are likely to be much larger, the autochamber/sampler system could prove very useful to quantify the effect.     

The Use of Mixtures of Sulfur and Bracken Litter to Reduce pH of Former Arable Soils and Control Ruderal Species

High soil pH has been highlighted as a constraint to the restoration of heathland on ex‐arable land. Previous studies at the Minsmere Reserve of the Royal Society for the Protection of Birds (RSPB) in England have shown that it is possible to acidify ex‐arable soils using elemental sulfur and bracken litter, although sulfur (S) is more effective. Current recommendations suggest that 4 tS/ha need to be applied to reduce soil pH below pH 4, control vigorous ruderal species, and create conditions suitable for Calluna vulgaris (heather) establishment. However, S is relatively expensive, and as bracken litter is moderately abundant within the reserve, it made economic sense to evaluate the potential for mixing S with bracken to see if adequate pH reductions could be achieved at lower S rates. Accordingly an experiment was designed to test the effects of combining S (0–8 t S/ha) and bracken litter (0–10 cm depth layers) on (1) soil pH, (2) cover of ruderal species, and (3) the developing plant community. Significant interactions were detected, especially in the period immediately after application. Where bracken litter was applied the soil pH fell immediately; in contrast, S took at least six months to start reducing pH. Where mixtures were applied there was a synergistic effect, which produced a lower pH than the S or bracken litter applied alone. These effects were most marked at low S application rates, between 0.5–4 t S/ha. The effects of the bracken litter addition also reduced the growth of ruderal species in the period immediately after application, probably through a combination of acidification and physical smothering. There is, therefore, a clear potential to acidify ex‐arable soils using combinations of S and bracken litter in schemes designed to restore Calluna heathland.     

Classical molecular interaction potentials: improved setup procedure in molecular dynamics simulations of proteins.

The latest version of the classical molecular interaction potential (CMIP) has the ability to predict the position of crystallographic waters in several proteins with great accuracy. This article analyzes the ability of the CMIP functional to improve the setup procedure of the molecular system in molecular dynamics (MD) simulations of proteins. To this end, the CMIP strategy is used to include both water molecules and counterions in different protein systems. The structural details of the configurations sampled from trajectories obtained using the CMIP setup procedure are compared with those obtained from trajectories derived from a standard equilibration process. The results show that standard MD simulations can lead to artifactual results, which are avoided using the CMIP setup procedure. Because the CMIP is easy to implement at a low computational cost, it can be very useful in obtaining reliable MD trajectories.     

GRACILARIA EDULIS (RHODOPHYTA) AS A BIOLOGICAL INDICATOR OF PULSED NUTRIENTS IN OLIGOTROPHIC WATERS

The response of the marine macroalga Gracilaria edulis (Gmelin) Silva to nutrient pulses of varying magnitude was investigated to test its applicability as a marine bioindicator at two oligotrophic locations. After exposure to nutrient pulses, algal amino acid, tissue nitrogen, and chlorophyll a content were assessed relative to algae incubated under control conditions (no nutrient enrichment). The smallest nutrient pulse involved a nutrient enrichment experiment conducted within a coral atoll, whereas two larger pulses resulted from sewage discharge to a tropical coastal bay. After exposure to the smallest nutrient pulse (10 × ambient), only changes in macroalgal amino acid concentration and composition were detected (mainly as increases in citrulline). At 100 × ambient concentrations, increases in tissue % nitrogen of the macroalgae were detected, in addition to responses in amino acids. Macroalgae exposed to the highest nutrient pulse (1000 × ambient) responded with increased chlorophyll a , tissue nitrogen, and amino acids within the three day incubation period. In contrast to these algal responses, analytical water sampling techniques failed to detect elevated nutrients when nutrient pulses were not occurring. The responses of this algal bioindicator to variable nutrient pulses may provide a useful tool for investigating the source and geographical extent of nutrients entering oligotrophic coastal waters.     

Estimation of Cd,Pb, and Zn Bioavailability in Smelter-Contaminated Soils by a Sequential Extraction Procedure

Chemical fractionation methods may be capable of providing an inexpensive estimate of contaminant bioavailability and risk in smelter-contaminated soil. In this study, the relationship between metal fractionation and methods used to estimate bioavailability of these metal contaminants in soil was evaluated. The Potentially BioAvailable Sequential Extraction (PBASE) was used for Cd, Pb, and Zn fractionation in 12 soils contaminated from Pb and Zn mining and smelting activities. The PBASE procedure is a four-step sequential extraction: extraction 1 (E1) is 0.5 M Ca(NO₃)₂, E2 is 1.0 M NaOAc, E3 is 0.1 M Na₂EDTA, and E4 is 4 M HNO₃. Metal bioavailability for two human exposure pathways, plant uptake (phytoavailability) and incidental ingestion (gastrointestinal, Gl, availability), was estimated using a lettuce (Lactuca sativa L.) bioassay and the in vitro-Gl Physiologically Based Extraction Test(PBET). Metal in the PBASE E1 fraction was correlated with lettuce Cd (P < 0.001) and Zn (P < 0.05) and was the best predictor of Cd and Zn phytoavailability. Only total metal content or the sum of all PBASE fractions, ΣE_1–4, were correlated (P < 0.001) with PBET gastric phase for Pb. The sum of the first two PBASE fractions, ΣE_1–2, was strongly correlated (P < 0.001) with Pb extracted by the PBET intestinal phase. The PBASE extraction method can provide information on Cd and Zn phytoavailability and Gl availability of Pb in smelter-contaminated soils.     

A Comparison of the Soil Water,Nutrient Status,and Litterfall Characteristics of Tropical Heath and Mixed‐Dipterocarp Forest Sites in Brunei1

Two of the main hypotheses to explain the distribution and special characteristics of tropical heath forest are nutrient and water limitation. A study was undertaken to investigate both factors on two sites under tropical heath forest (Badas Forest Reserve) and mixed‐dipterocarp forest (Andulau Forest Reserve) in Brunei. Soil water potentials were monitored at depths of 20, 50, and 90 cm over wet and dry periods for five months at each site. The results showed the mixed‐dipterocarp forest site to be drier at 50 cm depth compared to the tropical heath forest site. There was no significant difference in water potentials between sites at 20 or 90 cm. Nutrient concentrations in the soil solution were monitored at the same depths over a seven‐month period at the same sites. A 12‐month litterfall study was also undertaken to monitor nutrient returns from the canopy at each site. The results of both studies suggest that the tropical heath forest site is poorer in nitrogen, but richer in calcium, than the mixed‐dipterocarp forest site. The results for phosphorus are less clear, but do not suggest that its limitation is a significant factor at the tropical heath forest site compared to the mixed‐dipterocarp forest site. Phosphorus and magnesium concentrations in the soil solution showed a strong positive correlation with sliding 30‐day rainfall totals at both sites.     

Temperature- and moisture-induced changes in the structure of the nematode fauna of a semiarid grassland –- patterns and mechanisms

A field experiment was established to examine the effects of temperature and moisture modifications on the nematode fauna of a semiarid grassland. Several combinations of drying, wetting, warming and cooling were applied to plots and compared with untreated control plots. The experiment was performed from July to October 1996. A significant shift was observed in the structure of the nematode fauna between late summer and early autumn. This shift was manifested in the disappearance of four rare genera; Ecumenicus, Eucephalobus, Paraphelenchus and Pungentus. A significant decrease was found in the density of Acrobeles, Aphelenchoides, Ditylenchus and Prismatolaimus; in addition there was a significant increase in the density of Cephalobus, Helicotylenchus, Paratylenchus and Tylenchorhynchus. Community structural change was represented by an initial decrease in nematode generic richness of 30–50%, and in a statistically significant decrease of nematode diversity in the control and all treated plots. Thereafter, emergence of a new community was demonstrated. Data show that temperature manipulation was the main factor to influence nematode diversity, Maturity Index, and Plant Parasite Index. However, nematode population density was influenced predominantly by the soil moisture content. Coenological analysis of soil nematode fauna appears to be a useful tool for the biological monitoring of the effects of global change on semiarid grasslands.     

Top‐down effects on pelagic heterotrophic nanoflagellates (HNF) in a large river (River Rhine): do losses to the benthos play a role?

1. The abundance, production and control of pelagic heterotrophic nanoflagellates (HNF) in the Lower River Rhine (Germany) were investigated. Field samples (live‐counting technique) were taken at least every 2 weeks at Cologne (km 685) over a period of 20 months. In addition, Lagrangian sampling was carried out 160 km downstream of Cologne (Kalkar–Grieth, km 845) over a period of 12 months. Potential HNF growth rates and loss rates caused by planktonic predators were estimated in the laboratory (size fractionation experiments) and compared with the changes in HNF‐density in a water parcel flowing downstream. 2. Mean abundance (±SD) ranged from 7 ± 6 to 4890 ± 560 individuals mL^–1 and was positively correlated with discharge. Heterotrophic nanoflagellates abundance increased up to 30‐fold during flood events, whereas there were only minor changes in the taxonomic composition. 3. HNF growth rate ranged from 0.16 ± 0.12 to 1.98 ± 0.10 day^–1. Between 0 and 95% (mean: 32%) of the gross production was lost to planktonic predators; a larger portion between 0 and 195% (mean: 64%) of the HNF gross production was lost by other means. 4. There may be an important role for benthic predators in the control of pelagic HNF. First, production of HNF was high and grazing losses to planktonic predators low at times when HNF abundance was low. Secondly, high in situ loss rates (not explicable by planktonic predators), which were positively related to temperature, indicate the importance of biotic interactions. Thirdly, the dependence of HNF abundance on discharge indicated a decrease grazing intensity with rising water levels (increase in water volume/colonised river bed ratio). 5. The impact of discharge on planktonic HNF mediated by the grazing impact of benthic predators was modelled, showing a good fit with the field data.     

Comparison of bioassay and enzyme-linked immunosorbent assay for quantification of Spodoptera frugiperda nuclear polyhedrosis virus in soil

Standard curves with known amounts of Spodoptera frugiperda nuclear polyhedrosis virus (NPV) in soil were established with a bioassay and with an enzyme-linked immunosorbent assay (ELISA). The bioassay detected as few as 4 × 10⁴ polyhedral inclusion bodies (PIB)/g sandy soil and <10 PIB/g soils with large amounts of silt or clay. The ELISA detected as few as 360 PIB/g in all three soil types, and absorbance values were inversely related to the amount of clay. Results of the bioassay and ELISA were significantly (P < 0.01) correlated for natural NPV from field samples of silt (R = 0.961) and sandy soil (R = 0.723). Soil samples from Louisiana pastures and corn fields contain up to 7.6 × 10⁴ PIB/g, and 2× 10⁴ PIB/g are commonly present.