Spatial distribution of isopods in an oak–beech forest

Soil macroinvertebrates were studied in a Mediterranean-type forest on brown-pebble forest soils in southern Russia. At the site, 144 intact soil cores (76 cm² each) forming a grid of 24 × 6 units were taken in order to determine animal spatial distribution. Abundance of isopods was 166.3 ± 16.0 indiv. m^–2 and they constituted about 12% of the total macrofaunal abundance. Biomass of isopods was 3.5 g m^–2, or about 21% of the total biomass of macrofauna. Three woodlice genera (Armadillidium, Cylisticus, and Trachelipus) were found at the site. The two latter genera formed almost all (93%) of the isopod population. We found that spatial distribution of woodlice was heterogeneous: areas with 4–5 individuals per sample were neighboring those without animals. In order to study soil factors influencing isopod distribution in the brown-pebble forest soil, the size of a sample was artificially increased by combining adjacent sample units. Litter mass (r = 0.41) and loss on ignition (LOI) (r = –0.55) significantly influenced isopod distribution. Soil pH was near neutral (6.79), LOI was 8.39, and the water holding capacity was 70.9%. Pebbles comprised up to 84% of the sample's mass. Ca. 40 samples are recommended for estimation of isopod abundance in brown forest soil.     

The existence of an Allee effect in populations of Porcellio scaber (Isopoda: Oniscidea)

An Allee effect occurs when at low densities there is a positive relationship between density and fitness leading to a higher probability of extinction. One circumstance in which it may arise is when isolated individuals are less effective in modifying their environment in a favourable way than groups of individuals. In this paper we use Porcellio scaber to test the hypothesis that the fitness correlate of growth rate will vary as a function of density not just due to the negative effects of increasing interference competition whilst foraging at higher densities, but also positively at lower densities due to isolated individuals spending more time and energy searching for other individuals with which to aggregate whilst sheltering.Below densities of 70 m^–2 relative growth rates increased as a function of density, above 70 m^–2 growth rate decreased with increasing density. Time spent walking and searching increased at lower densities. These behavioural observations, together with the original observations on water loss and oxygen consumption made by Allee [1], are used to interpret the curvilinear growth rate response to density as an example of a component Allee effect, which is discussed in relation to the importance of aggregation as an adaptation to the terrestrial environment in the Oniscidea.     

Assessing spatial variability of soil enzyme activities in pasture topsoils using geostatistics

The purpose of this study was to assess the spatial variability of the activity of three hydrolytic enzymes, i.e. urease activity (UAc), alkaline phosphatase activity (APAc), and arylsulfatase activity (ASAc), in pasture topsoils using geostatistics. Enzyme activities along a transect in a 1.35-ha pasture were determined using 77 soil samples from the upper 20 cm of soil. UAc varied from 101.0 to 182.7 μg N g⁻¹ soil h⁻¹; APAc varied from 1.56 to 3.62 μg p-nitrophenol g⁻¹ soil h⁻¹; and ASAc varied from 1.50 to 3.26 μg p-nitrophenol g⁻¹ soil h⁻¹. The linear models fit the best semivariogram models for UAc, APAc, and ASAc. Semivariograms for enzyme activities exhibited spatial dependence with ranges of influence of approximately 124.7 m.     

Mineral nitrogen dynamics in poorly drained blanket peat

Summary Mineral-N dynamics have been measured over a period of 3 years in PK- and NPK-treated plots (4 m²) laid out on an area of poorly drained, reseeded, blanket peat in the north of Scotland. Mineral-N, present in the peat almost entirely as NH _in4 ^sup+ , accumulated in winter, reaching 42 kg N ha^–1 in the surface 10 cm in April before the application of 112.5 kg N ha^–1 as NH₄NO₃ or urea. In situ incubation of peat cores isolated to prevent leaching, and with grass tops removed, confirmed that net mineralization occurred between November and April, with the greatest rate, 1.2 kg N ha^–1 day^–1, recorded between March and April. During the period May to early June, immobilization of N predominated and rates of net immobilization ranged between 0.2 and 0.8 kg N ha^–1 day^–1. This coincided with a poor uptake into herbage, less than 16% of soil mineral N and fertilizer NH₄NO₃ in June of the first 2 years. The largest counts (most probable number) of ammonifying bacteria in the surface 5 cm were recorded in July for aerobes (27.1×10⁹ litre^–1) and August for anaerorbes (7.1×10⁹ litre^–1). N fertilizer increased these counts significantly (P<0.05) to 56×10⁹ aerobes and 13×10⁹ anaerobes. During July and August, in 2 out of the 3 years, mineralization predominated over immobilization and mean net rates of up to 0.9 kg N ha^–1 were recorded.     

Differences in the activity and bacterial community structure of drained grassland and forest peat soils

The microbial activity and bacterial community structure were investigated in two types of peat soil in a temperate marsh. The first, a drained grassland fen soil, has a neutral pH with partially degraded peat in the upper oxic soil horizons (16% soil organic carbon). The second, a bog soil, was sampled in a swampy forest and has a very high soil organic carbon content (45%), a low pH (4.5), and has occasional anoxic conditions in the upper soil horizons due to the high water table level. The microbial activity in the two soils was measured as the basal and substrate-induced respiration (SIR). Unexpectedly, the SIR (μl CO₂ g⁻¹ dry soil) was higher in the bog than in the fen soil, but lower when CO₂ production was expressed per volume of soil. This may be explained by the notable difference in the bulk densities of the two soils. The bacterial communities were assessed by terminal restriction fragment length polymorphism (T-RFLP) profiling of 16S rRNA genes and indicated differences between the two soils. The differences were determined by the soil characteristics rather than the season in which the soil was sampled. The 16S rRNA gene libraries, constructed from the two soils, revealed high proportions of sequences assigned to the Acidobacteria phylum. Each library contained a distinct set of phylogenetic subgroups of this important group of bacteria.     

Seasonal fluctuation in microbial biomass and activity along a natural nitrogen gradient in a drained peatland

The effects of peat total N on the dissolved N and C concentrations and microbial biomass and activity and their range of seasonal fluctuation were studied in a drained peatland forest in Finland. Seasonal fluctuations in the concentrations of extractable dissolved organic (DON) and inorganic nitrogen (DIN) compounds and extractable dissolved organic carbon (DOC), microbial C and N, ergosterol, net and gross N mineralisation rates were investigated during two growing seasons along a natural peat N gradient in a drained peatland. Significant seasonal fluctuations in NH₄⁺ and DOC concentrations, microbial C and N, but not in ergosterol or microbial C-to-N ratios in the peat, were observed during the 1999 and 2000 growing seasons. The peat total N concentration affected extractable DON and DOC, but not DIN concentrations in the peat. A negative correlation was found between total N concentration in peat and microbial N and C, and a positive correlation between total N and ergosterol, in peat with N concentrations of up to 2%. Gross mineralisation rates did not show any correlation, whereas net mineralisation rates showed a significant positive correlation with the total N concentration of the peat in both 1999 and 2000.     

No signs of thermal acclimation of heterotrophic respiration from peat soils exposed to different water levels

In a mesocosm experiment, with bare peat soils exposed to different water levels (WL), we examined whether heterotrophic respiration (R_h) acclimated to a 3 ^°C temperature increase. Across all WLs, R_h at 15 ^°C was never lower in the heated treatment than in the unheated treatment, indicating that R_h did not acclimate to the warmer conditions. We hypothesize that this lack of thermal acclimation is due to the unlimited substrate availability in these organic soils. These results imply that peat soils may exhibit a sustained positive feedback to global warming.     

The role of hydropedologic vegetation zones in greenhouse gas emissions for agricultural wetland landscapes

Net greenhouse gas (GHG) source strength for agricultural wetland ecosystems in the Prairie Pothole Region (PPR) is currently unknown. In particular, information is lacking to constrain spatial variability associated with GHG emissions (CH₄, CO₂, and N₂O). GHG fluxes typically vary with edaphic, hydrologic, biologic, and climatic factors. In the PPR, characteristic wetland plant communities integrate hydropedologic factors and may explain some variability associated with trace gas fluxes at ecosystem and landscape scales. We addressed this question for replicate wetland basins located in central North Dakota stratified by hydropedologic vegetation zone on Jul 12 and Aug 3, 2003. Data were collected at the soil-atmosphere interface for six plant zones: deep marsh, shallow marsh, wet meadow, low prairie, pasture, and cropland. Controlling for soil moisture and temperature, CH₄ fluxes varied significantly with zone (p < 0.05). Highest CH₄ emissions were found near the water in the deep marsh (277,800 μg m^− 2 d^− 1 CH₄), which declined with distance from water to − 730 μg m^− 2 d^− 1 CH₄ in the pasture. Carbon dioxide fluxes also varied significantly with zone. Nitrous oxide variability was greater within zones than between zones, with no significant effects of zone, moisture, or temperature. Data were extrapolated for a 205.6 km² landscape using a previously developed synoptic classification for PPR plant communities. For this landscape, we found croplands contributed the greatest proportion to the net GHG source strength on Jul 12 (45,700 kg d^− 1 GHG-C equivalents) while deep marsh zones contributed the greatest proportion on Aug 3 (26,145 kg d^− 1 GHG-C equivalents). This was driven by a 30-fold reduction in cropland N₂O–N emissions between dates. The overall landscape average for each date, weighted by zone, was 462.4 kg km^− 2 d^− 1 GHG-C equivalents on Jul 12 and 314.3 kg km^− 2 d^− 1 GHG-C equivalents on Aug 3. Results suggest GHG fluxes vary with hydropedologic soil zone, particularly for CH₄, and provide initial estimates of net GHG emissions for heterogeneous agricultural wetland landscapes.     

Physical properties of tsunami-affected soils in Aceh, Indonesia: 2½ years after the tsunami

On 26 December 2004, a tsunami caused extensive loss of life, damaged property and degraded agricultural land in the province of Aceh, Indonesia. While some of the associated soil chemical changes have been documented, information on soil physical properties is sparse. The objective of this study was to quantify physical properties of some tsunami-affected upland agricultural soils in Aceh, Indonesia. Soil was sampled approximately 21/2 years after the tsunami, from the 0–0.1 m, 0.1–0.3 m and 0.3–0.5 m depths in four sites in the villages of Kling Cot Aroun in Aceh Besar sub-district, Kuta Kruen in Aceh Utara sub-district, Udjong Blang Mesjid in Bireuen sub-district and Meue in Pidie Jaya sub-district on the east coast of Aceh. These sites were located within 1 km from the sea at elevations ranging from 0 to 5 m ASL. The soils were Ultisols except for Meue, which was an Entisol. Soil properties measured were bulk density, structural stability and particle size distribution. Soil water retention, pore-size distribution and saturated hydraulic conductivity were estimated by inserting the values of bulk density, clay, sand and silt contents into pedotransfer functions from the literature. The analyses conducted during this study did not permit us to ascertain what proportion of the soil particles were of tsunami-origin. Nonetheless, deposition of finer-textured material may have occurred in two of the sites. In comparison with the greyish-white, coarse textured soil in the rest of the profile, a finer-textured yellow horizon was present in the lower slopes of the Udjong Blang Mesjid site. At Meue, clay and silt contents were higher in the surface 0.3 m than in the 0.3–0.5 m depth, although a distinct horizon was absent. Particle size distribution in all sites was dominated by the sand fraction, although clay and silt contents were relatively high (20–30 g 100 g^− 1) at Kuta Kruen. Among the sand fractions, fine sand (0.02–0.25 mm) was highest at Kling Cot Aroun, Kuta Kruen and in the “yellow horizon” at Udjong Blang Mesjid, making them more prone to hardsetting and compaction after intensive tillage. Soil compaction was present in all sites with that in the “yellow horizon” at Udjong Blang Mesjid being highest. The relatively low porosity in this layer may be beneficial, as it is likely to reduce the high rates of water drainage and nutrient leaching in this sandy soil. The more compacted soils were characterised by higher numbers of micropores (r, pore radius < 4.3 μm), lower water retention at saturation, smaller numbers of macropores (r > 14.3 μm), lower hydraulic conductivity and intensive gleying, indicating frequent waterlogging. The soils in all depths from Kling Cot Aroun and the “yellow horizon” at Udjong Blang Mesjid were very dispersive, that at Meue moderately dispersive in the 0.3–0.5 m depth but stable in the 0–0.1 m depth, and at Kuta Kruen very stable in all depths. Soil physical degradation was a feature of the soils examined, and its amelioration will be the key to improving and sustaining crop yields in these soils. Possible management interventions include organic amendments such as compost or manure, and minimum tillage options such permanent beds or zero tillage with retention of crop residues as in situ mulch together with suitable cover crops.     

Denitrification rates in relation to groundwater level in a peat soil under grassland

In this study spatial and temporal relations between denitrification rates and groundwater levels were assessed for intensively managed grassland on peat soil where groundwater levels fluctuated between 0 and 1 m below the soil surface. Denitrification rates were measured every 3–4 weeks using the C₂H₂ inhibition technique for 2 years (2000–2002). Soil samples were taken every 10 cm until the groundwater level was reached. Annual N losses through denitrification averaged 87 kg N ha^-1 of which almost 70% originated from soil layers deeper than 20 cm below the soil surface. N losses through denitrification accounted for 16% of the N surplus at farm-level (including mineralization of peat), making it a key-process for the N efficiency of the present dairy farm. Potential denitrification rates exceeded actual denitrification rates at all depths, indicating that organic C was not limiting actual denitrification rates in this soil. The groundwater level appeared to determine the distribution of denitrification rates with depth. Our results were explained by the ample availability of an energy source (degradable C) throughout the soil profile of the peat soil.This revised version was published online November 2003 with corrections to Figure 4 and in February 2004 with corrections to Figure 2.     

Biological effects of native and exotic plant residues on plant growth, microbial biomass and N availability under controlled conditions

The leaf litter of six tropical tree species (Acacia holosericea, Acacia tortilis, Azadirachta indica, Casuarina equisetifolia, Cordyla pinnata and Faidherbia albida) frequently used in agroforestry plantations in Sahelian and Soudano-Sahelian areas were tested for their influence on soil nitrogen content, microbial biomass and plant growth under controlled greenhouse conditions. Half of the soil was planted with onion (Allium cepa L.) seedlings and the other half was not. Two herbaceous species, Andropogon gayanus and Eragrostis tremula, were also studied. Co-inertia analysis (CIA) and one-way analysis of variance (ANOVA) analysis showed that C. pinnata and F. albida leaf powder amendment induced the highest plant growth, whereas leaf powder of E. tremula is associated to higher microbial biomass and NH₄⁺ content. Higher onion seedlings growth is associated with higher concentration of nitrogen and lignin in leaf powders. Conversely, lower plant growth is associated to higher rates of cellulose, hemicellulose and phenols in leaves. Higher rates of cellulose and hemicellulose are associated with higher microbial biomass and NH₄⁺, whereas phenols are associated to lower microbial biomass. The results showed that amendment of A. holosericea leaf powder (high concentrations of phenol) to the soil resulted in a lower microbial biomass and lower onion seedlings growth. Data showed that the plant residue quality index (PRQI) could be a useful tool to predict the effects of litter materials on root growth in glasshouse conditions. The highest values on soil and plant parameters were recorded with C. pinnata litter. While powdered leaf material increased the accessibility of substrates to microbes, more research with C. pinnata leaf litter (under a wider range of ecological conditions) is needed. It could add deeper on its agronomic impact in the tropics.     

Ecophysiology of the cave isopod Mesoniscus graniger (Frivaldszky, 1865) (Crustacea: Isopoda)

Feeding biology and thermal adaptations of the terrestrial isopod Mesoniscus graniger were studied. M. graniger is a depigmented isopod mainly inhabiting cave systems, although it has also been reported in endogeic (soil) habitats. Physiological adaptations are expected to reflect the unique environmental characteristics of caves, including restricted food sources, and stable microclimate with temperatures not exceeding about 10 °C and high relative humidity. The M. graniger from Domica and Ardovská caves (Slovakian Karst) were investigated. We identified organic deposits with associated microorganisms as sources exploited by M. graniger and assessed how these might supply essential polyunsaturated fatty acids (PUFA) in its nutrition. Algae, fungi and bat guano were found as the most important potential resources of PUFA for isopods. Digestive enzymes amylase, trehalase, saccharase and maltase were confirmed in the whole body homogenates of isopods; neither cellulolytic activity nor activities against xylan, laminaran and lichenan were observed. Amylase, maltase and cellobiase activities were also observed in bacterial strains isolated and cultured from isopod midgut, and may account for the measured whole-animal activities. In an artificial temperature gradient, M. graniger selected temperature 3.5 ± 5.4 °C with lower and upper extremes of –1.5 and 18.5 °C. Respiration, as measured by VO₂, was almost independent of temperature between 5 and 10 °C, then increased between 10 and 30 °C. These ecophysiological measures are consistent with adaptation to a stenothermal environment.     

Preliminary observations on the natural variation in the endogenous rhythm of the desert Isopod Hemilepistus reaumurii

The desert woodlouse Hemilepistus reaumurii lives in monogamous pairs, together with their offspring, in self-dug burrows. Every burrow contains only one family. This animal is active on the surface from the beginning of spring to autumn. It remains in the burrow during the hottest part of the day, to avoid the heat, but is active at the surface during morning and evening. During the winter, low temperature prevents its above ground activity. Freshly collected animals, from the north of Kairouan (Tunisia) were transferred to a controlled environment cabinet in the laboratory and their locomotor activity monitored as individuals or in male/female pairs, in annular chambers equipped with an infrared activity recording system. For the first 7 days of recording, the animals were kept in a light–dark cycle in phase with the natural diel cycle. For the second 7 days, animals were maintained in constant darkness. The temperature was held constant at 18±1 °C. Analysis of preliminary data indicates that both individuals and pairs exhibit an endogenous rhythm of locomotor activity with a circadian periodicity. Activity was mostly confined to the hours of the photophase (1–7 days) or to those of subjective day (8–14 days). The activity pattern showed two main peaks, around dusk and dawn, respectively. This bimodal circadian rhythm persisted for as long as the experiments were run, and was clearer in individual recording than in pair recording. The characteristics and biological significance of the rhythm are discussed.     

Survival of faecal coliforms in four different types of sludge-amended soils in Botswana

This study aimed at identifying the factors that affected the survival of faecal coliforms as measured by E. coli in four types of soils in Botswana which were amended with sewage sludge. Physico-chemical and biochemical properties and coliform population in the different soils, sludge, and soil/sludge mixtures were determined after sampling, on composing the soil/sludge mixtures, and 90 days after composing. Coliform population in the different soil/sludge mixtures decreased by about 90% after 90 days. The age of the sludge used and the rate at which it was applied to the soils initially determined the population of E. coli in each soil/sludge mixture, but after 90 days, differences inherent in the different soil types were the main determinants of the E. coli population. Percentage reduction of coliforms in Type 1 sludge mixtures were lower (38%) than in Type 2 sludge mixtures (57%). Up to 79.8% of the reduction observed in E. coli population in the Barolong luvisol-sludge mixtures after 90 days was caused by reduction in pH and moisture content, while 72.6%, 84.5% and 55.1% of the reduction in E. coli population in Tuli luvisol-, arenosol- and vertisol-sludge mixtures, respectively, was accounted for by the reduction in moisture content and Olsen P concentration. Coliform survival rates varied with soil types being 12%, 6.4%, 5.3% and 5.8% for the vertisol, arenosol, Barolong luvisol and Tuli luvisol, respectively. A minimum period of 90 days should be allowed between when sludge is applied to similar soils and when seeds are sown. The exact period should, however, be determined by the properties of the soil with fine-textured soils requiring a longer period than coarse-textured soils.     

Influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol

In this study, the influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol from north east Scotland was investigated. Firstly, soil cores were incubated at 5, 10 and 15°C for up to 8 weeks. Net mineralisation was observed at all temperatures with larger rates observed at higher temperatures. In contrast, water extractable dissolved organic nitrogen (DON) displayed no clear trend with time and showed little response to temperature. Secondly, intact cores of the same soil, with and without vegetation, were leached with artificial rain for 6 weeks at 6.5 and 15°C. Temperature and the presence of vegetation interacted to have a significant (P<0.01) effect on the concentration of NO₃⁻ in leachates; highest concentrations were observed in leachates from cores without vegetation at 15°C, whereas lowest concentrations were observed in leachates from cores with vegetation at 6.5°C. In contrast, concentrations of DON and dissolved organic carbon (DOC) were significantly (P<0.001) higher in leachates from cores with vegetation than without vegetation and were not affected by temperature. The cumulative amounts of DON and DOC leached from the cores with vegetation were 4 and 2.5 times greater, respectively, than those leached from the cores without vegetation. Comparison of soil solution (extracted by centrifugation at 0–5 and 5–10 cm depth) after leaching for 6 weeks, showed that the upper layer contained more than twice the amount of DON than the 5–10 cm layer and that the difference in concentration between the two depths was enhanced in the presence of vegetation. The results indicate that vegetation is an important source of DON and DOC. However, the removal of vegetation did not lead to an increase in the quantity of total dissolved nitrogen (TDN) in soil water, but resulted in a change in the dominant N fraction from DON to NO₃⁻. In addition, the results show that DON, in both the incubated and leached cores, did not change as inorganic N was mineralised. This suggests that if water extractable DON was acting as a source of NH₄⁺ or NO₃⁻, then it was being replenished by, and in equilibrium with, a large reserve of organic N. Evidence of such a pool was indirect in the form of additional DON (equivalent to 2 g N m⁻²) being extracted by 0.5 M K₂SO₄.     

Role of cation exchange in preventing the decay of anoxic deep bog peat

The major bog systems in northern areas are dominated by Sphagnum species, the partially decomposed remains of which form the bulk of deep peat. By adding mono- and di-valent cations to deep peat cores (2.0-2.5 m) and measuring CH₄ and CO₂ concentrations in the manipulated peat cores using quadrupole mass spectrometry (QMS) we demonstrate that the lack of availability to microorganisms of essential cations is limiting decay in deep peat. The cations with the highest binding strength displaced the most cations and stimulated decay. Decay in deep peat cores was also stimulated by a C source (acetate), but not by NH₄⁺. The addition of cations and acetate resulted in a less than additive stimulation of decay. The stimulatory effect of acetate and copper decreased in the presence of ammonium. The addition of EDTA to surface bog peat (where cations are conserved) decreased decay rates in surface peat (0.0-0.5 m) to that of the untreated deep peat (2.0-2.5 m). Deep peat was unaffected by treatment with EDTA. The effect of adding Cu²⁺differed with the depth from which the peat was collected. Cu²⁺ did not stimulate decay in surface bog peat (0.0-1.5 m) but stimulated decay in peat from 1.5-3.5 m. Below 3.5 m to the bottom of the profile (5.0 m) no positive effect was observed. By comparing deep peat with surface peat we have shown that cation limitation because of high cation exchange capacity is specific to the main mass of deep bog peat and may explain differences in decay rates between anoxic surface peat and deep peat.     

Loss of low molecular weight dissolved organic carbon (DOC) and nitrogen (DON) in H2O and 0.5 M K2SO4 soil extracts

Soil extracts are routinely used to quantify dissolved organic nutrient concentrations in soil. Here we studied the loss and transformation of low molecular weight (LMW) components of DOC (¹⁴C-glucose, 1 and 100 μM) and DON (¹⁴C-amino acid mixture, 1 and 100 μM) during extraction of soil (0-6 h) with either distilled water or 0.5 M K₂SO₄. The extractions were performed at 20 °C, at 4 °C, or in the presence of an inhibitor of microbial activity (HgCl₂ and Na-azide). We showed that both glucose and amino acids became progressively lost from solution with increasing shaking time. The greatest loss was observed in H₂O extracts at 1 μM for both substances (>90% loss after 15 min). Lower temperature (4 °C) and presence of K₂SO₄ both resulted in reduced loss rates. The presence of microbial inhibitors effectively eliminated the loss of glucose and amino acids. We conclude that microbial transformation of LMW-DOC and DON during H₂O or K₂SO₄ extraction of soil may affect the estimation of their concentrations in soil. This finding has significant implications for methods that rely on chemical extractions to estimate LMW-C components of DOC and DON.     

A simultaneous model for ultrasonic aggregate stability assessment

Aggregate stability is a difficult to quantify, complex soil property. Ultrasonic processing of soil–water suspensions enables quantifiable and readily reproducible assessment of the level of mechanical energy applied to soil aggregates. Here, we present a method of investigating the stability and comminution of soil aggregates by simultaneously modeling the redistribution of particles throughout any arbitrarily-selected set of soil particle-size intervals as ultrasonic energy is applied to a soil–water suspension. Following model development, we demonstrate its application to 5 particle-size subgroups (0.04–2000 μm) of a Dystroxerept subject to 12 levels of ultrasonic energy between 0 and 5800 J g^− 1 (750 mL^− 1). Laser granulometry was used for particle-size distribution (PSD) analysis, providing precise, non-disruptive measurements of changes in the volume of PSD subgroups in both the microaggregate (< 250 μm; 3 subgroups) and macroaggregate (> 250 μm; 2 subgroups) fractions throughout ultrasonic treatment. Two groups of aggregates were detected exhibiting significantly (p < 0.05) different ultrasonic stability: a group composed exclusively of macroaggregates ranging 250–2000 μm in size, and a finer, relatively stable group ranging 20–1000 μm. The PSD of particles liberated from two aggregate groups significantly (p < 0.05) differed: the coarser, less-stable group liberated 13% clay (0.04–2 μm), 53% fine silt (2–20 μm), and 34% coarse silt and sand (20–250 μm); while the finer, more-stable group liberated 26% clay and 74% fine silt. The ultrasonic energy required to disrupt 25%, 50%, and 75% of all aggregates within a given PSD interval significantly (p < 0.05) differed between all selected intervals, showing a trend of declining stability with increasing particle-size. Both the flexibility of the proposed model and the extension of ultrasonic stability assessment to simultaneous analysis of both microaggregate and macroaggregate subgroups can facilitate broader application of ultrasonic methods to soil processes related research.     

Saturation of raised bog peat exchange sites by Pb and Al stimulates CH4 production

We examined the effect of cation treatments on methanogenic activity and nutrient release from exchange sites in raised bog and fen peats. Treatments consisted of cation chloride solutions (MgCl₂, AlCl₃ and PbCl₂) applied individually. In raised bog peat Al³⁺ and Pb²⁺ increased CH₄ production. A correlation was found between CH₄ production and the amount of micro- and macronutrient cations released by the treatments. In calcareous fen peat, such a stimulation was also found, but there was no correlation between CH₄ production and micro and macronutrient release. Direct nutrient and pH effects could not account for these observations. Thus the results support the hypothesis that the methanogenic community in the raised bog is limited by the availability of mineral nutrients and/or inactivity of exo-enzymes, both of which are bound onto exchange sites.     

Isotope (C and C) analysis of deep peat CO2 using a passive sampling technique

We developed and tested a new method to collect CO₂ from the surface to deep layers of a peatland for radiocarbon analysis. The method comprises two components: i) a probe equipped with a hydrophobic filter that allows entry of peat gases by diffusion, whilst simultaneously excluding water, and, ii) a cartridge containing zeolite molecular sieve that traps CO₂ passively. We field tested the method by sampling at depths of between 0.25 and 4 m at duplicate sites within a temperate raised peat bog. CO₂ was trapped at a depth-dependent rate of between ∼0.2 and 0.8 ml d⁻¹, enabling sufficient CO₂ for routine ¹⁴C analysis to be collected when left in place for several weeks. The age of peatland CO₂ increased with depth from modern to ∼170 BP for samples collected from 0.25 m, to ∼4000 BP at 4 m. The CO₂ was younger, but followed a similar trend to the age profile of bulk peat previously reported for the site (Langdon and Barber, 2005). δ¹³C values of recovered CO₂ increased with depth. CO₂ collected from the deepest sampling probes was considerably ¹³C-enriched (up to ∼+9‰) and agreed well with results reported for other peatlands where this phenomenon has been attributed to fermentation processes. CO₂ collected from plant-free static chambers at the surface of the mire was slightly ¹⁴C-enriched compared to the contemporary atmosphere, suggesting that surface CO₂ emissions were predominantly derived from carbon fixed during the post-bomb era. However, consistent trends of enriched ¹³C and depleted ¹⁴C in chamber CO₂ between autumn and winter samples were most likely explained by an increased contribution of deep peat CO₂ to the surface efflux in winter. The passive sampling technique is readily portable, easy to install and operate, causes minimal site disturbance, and can be reliably used to collect peatland CO₂ from a wide range of depths.