The effect of landscape processes upon gene flow and genetic diversity in an Australian freshwater fish, Neosilurus hyrtlii

1. Flow regime and riverine architecture are two important landscape characteristics that influence genetic diversity and gene flow in riverine species.
2. Using population genetic markers (mtDNA, microsatellites and allozymes), this study aimed to investigate genetic diversity and gene flow in the freshwater fish, Neosilurus hyrtlii , across two major drainage divisions in northern and central Australia (the Gulf of Carpentaria and Lake Eyre basins). These basins lie adjacent to each other and differ in their hydrological inputs and riverine structure, providing an ideal opportunity to identify the impact of landscape processes upon population dynamics of freshwater fish.
3. Populations were strongly structured among basins, among catchments within basins and were weakly structured within catchments in the Lake Eyre Basin, providing support for the Stream Hierarchy Model.
4. Interestingly, mtDNA and microsatellite diversity was much higher in the Gulf of Carpentaria Basin compared to the Lake Eyre Basin. It was concluded that this difference was due to the extreme hydrological variability in this basin and boom-bust population cycles resulting in smaller effective population sizes in the Lake Eyre Basin.     

Flying with the birds? Recent large-area dispersal of four Australian Limnadopsis species (Crustacea: Branchiopoda: Spinicaudata)

Temporary water bodies are important freshwater habitats in the arid zone of Australia. They harbor a distinct fauna and provide important feeding and breeding grounds for water birds. This paper assesses, on the basis of haplotype networks, analyses of molecular variation and relaxed molecular clock divergence time estimates, the phylogeographic history, and population structure of four common temporary water species of the Australian endemic clam shrimp taxon Limnadopsis in eastern and central Australia (an area of >1,350,000 km(2)). Mitochondrial cytochrome c oxidase subunit I sequences of 413 individuals and a subset of 63 nuclear internal transcribed spacer 2 sequences were analyzed. Genetic differentiation was observed between populations inhabiting southeastern and central Australia and those inhabiting the northern Lake Eyre Basin and Western Australia. However, over large parts of the study area and across river drainage systems in southeastern and central Australia (the Murray-Darling Basin, Bulloo River, and southern Lake Eyre Basin), no evidence of population subdivision was observed in any of the four Limnadopsis species. This indicates recent gene flow across an area of ～800,000 km(2). This finding contrasts with patterns observed in other Australian arid zone taxa, particularly freshwater species, whose populations are often structured according to drainage systems. The lack of genetic differentiation within the area in question may be linked to the huge number of highly nomadic water birds that potentially disperse the resting eggs of Limnadopsis among temporary water bodies. Genetically undifferentiated populations on a large geographic scale contrast starkly with findings for many other large branchiopods in other parts of the world, where pronounced genetic structure is often observed even in populations inhabiting pools separated by a few kilometers. Due to its divergent genetic lineages (up to 5.6% uncorrected p-distance) and the relaxed molecular clock divergence time estimates obtained, Limnadopsis parvispinus is assumed to have inhabited the Murray-Darling Basin continuously since the mid-Pliocene (～4 million years ago). This means that suitable temporary water bodies would have existed in this area throughout the wet-dry cycles of the Pleistocene.     

Evidence for multiple historical colonizations of an endoreic drainage basin by an Australian freshwater fish

The contemporary and historical colonization capacity of an Australian freshwater fish, north-west glassfish Ambassis sp., was tested using mtDNA sequence data and six newly developed microsatellite loci in an endoreic basin in central Australia. Overall, Ambassis sp. exhibited weak genetic structure within catchments, suggesting some capacity to recolonize extirpated waterholes after disturbance. Genetic structure revealed that the historical pattern of connectivity among catchments in the Lake Eyre Basin was dramatically different from other species studied in this region. Two highly divergent clades were detected in separate catchments in the basin. mtDNA from individuals sampled in catchments north of the Lake Eyre Basin suggest that Ambassis sp. has colonized on two separate occasions from catchments in northern Australia, subsequently generating two highly divergent lineages.     

Phylogeography of the widespread Australian freshwater prawn, Macrobrachium australiense (Decapoda, Palaemonidae)

Aim To investigate the phylogeographic structure of the widespread freshwater prawn, Macrobrachium australiense, within and between major Australian drainage basins using mitochondrial sequence data. This will enable the investigation of historical connections between major drainages and examination of hypotheses of biogeographic associations among Australian freshwater basins. Location Inland, eastern and northern Australia. Methods Sequencing 16S rRNA and ATPase 6 protein coding mitochondrial DNA genes from M. australiense from 19 locations from inland, eastern and northern Australia. Results Within drainage basins, haplotype trees are monophyletic, with the exception of the Finke River from the Lake Eyre Basin. Macrobrachium australiense from the two main inland drainages, the Murray–Darling and Lake Eyre Basin are divergent from each other and do not form a monophyletic group, instead the Murray–Darling Basin haplotypes clade with eastern coastal haplotypes. Haplotypes from neighbouring eastern coastal drainages were found to be quite divergent from each other. Main conclusions The phylogeographic relationships among M. australiense suggest that the two major inland drainages, the Murray–Darling Basin and the Lake Eyre Basin, are not biogeographically closely associated to each other. Instead the Murray–Darling Basin is more closely allied with the eastern coastal drainages across the Great Dividing Range. Despite their proximity the neighbouring southeast Queensland coastal Mary and Brisbane Rivers are also biogeographically divergent from each other. The results also indicate that the Finke River appears to have been isolated from the remainder of the Lake Eyre Basin catchment for a significant period of time.     

Sediment preferences and size-specific distribution of young-of-the-year Pacific halibut in an Alaska nursery

A combination of laboratory experiments and field surveys was used to test the hypotheses that responses to sediments change with fish size and that sediment grain-size is the predominant environmental factor affecting small-scale distribution in young-of-the-year (yoy) Pacific halibut Hippoglossus stenolepis . Laboratory tests showed that the smallest fish (31–40 mm L _T) chose fine sediments (muddy and fine sands), fish 51–70 mm had high selectivity (primarily medium sand), and the largest fish (80–150 mm) were not selective although they avoided the largest grain-sizes (pebbles and granules). Sediment preferences were correlated with size-dependent burial capabilities. Beam trawl collections were made over a 6 year period in Kachemak Bay, Alaska, to examine the distribution of yoy Pacific halibut (14–120 mm L _T) using small size classes (e.g. 10 mm intervals). Canonical correlation analysis showed that the per cent of sand in the sediment was a highly significant variable for all but one size and date combination. Catch per unit of effort (CPUE) for newly settled fish (<30 mm L _T) was highest on very fine sand, fish 41–80 mm were most abundant on fine sand, and the largest yoy fish (81–120 mm) were abundant over a range of sediments from fine sand to mud. Except for the smallest fish, Pacific halibut in the field were associated with sediments somewhat finer than predicted from the laboratory experiments; however, virtually all were captured where they could bury easily. The ability of flatfish to bury and shelter in sediment is related to fish size; consequently, habitat associations shift rapidly during the first year of life. Habitat models for yoy flatfishes should consider size-dependent shifts in capabilities and preferences.     

Did central Australian megafaunal extinction coincide with abrupt ecosystem collapse or gradual climate change?

Aim In central Australia, the giant flightless bird Genyornis newtoni disappeared about 45–50 thousand years ago (ka). It has been reported that coincident with this extinction the carbon isotopic composition of preserved eggshells of the extant emu (Dromaius novaehollandiae) shows an abrupt dietary shift from tropical grasses (C₄ photosynthesis) to temperate grasses and/or woody browse (C₃ photosynthesis). This abrupt shift has been interpreted as signalling ‘ecosystem collapse’ due to landscape burning by humans. We evaluate an alternative interpretation, that the shift in diet was not abrupt, but gradual, and caused by the weakening of the Australian monsoon. Location Lake Eyre, central Australia. Methods We re‐analysed a large, published dataset of emu diet δ¹³C (inferred from δ¹³C of preserved eggshells) spanning the last 140,000 years, using time‐series analysis. Using Akaike's information criterion, we compared two contrasting models: (1) there was an abrupt shift in δ¹³C coincident with the extinction of Genyornis, assumed 47.5 ka; and (2) there was a gradual shift in δ¹³C, correlated with reconstructed water level in Lake Eyre, a proxy for monsoon intensity. Results There was little evidence of an abrupt shift in emu diet δ¹³C about 45–50 ka, but δ¹³C appeared to steadily decrease between about 80 and 30 ka. Indeed, the model representing a correlation between δ¹³C and lake level was more than seven times more likely than the model representing an abrupt shift at 47.5 ka. Main conclusions The emu eggshell isotopic record from Lake Eyre does not support the hypothesis that landscape burning by humans transformed a savanna?grassland mosaic into the modern desert scrub, contributing to the extinction of Genyornis. While our findings cast strong doubt on the foremost line of evidence that landscape burning by humans caused the megafaunal extinctions, and suggest that central Australia was becoming increasingly arid in the Late Pleistocene, the relative roles of hunting by humans and climate change in the megafaunal extinctions remain unresolved.     

Biogeography of Nicotiana section Suaveolentes (Solanaceae) reveals geographical tracks in arid Australia

Aim Nicotiana section Suaveolentes is largely endemic to Australia but includes one species endemic to Africa, one to New Caledonia and Tongatupa, and one to the Marquesas Islands in the Pacific. Other sections of Nicotiana are found in the New World. In Australia, Suaveolentes is widespread across the continent, with many taxa adapted to the Eremean zone. We aim to analyse the biogeography of the Australian clade, both to shed light on the evolution of the group and to determine general area relationships that provide insight into the history of the arid‐zone biota. Location Mesic and arid regions of continental Australia, the Central–South Pacific and Namibia, Africa. Methods A phylogeny of Suaveolentes, based on morphology and molecular data, was used to analyse the relationships of areas in which the taxa occur. The section is monophyletic, and all but three taxa were included (25). The method of paralogy‐free subtree analysis was employed, with the basal taxon Nicotiana africana used as the outgroup. Results Paralogy‐free subtree analysis found five area subtrees that, when combined, resulted in a minimal area cladogram with six resolved nodes. Pacific and mesic eastern Australia (including Lord Howe Island) are at the base of the area cladogram, followed by the differentiation of North West Australia and later South East Australia. Arid regions of Australia are related, revealing three biogeographical tracks: a northern track including the Great Sandy Desert and Tanami, which are related to the Pilbara; a central track relating the Western Desert, Central Ranges, Eastern Desert and North East Interzone; and a southern track relating the South West Interzone, Nullarbor, Adelaide/Eyre and the South East Interzone. Plesiomorphic taxa with chromosome number n = 24–23 occur on the periphery of the continent, and derived taxa with n = 21, 20, 18, 16–15 identify the tracks across arid Australia. Main conclusions The patterns of distribution and differentiation of Suaveolentes in Australia show that the age of the clade is at least Early Miocene, dating to before the onset of aridification in Australia about 15 Ma. The patterns are also interpreted as evidence that it was vicariance that largely shaped speciation in the Eremean zone, with range expansion of some widespread taxa probably occurring in the most recent cycles of severe drying and mobilization of desert dune sands.     

The Permo-Carboniferous Dwyka Formation of Southern Africa: Deposition by a predominantly subpolar marine ice sheet

The 800-m thick glacigene Dwyka Formation was deposited along the northern margin of the Permo-Carboniferous Dwyka Basin which covered an area of approximately 2 × 10⁶ km² in southwestern Gondwana. The palaeogeographic setting, geochemical data of the mudrocks and diamictites, and the palaeontology indicate marine conditions during sedimentation. Ice lobes from spreading centres in the north, east and south coalesced in the basin to form an extensive ice cover from the Westphalian to the late Sakmarian.

Lodgement, rain-out and subaqueous debris-flow diamictons, subaqueous and subglacial melt-water sands, suspended mud, and turbidity current sands and silts accumulated in the Dwyka Basin. Sedimentation started on the continental shelf during a grounded ice sheet stage (predominantly lodgement processes), then a floating ice stage (predominantly debris rain-out), and finally an ice sheet disintegration stage (debris rain-out, sand fall-out and suspension settling of mud). The palaeogeographic setting, presence of marine conditions in the basin and the scale of glaciation indicate deposition from a predominantly mid-latitudinal marine ice sheet. The overall characteristics of the glacial sequence are neither typical of a polar nor a temperate setting and for such ancient glaciations a subpolar setting with the presence of unstable ice shelves is suggested.     

Distribution and bioavailability of heavy metals in different particle-size fractions of sediments in Taihu Lake,China

Abstract

Taihu Lake is one of the most important water sources in the economically developed central-eastern part of China, and metal pollution is a major concern for the lake. The distribution and bioavailability of Cd, Cr, Cu, Pb, Sb and Zn were analysed in undifferentiated bottom sediments and in various particle-size fractions of the sediment from different parts of the lake. The average concentration of total metals in undifferentiated sediments ranged from 0.86 mg kg^-1 (Cd) to 95.45 mg kg^-1 (Zn) for the entire lake, with the highest concentrations in Zhushan Bay. The concentration of heavy metals was higher in extremely fine sands (0.064–0.125 mm) and fine sands (0.125–0.25 mm) than in other fractions. Sequential extractions showed that Cu, Zn and Cd were the most bioavailable accounting for 55.6%, 38.7% and 30.0% of their total concentration, respectively. However, the bioavailable proportion of many metals was not significantly different between grain grades except for Cu and Zn, which were higher in silts (<0.064 mm) than in other grades. Compared with the background values of local soils, the concentration of Zn, Cd, Cu, Pb and Sb was higher, indicating enrichment in the sediment. From ecological safety concerns, Zn, Cd and Cu should be examined closely because of their higher bioavailabilty in the sediment.     

Faecal indicator bacteria enumeration in beach sand: a comparison study of extraction methods in medium to coarse sands

Aims:  The absence of standardized methods for quantifying faecal indicator bacteria (FIB) in sand hinders comparison of results across studies. The purpose of the study was to compare methods for extraction of faecal bacteria from sands and recommend a standardized extraction technique.
Methods and Results:  Twenty-two methods of extracting enterococci and Escherichia coli from sand were evaluated, including multiple permutations of hand shaking, mechanical shaking, blending, sonication, number of rinses, settling time, eluant-to-sand ratio, eluant composition, prefiltration and type of decantation. Tests were performed on sands from California, Florida and Lake Michigan. Most extraction parameters did not significantly affect bacterial enumeration. anova revealed significant effects of eluant composition and blending; with both sodium metaphosphate buffer and blending producing reduced counts.
Conclusions:  The simplest extraction method that produced the highest FIB recoveries consisted of 2 min of hand shaking in phosphate-buffered saline or deionized water, a 30-s settling time, one-rinse step and a 10 : 1 eluant volume to sand weight ratio. This result was consistent across the sand compositions tested in this study but could vary for other sand types.
Significance and Impact of the Study:  Method standardization will improve the understanding of how sands affect surface water quality.     

Urea hydrolysis and transformations in coastal dune sands and soil

Summary Urea hydrolysis was studied in samples taken from a coastal sand dune succession, from uncolonized sand; the rhizosphere ofAmmophila arenaria and soil from the mature dune. Comparisons were made with urea hydrolysis in a fertile loam soil. Urea was hydrolyzed in all sand and soil samples, with complete hydrolysis occurring after 6 and 3 weeks in the rhizosphere sand and dune soil compared with only 4 days in the fertile loam. A third of the added urea, however, was still present in the uncolonized sand samples 6 weeks after the beginning of the incubation period. Urea hydrolysis broadly correlated with urease activity.The liberated NH ₄ ⁺ was oxidized to NO ₃ ^– –N in all samples. Urea stimulated the release of N from native organic matter in the two soils, but not sands, due presumably to the low organic matter content of the latter. Nitrite accumulated in the dune sands and soil, but not in the fertile loam.Although N-Serve (Nitrapyrin) had no effect on urea hydrolysis in any of the treated samples, it inhibited the nitrification of released NH ₄ ⁺ –N. The relevance of these findings to the use of urea as a fertilizer to improve plant growth and dune stabilization is commented upon.     

Response of bacteria and phytoplankton to contaminated sediments from Trenton Channel,Detroit River

Several types of bioassays were used in 1986 and 1987 to investigate the effect of contaminated sediments on natural populations of bacteria and phytoplankton from the Trenton Channel, Detroit River. The approach included the measurement of uptake of ³H-glucose or ³H-adenine by bacteria and ¹⁴C-bicarbonate by phytoplankton in the presence of different amounts of Trenton Channel and Lake Michigan (control) sediments. Trenton Channel sediments are contaminated by high levels of toxic organic compounds and metals, especially zinc, lead, and copper. Because levels of biomass of bacteria and phytoplankton varied widely among the different bioassays, it was necessary to adjust uptake rates for biomass. Biomass adjustments were made using acridine orange counts for bacteria and chlorophyll measurements for phytoplankton. The results show a statistically significant suppression of uptake of substrates for both bacteria and phytoplankton with increasing amounts of sediment. Uptake was suppressed as much as 90 percent for bacteria and 93 percent for phytoplankton at 1200 mg l^-1 of Trenton Channel sediments compared to bioassays without sediment. Uncontaminated Lake Michigan sediment suppressed uptake much less than Detroit River sediment; the difference in suppression of uptake between the two types of sediment was statistically significant for both bacteria and phytoplankton.Contribution No. 518 of the Center for Great Lakes and Aquatic Sciences of the University of Michigan.     

Particulate matter filtration and seasonal nutrient dynamics in permeable carbonate and silicate sands of the Gulf of Aqaba,Red Sea

This study compares mineralization in permeable silicate and carbonate sands in the shallow shelf of the Gulf of Aqaba. From July 1999 to March 2000, we monitored concentrations of inorganic nutrients in water and pore water at two neighboring sites, one dominated by silicate, the other by carbonate sand. Although the carbonate was coarser than the quartz sand, organic matter, dissolved inorganic nitrogen (DIN), and ortho-phosphate concentrations in the biogenic carbonate sediment always exceeded those in the terrigenic silicate sands (factor 1.5–2.0 for organic matter, 1.7–14.0 for nutrients). Higher nutrient concentrations in the water column during winter months caused increases in pore-water nutrient concentrations in both sediments down to 10 cm depth with no significant delay, emphasizing the effect of advective transport of solutes and particles into permeable sands. An experiment was conducted where sieved clean quartz and carbonate sands of same grain size (250–500 µm) were incubated in-situ. Although exposed to the same water and boundary current conditions, the sieved carbonate sand accumulated more organic matter and developed higher nutrient concentrations than the incubated silicate sediment. We conclude that the mineralogical characteristics of the carbonate sand (higher porosity, sorption capacity and pH buffer capacity) enhance the filtration capacity, and the biocatalytic conversion efficiency relative to the smooth crystalline quartz grains.     

The diatom flora of Lake Eyre South: a large episodically filled salt lake in South Australia

Thirty-three diatom taxa were collected from the large terminal salt lake, Lake Eyre South, located in south-central Australia. The diatom assemblage included taxa that were widely distributed in marine and saline athalassic habitats in both northern and southern hemispheres, with the diatom flora of Lake Eyre South most similar to that of southern Africa. This study represents the first report on the diatom flora of Lake Eyre South and supports the findings that episodically filled lakes do not appear to be good evolutionary loci.     

Chemical limnology of Georgian Bay and the North Channel between 1974 and 1980

Intensive surveys of Georgian Bay and the North Channel were done in 1974 and 1980. This paper reviews the main findings of the surveys and describes the main factors that determine the chemistry of the two water bodies. Georgian Bay is oligotrophic. Its chemistry is determined chiefly by natural phenomena, specifically the geology of the drainage basin and the river runoff in the spring, and by exchange with Lake Huron and the North Channel. Localized eutrophication is found in the bays along the southern shore. The chemistry of the North Channel is determined by its very short residence time — about two years — caused by the large inflow from St. Mary's River. The water quality in the river reflects that of Lake Superior and the inputs of nutrients and contaminants from Sault Ste. Maire. Because of the short flushing time, the Channel remains oligotrophic despite the large loadings of phosphorus.     

鄱阳湖沙地蔓荆灌丛沙堆形态特征及空间分布格局

目前灌丛沙堆研究主要集中在干旱半干旱区的沙质草原和沙漠边缘,对亚热带湿润区灌丛沙堆的形成、演变过程并不清楚。以鄱阳湖沙地为研究区,通过样方调查和地统计学的方法,研究不同沙化程度下蔓荆灌丛沙堆的形态特征及分布格局。结果表明:鄱阳湖沙地蔓荆沙堆的形态以盾形为主,其冠幅变化幅度为1.2—18.2 m~2,固定和半固定沙地显著高于流动沙地;对灌丛沙堆的形态参数来说,其长轴与短轴在固定和半固定沙地上呈极显著的线性相关关系,流动沙地上呈二次函数关系;半固定和流动沙地上沙堆底面积与沙堆高度呈二次函数关系(r0.6);3种类型沙地上灌丛底面积与沙堆体积之间极显著线性相关,其中半固定沙地线性函数的斜率最大;除固定沙地的沙堆高度和半固定沙地的灌丛高度外,3种沙地上蔓荆灌丛与沙堆的其他形态参数间均极显著相关,说明随着沙地的固定,蔓荆灌丛有利于沙堆水平尺度的增长;3种沙地上蔓荆沙堆均呈随机分布。     

Influence of benthic organisms on solute transport in lake sediments

Increased inputs of nutrients into the waters of Lake Okeechobee has raised concern that the lake is becoming hypereutrophic. One aspect in understanding the overall cycling and dynamics of the nutrients in the system is the effect of benthic organisms on solute transport. Various diffusional models have been used to approximate the effect of benthic organisms on solute transport within sediments using diffusion coefficient values which represent the pooled contributions of molecular diffusion (D _s ) and enhanced solute mixing due to macrobenthos activity (D _i ). The objective of this study was to investigate the impact of benthic activity on solute transport by measuringD _s (i.e., no benthic activity) and an apparent-dispersion or mixing coefficientD _m (i.e., with benthic activity) for the four major sediment types of Lake Okeechobee, Florida using a reservoir technique. This method involved monitoring the depletion of a conservative tracer (tritiated water) from the overlying water (reservoir) resulting from transport into sediments using disturbed sediments repacked in cores (3.2 cm diam.) and undisturbed cores (3.2 to 12 cm diam.) obtained from the lake. Additional estimates ofD _m andD _s were also obtained by measuring tracer concentration profiles in the sediment cores at the end of a specified diffusion period. Molecular diffusion coefficients (D _s ) measured forrepacked cores of sand, littoral, mud and peat sediments ranged from 0.90 to 1.29 cm² d⁻¹, and estimates ofD _s were slightly higher in undisturbed cores without benthic organisms.D _m values for undisturbed cores of mud, sand and littoral sediments having macrobenthic populations ranged from 2.09 to 24.78 cm² d⁻¹; values that were 1.6 to 15 times higher than those in sediments without benthic activity. Undisturbed cores of varying diameter from mud sediments had similar estimates ofD _m for tritium; however, the undisturbed cores with larger diameters from littoral sediments yielded larger estimates ofD _m , reflecting the heterogeneity of benthic population densities and activity. Therefore,D _s estimates may not adequately represent transport processes for mud, sand and littoral sediments of Lake Okeechobee; hence careful consideration should be given to the role of benthic organisms in the overall transport of solutes across the sediment-water interface. A contribution of the Florida Agricultural Experiment Station Journal Series No. R-01150. A contribution of the Florida Agricultural Experiment Station Journal Series No. R-01150.     

Effects of microbial activity on the hydrochemistry and sedimentology of Lake Logipi,Kenya

Lake Logipi is a saline soda and alkaline lake which marks the northern termination of the Suguta River drainage system. It also receives waters from streams, possible seepage from Lake Turkana, and hot springs. Present hydrochemistry and sedimentology is controlled by numerous factors including seasonal variations, composition of incoming waters, water depth and, above all, bacterial activity. Given the scarcity of Ca²⁺ and Mg²⁺ in the lake waters, bacterial activity seems to intensify the alkalinization of the waters which inhibits the deposition of organic matter and leads to the genesis of a poorly organic, zeolitic mud that reaches 1.5 m in tickness in the deepest part of the lake. This black layer may be overlaid with thin crusts of trona and halite which prograde over the basin from its southern bank when the lake is drying out and which are dissolved in the lake waters during the rainy season.     

Oxidation of elemental-S in coastal-dune sands and soils

Summary S-oxidation was studied in samples of (a) coastal sands lacking vegetation; (b) sands from beneath isolated stands ofAmmophila arenaria andHippophaë rhamnoides; and (c) dune soils obtained from beneath vegetation growing on mature dunes. S-oxidation in samples taken from dune environments was compared with the process in a fertile garden soil.Elemental-S was oxidized to SO ₄ ^2– in all samples, with S₂O ₃ ^2– being formed as intermediates. S-oxidation was most pronounced in the dune soil, followed by the garden soil,Ammophila arenaria andH. rhamnoides rhizospheres and finally the non-vegetated sand. The rate of S-oxidation thus generally increased with increasing C and N content, increasing vegetation cover and decreasing soil-sand pH.Maximum S-oxidation occurred at 30–37°C, but some of the intermediates appeared even at 45°C, presumably indicating abiotic S-oxidation at high temperatures. S-oxidation decreased the pH of the two soils studied, but did not markedly acidify the unvegetated or rhizosphere sands.