Variability of the <Emphasis Type="Italic">Molinion</Emphasis> meadows in Slovakia

Nine vegetation types were distinguished using cluster analysis within Molinion meadows in Slovakia. Vegetation of cluster 1 occurs on most acidic soils and is characterized by the occurrence of species of the Caricion fuscae alliance and of the Nardus grasslands. Vegetation of cluster 2 is also found on rather acidic soils but in contrast to cluster 1 vegetation it contains species of base-rich sites, such as Betonica officinalis, Galium boreale or Serratula tinctoria. Vegetation of cluster 3 occurs in wet base-rich habitats and often contains species of the Caricion davallianae alliance. Species of dry and Nardus grasslands are typical for vegetation of cluster 4, which is found at the driest sites and is confined to oligotrophic sandy soils. Vegetation of clusters 5 and 6 occurs on moist mesotrophic soils. Their species composition is quite similar, the main difference being that the former includes species-poor relevés and the latter includes species-rich relevés. Relevés of cluster 7 include species of dry grasslands and some ruderal species and represent degraded types of inundated floodplain meadows of the Deschampsion alliance. Vegetation of clusters 8 is characterized by species of the Phragmito-Magnocaricetea class and of the Deschampsion alliance, and occurs in wet nutrient-rich habitats. Vegetation of cluster 9, which usually develops from vegetation of cluster 8 due to decrease in the ground-water table, often contains species of dry grasslands and mesic meadows. Except for relevés of clusters 1 and 7, all others can be assigned to the Molinietum caeruleae Koch 1926 association. Cluster 1 corresponds to the Junco effusi-Molinietum caeruleae Tüxen 1954 association. Average Ellenberg indicator values for relevés, which were passively projected on the ordination biplot of detrended correspondence analysis, showed that the first ordination axis correlates with nutrients, soil base status and temperature, and second axis with moisture.     

Soil Microbial Communities Associated with Douglas-fir and Red Alder Stands at High- and Low-Productivity Forest Sites in Oregon, USA

Communities of archaea, bacteria, and fungi were examined in forest soils located in the Oregon Coast Range and the inland Cascade Mountains. Soils from replicated plots of Douglas-fir (Pseudotsuga menziesii) and red alder (Alnus rubra) were characterized using fungal ITS (internal transcribed spacer region), eubacterial 16S rRNA, and archaeal 16S rRNA primers. Population size was measured with quantitative (Q)-PCR and composition was examined using length heterogeneity (LH)-PCR for fungal composition, terminal restriction fragment length (T-RFLP) profiles for bacterial and archaeal composition, and sequencing to identify dominant community members. Whereas fungal and archaeal composition varied between sites and dominant tree species, bacterial communities only varied between sites. The abundance of archaeal gene copy numbers was found to be greater in coastal compared to montane soils accounting for 11% of the prokaryotic community. Crenarchaea groups 1.1a-associated, 1.1b, 1.1c, and 1.1c-associated were putatively identified. A greater abundance of Crenarchaea 1.1b indicator fragments was found in acidic (pH 4) soils with low C:N ratios under red alder. In coastal soils, 25% of fungal sequences were putatively identified as basidiomycetous yeasts belonging to the genus Cryptococcus. Although the function of these yeasts in soil is not known, they could significantly contribute to decomposition processes in coastal soils distinguished by rapid tree growth, high N content, low pH, and frequent water-saturation events.     

Soil salinity and pH in Japanese mangrove forests and growth of cultivated mangrove plants in different soil conditions

Soil conditions of mangrove forests in southern Japan were found to correlate largely with zonal distributions of the species.Kandelia candel grew in soils with low salinity and low pH,Avicennia marina, Rhizophora stylosa andSonneratia alba in soils with high salinity and high pH, andBruguiera gymnorrhiza in soil with a wide range of pH but limited range of salinity.Lumnitzera racemosa colonized soil with a wide range of pH and medium salinity. Seedlings ofKandelia candel, Bruguiera gymnorrhiza andRhizophora stylosa were planted in soils with differing salinity and pH. Optimum seedling growth ofKandelia, Bruguiera andRhizophora occurred when plants were cultivated in soils similar to those of their natural habitats, suggesting that growth of mangrove species and their zonal distributions were regulated by salinity and soil pH.     

A comparison of distribution patterns in British and Irish mosses and liverworts

Abstract

We classified 747 species of British and Irish mosses into 10 clusters, based on their recorded distribution in 10×10 km grid squares (hectads). We generated the clusters in a two-stage process using the CLUSTASPEC program, the method that we had earlier used for British and Irish liverworts and hornworts. The clusters are named after the species with distributions which are most similar to those of the clusters as a whole. Clusters of widespread species (Bryum capillare), southern, lowland species (Rhynchostegium confertum), widespread calcifuges (Pleurozium schreberi), upland species (Blindia acuta), and montane calcifuges (Kiaeria falcata) closely match clusters recognised in the liverworts. The remaining clusters (Tortella flavovirens, Weissia longifolia, Mnium stellare, Encalypta alpina, Mnium lycopodioides) are less similar. The classification of mosses into 15 and 20 clusters generates additional clusters of hyperoceanic and montane mosses which also resemble liverwort clusters. The influence of calcareous bedrock has a more marked effect in determining moss distributions and, unlike the liverworts, the 10 moss clusters include one which is predominantly coastal. Mosses tend to be a less upland group than liverworts; a smaller proportion of their species have northern and western distributions and the lowland clusters are characterised by more extreme environmental conditions. As with the liverworts, geographically restricted clusters of species with predominantly Mediterranean-Atlantic, Arctic-montane and Boreo-arctic Montane world ranges include marked concentrations of threatened species, and species which are not recorded as fruiting in the British Isles.     

The influence of light and nutrient availability on herb layer species richness in oak-dominated forests in central Bohemia

In this study, we examined to what extent the internal site factors (light and soil conditions) are responsible for herb layer diversity in oak-dominated forest stands growing on different substrates in central Bohemia (Czech Republic). We collected data on herb layer diversity, light and nutrient availability at nine oak stands, representing the range of environmental variability for these types of forests in the region. We found that species richness increased with light availability, but only if the site occupied predominantly by fast-colonizing species was excluded from the analysis (P < 0.05). Species richness correlated positively with soil pH and negatively with nitrogen (N) concentration in humus (P < 0.05). The highest species richness was found at sites with not only low N soil concentration, but also simultaneously with high phosphorus (P) soil concentration. Despite this finding, however, herb layer diversity is evidently threatened much more in P-rich soils than in P-poor soils. It seems that the enhancement of N in an ecosystem due to litter accumulation and N deposition generally leads to only a minor increase in N availability at P-poor sites, but a considerable increase at P-rich sites. Therefore, species richness can be exceptionally high at P-rich sites, but only under conditions of strong N limitation.     

Studies on a climosequence of soils in tussock grasslands

Nematodes were extracted and identified from tussocks growing on eight soil types (Conroy, Cluden, Tawhiti, Lammerlaw, Carrick, McKerrow, Obelisk, Harihari) from altitudes of 80–1550 m. Most soils were from schist parent material in Otago. Samples from the bottom 5 cm of the leaves and the 0–5‐cm and 8–12‐cm soil strata were analysed separately. Altogether 4455 specimens of nematodes were identified, and the percentage composition in each stratum was calculated. Seventy species were found; only 1 occurred at all sites, 22 were found at only 1 site; 28 occurred in all 3 strata, 18 in only 1 stratum. A strong site factor was noted in species distribution, and there was distinct stratification. Correlation coefficients were calculated for nematode abundance and diversity with altitude, rainfall, and chemical factors (pH, organic C, and soil N); calculations were also made for only six sites by omitting Lammerlaw and Harihari, which did not reflect stages in the climosequence. Considering the six sites, total numbers of nematodes decline with altitude, but numbers of species do not; numbers in 0–5‐cm soil are significantly positively correlated with pH, and negatively with organic C. The correlation with pH is not causal, but reflects a negative association of pH with both precipitation and altitude which tends to obscure relationships between nematodes and environmental factors. Paratylenchus sp. was dominant at the two driest sites; at the next site it was mixed with Macroposthonia, which occurred at the subsequent wetter sites; this distribution apparently reflects the resistant preadult stage of Paratylenchus. Most Dorylai‐moidea had a wide distribution, but Belondiroidea and Mononchoidea showed a preference for the wetter sites. Of the 70 species found, 30 were named and 18 were previously known from New Zealand, 6 appear cosmopolitan in distribution, 2 have Australian relationships, and 1 has Southern Hemisphere relationships.     

Experimental evidence for an alkali ecotype of <Emphasis Type="Italic">Lolium multiflorum</Emphasis>, an exotic invasive annual grass in the Central Valley,CA, USA

One of the characteristics of highly invaded ecosystems is that exotic species are often poor invaders of edaphically severe sites, which become refuges for native flora. To investigate the invasive potential of Lolium multiflorum (Per.) into alkali sites in California, an ex-situ reciprocal transfer experiment was carried out using seeds from populations of L. multiflorum taken from three sites differing in alkalinity (and inundation), including alkali sink soils (pH 8.5) and sink matrix soils (pH 7.4) located within meters of each other, and non-sink soils (pH 5.0) located several km away. Survivorship, plant height, leaf number and seed production were assessed. In addition, a native composite, Hemizonia pungens (Hick.), commonly found on alkali sinks was also sampled at the sink and sink matrix microsites. Lolium multiflorum plants grown from alkali sink and sink matrix seeds produced fewer leaves and seeds but were taller than plants grown from non-alkali seeds, the latter perhaps an adaptation to frequently inundated soils. Non-alkali genotypes fared poorly in sink soils for all traits, both in comparison to their growth on non-sink soils, and in comparison to the sink and sink edge genotypes. This suggests the existence of L. multiflorum ecotypes adapted to inundated alkali sinks, a genotypic difference that occurs on a broad spatial scale (kilometers), but not so obviously on the micro-site scale (meters) between sink and sink matrix populations. These data suggest that the absence of exotic invasives from alkali sites may be temporary if they are evolving tolerance for these severe sites, and this may threaten the future of the native alkali specialists that currently find refuge in these sites.     

Growth and nitrogen fixation ofAzolla pinnata andAzolla caroliniana as affected by urea fertilizer and their influence on rice yield

Soils from avocado (Persea americana Mill.) orchards in Israel (IS) and California (CA), both sites with a Mediterranean climate, were sampled and analyzed for the species and quantities of vesicular-arbuscular mycorrhizal fungus (VAMF) spores in them, and for soil physical and chemical characteristics.Numbers of spores were similar in soil from IS and CA but the dominant VAMF species were very different. In IS the most common fungi were Sclerocystis sinuosa and Glomus macrocarpum. In CA, Gl. constrictum was present in every orchard examined and Gl. fasciculatum was nearly as widespread. Acaulospora spp. and other Glomus spp. also were found, including A. elegans which has never before been reported from CA.The differences in VAMF populations and species constituents found on two continents but in areas with similar climates and soil types may be due to host or edaphic factors. Different avocado rootstocks are used in the two countries and lower pH and higher soil fertility levels were present in CA soils.The total VAMF spore populations in each orchard was about 275 per 100 mL soil. The population level was not correlated with any of the soil physical or chemical characteristics examined nor with avocado cultivar or age. In IS no fungus spores were found in three orchards; available P, Ca, Mg and Cu levels were high in these soils.     

Evolutionary dynamics of 5S rDNA location in acridid grasshoppers and its relationship with H3 histone gene and 45S rDNA location

We analyze the chromosomal location of 5S rDNA clusters in 29 species of grasshoppers belonging to the family Acrididae. There was extensive variation among species for the number and location of 5S rDNA sites. Out of 148 sites detected, 75% were proximally located, 21.6% were interstitial, and only 3.4% were distal. The number of 5S rDNA sites per species varied from a single chromosome pair (in six species) to all chromosome pairs (in five species), with a range of intermediate situations. Thirteen chromosomes from eight species carried two 5S rDNA clusters. At intraspecific level, differences among populations were detected in Eyprepocnemis plorans, and some heteromorphisms have also been observed in some species. Double FISH for 5S rDNA and H3 histone gene DNA, performed on 17 of these 29 species, revealed that both markers are sometimes placed in a same chromosome but at different location, whereas they appeared to co-localize in five species (Calliptamus barbarus, Heteracris adpersa, Aiolopus strepens, Oedipoda charpentieri and O. coerulescens). Double fiber-FISH in A. strepens and O. coerulescens showed that the two DNAs are closely interspersed with variable relative amounts of both classes of DNA. Finally, no correlation was observed between the number of 5S and 45S rDNA clusters in 23 species where this information was available. These results are discussed in the light of possible mechanisms of spread that led to the extensive variation in the number of clusters observed for both rDNA types in acridid grasshoppers.     

Ecology of algal communities of different soil types from Cierva Point, Antarctic Peninsula

During summer 2005/2006, we characterized three sampling sites on mineral soils and four on ornithogenic soils from Cierva Point, Antarctic Peninsula, in terms of topographic and abiotic features (altitude, slope, magnetic direction, temperature, texture, pH, conductivity, organic matter, moisture and nutrient concentrations), and compared their microalgal communities through taxonomic composition, species richness, diversity, chlorophyll a content and their variation in time. Average values of pH, moisture, organic matter and nutrient concentrations were always significantly lower in mineral than in ornithogenic soils. Low N/P mass ratio showed potential N-limitation of biomass capacity in the former. On the other hand, the results suggested that physical stability is not as a key stress factor for mineral soil microalgae. Chlorophyll a concentration was not only higher in ornithogenic soils, but it also showed a wider range of values. As this parameter was positively correlated with temperature, pH, nutrients, organic matter and moisture, we cannot come to conclusions regarding the influence of each factor on algal growth. Communities of mineral soils were significantly more diverse than those of enriched ornithogenic soils due to higher species richness as well as higher equitability. Also, their structure was steadier over time, as shown by a cluster analysis based on relative frequency of algal taxa. Although Cyanobacteria and Bacillariophyceae dominated almost all samples, Chlorophyceae represented 34% of the 140 taxa recorded, and most of them observed only in cultures. The detection under controlled conditions of a high latent species richness in harsh mineral soil sites shows that the composition and equitability of these microalgal communities would be more prone to modification due to the manifold local consequences of climatic change than those of ornithogenic soils.     

Soil pH and organic matter content add explanatory power to Northern Lapwing Vanellus vanellus distribution models and suggest soil amendment as a conservation measure on upland farmland

Habitat associations of farmland birds are well studied, yet few have considered relationships between species distribution and soil properties. Charadriiform waders (shorebirds) depend upon penetrable soils, rich in invertebrate prey. Many species, such as the Northern Lapwing Vanellus vanellus, have undergone severe declines across Europe, despite being targeted by agri‐environment measures. This study assessed whether there were additive effects of soil variables (depth, pH and organic matter content) in explaining Lapwing distribution, after controlling for known habitat relationships, at 89 farmland sites across Scotland. The addition of these soil variables and their association with elevation improved model fit by 55%, in comparison with models containing only previously established habitat relationships. Lapwing density was greatest at sites at higher elevation, but only those with less peaty and less acidic soil. Lapwing distribution is being constrained between intensively managed lowland farmland with favourable soil conditions and upland sites where lower management intensity favours Lapwings but edaphic conditions limit their distribution. Trials of soil amendments such as liming are needed on higher elevation grassland sites to test whether they could contribute to conservation management for breeding Lapwings and other species of conservation concern that depend upon soil‐dwelling invertebrates in grassland soils, such as Eurasian Curlew Numenius arquata, Common Starling Sturnus vulgaris and Ring Ouzel Turdus torquatus. Results from such trials could support improvement and targeting of agri‐environment schemes and other conservation measures in upland grassland systems.     

Soil chemistry of local vegetation gradients in sandy calcareous grasslands

Acidification and N-deposition are continuous processes that alter the composition of plant communities. We investigated vegetation transitions in sandy grasslands and tested two hypotheses: (1) a shift from vegetation dominated by Koeleria glauca to one dominated by Corynephorus canescens is due to acidification and (2) a shift from vegetation dominated by K. glauca to one dominated by Arrhenatherum elatius is due to increased mineral-N. The occurrence of K. glauca and C. canescens followed pH shifts strikingly well. However, the pH varied considerably between sites, and we found that differences in preference for bare sand may be important for the coexistence of these two species at both high and low pH. In 75% of the gradients, the dominance of A. elatius was related to high content of mineral-N. Most species preferred lime-rich soils and we concluded that both N-enrichment and acidification may lead to loss of species in calcareous sandy grasslands.     

The distribution of keratinophilic fungi in soils from New Zealand,and from two polynesian Islands

Summary (1) A total of 312 soils, 271 from New Zealand, 24 from Rarotonga, Cook Islands and 17 from Tokelau Islands was examined by the hair-baiting technique.(2)Microsporon gypseum was isolated from 1.5% of New Zealand soils and from 36.6% of Pacific Island soils. The difference in incidence in the two areas is significant.(3)Keratinomyces ajelloi andMicrosporon cookei were recovered from 52.8% and 7.7% of New Zealand soils respectively but neither species was isolated from the Island samples.K. ajelloi was found to be significantly associated with strongly acid soils, andM. cookei with a farmyard substrate.(4)Trichophyton terrestre was isolated from 1.1% of New Zealand and from 12.2% of Pacific Island soils. Two strains ofChrysosporium keratinophilum were isolated from New Zealand soils and an unidentifiedChrysosporium from one Rarotongan sample.(5) The distribution of the different species is discussed and compared with those reported in surveys carried out in other countries of the Southern Hemisphere. The effects of ambient temperature, and source and pH of the soil, on the distribution of keratinophilic fungi are considered, but there is insufficient detailed evidence to determine the importance of these factors in the ecology of these organisms.     

Occurrence of two closely related ricefishes, Javanese medaka (Oryzias javanicus) and Indian medaka (O. dancena) at sites with different salinity in Peninsular Malaysia

Among ricefishes of the genus Oryzias, the Javanese medaka (O. javanicus) and the Indian medaka (O. dancena) are highly adaptable to seawater. Although wide distribution of the two species in the brackish waters of South and South-East Asia has been reported, their habitat preference remains unknown. We surveyed 12 sites in five estuarial areas of the west coast of Peninsular Malaysia from Kuala Gula to Tanjung Piai. Both species were found in all five areas, suggesting their distribution throughout the west coast of Peninsular Malaysia. This is the southernmost-recorded appearance of O. dancena, to the best of our knowledge. However, the habitats of the two species were essentially separated: of the 12 surveyed sites, the species were found in co-existence at only two sites, and one or the other species was found alone at the remaining 10 sites. We compared temperature, salinity, pH, and dissolved oxygen (DO) at the sampling sites and found that the habitat of O. javanicus is with higher salinity and DO. The salinity and DO at the sites of co-existence are near the lowest values found at the O. javanicus-only sites, and the highest values at the O. dancena-only sites. These results suggest that O. javanicus and O. dancena habitats are essentially separated; the former prefers hyperosmotic conditions while the latter prefers hypoosmotic conditions, and the latter may be more tolerant of hypoxia. The two sites of co-existence are points of contact between the species’ separate distribution areas.     

Nutrient requirements of exotic tree species in Zimbabwe

Pot and field experiments were conducted in the greenhouse and at three field sites (Marondera, Domboshawa and Makoholi) in Zimbabwe to examine the effects of soils and fertilizers on nutrient uptake and growth of 6 exotic tree species (Eucalyptus camaldulensis, E. grandis, E. tereticornis, Leucaena leucocephala, Casuarina cunninghamiana, and Acacia holosericea). Plant growth, N and P contents of all species were increased by the application of N, P, K and micronutrient fertilizers. The effect of individual nutrients (N, P, K and micronutrients) and their combination on Eucalyptus species was further investigated in a pot experiment using soil from Domboshawa. Eucalyptus species responded only to N application and no significant interactions were found between N and the other elements. Nutrient uptake results showed that E. camaldulensis and E. tereticornis removed more cations than the N-fixing trees but only in the fertilized treatments. L. leucocephala and C. cunninghamiana were higher in P, but no clear trends were observed for N. Plant growth and nutrient uptake by E. camaldulensis. C. cunninghamiana and A. holosericea were assessed in the field at the three sites. Plant species grown in the Marondera site had greater height and diameter at breast height (DBH) than those in the two other sites. These results followed trends in soil nutrient contents. The analysis of foliage revealed differences in the nutrient concentration of leaves from different trees, but no effect of site was found. The interrelationships between plant characteristics, soil and foliage nutrients were examined. In a pot experiment, mineral N was the only variable correlated with growth response and nutrient uptake, while in the field the only significant correlation was obtained with soil pH.     

A study of Chytridiales from Victorian and other Australian soils

Summary In a study of Australian soil Chytridiales, with particular emphasis on Victorian forms, there were further indications of a definite distribution pattern of these aquatic fungi related to soil conditions. A fairly prolific (in terms of number of species present) low pH micro-flora was obtained from krasnozem, podzol and rainforest soils, in contrast to a rather meagre high pH micro-flora obtained from black earth, mallee and red-brown earth soils. The ecology of the species Rhizophylyctis rosea was studied intensively using a crudely quantitative method which involved the addition of liquid nutrient amendments to the collected soil samples. The results indicated that R. rosea was very widespread but not ubiquitous in the natural soils investigated and that it was selectively favoured in soils which had received fertilizer (e.g. superphosphate) treatment.     

An assessment of macroinvertebrate functional feeding groups as water quality indicators in the Buffalo River, eastern Cape Province, South Africa

The aim of this paper was to investigate the potential for using functional feeding groups (FFGs) as indicators of water quality conditions in rivers, using the Buffalo River, South Africa, as a specific example. Multivariate classification and ordination techniques were used to investigate species and FFG distributions in relation to a number of physico-chemical variables at 16 sites from the headwaters to the estuary of the Buffalo River.Two-way indicator species analysis (TWINSPAN) of species composition ranked most of the sites sequentially down the river, irrespective of water quality conditions. Ordination of FFGs from a set of riffle samples collected in mid-late summer showed only weak relationships between FFG distribution and water quality changes, except where variables changed sequentially down the river (e.g. pH and temperature). Individual species responses to water quality gradients were examined for nine riffle-dwelling species representing diverse FFGs. Following correspondence analysis of a matrix of environmental variables and species frequencies, some species showed strong associations with defined ranges of some variables. In particular, Adenophlebia auriculata (Leptophlebiidae, Ephemeroptera) from the headwater sampling site, was associated with low pH and low temperature. Simulium damnosum occurred under conditions of high turbidity, while Afronurus harrisoni was found under high concentrations of potassium, ammonium and nitrite ions.We conclude that although there was a distinct headwaters fauna in the Buffalo River, and sequential downstream changes in species composition, most FFGs (apart from shredders) were represented down the whole length of the river. FFG classifications are therefore unlikely to provide useful indications of water quality conditions in the Buffalo River.Using a categorical approach to classifying water quality variables, and by applying correspondence analysis to the resulting matrix, we recognised nine species that could be used to define water quality. These indicator species can be used to define tolerance ranges of the fauna for water quality conditions in different parts of the Buffalo river.     

Growth of Trifolium alpinum: Effects of soil properties, symbionts and pathogens

The lowland cultivation of Trifolium alpinum, a clover species found on acid soils in the Alps and suitable for the restoration of erosion areas at high altitudes, failed repeatedly in previous experiments. Three experiments were carried out in a controlled environment to elucidate the reasons for the failure and to develop possible cultivation strategies. In experiment I, T. alpinum was grown in an autochthonous soil from the Alps (high elevation) and in two allochthonous soils, a grassland soil from the Hercynian mountains (medium elevation), and an arable soil (low elevation), in which the seed propagation of T. alpinum had failed previously. The two allochthonous soils had lower contents of soil organic C and ergosterol, an indicator for fungal biomass, than the autochthonous high-elevation soil, but higher levels of exchangeable Ca and extractable P. Plants grown in the allochthonous soils achieved higher biomass and total N amounts per plant than those from the high elevation soil if inoculated with this autochthonous material to establish rhizobial infection. In the allochthonous high elevation soil, the growth of T. alpinum was P-limited as shown in experiment II. In experiment I, plants grown in the low elevation soil had a lower biomass and smaller number of active leaves at 120 days after emergence than those grown on the medium elevation soil. This difference can be explained by strong colonization with the phytophagous nematode Pratylenchus sp., as demonstrated in experiment III by comparing plant growth either in untreated or in autoclaved low-elevation soil. Successful propagation of T. alpinum at low elevation may be achieved through suitable inoculation with autochthonous soil biota, especially Rhizobia, and avoidance of soils infested by Pratylenchus species by choosing sites with acidic soil and ensuring adequate P-availability.     

Genetic adaptation to soil acidification: experimental evidence from four grass species

Anthropogenic acidification has reduced soil pH and released potentially toxic aluminium (Al) ions in many regions. This investigation examines whether increased acidity has caused genetic adaptation to acidic conditions within the grass species Elymus caninus, Poa nemoralis, Deschampsia cespitosa and D. flexuosa. We sampled tussocks (genets) of each species in two regions of southern Sweden, differing in their exposure to acidifying deposition. The tolerance of the genets was tested in a solution experiment with different pH and Al concentrations. Our data suggest that species found at lower pH field locations have a greater tolerance to low pH and high Al levels than species found on less acidic soils. Analysis of variance showed a significant average effect of population and (or) genet in most species; however, we found little evidence of genetic adaptation to acidic conditions at the regional, population and micro-site level. In fact, there was no consistent change in the ranking of populations or genets with varying pH or Al concentration. Based on these results, we hypothesize that phenotypic plasticity rather than genetic adaptation has been favoured as the predominant mechanism to cope with soil acidity in the four grass species.     

Iron oxidation stimulates organic matter decomposition in humid tropical forest soils

Humid tropical forests have the fastest rates of organic matter decomposition globally, which often coincide with fluctuating oxygen (O₂) availability in surface soils. Microbial iron (Fe) reduction generates reduced iron [Fe(II)] under anaerobic conditions, which oxidizes to Fe(III) under subsequent aerobic conditions. We demonstrate that Fe (II) oxidation stimulates organic matter decomposition via two mechanisms: (i) organic matter oxidation, likely driven by reactive oxygen species; and (ii) increased dissolved organic carbon (DOC) availability, likely driven by acidification. Phenol oxidative activity increased linearly with Fe(II) concentrations (P < 0.0001, pseudo R² = 0.79) in soils sampled within and among five tropical forest sites. A similar pattern occurred in the absence of soil, suggesting an abiotic driver of this reaction. No phenol oxidative activity occurred in soils under anaerobic conditions, implying the importance of oxidants such as O₂ or hydrogen peroxide (H₂O₂) in addition to Fe(II). Reactions between Fe(II) and H₂O₂ generate hydroxyl radical, a strong nonselective oxidant of organic compounds. We found increasing consumption of H₂O₂ as soil Fe(II) concentrations increased, suggesting that reactive oxygen species produced by Fe(II) oxidation explained variation in phenol oxidative activity among samples. Amending soils with Fe(II) at field concentrations stimulated short‐term C mineralization by up to 270%, likely via a second mechanism. Oxidation of Fe(II) drove a decrease in pH and a monotonic increase in DOC; a decline of two pH units doubled DOC, likely stimulating microbial respiration. We obtained similar results by manipulating soil acidity independently of Fe(II), implying that Fe(II) oxidation affected C substrate availability via pH fluctuations, in addition to producing reactive oxygen species. Iron oxidation coupled to organic matter decomposition contributes to rapid rates of C cycling across humid tropical forests in spite of periodic O₂ limitation, and may help explain the rapid turnover of complex C molecules in these soils.