Bacterial communities and biogeochemical transformations of iron and sulfur in a high saltmarsh soil profile

Microbial community structure in saltmarsh soils is stratified by depth and availability of electron acceptors for respiration. However, the majority of the microbial species that are involved in the biogeochemical transformations of iron (Fe) and sulfur (S) in such environments are not known. Here we examined the structure of bacterial communities in a high saltmarsh soil profile and discuss their potential relationship with the geochemistry of Fe and S. Our data showed that the soil horizons Ag (oxic–suboxic), Bg (suboxic), Cr₁ (anoxic with low concentration of pyrite Fe) and Cr₂ (anoxic with high concentrations of pyrite Fe) have distinct geochemical and microbiological characteristics. In general, total S concentration increased with depth and was correlated with the presence of pyrite Fe. Soluble + exchangable-Fe, pyrite Fe and acid volatile sulfide Fe concentrations also increased with depth, whereas ascorbate extractable-Fe concentrations decreased. The occurrence of reduced forms of Fe in the horizon Ag and oxidized Fe in horizon Cr₂ suggests that the typical redox zonation, common to several marine sediments, does not occur in the saltmarsh soil profile studied. Overall, the bacterial community structure in the horizon Ag and Cr₂ shared low levels of similarity, as compared to their adjacent horizons, Bg and Cr₁, respectively. The phylogenetic analyses of bacterial 16S rRNA gene sequences from clone libraries showed that the predominant phylotypes in horizon Ag were related to Alphaproteobacteria and Bacteroidetes. In contrast, the most abundant phylotypes in horizon Cr₂ were related to Deltaproteobacteria, Chloroflexi, Deferribacteres and Nitrospira. The high frequency of sequences with low levels of similarity to known bacterial species in horizons Ag and Cr₂ indicates that the bacterial communities in both horizons are dominated by novel bacterial species.     

Distribution of microbial communities in a forest soil profile investigated by microbial biomass, soil respiration and DGGE of total and extracellular DNA

We studied the distribution of the indigenous bacterial and fungal communities in a forest soil profile. The composition of bacterial and fungal communities was assessed by denaturing gradient gel electrophoresis (DGGE) of total and extracellular DNA extracted from all the soil horizons. Microbial biomass C and basal respiration were also measured to assess changes in both microbial biomass and activity throughout the soil profile. The 16S rDNA-DGGE revealed composite banding patterns reflecting the high bacterial diversity as expected for a forest soil, whereas 18S rDNA-DGGE analysis showed a certain stability and a lower diversity in the fungal communities. The banding patterns of the different horizons reflected changes in the microbial community structure with increasing depth. In particular, the DGGE analysis evidenced complex banding patterns for the upper A1 and A2 horizons, and a less diverse microflora in the deeper horizons. The low diversity and the presence of specific microbial communities in the B horizons, and in particular in the deeper ones, can be attributed to the selective environment represented by this portion of the soil profile. The eubacterial profiles obtained from the extracellular DNA revealed the presence of some bands not present in the total DNA patterns. This could be interpreted as the remainders of bacteria not any more present in the soil because of changes of edaphic conditions and consequent shifting in the microbial composition. These characteristic bands, present in all the horizons with the exception of the A1, should support the concept that the extracellular DNA is able to persist within the soil. Furthermore, the comparison between the total and extracellular 16S rDNA-DGGE profiles suggested a downwards movement of the extracellular DNA.     

Reductosols: Natural soils and Technosols under reducing conditions without an aquic moisture regime

According to the German soil classification, Reductosols (German: “Reduktosole”) are soils with a redoximorphic color pattern, but without water saturation for lengthy periods. They are formed by reducing conditions due to oxygen deficiency caused by the accumulation of reduct gases such as CH₄, CO₂, NH₃, or H₂S in the soil atmosphere. Soil oxygen may have been displaced by the ascent of CO₂ from post‐volcanic mofettes or by CH₄ from sanitary landfills. Furthermore, a lack of oxygen causing redoximorphism can occur in unsaturated soils if they contain or receive large quantities of easily decomposable organic matter (i.e., organic urban waste or sludge). Under such circumstances, a gleyic color pattern (e.g., an oxidized Bg horizon above a Cr horizon with strongly reducing conditions) forms without the influence of an aquic moisture regime. Waste‐water and petrol infiltration in soil can form a stagnic color pattern without the stagnation of surface water. Such more or less well‐drained but strongly redoximorphic horizons should be named as Yg instead of Bg. In some cases, such soils only exist for a short time due to the loss of reduct gases or termination of infiltration of organic liquids. Reductosols are ecotops with oxygen deficiency. Natural Reductosols dominate in recent and former volcanic areas, whereas Reductic Technosols are formed in urban‐industrial agglomerations. Their morphology, chemistry, dynamics, genesis, and ecology is summarized and discussed in this paper. Natural gas and CO₂ gas are deposited in deeper zones of the earth crust since some time. Leakages of these depots let form reductosols as well.     

Soil–landscape relationships at the lower reaches of a watershed at Bear Creek near Oak Ridge,Tennessee

The watersheds at Bear Creek, Oak Ridge, TN, have similar soil–landscape relationships. The lower reaches of many of these watersheds consist of headwater riparian wetlands situated between sloping non-wetland upland zones. The objectives of this study are to examine the effects of (i) slope and geomorphic processes, (ii) human impacts, and (iii) particular characteristics of soils and saprolite that may effect drainage and water movement in the wetlands and adjacent landscapes in one of these watersheds. A transect was run from west to east in a hydrological monitored area at the lower reaches of a watershed on Bear Creek. This transect extended from a steep side slope position across a floodplain, a terrace, and a shoulder slope. On the upland positions of the Nolichucky Shale, mass wasting, overland flow and soil creep currently inhibit soil formation on the steep side slope position where a Typic Dystrudept is present, while soil stability on the shoulder slope has resulted in the formation of a well-developed Typic Hapludult. In these soils, argillic horizons occur above C horizons on less sloping gradients in comparison to steeper slopes, which have Bw horizons over Cr (saprolite) material. A riparian wetland area occupies the floodplain section, where a Typic Endoaquept is characterized by poorly drained conditions that led to the development of redoximorphic features (mottling), gleying, organic matter accumulation, and minimal development of subsurface horizons. A thin colluvial deposit overlies a thick well developed Aquic Hapludalf that formed in alluvial sediments on the terrace position. The colluvial deposit from the adjacent shoulder slope is thought to result from soil creep and anthropogenic erosion caused by past cultivation practices. Runoff from the adjacent sloping landscape and groundwater from the adjacent wetland area perhaps contribute to the somewhat poorly drained conditions of this profile. Perched watertables occur in upland positions due to dense saprolite and clay plugging in the shallow zones of the saprolite. However, no redoximorphic features are observed in the soil on the side slope due to high runoff. Remnants of the underlying shale saprolite, which occur as small discolored zones resembling mottles, are also present. The soils in the study have a CEC of <10 cmol kg⁻¹, silt loam textures and Fe_d values of 0.5–4.3%. These soils are also mainly acidic and low in total carbon.     

Depth‐related responses of soil microbial communities to experimental warming in an alpine meadow on the Qinghai‐Tibet Plateau

Although the effect of experimental warming on soil microorganisms has been well documented at surface horizons, less is known about its influence in subsurface horizons. An experiment was therefore carried out in an alpine meadow on the Qinghai‐Tibet Plateau to examine the responses of microbial communities to experimental warming at five soil depths (0–10, 10–20, 20–30, 30–40 and 40–50 cm). Plots were passively warmed for 3 years in open‐top chambers and compared with adjacent control plots at ambient temperature. Soil microbial communities were assessed by using phospholipid fatty acid (PLFA) analysis. Our results showed clearly that 3 years of experimental warming increased microbial biomass consistently and significantly throughout the upper 50‐cm soil profiles, as indicated by the changes in both microbial biomass carbon (C) and total PLFA contents. The composition of microbial communities was also affected significantly by warming, but its effect depended on soil depth. While warming induced a community shift towards bacteria at the 0–10‐cm depth, it tended to shift microbial communities towards fungi at the other, deeper, layers. These results indicate that warming had strong effects on soil microbial communities, including even those residing in subsurface horizons, which may help us to understand the microbial mediation of the feedback between terrestrial C cycling and climate warming.     

Adsorption of “real” dissolved organic matter on the clay and fine silt fractions of a forested Stagnic Gleysol

Dissolved organic matter (DOM) in soils is partially adsorbed when passing through a soil profile. In most adsorption studies, water soluble organic matter extracted by water or dilute salt solutions is used instead of real DOM gained in situ by lysimeters or ceramic suction cups. We investigated the adsorption of DOM gained in situ from three compartments (forest floor leachate and soil solution from 20 cm (Bg horizon) and 60 cm depth (2Bg horizon)) on the corresponding clay and fine silt fractions (< 6.3 μm, separated together from the bulk soil) of the horizons Ah, Bg, and 2Bg of a forested Stagnic Gleysol by batch experiments. An aliquot of each clay and fine silt fraction was treated with H₂O₂ to destroy soil organic matter. Before and after the experiments, the solutions were characterized by ultra‐violet and fluorescence spectroscopy and analyzed for sulfate, chloride, nitrate, and fluoride. The highest affinity for DOM was found for the Ah samples, and the affinity decreased in the sequence Ah > Bg > 2Bg. Dissolved organic matter in the 2Bg horizon can be regarded as slightly reactive, because adsorption was low. Desorption of DOM from the subsoil samples was reflected more realistically with a non‐linear regression approach than with initial mass isotherms. The results show that the extent of DOM adsorption especially in subsoils is controlled by the composition and by the origin of the DOM used as adsorptive rather than by the mineralogical composition of the soil or by contents of soil organic matter. We recommend to use DOM gained in situ when investigating the fate of DOM in subsoils.     

Do long-lived ants affect soil microbial communities?

This study was designed to test the hypothesis that desert ant species that build nests that remain viable at a particular point in space for more than a decade produce soil conditions that enhance microbial biomass and functional diversity. We studied the effects of a seed-harvester ant, Pogonomyrmex rugosus, and two generalist ant species, Aphaenogaster cockerelli and Myrmecocystus depilis, on soil microbial communities. Microbial biomass was higher in P. rugosus-modified soils than in reference soils when soil water content was higher than 3%. Microbial biomass was either higher in reference soils or exhibited no difference in reference soils and nest-modified soils of A. cockerelli and M. depilis. There were differences in microbial functional diversity and microbial community level physiological profiles (MicroResp method) between ant-nest-modified and reference soils of the three ant species on some sampling dates. Temporal patterns of soil microbial communities associated with the ant species resulted from differences in soil moisture, density, and species composition of the annual plant communities associated with the ant nests and in reference areas. Differences in annual plant communities associated with ant nests and surrounding areas resulted in different chemical inputs into the soil organic-matter pools. This study shows that generalizations about the effects of long-lived ant nests on soil biota in arid regions must consider feeding behaviors of the ant species and temporal patterns of rainfall.     

Experimental warming effects on the microbial community of a temperate mountain forest soil

Soil microbial communities mediate the decomposition of soil organic matter (SOM). The amount of carbon (C) that is respired leaves the soil as CO₂ (soil respiration) and causes one of the greatest fluxes in the global carbon cycle. How soil microbial communities will respond to global warming, however, is not well understood. To elucidate the effect of warming on the microbial community we analyzed soil from the soil warming experiment Achenkirch, Austria. Soil of a mature spruce forest was warmed by 4 °C during snow-free seasons since 2004. Repeated soil sampling from control and warmed plots took place from 2008 until 2010. We monitored microbial biomass C and nitrogen (N). Microbial community composition was assessed by phospholipid fatty acid analysis (PLFA) and by quantitative real time polymerase chain reaction (qPCR) of ribosomal RNA genes. Microbial metabolic activity was estimated by soil respiration to biomass ratios and RNA to DNA ratios. Soil warming did not affect microbial biomass, nor did warming affect the abundances of most microbial groups. Warming significantly enhanced microbial metabolic activity in terms of soil respiration per amount of microbial biomass C. Microbial stress biomarkers were elevated in warmed plots. In summary, the 4 °C increase in soil temperature during the snow-free season had no influence on microbial community composition and biomass but strongly increased microbial metabolic activity and hence reduced carbon use efficiency.     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.     

Microbial community composition on native and drastically disturbed serpentine soils

During construction of roads, entire hillsides can be cut away, dramatically disturbing the ecosystem. Microbial communities play important, but poorly understood roles in revegetating roadcuts because of the many functions they perform in nutrient cycling, plant symbioses, decomposition, and other ecosystem processes. Our objective was to determine relationships among microbial community composition, soil chemistry, and disturbance on a serpentine soil disturbed by a roadcut and then partially revegetated. We hypothesized that the adjacent undisturbed serpentine soil would have a different microbial community composition from barren and revegetated sections of the roadcut and that undisturbed soils would have the greatest microbial biomass and diversity. We measured phospholipid fatty acids (PLFA) and soil nutrient concentrations on barren and revegetated sections of the roadcut and on adjacent undisturbed serpentine and nonserpentine soils. Most roadcut samples had soil chemistry similar to the serpentine reference soil. The microbial biomass and diversity of barren sites was lower than that of revegetated or the serpentine reference site. The nonserpentine reference site had significantly (P≤0.05) greater microbial biomass than serpentine or disturbed sites but significantly lower relative proportions of actinomycetes, and slow growth biomarkers. The Barren site had the lowest microbial biomass and a significantly (P≤0.05) greater proportion of that biomass was fungi. Barren, revegetated, and serpentine sites all had dissimilar microbial community composition. The composition of the revegetated communities, however, was intermediate between the serpentine reference and barren soils, suggesting that community composition of revegetated soils is approaching that of an undisturbed site with similar soil chemistry.     

Organic matter quality of a forest soil subjected to repeated drying and different re‐wetting intensities

Extended drought periods followed by heavy rainfall may increase in many regions of the Earth, but the consequences for the quality of soil organic matter and soil microbial communities are poorly understood. Here, we investigated the effect of repeated drying and re‐wetting on microbial communities and the quality of particulate and dissolved organic matter in a Haplic Podzol from a Norway spruce stand. After air‐drying, undisturbed soil columns were re‐wetted at different intensities (8, 20 and 50 mm per day) and time intervals, so that all treatments received the same amount of water per cycle (100 mm). After the third cycle, SOM pools of the treatments were compared with those of non‐dried control columns. Lignin phenols were not systematically affected in the O horizons by the treatments whereas fewer lignin phenols were found in the A horizon of the 20‐ and 50‐mm treatments. Microbial biomass and the ratio of fungi to bacteria were generally not altered, suggesting that most soil microorganisms were well adapted to drying and re‐wetting in this soil. However, gram‐positive bacteria and actinomycetes were reduced whereas gram‐negative bacteria and protozoa were stimulated by the treatments. The increase in the (cy 17: 0 + cy 19: 0)/(16:1ω7c + 18:1ω7c) ratio indicates physiological or nutritional stress for the bacterial communities in the O, A and B horizons with increasing re‐wetting intensity. Drying and re‐wetting reduced the amount of hydrolysable plant and microbial sugars in all soil horizons. However, CO₂ and dissolved organic carbon fluxes could not explain these losses. We postulate that drying and re‐wetting triggered chemical alterations of hydrolysable sugar molecules in organic and mineral soil horizons.     

Microbial dynamics in Mediterranean Moder humus

There is a growing interest in the links between humus forms and soil biota, and little is known about these links in Mediterranean ecosystems. Culture-independent techniques, such as DNA extraction followed by DGGE and enzyme activities, allowed us to compare microbial communities in two horizons of a forest soil in different seasonal conditions. Direct in situ lysis was applied for extraction of DNA from soil; intracellular DNA was separated from extracellular and used to represent the composition of microflora. The aims were to describe how biochemical and microbiological parameters correlate with topsoil properties in typical Mediterranean Moder humus. Changes in bacterial and fungal community composition were evident from DGGE profiles. Degrees of similarity and clustering correlation coefficients showed that the seasonal conditions may affect the composition and activity of bacterial and fungal communities in the OH horizon, while in the E horizon the two communities were hardly modified. In the same season, OH and E horizons showed a different composition of bacterial and fungal communities and different enzyme activities, suggesting similar behaviour of eubacteria and fungi relatively to all the variables analysed. Evidently, different organic carbon content in soil horizons influenced microflora composition and microbial activities involved in the P and N cycles.     

Microbial community composition shapes enzyme patterns in topsoil and subsoil horizons along a latitudinal transect in Western Siberia

Soil horizons below 30 cm depth contain about 60% of the organic carbon stored in soils. Although insight into the physical and chemical stabilization of soil organic matter (SOM) and into microbial community composition in these horizons is being gained, information on microbial functions of subsoil microbial communities and on associated microbially-mediated processes remains sparse. To identify possible controls on enzyme patterns, we correlated enzyme patterns with biotic and abiotic soil parameters, as well as with microbial community composition, estimated using phospholipid fatty acid profiles. Enzyme patterns (i.e. distance-matrixes calculated from these enzyme activities) were calculated from the activities of six extracellular enzymes (cellobiohydrolase, leucine-amino-peptidase, N-acetylglucosaminidase, chitotriosidase, phosphatase and phenoloxidase), which had been measured in soil samples from organic topsoil horizons, mineral topsoil horizons, and mineral subsoil horizons from seven ecosystems along a 1500 km latitudinal transect in Western Siberia. We found that hydrolytic enzyme activities decreased rapidly with depth, whereas oxidative enzyme activities in mineral horizons were as high as, or higher than in organic topsoil horizons. Enzyme patterns varied more strongly between ecosystems in mineral subsoil horizons than in organic topsoils. The enzyme patterns in topsoil horizons were correlated with SOM content (i.e., C and N content) and microbial community composition. In contrast, the enzyme patterns in mineral subsoil horizons were related to water content, soil pH and microbial community composition. The lack of correlation between enzyme patterns and SOM quantity in the mineral subsoils suggests that SOM chemistry, spatial separation or physical stabilization of SOM rather than SOM content might determine substrate availability for enzymatic breakdown. The correlation of microbial community composition and enzyme patterns in all horizons, suggests that microbial community composition shapes enzyme patterns and might act as a modifier for the usual dependency of decomposition rates on SOM content or C/N ratios.     

Soil morphology,depth and grapevine root frequency influence microbial communities in a Pinot noir vineyard

The composition of microbial communities responds to soil resource availability, and has been shown to vary with increasing depth in the soil profile. Soil microorganisms partly rely on root-derived carbon (C) for growth and activity. Roots in woody perennial systems like vineyards have a deeper vertical distribution than grasslands and annual agriculture. Thus, we hypothesized that vineyard soil microbial communities along a vertical soil profile would differ from those observed in grassland and annual agricultural systems. In a Pinot noir vineyard, soil pits were excavated to ca. 1.6–2.5 m, and microbial community composition in ‘bulk’ (i.e., no roots) and ‘root’ (i.e., roots present) soil was described by phospholipid ester-linked fatty acids (PLFA). Utilization of soil taxonomy aided in understanding relationships between soil microbial communities, soil resources and other physical and chemical characteristics. Soil microbial communities in the Ap horizon were similar to each other, but greater variation in microbial communities was observed among the lower horizons. Soil resources (i.e., total PLFA, or labile C, soil C and nitrogen, and exchangeable potassium) were enriched in the surface horizons and significantly explained the distribution of soil microbial communities with depth. Soil chemical properties represented the secondary gradient explaining the differentiation between microbial communities in the B-horizons from the C-horizons. Relative abundance of Gram-positive bacteria and actinomycetes did not vary with depth, but were enriched in ‘root’ vs. ‘bulk’ soils. Fungal biomarkers increased with increasing depth in ‘root’ soils, differing from previous studies in grasslands and annual agricultural systems. This was dependent on the deep distribution of roots in the vineyard soil profile, suggesting that the distinct pattern in PLFA biomarkers may have been strongly affected by C derived from the grapevine roots. Gram-negative bacteria did not increase in concert with fungal abundance, suggesting that acidic pHs in lower soil horizons may have discouraged their growth. These results emphasize the importance of considering soil morphology and associated soil characteristics when investigating effects of depth and roots on soil microorganisms, and suggest that vineyard management practices and deep grapevine root distribution combine to cultivate a unique microbial community in these soil profiles.     

Mineralization of soil organic matter initiated by the application of an available substrate to the profiles of surface and buried podzolic soils

The priming effect (PE) initiated by the application of ¹⁴C-glucose was studied for copiotrophic microbial communities of organic horizons and for oligotrophic microbial communities of mineral soil horizons, as well as for mineral horizons of buried soils depleted in the input of fresh organic matter. The intensity of the PE depended on the reserves of C_org, the initial amount of the microbial biomass, and the enzymatic activity, which decreased from the organic to the mineral soil horizons. The ratio of the PE to the applied carbon was two times higher in the mineral horizons as compared with the organic horizons. This is explained by the predominance of K-strategists capable of decomposing difficultly available organic compounds in the mineral horizons, so that the turnover of the microbial biomass in the mineral horizons was more active than that in the organic horizons. The predominance of K-strategists was confirmed by the close correlation between the PE and the activity of the cellobiohydrolase enzyme decomposing cellulose (R = 0.96). In general, the absolute value of the PE was controlled by the soil organic matter content, whereas the specific PE was controlled by the functional features of the microorganisms. It was shown that the functional features of the soil microorganisms remained unchanged under the conditions of their preservation in the buried soil.     

Variations in microbial community composition through two soil depth profiles

Soil profiles are often many meters deep, but with the majority of studies in soil microbiology focusing exclusively on the soil surface, we know very little about the nature of the microbial communities inhabiting the deeper soil horizons. We used phospholipid fatty acid (PLFA) analysis to examine the vertical distribution of specific microbial groups and to identify the patterns of microbial abundance and community-level diversity within the soil profile. Samples were collected from the soil surface down to 2 m in depth from two unsaturated Mollisol profiles located near Santa Barbara, CA, USA. While the densities of microorganisms were generally one to two orders of magnitude lower in the deeper horizons of both profiles than at the soil surface, approximately 35% of the total quantity of microbial biomass found in the top 2 m of soil is found below a depth of 25 cm. Principal components analysis of the PLFA signatures indicates that the composition of the soil microbial communities changes significantly with soil depth. The differentiation of microbial communities within the two profiles coincides with an overall decline in microbial diversity. The number of individual PLFAs detected in soil samples decreased by about a third from the soil surface down to 2 m. The ratios of cyclopropyl/monoenoic precursors and total saturated/total monounsaturated fatty acids increased with soil depth, suggesting that the microbes inhabiting the deeper soil horizons are more carbon limited than surface-dwelling microbes. Using PLFAs as biomarkers, we show that Gram-positive bacteria and actinomycetes tended to increase in proportional abundance with increasing soil depth, while the abundances of Gram-negative bacteria, fungi, and protozoa were highest at the soil surface and substantially lower in the subsurface. The vertical distribution of these specific microbial groups can largely be attributed to the decline in carbon availability with soil depth.     

Refining the interpretation of orthic horizons in South Africa

The relationship between soil water regime and soil profile morphology could contribute towards improving catchment water yield. This paper proposes refinements to the interpretation of orthic A horizons (ochric), defined in Soil Classification — A Taxonomic System for South Africa, to aid interpretation for hydrological purposes. Soil water contents have been measured for six years in the Weatherley catchment in South Africa and used to classify orthic A horizons in terms of wetness classes defined in the Soil Survey Manual. The well drained class is applicable to orthic A horizons overlying red apedal B, yellow-brown apedal B (agric or ferralic) or neocutanic B (cambic) horizons. The moderate poorly and poorly drained classes are applicable to orthic A horizons overlying E (albic), soft plinthic B (ferric) or G (gleyic) horizons. The occurrence of mottles and matrix colour can be used for differentiation between the latter two wetness classes. Interpretation of orthic A horizons can therefore be enhanced by wetness classification, utilizing the nature of the underlying horizon, the occurrence of mottles and matrix colour.     

The effect of different tree species on the chemical and microbial properties of reclaimed mine soils

The chemical and microbial properties of afforested mine soils are likely to depend on the species composition of the introduced vegetation. This study compared the chemical and microbial properties of organic horizons and the uppermost mineral layers in mine soils under pure pine (Pinus sylvestris), birch (Betula pendula), larch (Larix decidua), alder (Alnus glutinosa), and mixed pine–alder and birch–alder forest stands. The studied properties included soil pH, content of organic C (C_org) and total N (N_t), microbial biomass (C_mic), basal respiration, nitrogen mineralization rate (Min-N), and the activities of dehydrogenase, acid phosphomonoesterase, and urease. Near-infrared spectroscopy (NIR) was used to detect differences in the chemical composition of soil organic matter under the studied forest stands. There were significant differences in C_org and N_t contents between stands in both O and mineral soil horizons and also in the chemical composition of the accumulated organic matter, as indicated by NIR spectra differences. Alder was associated with the largest C_org and N_t accumulation but also with a significant decrease of pH in the mineral soil. Microbial biomass, respiration, the percentage of C_org present as C_mic, Min-N, and dehydrogenase activity were the highest under the birch stand, indicating a positive effect of birch on soil microflora. Admixture of alder to coniferous stand increased basal respiration, Min-N, and activities of dehydrogenase and acid phosphomonoesterase as compared with the pure pine stand. In the O horizon, soil pH and N_t content had the most important effects on all microbial properties. In this horizon, the activities of urease and acid phosphomonoesterase did not depend on microbial biomass. In the mineral layer, however, the amount of accumulated C and microbial biomass were of primary importance for the enzyme activities.     

Distribution of microbial biomass and phospholipid fatty acids in Podzol profiles under coniferous forest

Microbial‐derived phospholipid fatty acids (PLFAs) can be used to characterize the microbial communities in soil without the need to isolate individual fungi and bacteria. They have been used to assess microbial communities of humus layers under coniferous forest, but nothing is known of their distribution in the deeper soil. To investigate the vertical distribution we sampled nine Podzol profiles on a 100‐m‐long transect in a coniferous forest and analysed for their microbial biomass and PLFA pattern to a depth of 0.4 m. The transect covered a fertility gradient from Vaccinium vitis‐idaea forest site type to Vaccinium myrtillus forest site type. The cores were divided into humus (O) and eluvial (E) layers and below that into 10‐cm sections and designated as either illuvial (B) or parent material (C), or as a combination (BC). Two measures of microbial biomass analyses were applied: substrate‐induced respiration (SIR) to determine microbial biomass C (C_mic), and the sum of the extracted microbial‐derived phospholipid fatty acids (totPLFA). The soil fertility had no effect on the results. The C_mic correlated well with totPLFA (r= 0.86). The microbial biomass decreased with increasing depth. In addition the PLFA pattern changed with increased depth as assessed with principal component analysis, indicating a change in the microbial community structure. The composition of the PLFAs in the O layer differed from that in the E layer and both differed from the upper part of the B layer and from the rest of the BC layers. The deeper parts of the B layer (BC₁, BC₂ and BC₃) were similar to one other. The O layer had more 18:2ω6, a PLFA indicator of fungi, whereas the E layer contained relatively more of the PLFAs 16:1ω9, 18:1ω7 and cy19:0 common in gram‐negative bacteria. With increased depth the relative amount of 10Me18:0, the PLFA indicator for actinomycetes, increased. We conclude that the PLFA method is a promising discriminator between the microbial community structures of the horizons in Podzols.     

Decoupling plant-growth from land-use legacies in soil microbial communities

Differences in soil microbial communities between ex-arable and undisturbed soils are often assumed to reflect long-term legacies of agricultural practices. Ex-arable soils, however, are commonly dominated by different plant species than undisturbed soils making it difficult to separate the importance of land-use and plant-growth legacies. In a system where non-native plants dominate ex-arable soils, we decoupled land-use (ex-arable, undisturbed) and plant-growth (native, non-native) effects on soil microbial communities using a factorial sampling design. Soils were removed from 14 sites that formed a 52-year chronosequence of agricultural abandonment. Microbial abundance and composition were measured using whole-soil phospholipid fatty acid analyses and microbial activity was measured in a subset of samples using sole-carbon-source utilization analyses. We found that both non-native-cultivated and ex-arable soils were independently associated with lower microbial abundance and diversity than native and undisturbed soils. We also found a correlation between microbial abundance and age-since-agricultural abandonment in ex-arable/non-native-cultivated soils suggesting that non-native plant effects accumulate over time. Microbial activity was consistent with microbial abundance; microbial communities in non-native-cultivated, ex-arable soils were slow to respire most carbon sources. Our data suggests that agricultural practices create soil conditions that favor non-native plant growth and non-native plants maintain these conditions. Potential mechanisms explaining how non-natives create soils with small microbial communities and how small microbial communities may benefit non-natives are discussed.