Responses of anaerobic bacteria to soil amendment with selenite

Soil (Haplic Luvisol) was incubated in anaerobic microcosms with and without addition of a small amount of selenite (2 mg Se kg⁻¹) and straw, and changes in both bacterial populations (fermentative and selenite-respiring) and selenium fractionation were assessed. Selenite caused an initial decrease in CO₂ emission (−98% alone and −60% with straw) but this effect decreased with time. Selenite and straw additions enhanced the dynamics of fermentative and selenite-respiring bacteria but their effect was not cumulative. Selenium became less easily extractable during incubation: the non-extractable fraction of added selenium increased from 22% to 68% (73% with straw).     

Immobilization and remineralization of N following addition of wheat straw into soil: determination of gross N transformation rates by N-ammonium isotope dilution technique

N dynamics in soil where wheat straw was incorporated were investigated by a soil incubation experiment using ¹⁵N-labelled nitrate or ¹⁵N-labelled wheat straw. The incubated soils were sampled after 7, 28, 54 days from the incorporation of wheat straw, respectively, and gross rates of N transformations including N remineralization and temporal changes in the amount of microbial biomass were determined.Following the addition of wheat straw into soils, rapid decrease of nitrate content in soil and increase of microbial biomass C and N occurred within the first week from onset of the experiment. Both the gross rates of mineralization and immobilization determined by ¹⁵N-ammonium isotope dilution technique were remarkably enhanced by the addition of wheat straw, and gradually decreased with time. Remineralization rate of N derived from ¹⁵N-labelled nitrate, and mineralization rate of N derived from ¹⁵N-labelled wheat straw was estimated by ¹⁵N isotope dilution technique using non-labelled ammonium. Remineralization rates of N derived from ¹⁵N-labelled nitrate were calculated to be 0.71 mg N kg⁻¹ d⁻¹ after 7 days, 0.55 mg N kg⁻¹ d⁻¹ after 28 days, and 0.29 mg N kg⁻¹ d⁻¹ after 54 days.Nearly 10% of the ¹⁵N-labelled N originally contained in the wheat straw was held in the microbial biomass irrespective of the sampling time. The amount of inorganic N in soil which was derived from ¹⁵N-labelled wheat straw ranged between 1.93 and 2.37 mg N kg⁻¹.Rates of N transformations in soil with ¹⁵N-labelled wheat straw were obtained by assuming that the k value was equal to the ¹⁵N abundance of biomass N, and the obtained values were considered to be valid.     

Manipulating the N release from N labelled celery residues by using straw and vinasses

The aim of this laboratory study was to investigate the effect of straw and vinasses on the nitrogen (N) mineralization-immobilization turnover of celery residues during two periods (each simulating a time period from autumn till spring) under laboratory conditions. During the first period (1-198 d), ¹⁵N-labelled celery residues (1.1 g dry matter (DM) kg⁻¹ soil) were incubated together with straw (8.1 g DM kg⁻¹ soil), aiming to immobilize the N released from celery residues, followed by an incorporation of vinasses (1.9 g DM kg⁻¹ soil) after 84 d, with a view to remineralizing the immobilized celery-N. During the second period (198-380 d), the experimental set-up was repeated, except that non-labelled celery residues were used. Total N, mineral N and their ¹⁵N enrichments as well as microbial biomass N were determined at regular time intervals. During both periods, mixing celery residues with straw significantly increased microbial biomass N (90.5 and 40.5 mg N kg⁻¹ extra compared to celery only treatment) and decreased the amount of mineral N (reduction of 56.1 and 45.9 mg N kg⁻¹ soil compared to celery only treatment) and the celery-derived mineral ¹⁵N (0% of mineral celery-derived ¹⁵N in straw treatment compared to 35% of mineral celery-derived ¹⁵N in celery only treatment). After maximum immobilization, a natural remineralization (without addition of vinasses) of 32.2 (at day 198) and 11.1 mg N kg⁻¹ soil (at day 380) occurred in the straw treatment, but the mineral N content remained significantly lower than in the celery only treatment during the complete experiment, and the amount of remineralized celery-¹⁵N was very low (5.4% of celery-derived ¹⁵N after 380 d). Vinasses caused no real priming effect, although it did slightly increase the amount of remineralized celery-¹⁵N (+6.4% of celery-derived ¹⁵N at day 380 compared to the straw treatment), probably due an apparent added N interaction caused by displacement reactions with the soil microbial biomass.     

Amino sugars and muramic acid—biomarkers for soil microbial community structure analysis

Characterizing functional and phylogenetic microbial community structure in soil is important for understanding the fate of microbially-derived compounds during the decomposition and turn-over of soil organic matter. This study was conducted to test whether amino sugars and muramic acid are suitable biomarkers to trace bacterial, fungal, and actinomycetal residues in soil. For this aim, we investigated the pattern, amounts, and dynamics of three amino sugars (glucosamine, mannosamine and galactosamine) and muramic acid in the total microbial biomass and selectively cultivated bacteria, fungi, and actinomycetes of five different soils amended with and without glucose. Our results revealed that total amino sugar and muramic acid concentrations in microbial biomass, extracted from soil after chloroform fumigation varied between 1 and 27 mg kg⁻¹ soil. In all soils investigated, glucose addition resulted in a 50-360% increase of these values. In reference to soil microbial biomass-C, the total amino sugar- and muramic acid-C concentrations ranged from 1-71 g C kg⁻¹ biomass-C. After an initial lag phase, the cultivated microbes revealed similar amino sugar concentrations of about 35, 27 and 17 g glucosamine-C kg⁻¹ TOC in bacteria, fungi, and actinomycetes, respectively. Mannosamine and galactosamine concentrations were lower than those for glucosamine. Mannosamine was not found in actinomycete cultures. The highest muramic acid concentrations were found in bacteria, but small amounts were also found in actinomycete cultures. The concentrations of the three amino sugars studied and muramic acid differed significantly between bacteria and the other phylogenetic microbial groups under investigation (fungi and actinomycetes). Comparison between the amino sugar and muramic acid concentrations in soil microbial biomass, extracted after chloroform fumigation, and total concentrations in the soil showed that living microbial biomass contributed negligible amounts to total amino sugar contents in the soil, being at least two orders of magnitude greater in the soils than in the soil inherent microbial biomass. Thus, amino sugars are significantly stabilized in soil.     

Biochemical characterization of urban soil profiles from Stuttgart, Germany

The knowledge of biochemical properties of urban soils can help to understand nutrient cycling in urban areas and provide a database for urban soil management. Soil samples were taken from 10 soil profiles in the city of Stuttgart, Germany, differing in land use—from an essentially undisturbed garden area to highly disturbed high-density and railway areas. A variety of soil biotic (microbial biomass, enzyme activities) and abiotic properties (total organic C, elemental C, total N) were measured up to 1.9 m depth. Soil organic matter was frequently enriched in the subsoil. Microbial biomass in the top horizons ranged from 0.17 to 1.64 g C kg⁻¹, and from 0.01 to 0.30 g N kg⁻¹, respectively. The deepest soil horizon at 170-190 cm, however, contained 0.12 g C kg⁻¹ and 0.05 kg N kg⁻¹ in the microbial biomass. In general, arylsulphatase and urease activity decreased with depth but in three profiles potentially mineralizable N in the deepest horizons was higher than in soil layers directly overlying. In deeply modified urban soils, subsoil beside topsoil properties have to be included in the evaluation of soil quality. This knowledge is essential because consumption of natural soils for housing and traffic has to be reduced by promoting inner city densification.     

Measuring rates of gross and net mineralisation of organic phosphorus in soils

The isotopic dilution method developed by Oehl et al. [2001b. Organic phosphorus mineralisation studies using isotopic dilution techniques. Soil Science Society of America Journal 65, 780-787] to measure gross mineralisation of soil organic phosphorus (P) was tested on a range of low-P sorbing soils. This isotopic dilution method relies on accurate prediction of radiolabel behaviour due to soil physicochemical processes. Based on experimental validation of the extrapolation for isotopic dilution due to physicochemical processes using autoclaved soils, a simple power function was used for extrapolation rather than the more complex equation used in the original method. For several soils, however, a potential overestimation of gross mineralisation by 0.1-2.0 mg P kg⁻¹ d⁻¹ was revealed. In addition, the detection limit of P mineralisation ranged between 0.6 and 2.6 mg P kg⁻¹ d⁻¹. The method is likely to be at the detection limit for soils that are high in available P and low in biological activity. The method was modified with respect to the extrapolation and successfully applied to a soil with relatively high microbial P (18 mg P kg⁻¹) and soil respiration rates (29 mg C kg⁻¹ d⁻¹), revealing gross mineralisation rates of organic P of 0.9-1.2 mg P kg⁻¹ d⁻¹. Measurement of uptake of ³²P by the microbial biomass allowed derivation of a net organic P mineralisation rate of 0.5-0.9 mg P kg⁻¹ d⁻¹.     

Hydrolase activity, microbial biomass and community structure in long-term Cd-contaminated soils

Long-term effects of high Cd concentrations on enzyme activities, microbial biomass and respiration and bacterial community structure of soils were assessed in sandy soils where Cd was added between 1988 and 1990 as Cd(NO₃)₂ to reach concentrations ranging from 0 to 0.36 mmol Cd kg⁻¹ dry weight soil. Soils were mantained under maize and grass cultivation, or ‘set-aside’ regimes, for 1 year. Solubility of Cd and its bioavailability were measured by chemical extractions or by the BIOMET bacterial biosensor system. Cadmium solubility was very low, and Cd bioavailability was barely detectable even in soils polluted with 0.36 mmol Cd kg⁻¹. Soil microbial biomass carbon (B_C) was slightly decreased and respiration was increased significantly even at the lower Cd concentration and as a consequence the metabolic quotient (qCO₂) was increased, indicating a stressful condition for soil microflora. However, Cd-contaminated soils also had a lower total organic C (TOC) content and thus the microbial biomass C-to-TOC ratio was unaffected by Cd. Alkaline phosphomonoesterase, arylsulphatase and protease activities were significantly reduced in all Cd-contaminated soils whereas acid phosphomonoesterase, β-glucosidase and urease activites were unaffected by Cd. Neither changes in physiological groups of bacteria, nor of Cd resistant bacteria could be detected in numbers of the culturable bacterial community. Denaturing gradient gel electrophoresis analysis of the bacterial community showed slight changes in maize cropped soils containing 0.18 and 0.36 mmol Cd kg⁻¹ soil as compared to the control. It was concluded that high Cd concentrations induced mainly physiological adaptations rather than selection for metal-resistant culturable soil microflora, regardless of Cd concentration, and that some biochemical parameters were more sensitive to stress than others.     

Long-term effects of metal-containing farmyard manure and sewage sludge on soil organic matter in a fluvisol

Our aim was to establish the long-term effects of repeated applications after 20 y of organic amendments (farmyard manure at 10 t ha⁻¹ y⁻¹, and urban sewage sludge at two different rates, 10 t ha⁻¹ y⁻¹ and 100 t ha⁻¹ every 2 y) on the quality of a sandy and poorly buffered soil (Fluvisol, pH 6). Chemical characteristics and biodegradability of the labile organic matter, which is mainly derived from microbial biomass and biodegradation products of organic residues, were chosen as indicators for soil quality. The organic C content had reached a maximal value (30.6 g C kg⁻¹ in the 100 t sludge-treated soil), i.e. about 2.5 times that in the control. Six years after the last application, the organic C content and the microbial biomass content remained higher in sludge-treated soils than in the control. In contrast, the proportion of labile organic matter was significantly lower in sludge-treated soils than in manure-treated and control soils. The labile organic matter of sludge extracts appeared less humified than that of manure-treated and control soils.     

C and N turnover in structurally intact soils of different texture

The turnover of native and applied C and N in undisturbed soil samples of different texture but similar mineralogical composition, origin and cropping history was evaluated at −10 kPa water potential. Cores of structurally intact soil with 108, 224 and 337 g clay kg⁻¹ were horizontially sliced and ¹⁵N-labelled sheep faeces was placed between the two halves of the intact core. The cores together with unamended treatments were incubated in the dark at 20 °C and the evolution of CO₂-C determined continuously for 177 d. Inorganic and microbial biomass N and ¹⁵N were determined periodically. Net nitrification was less in soil amended with faeces compared with unamended soil. When adjusted for the NO₃-N present in soil before faeces was applied, net nitrification became negative indicating that NO₃-N had been immobilized or denitrified. The soil most rich in clay nitrified least N and ¹⁵N. The amounts of N retained in the microbial biomass in unamended soils increased with clay content. A maximum of 13% of the faeces ¹⁵N was recovered in the microbial biomass in the amended soils. CO₂-C evolution increased with clay content in amended and unamended soils. CO₂-C evolution from the most sandy soil was reduced due to a low content of potentially mineralizable native soil C whereas the rate constant of C mineralization rate peaked in this soil. When the pool of potentially mineralizable native soil C was assumed proportional to volumetric water content, the three soils contained similar proportions of potentially mineralizable native soil C but the rate constant of C mineralization remained highest in the soil with least clay. Thus although a similar availability of water in the three soils was ensured by their identical matric potential, the actual volume of water seemed to determine the proportion of total C that was potentially mineralizable. The proportion of mineralizable C in the faeces was similar in the three soils (70% of total C), again with a higher rate constant of C mineralization in the soil with least clay. It is hypothesized that the pool of potentially mineralizable C and C rate constants fluctuate with the soil water content.     

Microbial community composition and substrate use in a highly weathered soil as affected by crop rotation and P fertilization

A better understanding of soil microbial processes is required to improve the synchrony between nutrient release from plant residues and crop demand. Phospholipid fatty acid analysis was used to investigate the effect of two crop rotations (continuous maize and maize-crotalaria rotation) and P fertilization (0 and 50 kg P ha⁻¹ yr⁻¹, applied as triple superphosphate) on microbial community composition in a highly weathered soil from western Kenya. Microbial substrate use in soils from the field experiment was compared in incubation experiments. Higher levels of soil organic matter and microbial biomass in the maize-crotalaria rotation were connected with higher total amounts of phospholipid fatty acids and an increase in the relative abundances of indicators for fungi and gram-negative bacteria. P fertilization changed the community profile only within the continuous maize treatment. The decomposition of glucose, cellulose and three plant residues (all added at 2.5 g C kg⁻¹ soil) proceeded faster in soil from the maize-crotalaria rotation, but differences were mostly transient. Microbial P and N uptake within one week increased with the water-soluble carbon content of added plant residues. More P and N were taken up by the greater microbial biomass in soil from the maize-crotalaria rotation than from continuous maize. Re-mineralization of nutrients during the decline of the microbial biomass increased also with the initial biological activity of the soil, but occurred only for a high quality plant residue within the half year incubation period. Compared to the effect of crop rotation, P fertilization had a minor effect on microbial community composition and substrate use.     

Do plant clumps constitute microbial hotspots in semiarid Mediterranean patchy landscapes?

Three semiarid Mediterranean patchy landscapes were investigated to test the existence of a microsite effect (i.e. plant canopy vs. inter-canopy) on soil microbial communities. Surface soil samples were independently taken from both microsites under naturally changing conditions of humidity and temperature through the year. In gypsiferous soils covered with a shrub steppe, improved physical and chemical soil properties were registered underneath the plant canopy, where the densest and most active microbial communities were also detected (e.g. microbial biomass C averaged 531 and 202 mg kg⁻¹ in canopy and inter-canopy areas, respectively). In calcareous perennial tussock grasslands, either growing on soils over limestones or alluvial deposits, the microsite effect was not so marked. Soil humidity, temperature and total organic C were homogeneously distributed over the landscape conditioning their uniform microbial activity under field moisture conditions (ATP content averaged 853 and 885 nmol kg⁻¹ in canopy and intercanopy areas, respectively). However, readily mineralizable C and microbial biomass C were preferentially accumulated in soils underneath the tussocks determining their larger potential microbial activity (e.g. C hydrolysis capacity under optimal conditions). In conclusion, plant clumps either functioned as microbial hotspots where enhanced microbially driven ecosystem processes took place or as microbial banks capable of undergoing a burst of activity under favourable climatic conditions. Our results provide experimental evidence of a non-patchy distribution of certain soil microbial properties in semi-arid Mediterranean patchy ecosystems.     

Response of microbial activity and microbial community composition in soils to long-term arsenic and cadmium exposure

Arsenic (As) and cadmium (Cd) in soils can affect soil microbial function and community composition and, therefore, may have effects on soil ecosystem functioning. The aim of our study was to assess the effects of long-term As and Cd contamination on soil microbial community composition and soil enzyme activities. We analyzed soils that have been contaminated 25 years ago and at present still show enhanced levels of either As, 18 and 39 mg kg⁻¹, or Cd, 34 and 134 mg kg⁻¹. Soil without heavy metal addition served as control. Polymerase chain reaction (PCR) followed by denaturing gradient gel electrophoresis (DGGE) showed that bacterial community composition in As and Cd contaminated soils differed from that in the control soil. The same was true for the microbial community composition assessed by analysis of respiratory quinones. Soil fungi and Proteobacteria appeared to be tolerant towards As and Cd, while other groups of bacteria were reduced. The decline in alkaline phosphatase, arylsulphatase, protease and urease activities in the As- and Cd-contaminated soils was correlated with a decrease of respiratory quinones occuring in Actinobacteria and Firmicutes. Xylanase activity was unaffected or elevated in the contaminated soils which was correlated with a higher abundance of fungal quinones, and quinones found in Proteobacteria.     

Microbial activity in pig slurry-amended soils under semiarid conditions

Soil amendment with manures from intensive animal industries is nowadays a common practice that may favorably or adversely affect several soil properties, including soil microbial activity. In this work, the effect of consecutive annual additions of pig slurry (PS) at rates of 30, 60, 90, 120 and 150 m³ ha⁻¹ y⁻¹ over a 4-year period on soil chemical properties and microbial activity was investigated and compared to that of an inorganic fertilization and a control (without amendment). Field plot experiment conducted under a continuous barley monoculture and semiarid conditions were used. Eight months after the fourth yearly PS and mineral fertilizer application (i.e. soon after the fourth barley harvest), surface soil samples (Ap horizon, 0-15 cm depth) from control and amended soils were collected and analysed for pH, electrical conductivity (EC), contents of total organic C, total N, available P and K, microbial biomass C, basal respiration and different enzymatic activities. The control soil had a slightly acidic pH (6.0), a small EC (0.07 dS m⁻¹), adequate levels of total N (1.2 g kg⁻¹) and available K (483 mg kg⁻¹) for barley growth, and small contents of total organic C (13.2 g kg⁻¹) and available P (52 mg kg⁻¹). With respect to the control and mineral fertilized soils, the PS-amended soils had greater pH values (around neutrality or slightly alkaline), electrical conductivities (still low) and contents of available P and K, and slightly larger total N contents. A significant decrease of total organic C was observed in soils amended at high slurry rate (12.3 g kg⁻¹). Compared with the control and mineral treatments, which produced almost similar results, the PS-amended soils were characterized by a higher microbial biomass C content (from 311 to 442 g kg⁻¹), microbial biomass C/total organic C ratio (from 2.3 to 3.6%) and dehydrogenase (from 35 to 173 μg INTF g⁻¹), catalase (from 5 to 24 μmol O₂ g⁻¹ min⁻¹), BAA-protease (from 0.7 to 1.9 μmol  g⁻¹ h⁻¹) and β-glucosidase (from 117 to 269 μmol PNP g⁻¹ h⁻¹) activities, similar basal respirations (from 48 to 77 μg C-CO₂ g⁻¹ d⁻¹) and urease activities (from 1.5 to 2.2 μmol  g⁻¹ h⁻¹), and smaller metabolic quotients (from 6.4 to 7.7 ng C-CO₂ μg⁻¹ biomass C h⁻¹) and phosphatese activities (from 374 to 159 μmol PNP g⁻¹ h⁻¹). For example, statistical analysis of experimental data showed that, with the exception of metabolic quotient and total organic C content, these effects generally increased with increasing cumulative amount of PS. In conclusion, cumulative PS application to soil over time under semiarid conditions may produce not only beneficial effects but also adverse effects on soil properties, such us the partial mineralization of soil organic C through extended microbial oxidation. Thus, PS should not be considered as a mature organic amendment and should be treated appropriately before it is applied to soil, so as to enhance its potential as a soil organic fertilizer.     

Does soil ergosterol concentration provide a reliable estimate of soil fungal biomass?

Our aim was to determine if soil ergosterol concentration provides a quantitative estimate of the soil fungal biomass concentration, as is usually assumed. This was done by comparing soil ergosterol measurements with soil fungal biomass (fungal biomass C) concentrations estimated by microscopic measurements and by the selective inhibition technique linked to substrate-induced respiration (SIR). The measurements were compared in a silty-clay loam soil given a range of previous treatments designed to increase or decrease the soil fungal biomass and so also to change the soil ergosterol concentration. The treatments used were ryegrass amendment, to increase the total and fungal biomass, and CHCl₃-fumigation and the addition of the biocides, captan, bronopol and dinoseb, to decrease both ergosterol and fungal biomass C concentrations. The mineralization of ergosterol following addition to sand innoculated with soil extract, and to a sandy loam soil, was also determined. The added ergosterol was little, if at all, degraded following addition to either sand or the unfumigated or fumigated soil during a 10 d aerobic incubation. Similarly, pesticide addition did not significantly change soil ergosterol concentrations yet the soil fungal biomass C concentration decreased significantly. Thus, the ratio: (soil ergosterol concentration/soil fungal biomass C concentration) was much higher in the pesticide-treated soils than the control soil. Following ryegrass amendment, soil ergosterol concentration increased from about 6-12 μg⁻¹ soil within 5 d and then decreased gradually to about 7 μg g⁻¹ soil by 20 d incubation. Changes in fungal biomass C (measured by direct microscopy) closely mirrored changes in soil ergosterol over this period. However, when the amended soil was fumigated and then incubated for a further 5 d, the initial ergosterol concentration declined from 7 to 5 μg g⁻¹ soil by 20 d incubation (a decline of about 0.4). The comparable decline in fungal biomass C was about eight-fold. Thus the ratio of ergosterol to fungal biomass C increased from 0.005 to about 0.01. There was a significant correlation (r>0.84, P<0.001) between soil ergosterol concentration and fungal biomass measured by either SIR or microscopy. However, three data points played a vital role in the correlation. When these points were excluded the relationship was very poor (r<0.4). Our results therefore suggest that substantial amounts of ergosterol may exist, other than in living cells, for considerable periods, with little, if any mineralization. Thus, these results indicate that ergosterol and fungal biomass C concentrations are not always closely correlated, due to the slow metabolism of ergosterol in recently dead fugal biomass and/or the existence of exocellular ergosterol in soil.     

Enzyme activities and microbial biomass in coastal soils of India

Soil salinity is a serious problem for agriculture in coastal regions, wherein salinity is temporal in nature. We studied the effect of salinity, in summer, monsoon and winter seasons, on microbial biomass carbon (MBC) and enzyme activities (EAs) of the salt-affected soils of the coastal region of the Bay of Bengal, Sundarbans, India. The average pH of soils collected from different sites, during different seasons varied from 4.8 to 7.8. The average organic C (OC) and total N (TN) content of the soils ranged between 5.2-14.1 and 0.6-1.4 g kg⁻¹, respectively. The electrical conductivity of the saturation extract (ECe) of soils, averaged over season, varied from 2.2 to 16.3 dSm⁻¹. The ECe of the soils increased five fold during the summer season (13.8 dSm⁻¹) than the monsoon season (2.7 dSm⁻¹). The major cation and anion detected were Na⁺ and Cl⁻, respectively. Seasonality exerted considerable effects on MBC and soil EAs, with the lowest values recorded during the summer season. The activities of β-glucosidase, urease, acid phosphatase and alkaline phosphatase were similar during the winter and monsoon season. The dehydrogenase activity of soils was higher in monsoon than in winter. Average MBC, dehydrogenase, β-glucosidase, urease, acid phosphatase and alkaline phosphatase activities of the saline soils ranged from 125 to 346 mg kg⁻¹ oven dry soil, 6-9.9 mg triphenyl formazan (TPF) kg⁻¹ oven dry soil h⁻¹, 18-53 mg p-nitro phenol (PNP) kg⁻¹ oven dry soil h⁻¹, 38-86 mg urea hydrolyzed kg⁻¹ oven dry soil h⁻¹, 213-584 mg PNP kg⁻¹ oven dry soil h⁻¹ and 176-362 mg PNP g⁻¹ oven dry soil h⁻¹, respectively. The same for the non-saline soils were 274-446 mg kg⁻¹ oven dry soil, 8.8-14.4 mg TPF kg⁻¹ oven dry soil h⁻¹, 41-80 mg PNP kg⁻¹ oven dry soil h⁻¹, 89-134 mg urea hydrolyzed kg⁻¹ oven dry soil h⁻¹, 219-287 mg PNP kg⁻¹ oven dry soil h⁻¹ and 407-417 mg PNP kg⁻¹ oven dry soil h⁻¹, respectively. About 48%, 82%, 48%, 63%, 40% and 48% variation in MBC, dehydrogenase activity, β-glucosidase activity, urease activity, acid phosphatase activity and alkaline phosphatase activity, respectively, could be explained by the variation in ECe of saline soils. Suppression of EAs of the coastal soils during summer due to salinity rise is of immense agronomic significance and needs suitable interventions for sustainable crop production.     

Earthworm-mycorrhiza interaction on Cd uptake and growth of ryegrass

The effects of inoculation of earthworms and arbuscular mycorrhiza separately, and in combination, on Cd uptake and growth of ryegrass were studied in soils contaminated with 0, 5, 10, 20 mg of Cd kg⁻¹ soil. Both earthworms and mycorrhiza were able to survive in all the treatments with added Cd. Earthworm activity significantly increased mycorrhizal infection rate of root and ryegrass shoot biomass. Earthworm activity decreased soil pH by about 0.2 units, and enhanced root Cd concentration and ryegrass Cd uptake. Mycorrhiza inoculation increased shoot and root Cd concentration substantially, and at the highest dosage of 20 mg Cd kg⁻¹ decreased biomass of ryegrass. Inoculation of both earthworms and mycorrhiza increased ryegrass shoot Cd uptake at low Cd concentrations (5 and 10 mg Cd kg⁻¹ soil), when compared with inoculation of earthworms or mycorrhiza alone. In conclusion, earthworm, mycorrhiza and their interaction may have a potential role in elevating phytoextraction efficiency in low to medium level metal contaminated soil.     

Influence of selenium on oxidoreductive enzymes activity in soil and in plants

The aim of this greenhouse experiment was the assessment of the influence of H₂SeO₃ at soil concentrations of 0.05, 0.15 and 0.45 mmol kg⁻¹, on the activity of selected oxidoreductive enzymes in wheat (Triticum aestivum). The wheat plants were grown in 2 dm³ pots filled with dust-silt black soil of pH 7.7. Applied H₂SeO₃ caused activation of plant nitrate reductase at all concentrations, but activation of plant polyphenol oxidase at only two lower concentrations. The highest concentration caused inhibition of polyphenol oxidase and peroxidase. Plant catalase activity decreased under the influence of 0.15 and 0.45 mmol kg⁻¹ concentration. After the final analysis Se was quantified in plants and soil. The amounts in plants were: control (unamended soil) 1.95 mg kg⁻¹; I dose (0.05 mmol kg⁻¹) 18.27 mg kg⁻¹; II dose (0.15 mmol kg⁻¹) 33.20 mg kg⁻¹ and III dose (0.45 mmol kg⁻¹) 38.37 mg kg⁻¹, in soil: 0.265 mg kg⁻¹; 3.61 mg kg⁻¹; 10.53 mg kg⁻¹; 30.53 mg kg⁻¹; respectively. Simultaneously, a laboratory experiment was performed, where the activity of soil catalase and peroxidase were tested after 1, 3, 7, 14, 28, 56, and 112 days after Se treatment. Peroxidase activity in soil decreased with increasing Se content, over the whole experiment. The lowest dose of Se caused activation a significant 10% increase in catalase activity, but the influence of others doses was unclear.     

Organic manure stimulates biological activity and barley growth in soil subject to secondary salinization

A pot experiment was performed to compare the impact of organic manure on soil enzymatic activity, respiration rate and the growth of two barley cultivars (Hordeum vulgare L.) differing in their salt tolerance under a simulated salinized environment. A plastic pot with a hole (2 cm in diameter) in the center of bottom was filled with an anthropogenic (paddy) soil and placed in a porcelain container containing NaCl solution (3.0 g L⁻¹) such that a secondary salinization process was simulated via upward capillary water movement along the soil profile. A treatment with neither organic manure nor simulated soil salinization was taken as a control (CK1). The organic manure was applied either inside or outside rhizobag made of nylon cloth (40 μm of pore size). The soil was treated with: 20 g kg⁻¹ rice straw (RS), 20 g kg⁻¹ pig manure (PM), or 10 g kg⁻¹ rice straw plus 10 g kg⁻¹ pig manure (RS+PM). No organic manure was added in an additional control treatment (CK2). The results indicated that the placement of organic manure both inside and outside rihzobags significantly increased the activity of urease, alkaline phosphatase and dehydrogenase, as well as respiration rate in both rhizosphere and bulk soils. Also, nutrient uptake by barley plants was enhanced in the treatments with organic manure amended either inside or outside rhizobags. The activity of these enzymes along with the respiration rate was higher in rhizosphere than in non-rhizosphere when organic manure was supplied inside rhizobags, while the opposite was found in the case of manure incorporated outside rhizobags. Among all the treatments, RS+PM treatment had most significant stimulating effects on enzymatic and microbial activity and shoot dry weight of barley, followed by PM and RS. Moreover, more significant stimulating effects on both enzyme activity and plant growth were achieved in the treatments with manure amended inside rhizobags than outside rhizobags. The results of the present study confirmed the view that incorporation of organic manure especially into soil-root zones is an effective low-input agro-technological approach to enhancing soil fertility and minimizing phytotoxicity induced by secondary salinization.     

Adenylates in the soil microbial biomass at different temperatures

Five soils from temperate sites (Germany; 2 arable and 3 grassland) were incubated aerobically at 5, 10, 15, 20, 25, 35, and 40 °C for 8 days. Soils were analysed for soil microbial biomass C, biomass N, AMP, ADP, and ATP to determine whether the increase in the ATP-to-microbial biomass C ratio with increasing temperature was either due to an increase in the adenylate energy charge (AEC) or de novo synthesis of ATP, or both. Around 80% of the variance in microbial biomass C and biomass N was explained by differences in soil properties, only 7% by the temperature treatments. Averaging the data of all 5 soils for each incubation temperature, the microbial biomass C content decreased with increasing temperature from 15 to 40 °C continuously by 2.5 μg g⁻¹ soil °C⁻¹ after 8-days' incubation. However, this decrease was not accompanied by a similar decrease in microbial biomass N. The average microbial biomass C/N ratio was 6.8. Between 54 and 76% of the variance in AMP, ADP, ATP and the sum of adenylates was explained by differences in soil properties and between 14 (ADP) and 27% (ATP) by the temperature treatments. However, temperature effects on AMP and ADP were variable and inconsistent. In contrast, ATP and consequently also the sum of adenylates increased continuously from 5 to 30 °C followed by a decline to 40 °C. The AEC showed similarly a small, but significant increase with increasing temperature from 0.73 to 0.85 at 30 °C. Consequently, the majority of the variance, i.e. roughly 60% in AEC values, but also in ATP-to-microbial biomass C ratios was explained by the incubation temperature. The mean ATP-to-microbial biomass C ratio increased from 4.7 μmol g⁻¹ at 5 °C to a 2.5 fold maximum of 12.0 μmol g⁻¹ at 35 °C. This increase was linear with a rate of 0.26 μmol ATP g⁻¹ microbial biomass C °C⁻¹. The energy for the extra ATP produced during temperature increase is probably derived from an accelerated turnover of endocellular C reserves in the microbial biomass.     

Microbial biomass and activity in alkalized magnesic soils under arid conditions

The effects of salinity and Mg²⁺ alkalinity on the size and activity of the soil microbial communities were investigated. The study was conducted along the border area of the alluvial fan of the Taolai River. Thirty soil samples were taken which had an electrical conductivity (EC) gradient of 0.93-29.60 mS cm⁻¹. Soil pH ranged from 8.60 to 9.33 and correlated positively with Mg²⁺/Ca²⁺ ratio, exchangeable Mg²⁺ percentage and HCO₃⁻+CO₃²⁻. Mg²⁺/Ca²⁺ varied considerably from 3.04 to 61.31, with an average of 23.03. Exchangeable Mg²⁺ percentage generally exceeded 60% and had a positive correlation with Mg²⁺/Ca²⁺. HCO₃⁻+CO₃²⁻ averaged 1.63 cmol kg⁻¹ and usually did not exceed 2.0 cmol kg⁻¹.Microbial biomass, indices of microbial activity and the activities of the hydrolases negatively correlated with Mg²⁺/Ca²⁺ or exchangeable Mg²⁺ percentage. Biomass C, biomass N, microbial quotient (the percentage of soil organic C present as biomass C), biomass N as a percentage of total N, potentially mineralizable N, FDA hydrolysis rate and arginine ammonification rate decreased exponentially with increasing EC. The biomass C/N tended to be lower in soils with higher salinity and Mg²⁺ alkalinity, probably reflecting the bacterial dominance in microbial biomass in alkalized magnesic soils. The metabolic quotient (qCO₂) positively correlated with salinity and Mg²⁺ alkalinity, and showed a quadratic relationship with EC, indicating that increasing salinity and Mg²⁺ alkalinity resulted in a progressively smaller, more stressed microbial communities which was less metabolically efficient. Consequently, our data suggest that salinity and Mg²⁺ alkalinity are stressful environments for soil microorganisms.