Microbial biomass and enzyme activities in coniferous forest soils as affected by lignite-derived deposition

A forest ecosystem study was conducted along a deposition gradient of air pollutants in old Scots pine stands located near the industrial belt around the city of Bitterfeld in northeast Germany from 1999 to 2000. In order to estimate the impact of different atmospheric deposition loads on microbial biomass and enzyme activities, samples were taken from the forest floor (L, F, and H horizon) and the mineral topsoil (0–10 cm). The emission-induced increases in ferromagnetic susceptibility, soil pH, concentrations of mobile (NH ₄NO ₃ extractable) Cr and Ni, effective cation exchange capacity, and base saturation in the humus layer along the 25-km long transect reflected that great portions of the past depositions were characterized by alkaline fly ash. Alkaline depositions significantly ( P <0.05) decreased the microbial biomass C and N contents, microbial biomass C-to-organic C ratios, and microbial respiration rates, but increased the metabolic quotient (qCO ₂) of the mineral topsoil and forest floor. Variations in microbial biomass and activity can mainly be predicted ( r ² =0.60) by the concentrations of Ca, Zn and Cd in these forest soils. The specific activities (activity kg ^-1 organic C) of l-asparaginase, l-glutaminase, arylsulfatase, and in part, acid phosphatase were significantly ( P <0.05) higher at forest sites receiving higher fly ash loads than those of the other sites, and thus followed the trend of the qCO ₂. In contrast, the specific activity of ß-glucosidase was significantly ( P <0.05) decreased at heavily affected sites compared to moderate and less affected sites, suggesting an inhibition of C mineralization in the forest floor of pine stands affected by predominantly alkaline emissions. A great portion ( r ²=0.91) of the variation in the specific enzyme activity data in forest soils in emission areas can be predicted from a linear combination of the variables total organic C and NH ₄Cl-extractable Ca, pH and effective cation exchange capacity.     

Arylsulfatase activity of soil microbial biomass along a Mediterranean-arid transect

The relationships between arylsulfatase and microbial activity were investigated in regional and microenvironmental scales, at three study sites in Israel, that represent different climatic regions—Mediterranean (sub-humid), mildly arid and arid.Total arylsulfatase activity was divided into extracellular and intracellular (microbial biomass enzyme) activities according to the chloroform-fumigation method. The results show that with increasing aridity, C_org (soil organic carbon), C_mic (soil microbial biomass carbon), N_mic (soil microbial biomass nitrogen) and respiration rate decreased, while C_mic/C_org and metabolic quotient (qCO₂) increased. Total, extracellular and microbial biomass arylsulfatase activities decreased with aridity. Expressed as percentage of total activity, the arylsulfatase activity of microbial biomass in the soil, at 0-2 cm and 5-10 cm depths, accounted for more than 50% of the total, in most measurements. This activity was significantly higher in the arid sites than that found in the Mediterranean one for the 0-2 cm soil. The results indicate the importance of the microflora as an enzyme source in soils, especially in arid climate conditions.Enzyme activity in the different study sites was found to be influenced by microenvironmental conditions. The Mediterranean site showed a much higher enzyme activity under shrubs than that under rock fragments and in bare soil. In the arid site rock fragments created a favorable microenvironment for microbial activity on soil surface, which resulted in a much higher microbial biomass and arylsulfatase activity than that in bare soil.The total, extracellular and intracellular arylsulfatase activities, were significantly correlated with C_org, C_mic, N_mic and respiration rate (p<0.05) at all study sites. The correlation coefficients between microbial biomass and arylsulfatase activity were usually higher than those between organic carbon and enzyme activity, especially in the arid sites. Close relationships between microbial biomass and arylsulfatase activities in all the studied sites supported the hypothesis that C_org content and enzyme activities should be related to each other via microbial biomass. Arylsulfatase activity was found to be a good indicator of microbial one. The regression equations between these factors can be incorporated into models of biogeochemical cycling for their easy method of analysis.     

Adenylates as an estimate of microbial biomass C in different soil groups

Adenylate (i.e. adenosine tri- (ATP), di- (ADP) and monophosphates (AMP)) and microbial biomass C data were collected over a wide range of sites including forest floor layers and forest, grassland and arable soils. Microbial biomass C was measured by fumigation extraction and adenylates after alkaline Na₃PO₄/DMSO/EDTA extraction and HPLC detection. Our aims were (1) to test whether the sum of adenylates is a better estimate for microbial biomass than the determination of ATP, (2) to compare our conversion values with those proposed by others, and (3) to analyse whether soil properties or land use form affect the relationships between ATP, adenylates and microbial biomass C. A close relationship was found between microbial biomass C and ATP (r=0.96), but also with the sum of adenylates (r=0.96) within all appropriately conditioned soil samples (n=112). In the mineral soil (n=98), the geometric means of the ATP-to-microbial biomass C ratio and the adenylates-to-microbial biomass C ratio were 7.4 and 11.4 μmol g⁻¹, respectively. The mean ratios did not differ significantly between the different texture classes and land use forms. In the forest floor, the ATP-to-microbial biomass C ratio and the adenylates-to-microbial biomass C ratio were both roughly two-thirds of those of the mineral soil. The average adenylate energy charge (AEC) of all soil samples was 0.79 and showed a strong negative relationship with the soil pH (r=−0.69). However, the AEC is presumably only indirectly affected by the soil pH.     

The adverse effects of soil salinization on the growth of Trifolium alexandrinum L. and associated microbial and biochemical properties in a soil from Iran

The aim of this study was to determine the effects of increasing concentrations of salt solutions (including 0.12, 2, 6, and 10 dS m⁻¹) on the growth of berseem clover (Trifolium alexandrinum L.) and related soil microbial activity, biomass and enzyme activities. Results showed that the dry weights of root and shoot decreased with an increase in the concentrations of salt solutions. Soil salinization depressed the microbiological activities including soil respiration and enzyme activities. Substrate-induced respiration was consistently lower in salinized soils, whereas microbial biomass C did not vary among salinity levels. Higher metabolic quotients (qCO₂) and unaffected microbial biomass C at high EC values may indicate that salinity is a stressful factor, inducing either a shift in the microbial community with less catabolic activity or reduced efficiency of substrate utilization. Acid phosphatase and alkaline phosphatase activities decreased with increasing soil salinity. We found significant, positive correlations between the activities of phosphatase enzymes and plant's root mass, suggesting that any decrease in the activities of the two enzymes could be attributed to the reduced root biomass under saline conditions.     

Effects of tree harvesting, forest floor removal, and compaction on soil microbial biomass, microbial respiration, and N availability in a boreal aspen forest in British Columbia

The effects of timber harvesting and the resultant soil disturbances (compaction and forest floor removal) on relative soil water content, microbial biomass C and N contents (C_mic and N_mic), microbial biomass C:N ratio (C_mic-to-N_mic), microbial respiration, metabolic quotient (qCO₂), and available N content in the forest floor and the uppermost mineral soil (0-3 cm) were assessed in a long-term soil productivity (LTSP) site and adjacent mature forest stands in northeastern British Columbia (Canada). A combination of principal component analysis and redundancy analysis was used to test the effects of stem-only harvest, whole tree harvest plus forest floor removal, and soil compaction on the studied variables. Those properties in the forest floor were not affected by timber harvesting or soil compaction. In the mineral soil, compaction increased soil total C and N contents, relative water content, and N_mic by 45%, 40%, 34% and 72%, respectively, and decreased C_mic-to-N_mic ratio by 29%. However, these parameters were not affected by stem only harvesting or whole tree harvesting plus forest floor removal, contrasting the reduction of white spruce and aspen growth following forest floor removal and soil compaction reported in an earlier study. Those results suggest that at the study site the short-term effects of timber harvesting, forest floor removal, and soil compaction are rather complex and that microbial populations might not be affected by the perturbations in the same way as trees, at least not in the short term.     

Attempts to derive EC50 values for heavy metals from land-applied Cu-, Ni-, and Zn-spiked sewage sludge

We established a field trial to assess the impacts on soil biological properties of application of heavy metal-spiked sewage sludge, with the aim of determining toxicity threshold concentrations of heavy metals in soil. Plots were treated with sludges containing increasing concentrations of Cu, Ni and Zn in order to raise the metal concentrations in the soil by 0-200 mg Cu kg⁻¹, 0-60 mg Ni kg⁻¹ and 0-400 mg Zn kg⁻¹, and were then cultivated and sown in ryegrass-clover pasture and monitored annually for 6 years. All biological properties measured (soil basal respiration, microbial biomass C, and sulphatase enzyme activities), except phosphatase activity, increased in all plots over the duration of the experiment. Consequently, it was only possible to assess effects of heavy metals across time if, each year, all data for each metal were normalised by expressing them as percentages of the activities measured in an un-sludged control plot. When this was done, no significant effects of increasing heavy-metal concentrations on basal respiration, microbial biomass C or respiratory quotient (qCO₂) were observed, although total Cu and soil solution Cu were significantly negatively related to microbial biomass C when it was expressed as a proportion of soil total C. None of the properties measured were affected by increasing Ni concentrations. Phosphatase and sulphatase activities were significantly negatively related to increasing Zn concentrations, but not usually to increasing Cu unless they were expressed as a proportion of total C. A sigmoidal dose-response model was used to calculate EC₂₀ and EC₅₀ values using the normalised data, but generally, the model parameters had very large 95% confidence intervals and/or the fits to the model had small R² values. The factors primarily responsible for confounding these results were site and sample variations not accounted for by the normalisation process and the absence of any data points at metal concentrations beyond the calculated EC₅₀ values. In the few instances where reasonable EC₂₀ values could be calculated, they were relatively consistent across properties, e.g., EC₂₀ for total Zn and phosphatase (330 mg kg⁻¹), total Zn and sulphatase (310 mg kg⁻¹), and EC₂₀ for total Cu and sulphatase (140 mg kg⁻¹) and total Cu and microbial biomass C (140 mg kg⁻¹), when both sulphatase and microbial biomass C were expressed as a proportion of total C. Our results suggest that Cu and Zn at the upper concentrations used in this experiment were possibly having adverse effects on some soil biological properties. However, much higher metal concentrations will be needed to accurately calculate EC₂₀ and EC₅₀ and this may not be easily achievable without many applications of sewage sludge, even if the sludge is spiked with heavy metals.     

Microbial performance under increasing nitrogen availability in a Mediterranean forest soil

In forest ecosystems, the external nitrogen (N) inputs mainly involve wet and dry depositions that potentially alter inorganic N availability in the soil and carbon (C) turnover. This study assesses the effect of a slow increase of inorganic N availability on microbial community activity and functionality in a Mediterranean forest soil. A four-month incubation experiment was performed with soil collected from the organic layer of a forest site and fertilized with a solution of ammonium nitrate. The fertilizer was supplied at an equivalent of 0, 10, 25, 50 and 75 kg N ha⁻¹ (0, 0.3, 0.7, 1.3 and 2 mg N g⁻¹ for control N0 and treatments N1, N2, N3 and N4, respectively). The incubation was carried out under optimal conditions, with the addition of the nutritive solution in small aliquots once a week to mimic the phenomenon of N deposition. In order to isolate the effect of N, the pH of the NH₄NO₃ solutions was adjusted to soil pH, and phosphorus was added in order to prevent any nutrient limitation effect. Inorganic N, C-mineralization, the activity of one oxidative enzyme (o-diphenol oxidase) and 8 hydrolitic enzymes (α-glucosidase, β-glucosidase, N-acetyl-β-d-glucosaminidase, cellulase, leucine amino-peptidase, acid phosphatase, butyric esterase and β-xylosidase) and the community level physiological profile (CLPP) were measured and analyzed during the whole incubation and at the end of the experiment as a proxy for microbial decomposition activity. In the first month, the highest N availability (N4) repressed the microbial respiration activity but stimulated microbial enzymatic activity, suggesting a change of C-pathways from spilling to enzymes and biomass investment. The treatments N1, N2 and N3 had no effect in the same period. Throughout the incubation, a general stress condition affected all the treated soils. As a consequence, treated soils exhibited higher respiration rates than the control. This was accompanied by a loss of functional diversity and an end-detected decline in biomass C. Although at the end of incubation most of the soil features showed a clear correlation with the inorganic N pool, the organic C content was strongly affected by different patterns of microbial activity during the experiment: the highest N treatment (N4) showed a lower C loss than the N3 treatment. Overall, the experiment showed how inorganic N availability can potentially alter the C cycle in a Mediterranean forest soil. The effect is non linear, depending on microbial community dynamics, on the community’s ability to adapt given the time scale of the process, and on N supply amount. Our study also revealed a common pattern in the short-term response to N addition in other, similar ecosystems with different climatic conditions.     

Effects of root and litter exclusion on soil CO2 efflux and microbial biomass in wet tropical forests

We examined the effects of root and litter exclusion on the rate of soil CO₂ efflux and microbial biomass at a soil depth of 25 cm in a secondary forest (dominated by Tabebuia heterophylla) and a pine (Pinus caribaea) plantation in the Luquillo Experimental Forest in Puerto Rico. The experimental plots were initially established in 1990, when root, forest floor mass and new litterfall were excluded for 7 y since then. Soil respiration was significantly reduced in the litter and root exclusion plots in both the secondary forest and the pine plantation compared with the control. Root exclusion had a greater effect on soil CO₂ efflux than the litter exclusion in the plantation, whereas a reversed pattern was observed in the secondary forest. The reduction of microbial biomass in the root exclusion plot was greater in the secondary forest (59%) than in the plantation (31%), while there was no difference of the reduction in the litter exclusion plots between these forests. Our results suggest that above-ground input and roots (root litter and exudates) differentially affect soil CO₂ efflux under different vegetation types.     

Animal manure and anhydrous ammonia amendment alter microbial carbon use efficiency,microbial biomass,and activities of dehydrogenase and amidohydrolases in semiarid agroecosystems

The potential negative impact of agricultural practices on soil and water quality is of environmental concern. The associated nutrient transformations and movements that lead to environmental concerns are inseparable from microbial and biochemical activities. Therefore, biochemical and microbiological parameters directing nitrogen (N) transformations in soils amended with different animal manures or inorganic N fertilizers were investigated. Soils under continuous corn cultivation were treated with N annually for 5 years at 56, 168, and 504 kg N ha⁻¹ in the form of swine effluent, beef manure, or anhydrous ammonia. Animal manure treatments increased dehydrogenase activity, microbial biomass carbon (C_mic) and N (N_mic) contents, and activities of amidohydrolases, including l-asparaginase, urease, l-glutaminase, amidase, and β-glucosaminidase. Soils receiving anhydrous ammonia demonstrated increased nitrate contents, but reduced microbiological and biochemical activities. All treatments decreased C_mic:organic C (C_org) ratios compared with the control, indicating reduced microbial C use efficiency and disturbance of C equilibrium in these soil environments. Activities of all enzymes tested were significantly correlated with soil C_org contents (P < 0.001, n = 108), but little correlation (r = 0.03, n = 36) was detected between C_mic and C_org. Activities of amidase and β-glucosaminidase were dominated by accumulated enzymes that were free of microbial cells, while activities of asparaginase and glutaminase were originated predominately from intracellular enzymes. Results indicated that soil microbial and biochemical activities are sensitive indicators of processes involved in N flow and C use efficiency in semiarid agroecosystems.     

Changes in microbial biomass,CH4 and CO2 emissions,and soil carbon content by fly ash co-applied with organic inputs with contrasting substrate quality under changing water regimes

Application of fine-textured and Ca-rich fly ash may be helpful in enhancing soil carbon content via protecting soil organic C (SOC) by organo-mineral complexation and via reducing CO₂ emission by carbonation (e.g. formation of CaCO₃). However, very limited information is available on the effects of fly ash application on gases loss of C and soil C content. In this study, to estimate the potential use of fly ash as a soil amendment for SOC enhancement purposes, the effects of fly ash application (0, 5, and 10 w/w %) on microbial biomass C (MBC), CH₄ and CO₂ emissions, and on soil C content were investigated. A 60-days incubation experiment was conducted with an acidic soil in the presence of organic input (pig manure compost, PMC; hairy vetch, HV) with contrasting substrate quality under changing water regime from water-logged to unsaturated via a transition period. Fly ash application did not affect MBC under water-unsaturated conditions, but reduced (P < 0.01) microbial growth under water-logged conditions, probably due to the increased solubility of a certain toxic element such as arsenic under the anaerobic conditions. Across the 60 days of incubation, the CO₂ emission was reduced by fly ash regardless of organic input by 20.5–41.3%; meanwhile, a decline of CH₄ emission by fly ash application was significant (P < 0.05) only in the HV treatment. Overall, fly ash application slowed down gases C loss and increased soil C content, probably due to the retardation of CH₄ and CO₂ emission as well as the addition of C contained in the fly ash. Biochemical (inhibition of microbial activity), chemical (formation of CaCO₃ via carbonation), and physical (restriction of gas diffusion) mechanisms were suggested for the fly ash effects.     

Ratios between estimates of microbial biomass content and microbial activity in soils

The content levels and activities of the microbiota were estimated in topsoils and in one soil profile at agricultural and forest sites of the Bornhöved Lake district in northern Germany. Discrepancies between data achieved by fumigation-extraction (FE) and substrate-induced respiration (SIR), both used for the quantification of microbial biomass, were attributed to the composition of the microbial populations in the soils. In the topsoils, the active, glucose-responsive (SIR) versus the total, chloroform-sensitive microbial (FE) biomass decreased in the order; field maize monoculture (field-MM)>field crop rotation (field-CR) and dry grassland>beech forest. This ratio decreased within the soil profile of the beech forest from the litter horizon down to the topsoil. Differences between microbial biomass and activities suggested varying biomass-specific transformation intensities in the soils. The metabolic quotient (qCO₂), defined as the respiration rate per unit of biomass, indicates the efficiency in acquiring organic C and the intensity of C mineralization, while biomass-specific arginine-ammonification (arginine-ammonification rate related to microbial biomass content) seems to be dependent on N availability. The qCO₂, calculated on the basis of the total microbial biomass, decreased for the topsoils in the same order as did the ratio between the active, glucose-responsive microbial biomass to the total, chloroform-sensitive microbial biomass, in contrast to qCO₂ values based on the glucose-responsive microbial biomass, which did not. There was no difference between the levels of biomass-specific arginine-ammonification in topsoils of the fertilized field-CR, fertilized field-MM, fertilized dry grassland and eutric alder forest, but levels were lower in the beech forest, dystric alder forest, and unfertilized wet grassland topsoils. Ratios between values of different microbiological features are suggested to be more useful than microbiological features related to soil weight when evaluating microbial populations and microbially mediated processes in soils.     

The relationships among microbial parameters and the rate of organic matter mineralization in forest soils,as influenced by forest type

Vegetation type influences the rate of accumulation and mineralization of organic matter in forest soil, mainly through its effect on soil microorganisms. We investigated the relationships among forest types and microbial biomass C (MBC), basal respiration (R_B), substrate-induced respiration (R_S), N mineralization (N_min), specific growth rate μ, microbial eco-physiology and activities of seven hydrolytic enzymes, in samples taken from 25 stands on acidic soils and one stand on limestone, covering typical types of coniferous and deciduous forests in Central Europe. Soils under deciduous trees were less acidic than soils of coniferous forests, which led to increased mineralizing activities R_B and N_min, and a higher proportion of active microbial biomass (R_S/MBC) in the Of horizon. This resulted in more extractable organic C (0.5 M K₂SO₄) in soils of deciduous forests and a higher accumulation of soil organic matter (SOM) in coniferous forest soil. No effect of forest type on the microbial properties was detected in the Oh horizon and in the 0–10 cm layer. The microbial quotient (MBC/C_org), reflecting the quality of organic matter used for microbial growth, was higher in deciduous forests in all three layers. The metabolic quotient qCO₂ (R_B/MBC) and the specific growth rate μ, estimated using respiration growth curves, did not differ in soils of both forest types. Our results showed that the quality of SOM in coniferous forests supported microorganisms with higher activities of β-glucosidase, cellobiosidase and β-xylosidase, which suggested the key importance of fungi in these soils. Processes mediated by bacteria were probably more important in deciduous forest soils with higher activities of arylsulphatase and urease. The results from the stand on limestone showed that pH had a positive effect on microbial biomass and SOM mineralization.     

Soil microbial biomass, activity and nitrogen transformations in a turfgrass chronosequence

Understanding the chronological changes in soil microbial properties of turfgrass ecosystems is important from both the ecological and management perspectives. We examined soil microbial biomass, activity and N transformations in a chronosequence of turfgrass systems (i.e. 1, 6, 23 and 95 yr golf courses) and assessed soil microbial properties in turfgrass systems against those in adjacent native pines. We observed age-associated changes in soil microbial biomass, CO₂ respiration, net and gross N mineralization, and nitrification potential. Changes were more evident in soil samples collected from 0 to 5 cm than the 5 to 15 cm soil depth. While microbial biomass, activity and N transformations per unit soil weight were similar between the youngest turfgrass system and the adjacent native pines, microbial biomass C and N were approximately six times greater in the oldest turfgrass system compared to the adjacent native pines. Potential C and N mineralization also increased with turfgrass age and were three to four times greater in the oldest vs. the youngest turfgrass system. However, microbial biomass and potential mineralization per unit soil C or N decreased with turfgrass age. These reductions were accompanied by increases in microbial C and N use efficiency, as indicated by the significant reduction in microbial C quotient (qCO₂) and N quotient (qN) in older turfgrass systems. Independent of turfgrass age, microbial biomass N turnover was rapid, averaging approximately 3 weeks. Similarly, net N mineralization was ∼12% of gross mineralization regardless of turfgrass age. Our results indicate that soil microbial properties are not negatively affected by long-term management practices in turfgrass systems. A tight coupling between N mineralization and immobilization could be sustained in mature turfgrass systems due to its increased microbial C and N use efficiency.     

Simultaneous measurement of S, macronutrients, and heavy metals in the soil microbial biomass with CHCl3 fumigation and NH4NO3 extraction

The study was carried out to investigate whether 1 M NH₄NO₃ extraction is a useful alternative to 10 mM CaCl₂ extraction for estimating soil microbial biomass S and whether the data of CHCl₃-labile NH₄NO₃-extractable macronutrients and heavy metals are useful and in agreement with the available data on element concentrations in soil microorganisms. Microbial biomass C was followed by microbial biomass S after CaCl₂ extraction with an average C/S ratio of 82, and by microbial biomass S after NH₄NO₃ extraction with an average C/S ratio of 57. The mean contribution of CHCl₃-labile metals in relation to the NH₄NO₃-extractable fraction from non-fumigated soils ranged from 0.1 to 112% in the order potassium < magnesium < cadmium < sodium < zinc + nickel < manganese < copper. The mean contribution of CHCl₃-labile metals in relation to the microbial biomass C ranged from 0.03 to 22‰ in the order cadmium < nickel < zinc < manganese < magnesium < copper < sodium < potassium. These relative contributions varied within the different metals from a 4-fold (Na⁺) to a more than 200-fold range (Cu²⁺). Significant positive correlations with microbial biomass C were observed for CHCl₃-labile zinc, sodium and especially potassium. The concentration of all elements except copper in relation to microbial biomass C were in the range known from the limited literature on fungi grown on heavy metal contaminated soils.     

Microbial communities, biomass, and activities in soils as affected by freeze thaw cycles

Two Finnish agricultural soils (peat soil and loamy sand) were exposed to four freeze-thaw cycles (FTC), with a temperature change from −17.3±0.4 °C to +4.1±0.4 °C. Control cores from both soils were kept at constant temperature (+6.6±2.0 °C) without FTCs. Soil N₂O and CO₂ emissions were monitored during soil thawing, and the effects of FTCs on soil microbes were studied. N₂O emissions were extremely low in peat soil, possibly due to low soil water content. Loamy sand had high N₂O emission, with the highest emission after the second FTC. Soil freeze-thaw increased anaerobic respiration in both soil types during the first 3-4 FTCs, and this increase was higher in the peat soil. The microbial community structure and biomass analysed with lipid biomarkers (phospholipid fatty acids, 3- and 2- hydroxy fatty acids) were not affected by freezing-thawing cycles, nor was soil microbial biomass carbon (MIB-C). Molecular analysis of the microbial community structure with temperature gradient gel electrophoresis (TGGE) also showed no changes due the FTCs. These results show that freezing and thawing of boreal soils does not have a strong effect on microbial biomass or community structure.     

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.     

Elevated CO2 differentially alters belowground plant and soil microbial community structure in reed canary grass-invaded experimental wetlands

Several recent studies have indicated that an enriched atmosphere of carbon dioxide (CO₂) could exacerbate the intensity of plant invasions within natural ecosystems, but little is known of how rising CO₂ impacts the belowground characteristics of these invaded systems. In this study, we examined the effects of elevated CO₂ and nitrogen (N) inputs on plant and soil microbial community characteristics of plant communities invaded by reed canary grass, Phalaris arundinacea L. We grew the invasive grass under two levels of invasion: the invader was either dominant (high invasion) at >90% plant cover or sub-dominant (low invasion) at <50% plant cover. Experimental wetland communities were grown for four months in greenhouses that received either 600 or 365 μl l⁻¹ (ambient) CO₂. Within each of three replicate rooms per CO₂ treatment, the plant communities were grown under high (30 mg l⁻¹) or low (5 mg l⁻¹) N. In contrast to what is often predicted under N limitation, we found that elevated CO₂ increased native graminoid biomass at low N, but not at high N. The aboveground biomass of reed canary grass did not respond to elevated CO₂, despite it being a fast-growing C3 species. Although elevated CO₂ had no impact on the plant biomass of heavily invaded communities, the relative abundance of several soil microbial indicators increased. In contrast, the moderately invaded plant communities displayed increased total root biomass under elevated CO_2, while little impact occurred on the relative abundance of soil microbial indicators. Principal components analysis indicated that overall soil microbial community structure was distinct by CO₂ level for the varying N and invasion treatments. This study demonstrates that even when elevated CO₂ does not have visible effects on aboveground plant biomass, it can have large impacts belowground.     

Microbial biomass and C and N transformations in forest floors under European beech, sessile oak, Norway spruce and Douglas-fir at four temperate forest sites

The purpose of this research was to compare soil chemistry, microbially mediated carbon (C) and nitrogen (N) transformations and microbial biomass in forest floors under European beech (Fagus sylvatica L.), sessile oak (Quercus petraea (Mattuschka) Lieblein), Norway spruce (Picea abies (L.) Karst) and Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) at four study sites. We measured soil chemical characteristics, net N mineralization, potential and relative nitrification, basal respiration, microbial and metabolic quotient and microbial biomass C and N under monoculture stands at all sites (one mixed stand). Tree species affected soil chemistry, microbial activities and biomass, but these effects varied between sites. Our results indicated that the effect of tree species on net N mineralization was likely to be mediated through their effect on soil microbial biomass, reflecting their influence on organic matter content and carbon availability. Differences in potential nitrification and relative nitrification might be related to the presence of ground vegetation through its influence on soil NH₄ and labile C availability. Our findings highlight the need to study the effects of tree species on microbial activities at several sites to elucidate complex N cycle interactions between tree species, ground vegetation, soil characteristics and microbial processes.     

Carbon stocks, soil respiration and microbial biomass in fire-prone tropical grassland, woodland and forest ecosystems

A thorough understanding of the role of microbes in C cycling in relation to fire is important for estimation of C emissions and for development of guidelines for sustainable management of dry ecosystems. We investigated the seasonal changes and spatial distribution of soil total, dissolved organic C (DOC) and microbial biomass C during 18 months, quantified the soil CO₂ emission in the beginning of the rainy season, and related these variables to the fire frequency in important dry vegetation types grassland, woodland and dry forest in Ethiopia. The soil C isotope ratios (δ¹³C) reflected the 15-fold decrease in the grass biomass along the vegetation gradient and the 12-fold increase in woody biomass in the opposite direction. Changes in δ¹³C down the soil profiles also suggested that in two of the grass-dominated sites woody plants were more frequent in the past. The soil C stock ranged from being 2.5 (dry forest) to 48 times (grassland) higher than the C stock in the aboveground plant biomass. The influence of fire in frequently burnt wooded grassland was evident as an unchanged or increasing total C content down the soil profile. DOC and microbial biomass measured with the fumigation-extraction method (C_mic) reflected the vertical distribution of soil organic matter (SOM). However, although SOM was stable throughout the year, seasonal fluctuations in C_mic and substrate-induced respiration (SIR) were large. In woodland and woodland-wooded grassland C_mic and SIR increased in the dry season, and gradually decreased during the following rainy season, confirming previous suggestions that microbes may play an important role in nutrient retention in the dry season. However, in dry forest and two wooded grasslands C_mic and SIR was stable throughout the rainy season, or even increased in this period, which could lead to enhanced competition with plants for nutrients. Both the range and the seasonal changes in soil microbial biomass C in dry tropical ecosystems may be wider than previously assumed. Neither SIR nor C_mic were good predictors of in situ soil respiration. The soil respiration was relatively high in infrequently burnt forest and woodland, while frequently burnt grasslands had lower rates, presumably because most C is released through dry season burning and not through decomposition in fire-prone systems. Shifts in the relative importance of the two pathways for C release from organic matter may have strong implications for C and nutrient cycling in seasonally dry tropical ecosystems.     

Hydrolase activities, microbial biomass and bacterial community in a soil after long-term amendment with different composts

The use of composts in agricultural soils is a widespread practice and the positive effects on soil and plants are known from numerous studies. However, there have been few attempts to compare the effects of different kinds of composts in one single study. The aim of this paper is to investigate to what extent and to which soil depth four major types of composts would affect the soil and its microbiota.In a crop-rotation field experiment, composts produced from (i) urban organic wastes, (ii) green wastes, (iii) manure and (iv) sewage sludge were applied at a rate equivalent to 175 kg N ha⁻¹ yr⁻¹ for 12 years. General (total organic C (C_org), total N (N_t), microbial biomass C (C_mic), and basal respiration), specific (enzyme activities related to C, N and P cycles), biochemical properties and bacterial genetic diversity (based on DGGE analysis of 16S rDNA) were analyzed at different depths (0-10, 10-20 and 20-30 cm).Compost treatment increased C_org at all depths from 11 g kg⁻¹ for control soil to 16.7 g kg⁻¹ for the case of sewage sludge compost. Total N increased with compost treatment at 0-10 cm and 10-20 cm depths, but not at 20-30 cm. Basal respiration and C_mic declined with depth, and the composts resulted in an increase of C_mic and basal respiration. Enzyme activities were different depend on the enzyme and among compost treatments, but in general, the enzyme activities were higher in the upper layers (0-10 and 10-20 cm) than in the 20-30 cm layer. Diversity of ammonia oxidizers and bacteria was lower in the control than in the compost soils. The type of compost had less influence on the composition of the microbial communities than did soil depth.Some of the properties were sensitive enough to distinguish between different compost, while others were not. This stresses the need of multi-parameter approaches when investigating treatment effects on the soil microbial community. In general, with respect to measures of activity, biomass and community diversity, differences down the soil profile were more pronounced than those due to the compost treatments.