Changes in the fungal-to-bacterial respiratory ratio and microbial biomass in agriculturally managed soils under free-air CO2 enrichment (FACE) - A six-year survey of a field study

In soil ecology, microbial parameters have been identified as sensitive indicators of changes in the soil environment. The Braunschweig FACE project provided the opportunity to study the effects of elevated CO₂ (550 μmol mol⁻¹) as compared to ambient CO₂ (370 μmol mol⁻¹) on total microbial biomass (C_mic), C_mic-to-C_org ratio and the fungal-to-bacterial respiratory ratio together with total C_org, N_t, C:N ratio and pH over a six-year period. Field management followed a typical crop rotation system of this region with either a crop-related full nitrogen supply (N100) or 50% reduced N supply (N50). The soil microbial parameters responded to the elevated CO₂ treatment in varying intensities and time spans. The fungal-to-bacterial respiratory ratio was the most sensitive parameter in responding to an elevated CO₂ treatment with highly significant differences to ambient CO₂-treated control plots in the third year of CO₂ fumigation. After six years bacterial respiratory activity had increased in ascending order to 34% in FACE-treated plots (N50 and N100) as compared to control plots. Soil microbial biomass (C_mic) responded more slowly to the FACE treatment with highly significant increases of >12% after the fourth year of CO₂ fumigation. The C_mic-to-C_org ratio responded very late in the last two years of the CO₂ treatment with a significant increase of >7.0% only in the N100 variant. Total C_org and N_t were slightly but significantly increased under FACE around 10.0% with ascending tendency over time starting with the second year of CO₂ treatment. No significant FACE effects could be recorded for the C:N ratio or pH.These results suggest that under FACE treatment changes in the soil microbial community will occur. In our study the fungal-to-bacterial respiratory ratio was superior to total C_mic as microbial bioindicators in reflecting changes in the soil organic matter composition.     

Effect of forest and soil type on microbial biomass carbon and respiration

The aim of study was to evaluate the variation of soil microbial biomass carbon (C_mic) and microbial respiration (M_R) in three types soil (Chromic Cambisols, Chromic Luvisols and Eutric Leptosols) of mixed beech forest (Beech- Hornbeam and Beech- Maple). Soil was randomly sampled from 0–10 cm layer (plant litter removed), 90 soil samples were taken. C_mic determined by the fumigation-extraction method and M_R by closed bottle method. Soil C_org, N_tot and pH were measured. There are significant differences between the soil types concerning the C_mic content and M_R. These parameters were highest in Chromic Cambisols following Chromic Luvisols, while the lowest were in Eutric Leptosols. A similar trend of C_org and N_tot was observed in studied soils. Two-way ANOVA indicated that soil type and forest type have significantly effect on the most soil characteristics. Chromic Cambisols shows a productive soil due to have the maximum C_mic, M_R, C_org and N_tot. In Cambisols under Beech- Maple forest the C_mic value and soil C/N ratio were higher compared to Beech-Hornbeam (19.5 and 4.1 mg C g^–1, and 16.3 and 3.3, respectively). This fact might be indicated that Maple litter had more easy decomposable organic compounds than Hornbeam. According to regression analysis, 89 and 68 percentage of C_mic variability could explain by soil C_org and N_tot respectively.     

Soil microbial biomass carbon and nitrogen as affected by cropping systems

 The impacts of crop rotations and N fertilization on microbial biomass C (C_mic) and N (N_mic) were studied in soils of two long-term field experiments initiated in 1978 at the Northeast Research Center (NERC) and in 1954 at the Clarion-Webster Research Center (CWRC), both in Iowa. Surface soil samples were taken in 1996 and 1997 from plots of corn (Zea mays L.), soybeans (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) that had received 0 or 180 kg N ha^–1 before corn and an annual application of 20 kg P and 56 kg K ha^–1. The C_mic and N_mic values were determined by the chloroform-fumigation-extraction method and the chloroform-fumigation-incubation method, respectively. The C_mic and N_mic values were significantly affected (P<0.05) by crop rotation and plant cover at time of sampling, but not by N fertilization. In general, the highest C_mic and N_mic contents were found in the multicropping systems (4-year rotations) taken in oats or meadow plots, and the lowest values were found in continuous corn and soybean systems. On average, C_mic made up about 1.0% of the organic C (C_org), and N_mic contributed about 2.4% of the total N (N_tot) in soils at both sites and years of sampling. The C_mic values were significantly correlated with C_org contents (r≥0.41**), whereas the relationship between C_mic and N_tot was significant (r≤0.53***) only for the samples taken in 1996 at the NERC site. The C_mic : N_mic ratios were, on average, 4.3 and 6.4 in 1996, and 7.6 and 11.4 in 1997 at the NERC and CWRC sites, respectively. Crop rotation significantly (P<0.05) affected this ratio only at the NERC site, and N fertilization showed no effect at either site. In general, multicropping systems resulted in greater C_mic : C_org (1.1%) and N_mic : N_tot (2.6%) ratios than monocropping systems (0.8% and 2.1%, respectively). Received: 9 February 1999     

Acquisition of microbial communities and enhanced availability of soil nutrients by the isopod Porcellio scaber (Latr.) (Isopoda: Oniscidea)

 In microcosm experiments Porcellio scaber increased litter disappearance of oak and alder litter. Alder litter disappeared at more than twice the rate of oak litter. Soil texture did not influence the disappearance of oak litter; however, disappearance of alder litter was enhanced on silt rather than on sand. P. scaber enhanced microbial communities (i.e. microbial respiration and microbial biomass) on both silt and sand when feeding on either alder or oak. Overall, microbial respiration increased 10-fold when isopods fed on oak litter on sand and 20-fold when isopods fed on alder litter on sand. On silt, the initially high microbial respiration remained constant when isopods fed on oak and doubled when they fed on alder litter. In all treatments without P. scaber there was a decrease in microbial respiration over the 12-week experimental period. The availability of macronutrients (C_org, N_tot, P, K, Mg, Ca) in the topsoil was increased when P. scaber fed on alder litter but less pronounced when P. scaber fed on oak litter. Using sand as a substrate, there was an apparent increase only for C_org, Mg and Ca; on silt, increases in C_org, N_tot and P were measured. Under field conditions the contribution of P. scaber to nutrient fluxes will be higher on sand than on silt. Received: 1 July 1999     

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.     

Microbial diversity in three floodplain soils at the Elbe River (Germany)

Microbial communities in floodplain soils are exposed to periodical flooding. A long-term submerged Eutric Gleysol (GLe), an intermediate flooded Eutric Fluvisol (FLe), and a short-time flooded Mollic Fluvisol (FLm) at the Elbe River (Germany) with similar organic carbon contents (C_org) between 8.1% and 8.9% were selected to test the quality of phospholipid fatty acids (PLFA), soil microbial carbon (C_mic), basal respiration (BR), metabolic quotient (qCO₂), and C_mic/C_org ratio to characterize and discriminate these soils with microbial parameters.The three floodplain soils can be differentiated by C_mic and by total PLFA-biomass. Due to the different flooding durations and the time since the soils were last flooded C_mic and PLFA-biomass increase in the order GLe<FLe<FLm. Both parameters correlate significantly (r=0.999;p<0.05). The C_mic/C_org ratios are low in comparison to terrestrial soils and revealed the same ranking over the three soils like C_mic. Contrary, qCO₂ and BR are highest in GLe and lowest in FLm according to inundation regime. The diminished C_mic, high BR, and high qCO₂ values in GLe seem to be an unspecific response of aerobic soil microorganisms on the long flooding period and the resulting short time for developing after last flooding as well as the low pH value. Different plant communities and their residues may influence the microbial diversity additionally.The PLFA profiles were dominated by the group of saturated fatty acids that together constituted almost 62-72% of the total fatty acids identified in the soils. In GLe all groups of PLFA, inclusive monounsaturated fatty acids, are lowest and in FLm highest, while in FLe the PLFA fractions show an intermediary amount of the three soils. The FLm had most of the time aerobic conditions and revealed therefore the highest C_mic, PLFA-biomass, especially monounsaturated fatty acids, C_mic/C_org ratio as well as relatively low BR and qCO₂ value. These indicate that microorganisms in FLm are more efficiently in using carbon sources than those in GLe and FLe.All 26 identified PLFA were found in FLe and FLm, while the polyunsaturated fungi biomarker 18:2ω6,9c could not be detected in GLe. In this long-time submerged soil the environmental conditions which microorganisms are exposed might be disadvantageous for fungi.     

Soil microbial and extractable C and N after wildfire

 The effect of wildfire on soil microbes and extractable C (C_ext) and N (N_ext) changed with respect to the time from burning and soil depth. Initially, microbial biomass C (C_mic) and N (N_mic) were drastically reduced in the soil surface layer (0–5 cm) and reduced by 50% in the subsurface (5–10 cm), whereas C_ext increased by 62% in the surface layer and did not significantly change in the subsurface. These parameters were affected for the following 4 years, during which the average reductions in the soil surface and subsurface layers were, respectively, 60% and 50% for C_mic, 70% and 45% for N_mic, 60% and 40% for the ratio C_mic: organic C (C_org) and 70% and 30% for the ratio N_mic: total N (N_tot), while for C_ext the surface layer was the only zone consistently affected and C_ext decreased by up to 59%. Immediately after a fire, the C_ext : C_org ratio increased by 3.5-fold and 2-fold in the surface and subsurface layers, respectively; thereafter for 2 years, it decreased in the surface layer (by up to 45%) while the effect on the subsurface layer was not consistent. The effect of burning on N_ext lasted 1 year, in which N_ext increased by up to 7- and 3-fold in the surface and subsurface layers, respectively, while the average N_ext : N_tot ratio doubled in the surface layer and increased by 34% in the subsurface. During the time in which each parameter was affected by burning, the soil factor explained a high percentage of variance in the fluctuations of C_mic, N_mic, C_mic : C_org and N_mic : N_tot, while those of N_ext and N_ext : N_tot, but not those of C_ext and C_ext : C_org depended on both the soil and its depth. In the burned soils similar patterns of response were found between the following parameters listed in pairs: C_mic and N_mic; C_mic : C_org and N_mic : N_tot; C_ext and N_ext; and C_ext : C_org and N_ext : N_tot. However, after the fire relationships found previously between the parameters studied and many other soils properties were either no longer evident, or were inverted. Although the addition of cellulose to the burned soil favoured fungal mycelium development and increased C_mic and C_ext contents, the negative effect of burning on the microbial biomass and the C_ext was not counteracted even under incubation conditions suitable for both microbial growth and C mineralization. Received: 28 May 1997     

Soil microbial biomass C and N changes in relation to forest conversion in the Northwestern Turkey

Tree species differ in their effect on soil development and nutrient cycling. Conversion of beech coppice to pine plantations can alter soil physical and chemical properties, which in turn may have significant impacts on soil microbial biomass C and N (C_mic, N_mic). The major objective of this study was to evaluate soil quality changes associated with the forest conversion in humid NW Turkey. Results from this study showed that levels of soil organic carbon (C_org), total nitrogen (N_t), moisture, C_mic and N_mic under beech coppice were consistently higher but levels of pH, CaCO₃ and EC were lower compared to pine plantation. Differences between the forest stands in C_mic and N_mic were mainly related to the size of the C_org stores in soil and to tree species. In addition, high level of CaCO₃ is likely to reduce pools of soil organic C and possibly even microbial biomass C and N in pine forests. The average C_mic:N_mic ratios were higher in soils under beech coppice than pine plantation, while C_mic:C_org and N_mic:N_t percentages were similar in both forest types. These results revealed the differences in microbial community structure associated with different tree species and the complex interrelationships between microbial biomass, soil characteristics, litter quantity and quality. Copyright © 2008 John Wiley & Sons, Ltd.     

蒙古高原草原土壤微生物量碳氮特征

沿着水分梯度采集了蒙古高原不同草原类型表层土壤样品 1 44个 ,分析了土壤微生物量C、N含量及其与年平均温度和降雨量的关系。结果表明 :蒙古高原草原土壤微生物量C、N与土壤有机C、全N、降雨量、温度均表现出了很好的相关性。微生物量C变化在5 1 7～ 797mgkg- 1之间 ,微生物量N变化在 1 1 0～ 1 1 8 6mgkg- 1之间。微生物量C∶N比变化在 5～ 9之间。土壤微生物量碳 (Cmic)占土壤有机碳 (Corg)的比例 (Cmic Corg)变化在 1 1 5 %～ 4 1 %之间 ,Cmic Corg与土壤有机C、全N、降雨量均成显著的负相关。土壤呼吸表现为草甸草原土壤 >典型草原 >荒漠草原 ,土壤呼吸与降雨量显著正相关 ,与温度显著负相关。呼吸熵 (QCO2 )与降雨量成二次抛物线关系。放牧对微生物量的影响与不同草原类型和放牧率有关。     

Seasonal changes in microbial nitrogen in an old broadleaf forest and in a neighbouring young plantation

Soil microorganisms are actively involved in many processes of the soil N cycle and are strong competitors with plants for soil N. Therefore, microbial dynamics are important factors in controlling forest productivity. Nevertheless, they are poorly studied especially in relation to forest age, which can produce strong effects on the microbial community by affecting the forest floor environment. In the present study, seasonal variations of soil microbial N (N_mic) were monitored in an old floodplain hardwood forest (270 years) and in a young hardwood plantation (19 years) in two soil horizons (0–15 and 15–30 cm). Although the differences according to time of sampling and soil horizon were statistically significant, N_mic was significantly higher in old than in young forest, especially for the deeper soil layer. However, the highest percentage of total N (N_tot) immobilised in microbial biomass was found in the surface soil layer of the young plantation. Soil organic C (C_org) explained 23% of the spatial–temporal variation of N_mic over all sampling periods in the old forest, whereas the linear combination of N_tot, total extractable soil N (N_totex) and the C/N ratio explained 59% of variation in N_mic when considering only the growing season. In contrast, C_org and N_totex explained 59% of variation in N_mic in the young stand when considering all sampling periods and 75% when the analysis was limited to growing season. Soil moisture did not show any significant correlation with N_mic in either site. The sensitivity of N_mic to variation in C_org and N_tot seems to be affected by forest age, being higher in young than in old forest. Finally our results indicate that during the growing season, when the N_totex availability is low, the dynamics of N_mic and N_totex are temporally interdependent, suggesting the existence of a reciprocal control whose mechanisms deserve to be elucidated.     

Soil microbial biomass C and N along a climatic transect in the Mongolian steppe

The relationships of soil microbial biomass C (C_mic) or N (N_mic) with mean annual precipitation and temperature were studied along a climatic transect in the Mongolian steppe. Soil organic C (C_org) and total N (N_t), respiration rate, C_mic and N_mic at depths of 0–5 and 5–10 cm decreased with increasing aridity. The contents of C_org and N_t in the 0- to 5-cm soil layers decreased linearly with precipitation reduction along the transect. C_mic and N_mic changes with precipitation were not linear, with higher changes between 330 and 128 mm mean annual precipitation. C_mic/C_org and N_mic/N_t increased with increasing aridity. The metabolic quotient qCO₂ of 0- to 5-cm soil layers was low between 330 and 273 mm precipitation. The relationship between the qCO₂ of the 0- to 5-cm soil layers and the mean annual precipitation was well fitted with a quadratic function y =0.0006x² –0.40x +86.0, where y is the qCO₂ (µmol CO₂-C mmol^–1 C_mic) and x is the mean annual precipitation (mm). C_org, N_t, C_mic, N_mic and respiration rate decreased exponentially with increasing mean annual temperature in both the 0- to 5- and 5- to 10-cm soil layers, and change rate was lower when the mean annual temperature was higher than 2.6°C. The close relationships of the mean annual precipitation or temperature with soil C_org, N_t, C_mic, N_mic, C_mic/C_org and qCO₂ indicate that each parameter can be calculated by determining the other parameters in this specific climatic range.     

Microbial C,N, and P relationships in moisture‐stressed soils of Potohar,Pakistan

In 11 rain‐fed arable soils of the Potohar plateau, Pakistan, the amounts of microbial‐biomass C (C_mic), biomass N (N_mic), and biomass P (P_mic) were analyzed in relation to the element‐specific total storage compartment, i.e., soil C_org, N_t, and P_t. The effects of climatic conditions and soil physico‐chemical properties on these relationships were highlighted with special respect to crop yield levels. Average contents of soil C_org, N_t, and P_t were 3.9, 0.32, and 0.61 mg (g soil)^–1, respectively. Less than 1% of P_t was extractable with 0.5 M NaHCO₃. Mean contents of C_mic, N_mic, and P_mic were 118.4, 12.0, and 3.9 µg (g soil)^–1. Values of C_mic, N_mic, P_mic, soil C_org, and N_t were all highly significantly interrelated. The mean crop yield level was closely connected with all soil organic matter– and microbial biomass–related properties, but showed also some influence by the amount of precipitation from September to June. Also the fraction of NaHCO₃‐extractable P was closely related to soil organic matter, soil microbial biomass, and crop yield level. This reveals the overwhelming importance of biological processes for P turnover in alkaline soils.     

Specific features of the ecological functioning of urban soils in Moscow and Moscow oblast

Urban soils (constructozems) were studied in Moscow and several cities (Dubna, Pushchino, and Serebryanye Prudy) of Moscow oblast. The soil sampling from the upper 10-cm-thick layer was performed in the industrial, residential, and recreational functional zones of these cities. The biological (the carbon of the microbial biomass carbon, C_mic and the microbial (basal) respiration, BR) and chemical (pH_water and the contents of C_org, heavy metals, and NPK) indices were determined in the samples. The ratios of BR to C_mic (the microbial respiration quotient, qCO₂) and of C_mic to C_org were calculated. The C_mic varied from 120 to 738 μg C/g soil; the BR, from 0.39 to 1.94 μg CO₂-C/g soil per hour; the C_org, from 2.52 to 5.67%; the qCO₂, from 1.24 to 5.28 μg CO₂-C/mg C_mic/g soil per h; and the C_mic/C_org, from 0.40 to 1.55%. Reliable positive correlations were found between the C_mic and BR, the C_mic and C_mic/C_org, and the C_mic and C_org values (r = 0.75, 0.95, and 0.61, respectively), as well as between the BR and C_mic/C_org values (r = 0.68). The correlation between the C_mic/C_org and qCO₂ values was negative (r = −0.70). The values of C_mic, BR, C_org, and C_mic/C_org were found to correlate with the ammonium nitrogen content. No correlative relationships were revealed between the determined indices and the climatic characteristics. The principal component analysis described 86% of the variances for all the experimental data and clearly subdivided the locations of the studied soil objects. The ANOVA showed that the variances of C_mic, C_org, and BR are controlled by the site location factor by 66, 63, and 35%, respectively. The specificity of the functioning of the anthropogenic soils as compared with their natural analogues was clearly demonstrated. As shown in this study, measurable biological indices might be applied to characterize the ecological, environmental-regulating, and productive functions of soils, including urban soils.     

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.     

Spatial variability in the carbon of microbial biomass and microbial respiration in soils of the south of Moscow oblast

Soil samples from the upper 10-cm-thick layer of the humus horizon (without forest litter) were taken in Podol’sk and Serpukhov districts (1130 and 1080 km², respectively) of Moscow oblast. At each sampling site, ecosystem (forest, plowland, or fallow), soil (soddy-podzolic, soddy-gley, bog-podzolic, meadow alluvial, gray forest, and anthropogenically transformed soils of lawns and industrial zones), predominant vegetation, and topography (floodplain and low, medium, and upper parts of watersheds) were determined. The carbon content of the microbial biomass (C_mic) was determined by the method of substrate-induced respiration; we also determined the rate of basal (microbial) respiration (BR) and the organic carbon content, pH, and particle-size distribution. Overall, 237 samples from Serpukhov district and 45 samples from Podol’sk district were analyzed. The BR/C_mic ratios (respiration quotient qCO₂) and C_mic/C_org ratios were calculated. The C_mic content in the soils ranged from 43 to 1394 μg C/kg; the BR varied from 0.06 to 25 μg CO₂-C/g per h, qCO₂, from 0.34 to 6.52 μg CO₂-C/mg C_mic per h; and the C_mic/C_org ratio, from 0.19 to 10.65%. It was found that the most significant factors affecting the variability of the C_mic and BR are the parameters of ecosystem (50% and 80%, respectively) and soil (30% and 9%, respectively). The most significant variability of these indices was found in forest soils; it was mainly controlled by the soil texture (33 and 23%) and the C_org content (19 and 24%). The C_mic parameter made it possible to differentiate the soils of the territory for the purposes of their evaluation, monitoring, and biological assessment more clearly than the BR value and the soil chemical characteristics.     

Utilization of Distillation Waste–Based Vermicompost and Other Organic and Inorganic Fertilizers on Improving Production Potential in Geranium and Soil Health

Vermicompost (VC) produced from distillation waste of geranium (Pelargonium graveolens), farmyard manure (FYM) produced from animal excreta mixed with pine needle (Pinus sp.), and biofertilzer (Azotobacter) were utilized for this experiment. The plant growth attributes, biomass, and oil yield of geranium were significantly increased with integrated nutrient supply, and maximum increase was found in T₈ treatments (N₁₀₀P₆₀ K₆₀ + 5t VC). Soil organic carbon (C_org) significantly increased by 4.2% to 81.8% in T₄ and T₈ treatments, respectively, over the control. Data obtained on total nitrogen (N_t) and available N, phosphorus (P), and potassium (K) clearly showed that the integrated nutrient supply considerably improved the soil health and sustainability. The soil respiration and microbial biomass C (C_mic) and N (N_mic) were increased by the manures according to the application rate. The C_mic accounted for 1.8 to 2.7% of the soil C_org content and microbial N accounted for 3.9 to 5.8 % of N_t under different treatment combinations.     

Medium-term effects of a single application of mustard residues on soil microbiota and C content of vertisols

 A study of the effects of different qualities (fresh and composted) and rates (equivalent to 120, 240, and 360 kg N ha^–1) of mustard meal application on wheat yields on humid tropical vertisol was started in 1990 at Ginchi Research Station in Ethiopia. After continuous wheat cropping for 7 years and without any further fertilisation, soil microbial parameters (basal respiration, microbial biomass-C and N, organic-C, and ecophysiological quotients) were studied during one growth period. After 7 years of application, mustard meal still exerted a significant positive effect on microbial biomass, basal respiration, organic-C, C_mic : N_mic ratio, and metabolic quotient (qCO₂). Organic-C, qCO₂ and C_mic : N_mic ratios were higher for the compost-amended plots than plots amended with fresh mustard meal. Basal respiration, C_mic, and C_mic : N_mic ratio showed a clear seasonality, but only in manured plots. The data indicate shifts in microbial community structure (from bacteria to fungi and from r to K strategists) and suggest positive medium-term effects of mustard meal on humid tropical vertisol biological qualities. Received: 25 May 1999     

Nitrogen dynamics and microbial response in soil amended with either olive pulp or its by-products after biogas production

Olive pulp (OP), the residual material of a two-phase olive oil extraction system, and effluents from hydrogen (EH₂) and methane (ECH₄) production, have been evaluated as soil amendments particularly for their impact on soil mineral nitrogen (N) dynamics, gross N mineralization, and soil microbial biomass N (N_mic). Both N transformation and microbial growth were mainly influenced by the amount and quality of added organic carbon (C). Both OP and EH₂, which contain more carbohydrates and lipids than polyphenolic compounds, stimulated NO₃ ⁻ immobilization during the early incubation period and increased N_mic, saprophytic fungi, and N mineralization. On the contrary, soil amended with ECH₄, which is characterized by the lowest C content but the highest content of polyphenolic compounds, behaved as the control; neither NO₃ ⁻ immobilization nor microbial growth were observed and gross N mineralization was stimulated only at the beginning of the incubation period. Bacterial plate count was significantly correlated with direct bacterial count and fungal count was correlated with N_mic. Therefore, it is suggested that both bacterial and fungal plate counts may be used as indicators of the overall bacterial and fungal populations inhabiting soil, respectively. The knowledge of the impact of these materials on soil N dynamics is crucial for their correct use in agriculture because it has been shown that NO₃ ⁻ availability can be strongly influenced by the addition of different amounts and quality of organic amendment.     

Soil microbiological and biochemical properties affected by plant growth and different long-term fertilisation

The purpose of this study was to measure the effects of plant growth on soil microbial biomass C (C_mic) and soil enzyme activities. In a pot experiment using spring barley and sugar beet, we investigated the response of C_mic, hot water extractable C and N fractions (C_hwe, N_hwe), and enzyme activities involved in C, N and P cycling in a loess-derived Chernozem from Bad Lauchstädt (Central Germany). The study site has been receiving the same fertilisation treatments for 100 years. The soil originated from plots fertilised with 15 t ha⁻¹ farmyard manure (FYM) year⁻¹ + mineral fertiliser (NPK), or 15 t ha⁻¹ FYM year⁻¹, or NPK or from an unfertilised control. Pots were sampled monthly, and alkaline phosphatase- (AP), β-glucosidase- and protease-activities were analysed. At the beginning and the end of study, levels of C_mic, C_hwe and N_hwe were also measured. All three enzyme activities and C_mic were significantly and positively correlated with C_org and C_hwe. Results suggest that the enzyme activities measured originated mostly from microorganisms and that C_hwe is an important C source for soil microorganisms. β-Glucosidase and AP activities were higher in summer months than at other times. In contrast, protease activity changed only slightly during the growing period.     

Near-infrared spectroscopy for analysis of chemical and microbiological properties of forest soil organic horizons in a heavy-metal-polluted area

In industrial areas, heavy metals may accumulate in forest soil organic horizons, affecting soil microorganisms and causing changes in the chemical composition of the accumulated organic matter. The objectives of this study were to test the ability of near-infrared spectroscopy (NIRS) to detect heavy metal effects on the chemical composition of forest soil O horizons and to test whether NIRS may be used to quantitatively determine total and exchangeable concentrations of Zn and Pb (Zn_t, Pb_t, Zn_ex, Pb_ex) and other chemical and microbial properties in forest soil O horizons polluted with heavy metals. The samples of O horizons (n = 79) were analyzed for organic C (C_org), total N and S (N_t, S_t), Zn_t, Pb_t, Zn_ex, Pb_ex, basal respiration (BR), microbial biomass (C_mic) and C_mic-to-C_org ratio. Spectra of the samples were recorded in the Vis-NIR range (400–2,500 nm). To detect heavy-metal-induced changes in the chemical composition of O horizons principal components (PC1–PC7) based on the spectral data were regressed against Zn_t + Pb_t values. A modified partial least squares method was used to develop calibration models for prediction of various chemical and microbial properties of the samples from their spectra. Regression analysis revealed a significant relationship between PC3 and PC5 (r = −0.27 and −0.34, respectively) and Zn_t + Pb_t values, indicating an effect of heavy metal pollution on the spectral properties of the O horizons and thus on their chemical composition. For quantitative estimations, the best calibration model was obtained for C_org-to-N_t ratio (r = 0.98). The models for C_org, N_t, and microbial properties were satisfactory but less accurate. NIRS failed to accurately predict S_t, C_org-to-S_t, Zn_t, Pb_t, Zn_ex, and Pb_ex.