Is the soil microbial community related to the basal area of trees in a Scots pine stand?

The main energy sources of soil microorganisms are litter fall, root litter and exudation. The amount on these carbon inputs vary according to basal area of the forest stand. We hypothesized that soil microbes utilizing these soil carbon sources relate to the basal area of trees. We measured the amount of soil microbial biomass, soil respiration and microbial community structure as determined by phospholipid fatty acid (PLFA) profiles in the humus layer (FH) of an even-aged stand of Scots pine (Pinus sylvestris L.) with four different basal area levels ranging from 19.9 m² ha⁻¹ in the study plot Kasper 1 to 35.7 m² ha⁻¹ in Kasper 4. Increasing trend in basal respiration, total PLFAs and fungal-to-bacterial ratio was observed from Kasper 1 to Kasper 3 (basal area 29.2 m² ha⁻¹). The soil microbial community structure in Kasper 3 differed from that of the other study plots.     

Soil microbial biomass and fluorescein diacetate hydrolytic activity in Japanese acidic tea field soils

The soil microbial biomass (SME) content and fluorescein diacetate (FDA) hydrolytic activity in 21 acidic tea field soils in Japan were determined. SM 3 content in the tea field soils was quantitatively similar to that in 13 arable soils with neutral soil pH previously reported. However, the ratio of the SMB content to organic matter content in the tea field soils classified as red-yellow soil, brown forest soil, and lithosol was clearly lower than that in the neutral arable soils classified as non-volcanic ash soil. FDA hydrolytic activity in the tea field soils was higher than the activity in various soils with neutral soil pH and showed a negative relationship with the soil pH.     

Changes in the soil microbial community structure with latitude in eastern China, based on phospholipid fatty acid analysis

Phospholipid fatty acid (PLFA) profiles were measured in soils from 14 sites in eastern China representing typical geographic zones of varying latitude from north (47.4°N) to south (21.4°N). Amounts of soil microbial biomass, measured as total amounts of PLFAs, showed no regular trend with latitude, but were positively correlated with soil organic carbon content, the concentration of humic acid and amorphous iron oxide. Soil microbial community structure showed some biogeographical distribution trends and was separated into three groups in a cluster analysis and principal coordinate analysis of log transformed PLFA concentrations (mol%). Soils in the first group came from northern China with medium mean annual temperature (1.2–15.7 °C) and rainfall (550–1021 mm). Soils in the second group originated from southern China with a relatively higher mean annual temperature (15.7–21.2 °C) and rainfall (1021–1690 mm). Soils clustered in the third group originated from the most southerly region. The northern soils contained relatively more bacteria and Gram-negative PLFAs, while the southern soils had more fungi and pressure indexed PLFAs. These differences in soil microbial community structure were largely explained by soil pH, while other site and soil characteristics were less important.     

Soil microbial activity and biomass is stimulated by leguminous cover crops

The leguminous cover crops Atylosia scarabaeoides (L.) Benth., Centrosema pubescens Benth., and Pueraria phaseoloides (Roxb.) Benth., were grown in the interspaces of a 19 y–old coconut plantation and incorporated into the soil at the end of the monsoon season every year. At the end of the 12th year, soils from different depths were collected and analyzed for various microbial indices and their interrelationships. The objectives were to assess the effects of long‐term cover cropping on microbial biomass and microbial‐community structure successively down the soil profile. In general, total N (TN), organic C (OC), inorganic N, extractable P, and the levels of biological substrates viz., dissolved organic C (DOC) and N (DON), labile organic N (LON), and light‐fraction organic matter (LFOM) C and N decreased with depth at all the sites. Among sites, the cover‐cropped (CC) sites possessed significantly greater levels of TN, OC, DOC, DON, and LON compared to the control. Consequently, microbial biomass C (MBC), N (MBN), and P (MBP), CO₂ evolution, and ATP levels, in general, decreased with depth at all sites and were also significantly higher in the CC sites. Among the ratios of various microbial indices, the ratio of MBC to OC and metabolic quotient (qCO₂) declined with depth. Higher MBC‐to‐OC ratios and large qCO₂ levels in the surface soils could be ascribed to greater levels of readily degradable C content and indicated short turnover times of the microbial biomass. In contrast, the ratios of MBC to MBN and MBC to MBP increased with depth due to low N/P availability and relatively higher C availability in the subsoils. Cover cropping tended to enhance the ratios of MBC to OC, MBC to MBN, MBC to MBP, and ergosterol to MBC and decreased the ATP‐to‐MBC ratio at all depths. The relatively lower ATP‐to‐MBC ratios in the CC site, especially in the subsoil indicated microbial‐community structure possibly dominated by fungi. By converting the ergosterol content to fungal biomass, it was observed that fungi constituted 52%–63% of total biomass C at the CC site, but only 33%–40% of total biomass C at the control site. Overall, the study indicated that leguminous cover crops like P. phaseoloides or A. scarabaeoides significantly enhanced the levels of OC, N and microbial activity in the soils, even down to 50 cm soil depth.     

不同林龄马尾松人工林土壤微生物群落结构和功能多样性演变

土壤微生物在森林生态系统中起着至关重要的作用,研究人工林演变中土壤微生物群落结构特征,对评价人工林土壤质量动态变化和维持土壤微生态平衡具有重要意义。以亚热带地区马尾松人工林为研究对象,采用磷脂脂肪酸(Phospholipid fatty acid,PLFA)和BIOLOG技术研究不同林龄(13 a,25 a,38 a和58 a)对土壤微生物群落结构和代谢功能多样性的影响。结果表明:不同林龄土壤微生物类群均以细菌为主,其次为真菌和放线菌,最后为原生动物;土壤微生物总PLFAs量、真菌数量和真菌/细菌均表现为13 a最高,38 a最低;土壤细菌、革兰氏阳性细菌(G+)、革兰氏阴性细菌(G–)和放线菌数量均25a最高。层次聚类和主成分分析(PCA)结果表明,林龄对土壤微生物群落结构产生显著影响,13 a和25 a林龄分别与38 a和58 a林龄的土壤微生物群落结构差异较大。冗余分析表明,有机碳、全氮和pH是土壤微生物群落结构的主要影响因素。不同林龄土壤平均颜色变化率(AWCD)和微生物功能多样性指数(香农指数、辛普森指数和McIntosh指数)总体表现为25 a>13 a>58 ...     

Soil texture affects soil microbial and structural recovery during grassland restoration

Many biotic and abiotic factors influence recovery of soil communities following prolonged disturbance. We investigated the role of soil texture in the recovery of soil microbial community structure and changes in microbial stress, as indexed by phospholipid fatty acid (PLFA) profiles, using two chronosequences of grasslands restored from 0 to 19 years on silty clay loam and loamy fine sand soils in Nebraska, USA. All restorations were formerly cultivated fields seeded to native warm-season grasses through the USDA’s Conservation Reserve Program. Increases in many PLFA concentrations occurred across the silty clay loam chronosequence including total PLFA biomass, richness, fungi, arbuscular mycorrhizal fungi, Gram-positive bacteria, Gram-negative bacteria, and actinomycetes. Ratios of saturated:monounsaturated and iso:anteiso PLFAs decreased across the silty clay loam chronosequence indicating reduction in nutrient stress of the microbial community as grassland established. Multivariate analysis of entire PLFA profiles across the silty clay loam chronosequence showed recovery of microbial community structure on the trajectory toward native prairie. Conversely, no microbial groups exhibited a directional change across the loamy fine sand chronosequence. Changes in soil structure were also only observed across the silty clay loam chronosequence. Aggregate mean weighted diameter (MWD) exhibited an exponential rise to maximum resulting from an exponential rise to maximum in the proportion of large macroaggregates (>2000 μm) and exponential decay in microaggregates (<250 μm and >53 μm) and the silt and clay fraction (<53 μm). Across both chronosequences, MWD was highly correlated with total PLFA biomass and the biomass of many microbial groups. Strong correlations between many PLFA groups and the MWD of aggregates underscore the interdependence between the recovery of soil microbial communities and soil structure that may explain more variation than time for some soils (i.e., loamy fine sand). This study demonstrates that soil microbial responses to grassland restoration are modulated by soil texture with implications for estimating the true capacity of restoration efforts to rehabilitate ecosystem functions.     

茶园土壤微生物量碳的质量分数及其影响因素的研究

对148个茶园土壤的微生物量碳(MBC)质量分数及其影响因素进行了研究.结果表明,茶园土壤MBC的质量分数(ωmic)在38.1～680.2 mg·kg-1之间,平均为246.0 mg·kg-1,其含量变化幅度较大,主要受到土壤有机碳、pH、茶园管理方式、植茶年限、成土母质以及全氮量的影响.(1)茶园土壤的ωmic与有...     

Can the extent of degradation of soil fungal mycelium during soil incubation be used to estimate ectomycorrhizal biomass in soil?

The extent of degradation of the fungal biomass in forest soil during laboratory incubation was investigated as a measure of ectomycorrhizal (EM) biomass. The method simulates the disappearance of fungal mycelium after root trenching, where the EM fungi, deprived of its energy source (the tree), will start to die off. Incubating a forest humus soil at 25 °C resulted in a decrease in the relative proportion (mol%) of the phospholipid fatty acid 18:2ω6,9 (a fungal marker molecule) within 3-6 months, indicating that fungal biomass was disappearing. Incubation at 5 °C resulted in essentially no change in the amount of 18:2ω6,9. The measurement of ergosterol, another fungal marker molecule, gave similar results. Incubation of different forest soils (pine, spruce and spruce/oak), and assuming that the disappearance of fungal biomass during this period of time was entirely due to EM fungi, resulted in an estimation of EM biomass of between 47 and 84% of the total fungal biomass in these soils. The humus layer had more EM biomass than deeper mineral layers.     

氮沉降对亚热带常绿阔叶天然林不同季节土壤微生物群落结构的影响

以亚热带地区细柄阿丁枫(Altingia gracilipes)天然林为研究对象,开展2年(2010—2011年)的原位模拟大气氮沉降试验,设置3个氮水平(以氮(N)含量计算):对照(CK, 0 kg·hm~(-2)·a~(-1))、低氮(LN, 50 kg·hm~(-2)·a~(-1))、高氮(HN,100 kg·hm~(-2)·a~(-1))。采用磷脂脂肪酸(PLFA)技术探讨常绿阔叶天然林土壤微生物群落结构对氮沉降的响应。结果表明,天然林土壤微生物以细菌为优势类群,占微生物总PLFA含量的78.3%。夏季土壤微生物群落结构与其他季节发生显著差异;LN仅显著改变夏季土壤微生物群落结构;HN导致春季和冬季土壤微生物群落结构发生显著分异,且在不同季节对土壤微生物变化类群的影响不同。此外,氮沉降未使真菌与细菌比(F∶B)和革兰氏阳性菌与革兰氏阴性菌比(G+∶G-)发生显著改变,而使不同结构的环丙烷脂肪酸或异构脂肪酸等特殊脂肪酸比值发生改变,这表明短期氮沉降虽然改变了土壤微生物的群落结构,但并未影响微生物对环境变化的响应力,微生物可能通过改变特殊脂肪酸含量应对短期氮沉降,不同季节的应对策略有一定差异。因此,在探讨氮沉降对亚热带地区森林生态系统土壤微生物群落结构的影响时有必要考虑季节因素。     

Plant species richness does not attenuate responses of soil microbial and nematode communities to a flood event

Human activities are causing climatic changes and alter the composition and biodiversity of ecosystems. Climate change has been and will be increasing the frequency and severity of extreme climate events and natural disasters like floods in many ecosystems. Therefore, it is important to investigate the effects of disturbances on ecosystems and identify potential stabilizing features of ecological communities. In this study, soil microbial and nematode communities were investigated in a grassland biodiversity experiment after a natural flood to investigate if plant diversity is able to attenuate or reinforce the magnitude of effects of the disturbance on soil food webs. In addition to community analyses of soil microorganisms and nematodes, the stability indices proportional resilience, proportional recovery, and proportional resistance were calculated. Generally, soil microbial biomass decreased significantly due to the flood with the strongest reduction in gram-negative bacteria, while gram-positive bacteria were less affected by flooding. Fungal biomass increased significantly three months after the flood compared to few days before the flood, reflecting elevated availability of dead plant biomass in response to the flood. Similar to the soil microbial community, nematode community structure changed considerably due to the flood by favoring colonizers (in the broadest sense r-strategists; c–p 1, 2 nematodes), particularly so at high plant diversity. None of the soil microbial community stability indices and few of the nematode stability indices were significantly affected by plant diversity, indicating limited potential of plant diversity to buffer soil food webs against flooding disturbance. However, plant diversity destabilized colonizer populations, while persister populations (in the broadest sense K-strategists; c–p 4 nematodes) were stabilized, suggesting that plant diversity can stabilize and destabilize populations depending on the ecology of the focal taxa. The present study shows that changes in plant diversity and subsequent alterations in resource availability may significantly modify the compositional shifts of soil food webs in response to disturbances.     

Soil microbial communities in (sub)alpine grasslands indicate a moderate shift towards new environmental conditions 11 years after soil translocation

A change in environmental conditions may result in altered soil microbial communities in alpine grasslands but the extent and direction of the change is largely unknown. The aim of our study was to investigate (i) differences in soil microbial communities across an elevation gradient of (sub)alpine grassland soils in the Swiss Alps, and (ii) the long-term effect of translocation of soil cores from a higher to a lower elevation site. The translocation of undisturbed soil cores from a high alpine site (2525 m asl) to a subalpine site near the timberline (1895 m asl) induced an effective artificial warming of 3.3 °C. We hypothesized that after longer than a decade, soil microbial community in translocated cores would differ from that at the original site but resemble the community at the new site. Results from soil phospholipid fatty acid (PLFA) analysis confirm significant differences in microbial communities between sites and a shift in total microbial biomass (TMB) and proportional distribution of structural groups in the translocated cores towards the lower elevation community. Patterns related to translocation were also observed as shifts in the fractional biomass of ectomycorrhizal and arbuscular fungi, and in relative contents of several structural groups. Hence, soil microbial community activity and diversity indicate a moderate shift towards new site conditions after 11 years and therefore, our data suggest slow responses of microbial communities to environmental changes in alpine soils.     

Evidence that plant diversity and site productivity confer stability to forest floor microbial biomass

The ability of soil microbial communities to withstand punctual disturbance or chronic stress is important for the stability of ecosystem processes. Factors controlling microbial community composition or soil resource availability should be regarded as potential determinants of this stability. Here, we explored the effects of three stand types (jack pine, aspen and mixed-wood) and two geologic parent materials (clay and till), on the stability of the microbial biomass in the forest floor. We hypothesised that microbial communities in mixed-wood stands or on the clay soil would show greater resistance to, and resilience from, a dry-wet disturbance, and a higher tolerance to incremental additions of HCl or Cu, than microbial communities in mono-specific stands or on the till soil. We also surveyed the understory vegetation, and measured chemical properties and microbial phospholipid fatty acid profiles in the forest floor, so as to gain insights into the factors regulating microbial stability. Microbial resistance to disturbance was found to be higher in mixed-wood than in mono-specific stands. Microbial communities from mixed-wood stands also showed a high tolerance to HCl and Cu stress over both geologic parent materials, as opposed to those in mono-specific stands that showed a high tolerance to stress on only one type of parent material. Some forest floor properties in mixed-wood stands (e.g. Ca on clay, mineralisable N and C/N ratio on till) were more similar to the more productive aspen, than to jack pine stands. Other properties (understory plant communities, pH, actinomycete and arbuscular mycorrhizae) of mixed-wood stands were transitional between those in aspen and jack pine stands, suggesting that both tree species contribute in structuring the forest floor microbial pool in mixed-wood stands. We put forward that this may provide a more diverse capability to resist disturbance and tolerate stress than in mono-specific stands. We found no effect of stand type on microbial resilience to disturbance, but resilience was higher on clay than on till plots. This could be due to a higher fungal/bacterial ratio on till plots, as slower fungal growth rates may hinder resilience, or to lower carbon and nutrient availability limiting the growth rate of resistant microbial cells. We conclude that plant diversity and site productivity are important drivers of forest floor microbial stability in the southern boreal forest of eastern Canada.     

Forest floor microbial communities in relation to stand composition and timber harvesting in northern Alberta

With the growing interest in silvicultural techniques that more closely emulate natural disturbance regimes, there is a need to better understand how partial harvesting affects the soil microbial community in stands with varying ecological characteristics, e.g., tree species composition. Four and a half and 5.5 years post-harvest, we used phospholipid fatty acid (PLFA) and substrate-induced respiration (SIR) analyses to compare the microbial biomass and microbial community structure of forest floors from stands dominated by white spruce (Picea glauca; SPRUCE) or by trembling aspen (Populus tremuloides; ASPEN) and from mixed-species (MIXED) stands in northern Alberta, Canada, that had been clearcut, partial-cut with 20% retention, partial-cut with 50% retention or left uncut (controls). PLFA and SIR analyses revealed that ASPEN forest floors supported a larger microbial biomass with a very different community structure than MIXED or SPRUCE forest floors. The microbial community structure of these soils appeared to be strongly affected by the presence of white spruce and the composition of the understory vegetation. There were no effects of timber harvesting detected within or across stand types on any of the variables measured, with the exception of the PLFA 16:1ω5, which was relatively more abundant in the clearcuts and 50% retention treatments than in the uncut controls, perhaps in response to an increased forest floor pH and grass cover in the disturbed areas. The resilience to timber harvesting of the forest floors from these stands may be the result of efforts to minimize soil disturbance during harvesting and to allow vegetation to regenerate naturally. From the perspective of the forest floor microbial community, partial harvesting does not appear to have any benefit over clearcut harvesting at these boreal forest sites.     

不同林龄杉木根际与非根际土壤微生物群落特征

为探究中亚热带杉木土壤微生物群落随林龄变化特征,以中亚热带7,24,34 a生杉木人工林为研究对象,采用磷脂脂肪酸(PLFA)法分析其根际和非根际土壤微生物数量和群落结构及驱动土壤微生物变化的主要土壤环境因子。结果表明：随着杉木林龄的增长,非根际土壤各类微生物数量不断减少,根际土壤微生物数量不断增加,34 a生杉木人工林细菌含量、革兰氏阴性菌含量、Cy∶MONO根际土壤显著高于非根际土壤,而其他各类微生物在根际和非根际土壤间均没有显著差异。相关分析和冗余分析结果表明：土壤环境因子对杉木土壤微生物群落有显著影响,其中有效磷和铵态氮含量对土壤微生物群落的影响较大,有效磷含量与土壤微生物群落呈正相关,土壤铵态氮含量与其呈负相关。因此,在杉木人工林管理过程中,可适当增加磷的输入,以增加土壤微生物数量,提高土壤质量,促进杉木的生长。     

不同海拔茶树根系土壤微生物群落多样性分析

从福建省泉州市安溪县不同海拔铁观音茶园采集了根系区域土壤, 利用磷脂脂肪酸(PLFAs)生物标记法研究了不同海拔茶园根系土壤微生物群落多样性。结果表明, 不同海拔茶树根系土壤共检测22个PLFAs, 不同的PLFAs在不同海拔茶树根系土壤分布差异明显, 可分为完全分布和不完全分布两种类型。高海拔(834 m)茶树根系土壤分布的PLFAs种类和数量都最多, 分布最高的前3个生物标记为16:0(指示细菌)、18:3ω6c(6,9,12)(指示真菌)、18:00(指示嗜热解氢杆菌)。对不同海拔茶树根系土壤特征微生物PLFAs分析表明, 代表细菌的16:0、代表真菌的18:3ω6c(6,9,12)和代表放线菌的10Me 16:0相对生物量在不同海拔茶树根系土壤中分布量不同, 细菌分布量最大, 其次是放线菌, 真菌分布量最小。微生物群落PLFAs标记聚类结果显示, 当欧氏距离为11.51时, 可将不同海拔茶园根系土壤的PLFAs生物标记分成3个大的类群, 类群I的PLFAs生物标记为完全分布且分布量中等, 类群Ⅱ的PLFAs生物标记为完全分布且分布量较高, 类群Ⅲ的PLFAs生物标记为不完全分布且分布量较低。分析多样性指数表明, Shannon-Wiener指数、Simpson指数和Pielou指数均是海拔834 m处最大, 海拔717 m处最低。     

Variations in soil microbial biomass and crop roots due to differing resource quality inputs in a tropical dryland agroecosystem

The influence of exogenous organic inputs on soil microbial biomass dynamics and crop root biomass was studied through two annual cycles in rice-barley rotation in a tropical dryland agroecosystem. The treatments involved addition of equivalent amount of N (80 kg N ha⁻¹) through chemical fertilizer and three organic inputs at the beginning of each annual cycle: Sesbania shoot (high-quality resource, C:N 16, lignin:N 3.2, polyphenol+lignin:N 4.2), wheat straw (low-quality resource, C:N 82, lignin:N 34.8, polyphenol+lignin:N 36.8) and Sesbania+wheat straw (high-and low-quality resources combined), besides control. The decomposition rates of various inputs and crop roots were determined in field conditions by mass loss method. Sesbania (decay constant, k=0.028) decomposed much faster than wheat straw (k=0.0025); decomposition rate of Sesbania+wheat straw was twice as fast compared to wheat straw. On average, soil microbial biomass levels were: rice period, Sesbania?Sesbania+wheat straw>wheat straw?fertilizer; barley period, Sesbania+wheat straw>Sesbania?wheat straw?fertilizer; summer fallow, Sesbania+wheat straw>Sesbania>wheat straw?fertilizer. Soil microbial biomass increased through rice and barley crop periods to summer fallow; however, in Sesbania shoot application a strong peak was obtained during rice crop period. In both crops soil microbial biomass C and N decreased distinctly from seedling to grain-forming stages, and then increased to the maximum at crop maturity. Crop roots, however, showed reverse trend through the cropping period, suggesting strong competition between microbial biomass and crop roots for available nutrients. It is concluded that both resource quality and crop roots had distinct effect on soil microbial biomass and combined application of Sesbania shoot and wheat straw was most effective in sustained build up of microbial biomass through the annual cycle.     

Structure and function of the soil microbial community in microhabitats of a heavy metal polluted soil

 Particle-size fractionation of a heavy metal polluted soil was performed to study the influence of environmental pollution on microbial community structure, microbial biomass, microbial residues and enzyme activities in microhabitats of a Calcaric Phaeocem. In 1987, the soil was experimentally contaminated with four heavy metal loads: (1) uncontaminated controls; (2) light (300 ppm Zn, 100 ppm Cu, 50 ppm Ni, 50 ppm V and 3 ppm Cd); (3) medium; and (4) heavy pollution (two- and threefold the light load, respectively). After 10 years of exposure, the highest concentrations of microbial ninhydrin-reactive nitrogen were found in the clay (2–0.1 μm) and silt fractions (63–2 μm), and the lowest were found in the coarse sand fraction (2,000–250 μm). The phospholipid fatty acid analyses (PLFA) and denaturing gradient gel electrophoresis (DGGE) separation of 16S rRNA gene fragments revealed that the microbial biomass within the clay fraction was predominantly due to soil bacteria. In contrast, a high percentage of fungal-derived PLFA 18 : 2ω6 was found in the coarse sand fraction. Bacterial residues such as muramic acid accumulated in the finer fractions in relation to fungal residues. The fractions also differed with respect to substrate utilization: Urease was located mainly in the <2 μm fraction, alkaline phosphatase and arylsulfatase in the 2–63 μm fraction, and xylanase activity was equally distributed in all fractions. Heavy metal pollution significantly decreased the concentration of ninhydrin-reactive nitrogen of soil microorganisms in the silt and clay fraction and thus in the bulk soil. Soil enzyme activity was reduced significantly in all fractions subjected to heavy metal pollution in the order arylsulfatase >phosphatase >urease >xylanase. Heavy metal pollution did not markedly change the similarity pattern of the DGGE profiles and amino sugar concentrations. Therefore, microbial biomass and enzyme activities seem to be more sensitive than 16S rRNA gene fragments and microbial amino-sugar-N to heavy metal treatment. Received: 21 January 2000     

Influence of lanthanum on microbial biomass C,P and C- and P-cycling enzyme activities in tea garden soil

At present, reports of the effect of lanthanum on nitrogen cycling in the soil are very detailed, but information on carbon (C) and phosphorus (P) cycling is less reported. We present an investigation into the effects of lanthanum on the pH, microbial biomass C and P and enzyme activities (such as that of β-glucosidase, peroxidase, polyphenol oxidase, acid phosphomonoesterase, phosphodiesterase and phosphotriesterase) in soil in an indoor culture experiment. The results show that the application of lanthanum decreased the pH and had an inhibitory effect on microbial biomass C and P throughout the experiment. The application of lanthanum significantly inhibited most of enzyme activities at the 14th day of the experiment. However, soil samples treated with 100 mg kg^?1 of lanthanum significantly stimulated the activity of polyphenol oxidase at the 14th day, and the range 100–300 mg kg^?1 stimulated the activities during the 28th day to the 56th day. From the 42nd to the 56th day, the pH and all enzyme activities gradually increased. We inferred that the lanthanum had different effects on microbial biomass C and P and enzymes. We recommended that the amount of the rare earth element applied in a tea garden should be lower than 100 mg kg^?1.     

Soil microbial biomass: The eco-physiological approach

In the 1980s ecosystem research projects were implemented world-wide since there was a pressing need to quantify the impacts of anthropogenic pollutants. Soil ecosystem analyses concentrated first on the quantification of the element and energy transfer between pools. Since mineralization of organic substrates and the release of nutrients and elements are due to the heterotrophic activity of the microbial decomposer compartment, this subsystem of terrestrial ecosystems gained importance. Direct microscopic observation methods were inadequate for the quantification of environmental impacts on the microflora. We adopted the maintenance requirement concept for the quantification of environmental impacts or stress effects on the soil microbial community. The paper gives a brief inside to the concept of maintenance from autecological studies and describes the underlying points which lead to our experimental approach of its application at the synecological level (i.e., microbial biomass as a single ecological entity) - a process which rested on long-term continuous research.     

Influence of polychlorinated biphenyls on microbial biomass and its activity in grassland soil

Microbial biomass content, soil respiration and biomass specific respiration rate were measured in two parts of an area polluted by a municipal waste incinerator [polychlorinated biphenyls (PCBs) from combustion processes]. The soils in the studied parts differed significantly only in their levels of PCBs. The concentration of PCBs found in a control plot (4.4 ng g^-1 soil) can be regarded as a background value while the polluted plot contained an increased amount of PCBs (14.0 ng g^-1 soil). A significantly lower microbial biomass (decreased by 23%, based on the chloroform-fumigation extraction technique) and a lower specific respiration rate (decreased by 14%) were observed in the polluted plot in comparison with the control plot at the end of experimental period (1992–1994). Furthermore, a lower ability of microorganisms in the polluted plot to convert available C_org into new biomass was found in laboratory incubations with glucose-amended samples.