Characterization of the forest humus microbial community in a heavy metal polluted area

Various parameters of the soil microbial community may be used in soil quality evaluation and environmental risk assessment. The objectives of this study were to assess the effects of different environmental factors on the characteristics of forest humus microbial communities, and to test which environmental factors most affect the gross microbial indices and physiological profiles of these communities. Samples were taken at 71 plots located in a heavily polluted area of the Krakowsko-Cze¸stochowska upland in southern Poland. The samples were analyzed for pH in KCl (pH_KCl), organic C (C_org), total N (N_t) and S (S_t), and for total and soluble Zn, Pb and Cd concentrations. The considered microbial parameters included basal respiration (BAS), microbial biomass (C_mic), C_mic-to-C_org ratio, and community-level physiological profiles (CLPPs) studied using BIOLOG^® Ecoplates. Multiple regression analysis was used to estimate the effects of humus properties on the microbial parameters. It indicated that S_t and C_org-to-N_t ratio were the most important factors positively affecting C_mic (β=0.15 and 0.11, respectively) and BAS (β=0.13 and 0.08, respectively). The C_mic-to-C_org ratio was related positively to S_t (β=0.12) but negatively to N_t (β=−0.08). The effects of pH_KCl and heavy metals on the gross microbial indices were significant but less important. The most important effect on microbial activity on BIOLOG^® plates and CLPPs was from pH_KCl. The other significant variables included S_t, C_org-to-N_t and interactions of heavy metals with pH_KCl. It was concluded that C_mic, C_mic-to-C_org and BAS might be good indicators of the general status of soil microbial communities, but their use in studying heavy metal effects may entail difficulties in separating the effects of other factors. The sensitivity of the BIOLOG^® test to pH_KCl suggests that it may be useful for studying the effects of acidification or liming on soil microbial communities. The significant effect of the interactions between heavy metals and other variables on physiological profiles indicated that high heavy metal content affects the metabolic functions of soil microbial populations.     

Spatial structure of microbial biomass and activity in prairie soil ecosystems

Management of soil ecosystems requires assessment of key soil physicochemical and microbial properties and the spatial scale over which they operate. The objectives were to determine the spatial structure of microbial biomass and activity and related soil properties, and to identify spatial relationships of these properties in prairie soils under different management histories. Soil were sampled along a transect at 0.2 m intervals in each of five long-term treatments, namely, undisturbed, cattle grazed at two intensities, and cultivated with either wheat (Triticum aestivum L.) or cotton (Gossypium hirsutum L.). Contents of organic carbon (C_org), dissolved organic C (DOC), soluble nitrogen (N_sol), and microbial biomass C (C_mic) and N (N_mic) as well as dehydrogenase activity (DH) in 70 samples were evaluated. Results showed that long-term soil management altered the spatial structure and dependence of C_org and microbial biomass and activity. Cultivation has contributed to high nugget variance for C_org, C_mic, N_mic and DH which interfered with detection of spatial structure at the sampling scale used. Contents of C_org were spatially connected to microbial biomass and activity and to DOC in the uncultivated but not in the cultivated soils, indicating that various factors affected by management may operate at different spatial scales.     

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.     

Soil microbial activity and free-living nematode community in the upper soil layer of the anticline erosional cirque,Makhtesh Ramon,Israel

The physical–chemical peculiarity of soil rock formations is one of the leading factors determining diversity and abundance of soil biota. The main aim of the present research was to study soil microbial and free-living nematode abundance and diversity on different soil rock formations (basalt, sandstone, limestone, granite and gypsum) of the Makhtesh Ramon erosional cirque. The obtained results showed the strong effect of soil features of different soil formations on microbial biomass and respiration as well as on the soil free-living nematode communities and its trophic and species composition. The Sorenson-Czenkanowski similarity index indicated significant differences between soil properties as well as between soil biota in observed soil formations. The qCO₂, which is known to increase according to the level of environmental stress, reached maximal values in the sandstone soil formation. The values of ecological indices such as Simpson's dominance index, maturity index and modification and species richness pointed to a specific ecological condition in the studied soil formations dependent on low content of an essential soil matter as soil moisture, organic matter and cations.     

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.     

Responses of soil microbial biomass and activity for practices of organic and conventional farming systems in Piauí state,Brazil

The aim of this work was to investigate the response of soil microbial biomass and activity to practices in organic and conventional farming systems. The study was carried out at the Irrigation District of Piauí, Brazil. Five different plots planted with “acerola” orchard (Malpighia glaba) and established at the following management were evaluated: (1) under 12 months of soil conventional management (CNV); (2) under six months of soil organic management (ORG6); (3) under 12 months of soil organic management (ORG12); (4) under 18 months of soil organic management (ORG18); and (5) under 24 months of soil organic management (ORG24). Soil microbial biomass C (C_mic), basal respiration, organic carbon (C_org), C_mic-to-C_org ratio and metabolic quotient (qCO₂) were evaluated in soil samples collected at 0–10 cm depth. The highest C_org and C_mic levels occurred in organic system plots ORG18 and ORG24 compared to the conventional system. Soil respiration and C_mic-to-C_org ratio were significantly enhanced by the organic system plots. The qCO₂ was greater in conventional than in organic system. These results indicate that the organic practices rapidly improved soil microbial characteristics and slowly increase soil organic C.     

Microbial biomass carbon in the profiles of forest soils of the southern taiga zone

In the mineral horizons of the soils under different southern taiga forests (oak, archangel spruce, and aspen in the Kaluzhskie Zaseki Reserve of Kaluga region and the green moss spruce and spruce-broadleaved forests of the Zvenigorod Biological Station of Moscow State University in Moscow region), the carbon content in the microbial biomass (C_mic), the rate of the basal respiration (BR), and the specific microbial respiration (qCO₂= BR/C_mic) were determined. The C_mic content was measured using the method of substrate-induced respiration (SIR). In the upper humus horizons of the soils, the C_mic content amounted to 762–2545 μg/g and the BR ranged from 1.59 to 7.55 μg CO₂-C/g per h. The values of these parameters essentially decreased down the soil profiles. The portion of C_mic in the organic carbon of the humus horizons of the forest soils was 4.4 to 13.2%. The qCO₂values increased with the depth in the soils of the Biological Station and did not change in the soils of the Reserve. The pool of C_mic and C_org and the microbial production of CO₂ (BR) within the forest soil profiles are presented.     

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.     

Long-term effects of mineral fertilizers on soil microorganisms – A review

Increasing nutrient inputs into terrestrial ecosystems affect not only plant communities but also associated soil microbial communities. Studies carried out in predominantly unmanaged ecosystems have found that increasing nitrogen (N) inputs generally decrease soil microbial biomass; less is known about long-term impacts in managed systems such as agroecosystems. The objective of this paper was to analyze the responses of soil microorganisms to mineral fertilizer using data from long-term fertilization trials in cropping systems. A meta-analysis based on 107 datasets from 64 long-term trials from around the world revealed that mineral fertilizer application led to a 15.1% increase in the microbial biomass (C_mic) above levels in unfertilized control treatments. Mineral fertilization also increased soil organic carbon (C_org) content and our results suggest that C_org is a major factor contributing to the overall increase in C_mic with mineral fertilization. The magnitude of the effect of fertilization on C_mic was pH dependent. While fertilization tended to reduce C_mic in soils with a pH below 5 in the fertilized treatment, it had a significantly positive effect at higher soil pH values. Duration of the trial also affected the response of C_mic to fertilization, with increases in C_mic most pronounced in studies with a duration of at least 20 years. The input of N per se does not seem to negatively affect C_mic in cropping systems. The application of urea and ammonia fertilizers, however, can temporarily increase pH, osmotic potential and ammonia concentrations to levels inhibitory to microbial communities. Even though impacts of fertilizers are spatially limited, they may strongly affect soil microbial biomass and community composition in the short term. Long-term repeated mineral N applications may alter microbial community composition even when pH changes are small. How specific microbial groups respond to repeated applications of mineral fertilizers, however, varies considerably and seems to depend on environmental and crop management related factors.     

Eucalyptus grandis and Acacia mangium in monoculture and intercropped plantations: Evolution of soil and litter microbial and chemical attributes during early stages of plant development

Soil microorganisms and microbial processes are influenced by the quality and quantity of plant waste entering the soil, by its seasonal and spatial distribution, by the ratio of above- to below-ground inputs, and by changes in nutrient inputs. Soil management strategies sometimes promote mixed-species plantations to mitigate the loss of soil nutrients and improve biogeochemical cycling. The objective of this study was to explore changes in microbiological and chemical attributes of soils and litter in the early stages of the second rotation of mixed and pure plantations of Eucalyptus grandis and Acacia mangium, and to look for correlations between attributes. Soil samples at 0–10 cm depth were collected two, seven, 14, and 20 months after planting in the following treatments: monocultures of A. mangium and E. grandis, a monoculture of E. grandis with N-fertilizer, and an intercropped plantation with E. grandis and A. mangium. Microbial soil attributes varied dramatically between treatments 20 months after planting. Total C, N and P contents in litter showed the strongest correlations with microbial biomass C and N (C_mic and N_mic), microbial respiration, and dehydrogenase activity in all sampling periods. Lower C/N and C/P ratios in litter and lower C/N and C_mic/tC ratios in soils after 20 months in the intercropped plantation illustrated the system's capacity for supplying inputs of high-quality organic matter rich in N and P, but this did not result in higher contents of these elements or greater microbial activity in soils. An implication of this finding is that, at least in the initial growth phase of these plantations, chemical attributes of the litter and variation in those attributes govern microbial processes and, consequently, are mostly responsible for plant development. Canonical discriminant analysis revealed changes in the microbiological and chemical attributes of soil in the intercropped plantation due to the plants growth and the leaf litter accumulation. Twenty months after planting, the different plantations could be discriminated by differences in litter chemistry (C, N, and P), total soil C, N_mic, and dehydrogenase activity, which were very similar in intercropped plantations and E. grandis with N-fertilizer. These results from the early stages of plantation development are important for understanding the dynamics of soil attributes in these systems, and especially in intercropped plantations. In intercropped areas the cumulative effect of microbial attributes reflects a more sustainable system. Long-term studies are needed to identify patterns that develop after 20 months, during the growth period of these plantations.     

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.     

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.     

Indications for soil organic matter quality in soils under different management

Changes in management practice are reflected by soil carbon and nitrogen status, in particular by the proportion of soil organic matter (SOM) being easily transformed (active SOM). We describe SOM quality for three management practices, Organic Farming system (OF), Integrated Crop Production (ICP) and pasture sites (G), which intend to achieve sustainable management practice. The experimental sites were conventionally farmed until 1992. SOM quality was examined by describing active SOM pools, such as the decomposed ‘young soil organic matter’ (YSOM), ratio of microbial biomass carbon (C_mic) to organic carbon (C_org), ecophysiological status of the microbial biomass (qCO₂), and the ratio of light particulate organic matter (POM-LF) to C_org. Ratios of soil microbial biomass (C_mic/C_org) and POM-LF (POM-LF/C_org) and the amount of decomposed YSOM were relatively similar to each other, despite differences in management practice and soil texture. Soil microbial parameters (C_mic, C_mic/C_org and qCO₂) were significantly (p<0.05) affected by the amount of decomposed YSOM and the silt content in the OF. In the ICP, soil microbial parameters depended only on the amount of decomposed YSOM, which was considered to be a consequence of the more heterogeneous texture at the OF-sites. Management effects were detectable for no-tillage in the ICP leading to an accumulation of active SOM in the surface soil (0–10 cm). The ratio POM-LF/C_org showed no difference between G and OF despite markedly higher C_org-contents at the G-sites. Conclusively, all methods used indicate comparable SOM qualities for the three management systems, despite differences in soil texture and soil management during 7 years. Management practices seem to be well adapted to the site conditions.     

Microbial enzyme activities in leaf litter, humus and mineral soil layers of European forests

The rationale of the study was to investigate microbial activity in different soil horizons in European forests. Hence, activities of chitinase and cellulase, microbial biomass carbon (C_mic) and basal respiration were measured in litter, fragmentation, humus and mineral soil layers collected several times from various beech and spruce forests. Sites were selected to form a gradient in N availability. Analyses were also performed on beech litter from a litterbag transplant experiment. Furthermore, microbiological parameters were measured in horizons of beech and spruce chronosequence sites with different stand age in order to investigate the influence of forest rotation, and hence changes in soil organic matter (SOM) dynamics, on microbial activity. Finally in horizons of one beech forest, the seasonal variation of selected microbiological parameters was measured more intensively. β-Glucosaminidase and cellobiohydrolase activities were measured using fluorogenic 4-methylumbelliferyl substrates to estimate chitinase and cellulase activities, respectively. On a spatial scale, chitinase and cellulase activities, C_mic determined by substrate induced respiration, and basal respiration ranged from 144 to 1924 and 6-177 nmol 4-MU g⁻¹ org-C h⁻¹, 8-48 mg C g⁻¹ org-C and 11-149 μg CO₂-C g⁻¹ org-C h⁻¹, respectively; in general values were significantly lower in layers of humus and mineral soil than of litter. Chitinase activity, C_mic and basal respiration from humus and mineral soil layers, together, correlated positively, while none correlated with cellulase activity. Similarly in the litter layer, no correlations were found between the microbiological parameters. On a seasonal scale, a time lag between a burst in basal respiration rate and activities of both enzymes were observed. In general, activities of cellulase and chitinase, C_mic and basal respiration, did not change with stand age, except in the humus layer in the spruce chronosequence, where C_mic decreased with stand age. In the litter layer, cellulase activity was significantly and positively related to the C:N ratio, while only a tendency for chitinase activity was shown, indicating that enzyme activities decreased with increasing N availability. In accordance, the enzyme activities and C_mic decreased significantly with increasing chronic N deposition in the humus layer, while basal respiration only tended to decrease with increasing N deposition. In contrast, enzyme activities in beech litter from litterbags after 2 years of incubation were generally higher at sites with higher N deposition. The results show different layer-specific responses of enzyme activities to changes in N availability, indicating different impacts of N availability on decomposition of SOM and stage of litter decomposition.     

Effect of phosphate rock, lime and cellulose on soil microbial biomass in acidic forest soil and its significance in carbon cycling

Phosphate rock (PR), limestone, coal combustion by-product (CCBP) high in Ca and high organic manures are potential amendments for increasing agricultural production in the acidic soils of the Appalachian region. The objective of this study was to examine effects of PR, CCBP and cellulose addition on soil microbial biomass in an acidic soil based on the measurement of soil microbial biomass P (P _mic) and on the mineralization of organic matter. Application of PR alone or in combination with CCBP increased P _mic. The P _mic was far less when the soil received PR in combination with limestone than with PR application alone or PR in combination with CCBP. Either CCBP or limestone application alone considerably decreased P _mic in the soil due to reduced P solubility. Cellulose addition alone did not increase P _mic, but P _mic was significantly increased when the soil was amended with cellulose in combination with PR. The decomposition of added cellulose was very slow in the soil without PR amendment. However, mineralization of both native organic matter and added cellulose was enhanced by PR application. Mineralization of organic matter was less when the soil was amended with PR in combination with high rates of CCBP (> 2.5%) because PR dissolution varied inversely with amount of CCBP addition. Overall, CCBP had no detrimental effect on soil microbial biomass at low application rates, although, like limestone, CCBP at a high rate may decrease P _mic in P-deficient soils through its influence on increased soil pH and decreased P bioavailability in the soil. Application of PR to an acidic soil considerably enhanced the microbial activity, thereby promoting the cycling of carbon and other nutrients. Received: 11 December 1995     

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.     

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.     

Microbial community structure and characteristics of the organic matter in soils under Pinus sylvestris, Picea abies and Betula pendula at two forest sites

 Microbial biomass C (C_mic), C mineralization rate, phospholipid fatty acid (PLFA) profiles and community level physiological profiles (CLPPs) using Biolog were determined from the humus and mineral soil layers in adjacent stands of Scots pine (Pinus sylvestris L.), Norway spruce [Picea abies (L.) Karst.] and silver birch (Betula pendula Roth) at two forest sites of different fertility. In addition, the Fourier-transformed infrared (FTIR) spectra were run on the samples for characterization of the organic matter. C_mic and C mineralization rate tended to be lowest under spruce and highest under birch, at the fertile site in all soil layers and at the less fertile site in the humus layer. There were also differences in microbial community structure in soils under different tree species. In the humus layer the PLFAs separated all tree species and in the mineral soil spruce was distinct from pine and birch. CLPPs did not distinguish microbial communities from the different tree species. The FTIR spectra did not separate the tree species, but clearly separated the two sites. Received: 3 December 1999     

水分含量对秸秆还田土壤碳矿化和微生物特性的影响

An 80-d incubation experiment was conducted to investigate straw decomposition,the priming effect and microbial characteristics in a non-fertilized soil(soil 1) and a long-term organic manure-fertilized soil(soil 2) with and without13 C-labeled maize straw amendment under different moisture levels. The soil 2 showed a markedly higher priming effect,microbial biomass C(Cmic),and β-glucosidase activity,and more abundant populations of bacteria and fungi than the soil 1. Also,soil CO2 emission,Cmic,β-glucosidase activity,and bacterial and fungal population sizes were substantially enhanced by straw amendment. In the presence of straw,the amount of straw mineralization and assimilation by microbes in the soil at 55% of water holding capacity(WHC) were significantly higher by 31% and 17%,respectively,compared to those at 25% of WHC. In contrast,β-glucosidase activity and fungal population size were both enhanced as the moisture content decreased. Cmicdecreased as straw availability decreased,which was mainly attributed to the reduction of straw-derived Cmic. Amended soils,except the amended soil 2 at 25% of WHC,had a more abundant fungal population as straw availability decreased,indicating that fungal decomposability of added straw was independent of straw availability. Non-metric multidimensional scaling analysis based on fungal denatured gradient gel electrophoresis band patterns showed that shifts in the fungal community structure occurred as water and straw availability varied. The results indirectly suggest that soil fungi are able to adjust their degradation activity to water and straw availability by regulating their community structure.     

Long-term application of compost influences microbial biomass and enzyme activities in a tropical Aeric Endoaquept planted to rice under flooded condition

In a field study, long-term application of compost to a tropical Aeric Endoaquept under continuous rice growing in a rice-rice-fallow sequence resulted in the stimulation of microbial biomass and select soil enzyme activities. Mean seasonal soil microbial biomass-C (C_mic) increased by 42%, 39% and 89% in inorganic fertilizer, compost and compost+inorganic fertilizer treatments, respectively, over the unamended control. C_mic content was also influenced by the rice crop growth stage and was highest at maximum tillering stage irrespective of treatments and declined thereafter. Soil organic C (C_org) content showed highly significant positive correlation with dehydrogenase, urease, cellulase, β-glucosidase and fluorescein di-acetate (FDA) hydrolysis activity, and a positive but not significant correlation with invertase and amidase activity. C/N ratio which was lowest in unamended control plots showed a significant positive relationship with only the enzymes involved in C cycle. Stepwise regression analysis revealed that for prediction of both total organic C and total N, FDA hydrolysis activity contributed significantly for the variance and explained up to 85-96% variability. Results demonstrated that microbial biomass and soil enzyme activity is sensitive in discriminating between long-term organic residue amendment practices.