Measurement Of Microbial Biomass Research Articles

Relationship of Soil Respiration to Crop and Landscape in the Walnut Creek Watershed

Abstract Soil respiration is an important component of the carbon dynamics of terrestrial ecosystems. Many factors exert controls on soil respiration, including temperature, soil water content, organic matter, soil texture, and plant root activity. This study was conducted to quantify soil respiration in the Walnut Creek watershed in central Iowa, and to investigate the factors controlling this process. Six agricultural fields were identified for this investigation: three of the fields were cropped with soybean [Glycine max (L.) Merr.] and three were cropped with corn (Zea mays L.). Within each field, soil respiration was measured at nine locations, with each location corresponding to one of three general landscape positions (summit, side slope, and depression). Soil respiration was measured using a portable vented chamber connected to an infrared gas analyzer. Soil samples were collected at each location for the measurement of soil water content, pH, texture, microbial biomass, and respiration potential. Field respiration rates did not show a significant landscape effect. However, there was a significant crop effect, with respiration from cornfields averaging 37.5 g CO2 m−2 day−1 versus an average respiration of 13.1 g CO2 m−2 day−1 in soybean fields. In contrast, laboratory measurements of soil respiration potential, which did not include plant roots, showed a significant landscape effect and an insignificant cropping system effect. Similar relationships were observed for soil organic C and microbial biomass. Additional analyses indicate that corn roots may be more important than soybean roots in their contribution to surface CO2 flux, and that root respiration masked landscape effects on total soil respiration. Also, the failure to account for soil respiration may lead to biased estimates of net primary production measured by eddy covariance.

Microbial Community Structure and Density Under Different Tree Species in an Acid Forest Soil (Morvan, France)

Overexploitation of forests to increase wood production has led to the replacement of native forest by large areas of monospecific tree plantations. In the present study, the effects of different monospecific tree cover plantations on density and composition of the indigenous soil microbial community are described. The experimental site of "Breuil-Chenue" in the Morvan (France) was the site of a comparison of a similar mineral soil under Norway spruce (Picea abies), Douglas fir (Pseudotuga menziesii), oak (Quercus sessiflora), and native forest [mixed stand dominated by oak and beech (Fagus sylvatica)]. Sampling was performed during winter (February) at three depths (0-5, 5-10, and 10-15 cm). Abundance of microorganisms was estimated via microbial biomass measurements, using the fumigation-extraction method. The genetic structure of microbial communities was investigated using the bacterial- and fungal-automated ribosomal intergenic spacer analysis (B-ARISA and F-ARISA, respectively) DNA fingerprint. Only small differences in microbial biomass were observed between tree species, the highest values being recorded under oak forest and the lowest under Douglas fir. B- and F-ARISA community profiles of the different tree covers clustered separately, but noticeable similarities were observed for soils under Douglas fir and oak. A significant stratification was revealed under each tree species by a decrease in microbial biomass with increasing depths and by distinct microbial communities for each soil layer. Differences in density and community composition according to tree species and depth were related to soil physicochemical characteristics and organic matter composition.

The use of microorganisms in ecological soil classification and assessment concepts

Microbial communities are integral parts of soil and their activity is very important to the functioning of soil. Therefore, microorganisms should be included in soil quality classification and assessment concepts. The challenges of using microbial indicators are to identify the best choice among the many techniques available to assess soil quality and to convert the information obtained from the microbial indicator into a form relevant for policy makers. In this article, we present a wide range of possible microbial indicators, some of them standardized; each provides slightly different information on soil quality. Experience with the use of indicators for assessment of microbial communities and soil quality is discussed. At present, as many microbial indicators as possible should be included to gain experience. At a minimum, measures of microbial biomass, respiration, and N mineralization and a community profiling method (e.g., DGGE, PLFA, or CLPP) should be included.

Analysis of manure and soil nitrogen mineralization during incubation

Understanding the N-cycling processes that ensue after manuring soil is essential in order to estimate the value of manure as an N fertilizer. A laboratory incubation of manured soil was carried out in order to study N mineralization, gas fluxes, denitrification, and microbial N immobilization after manure application. Four different manures were enclosed in mesh bags to allow for the separate analysis of manure and soil. The soils received 0.15 mg manure N g−1 soil, and the microcosms were incubated aerobically and sampled throughout a 10-week period. Manure addition resulted in initial NH4-N concentrations of 22.1 to 36.6 mg kg−1 in the microcosms. All manured microcosms had net declines in soil mineral N. Denitrification resulted in the loss of 14.7 to 39.2% of the added manure N, and the largest N losses occurred in manures with high NH4-N content. Increased soil microbial biomass N amounted to 6.0 to 8.6% of the added manure N. While the microcosms as a whole had negative N mineralization, all microcosms had positive net nitrification within the manure bags. Gas fluxes of N2O and CO2 increased in all manured soils relative to the controls. Our results show that measurement of microbial biomass N and denitrification is important to understand the fate of manure N upon soil application.

Comparação de métodos para determinação de biomassa microbiana em dois solos

O potencial de medidas de biomassa microbiana do solo como indicadoras da bioatividade e das interferências nesse ambiente pode ser verificado pela quantidade de trabalhos publicados a esse respeito. Entretanto, a comparação entre dados de diferentes autores, localidades e solos só é possível quando o método de determinação é o mesmo. No entanto, há numerosos trabalhos que propõem diferentes métodos e cálculos. Este trabalho avaliou resultados de determinação de C microbiano obtidos por diferentes métodos de extração e digestão de amostras de dois solos e cálculos de biomassa; além de diferentes cálculos para transformação indireta de CO2 produzido por respiração basal e induzida em C microbiano, nos dois diferentes solos. Os resultados variaram bastante entre métodos e cálculos e concluiu-se que o método de Vance et al. (1987), mais freqüentemente citado na literatura, deve ser a opção para boa comparação da quantidade de C microbiano entre solos e dados da literatura.

Modeling changes in soil organic matter in Amazon forest to pasture conversion with the Century model

AbstractLand use and land cover changes in the Brazilian Amazon region have major implications for regional and even global carbon cycling. We analyzed the effects of the predominant land use change, conversion of tropical forest to pasture, on total soil C and N, using the Century ecosystem model and data collected from the Nova Vida ranch, Western Brazilian Amazon. We estimated equilibrium organic matter levels, plant productivity and residue carbon inputs under native forest conditions, then simulated deforestation following the slash and burn procedure. Soil organic matter dynamics were simulated for pastures established in 1989, 1987, 1983, 1979, 1972, 1951, and 1911. Using input data from the Nova Vida ranch, the Century model predicted that forest clearance and conversion to pasture would cause an initial decline in soil C and N stocks, followed by a slow rise to levels exceeding those under native forest. Simulated soil total C and N levels (2500 g C m−2 and 245 g N m−2 in the 0–20 cm layer) prior to conversion to pasture were close to those measured in the native forest. Simulated above‐ and below‐ground biomass for the forest and pasture were comparable with literature values from this region. The model predicted the long‐term changes in soil C and N under pasture inferred from the pasture chronosequence, but there was considerable variation in soil C stocks for pastures <20 years in age. Differences in soil texture between pastures were relatively small and could not account for much of the variability between different pastures of similar ages, in either the measured or simulated data. It is likely that much of the variability in C stocks between pastures of similar ages is related to initial C stocks immediately following deforestation and that this was the largest source of variability in the chronosequence. Internal C cycling processes in Century were evaluated using measurements of microbial biomass and soil δ13C. The relative magnitude and long‐term trend in microbial biomass simulated by the model were consistent with measurements. The close fit of simulated to measured values of δ13C over time suggests that the relative loss of forest‐derived C and its replacement by pasture‐derived C was accurately predicted by the model. After 80 years, almost 90% of the organic matter in the top 20 cm was pasture derived. While our analysis represents a single ‘case study’ of pasture conversion, our results suggest that modeling studies in these pasture systems can help to evaluate the magnitude of impacts on C and N cycling, and determine the effect of management strategies on pasture sustainability.

Comparison of chloroform fumigation-extraction, phospholipid fatty acid, and DNA methods to determine microbial biomass in forest humus

Techniques developed to measure microbial biomass in mineral soils may not give reliable results in humus. We evaluated the relationships between three techniques to estimate microbial biomass in forest humus: chloroform fumigation-extraction (CFE), total extractable phospholipid fatty acids (PLFA), and extractable DNA. There was a good relationship between PLFA and CFE ( R 2=0.96), with a slope slightly different from that previously reported for mineral soils (1 nmol PLFA corresponded to a flush of 3.2 μg C released by fumigation in humus cf. 2.4 μg C in mineral soil). There was no relationship between DNA concentration and the other two measurements of microbial biomass. This may be due, in part, to the high fungal biomass in forest humus, as DNA concentration per unit biomass is much more variable for fungi than bacteria.

Evaluation of the Capability of Aggregated Oil Sands Mine Tailings: Biological Indicators

Abstract An experiment was initiated in 1997 in northeast Alberta at the Syncrude Canada Ltd. Mildred Lake site to field test an innovative technique for reclamation of oil sand mine tailings. This technique was used to create an aggregated soil material from oil sand tailings. A plant community was successfully established on the soil material created by this technique. However, whether the site would be capable of supporting a self-sustainable ecosystem for the long-term remained a challenging issue. We evaluated the capability of these aggregated oil sand tailings by using biological indicators of the abundance and diversity of soil microbial biomass. Soil respiration rates and soil microbial biomass measurements were used to assess the abundance and activities of soil microbial communities. The ability of soil microbial biomass to utilize a diverse range of carbon substrates was used to assess the diversity of soil microbial communities. Soil biological activity increased with increasing growth of plant biomass and over time. Increasing the amount of peat moss or muskeg incorporated into the soil during reclamation resulted in higher organic carbon and nitrogen content and caused an increase in abundance and diversity of soil microbial biomass. These results indicate that measurements of soil respiration and substrate utilization by soil microbial communities may be used as biological indicators for assessing the capability of reclaimed soils. Introduction A critical component in the reclamation of oil sand tailings is to create soil materials conducive to the growth of soil microorganisms and, as a result, to stimulate the soil microbial biomass mediated nutrient cycling process following initial reclamation. Soil organic carbon dynamics is at the centre of these processes. Soil microbial biomass is largely responsible for the decomposition of soil organic matter and litters that contribute to soil nutrient pools through mineralization. Additionally, certain soil microorganisms form symbiotic associations (mycorrhizae, nodules) with plants and contribute to the overall success of plant growth. Thus the success of planting for reclamation purposes is affected by, and may be contingent upon, the quality and quantity of soil microbial communities and their activities. This paper presents the use of a few measurements of soil microbial activities as biological indicators for evaluating the capability of reclaimed soil. Materials and Methods Field Site The site, located at Syncrude Canada Ltd. Mildred Lake in northeastern Alberta, was established in 1997. Composite tailings (CT) were used as a sub-material. Five treatments were used for the top 20 cm layer. The composite tailings, amended with various amounts of peat moss, were aggregated using an aggregation technology(1). The control consisted of standard reclamation material (RM) placed over the CT base. The properties of the materials used on this site, and the treatments used, are described in Table 1. Each of the five treatments was replicated four times. A total of 20 (10 m by 10 m) plots arranged in four blocks were set up as a randomized complete block design. After the site was established, the following tree/shrub seedlings were transplanted:

Novel approach to monitoring of the soil biological quality

In this study, a new approach to interpretation of results of the simple microbial biomass and respiration measurements in the soil microbiology is proposed. The principle is based on eight basal and derived microbial parameters, which are standardized and then plotted into sunray plots. The output is visual presentation of one plot for each soil, which makes possible the relative comparison and evaluation of soils in the monitored set. Problems of soil microbiology, such as the lack of benchmarking and reference values, can be avoided by using the proposed method. We found that eight parameters provide enough information for evaluation of the status of the soil microorganisms and, thus, for evaluation of the soil biological quality. The usage of rare parameters (potential respiration PR, ratio of potential and basal respiration PR/BR, biomass-specific potential respiration PR/C bio, available organic carbon C ext, and biomass-specific available organic carbon C ext/C bio) can be recommended, besides classical and well-known parameters (microbial biomass C bio, basal respiration BR, metabolic coefficient qCO 2). The combination of basal parameters and derived coefficients can also extend our knowledge about the condition of the soil microorganisms. In monitoring the case studies presented, we observed that soils evaluated to possess good biological quality displayed generally higher values of organic carbon, total nitrogen, clay, and cation exchange capacity. The soils of good biological quality can display higher levels of contaminants. This is probably related with the higher content of organic carbon and clay in these soils.

Soil microbial community composition and land use history in cultivated and grassland ecosystems of coastal California

Phospholipid ester-linked fatty acid (PLFA) profiles were used to evaluate soil microbial community composition for 9 land use types in two coastal valleys in California. These included irrigated and non-irrigated agricultural sites, non-native annual grasslands and relict, never-tilled or old field perennial grasslands. All 42 sites were on loams or sandy loams of similar soil taxa derived from granitic and alluvial material. We hypothesized that land use history and its associated management inputs and practices may produce a unique soil environment, for which microbes with specific environmental requirements may be selected and supported. We investigated the relationship between soil physical and chemical characteristics, management factors, and vegetation type with microbial community composition. Higher values of total soil C, N, and microbial biomass (total PLFA) and lower values of soil pH occurred in the grassland than cultivated soils. The correspondence analysis (CA) of the PLFA profiles and the canonical correspondence analysis (CCA) of PLFA profiles, soil characteristics, and site and management factors showed distinct groupings for land use types. A given land use type could thus be identified by soil microbial community composition as well as similar soil characteristics and management factors. Differences in soil microbial community composition were highly associated with total PLFA, a measure of soil microbial biomass, suggesting that labile soil organic matter affects microbial composition. Management inputs, such as fertilizer, herbicide, and irrigation, also were associated with the distinctive microbial community composition of the different cultivated land use types.

Experimental investigation of the control of bacterial community composition in macrofaunal burrows

The burrows and tubes of infauna are complex and dynamic systems that promote sig- nificant heterogeneity in the ecological and geochemical properties of benthic systems. We experi- mentally evaluated the extent to which behaviors of macrofauna, including ventilation activity, the residence times of tubes in sediments and tube composition, influence the development and succes- sion of microbial communities which line macrofaunal burrows (i.e. the burrow microbial biofilm). Our experiments used a burrow mimic system, which allowed manipulation of some biological and environmental factors while holding other factors constant. The biofilm microbial community was assessed using phospholipid fatty acid (PLFA) analysis, a sensitive measure of microbial biomass and a significant tool in the identification of some taxonomic and functional groups of microorganisms in marine systems. Results suggest that the burrow mimics supported the development of microbial assemblages with phospholipid signatures that were both similar to those found in natural burrows and responsive to experimental manipulation. Burrow mimic composition significantly affected PLFA signatures, within the range of compounds and concentrations observed in natural systems. Tubes that were coated with an organic polymer were characterized by lower microbial biomass than uncoated tubes and also exhibited a substantially different PLFA signature. The residence time of tube mimics in sediments also affected the PLFA signature. Tubes with a longer residence time were characterized by higher microbial biomass, and a distinct anaerobe signature, relative to tube mimics having shorter residence times. Thus, a distinct successional sequence was observed. Surprisingly, irrigation frequency, i.e. the rate at which burrows are renewed with fresh, oxic overlying water and rid of metabolic byproducts and other solutes from the host organism and surrounding sediments, had minor effects on PLFA signature within the burrow biofilm. Our results suggest that the proper- ties of the burrow microenvironment exert fundamental controls on microbial community composi- tion in burrow biofilms and are likely to influence biogeochemical processes over short length scales in nearshore systems.

Responses of soil microbial biomass, nematode trophic groups, N-mineralization, and litter decomposition to disturbance events in the southern Appalachians

Seasonal measurements of soil microbial biomass, nematode trophic groups, net N-mineralization, net nitrification, net ammonification, litter decomposition, and soil respiration rates were examined to assess the effects of rhododendron ( Rhododendron maximum L.) removal and hurricane windthrow upon these factors. Comparisons of pre- and post-disturbance quantities of soil microbial biomass carbon ranged from 503 to 1080 μg C g soil −1 and microbial biomass nitrogen ranged from 67 to 169 μg total persulfate nitrogen g soil −1, showing no significant differences or trends in response to these disturbance events. Total nematode abundance ranged from 13.05 to 54.31 nematodes g soil −1 and showed no significant differences in response to these disturbance events, although overall nematode abundance was declining in the disturbed plots at the end of the study. Nematode trophic structure was fairly consistent across both sites and all sampling dates and exhibited no discernible trends in response to these disturbance events. Rates of net N-mineralization, nitrification, and ammonification exhibited greater variability, post-disturbance. Litter decomposition rates and soil respiration rates decreased following the disturbance events, with respiration rates in the Cut plot being significantly higher across years. Overall, the effects of these disturbance events upon soil properties were highly variable and were unrelated to the major increases in N loss that were detected from deeper soil horizons following the hurricane windthrow event.

Measurement of microbial numbers and biomass contained in thermophilic anaerobic reactors.

This paper describes the determination of the microbial population in terms of the number, biomass, and composition of single- and two-phase, laboratory-scale thermophilic (55 degrees C) anaerobic reactors under steady-state conditions. Epifluorescence microscopy with 4',6-diamidine-2-phenylindole (DAPI) as fluorochrome was used to determine the total number of microorganisms in the reactors, and autofluorescence microscopy was used to determine the total methanogenic bacteria populations. The results obtained by the direct count methods were compared to the quantity of biomass contained in the system, which was determined by volatile suspended solids. The concentration of acidogenic bacteria was estimated by subtraction of the autofluorescence results from those of the DAPI epifluorescence microscopy. The viable bacterial population was determined by plating techniques using an anaerobic chamber. The total bacterial and methanogenic populations of single-stage digesters increase when the hydraulic retention time decreases; nevertheless, the percentages of the principal bacterial groups (acidogenic and methanogenic) remain constant at 87% and 13%, respectively. In the two-stage reactors, the percentages of the acidogenic and methanogenic groups are 99% and 26% of the total population in the acidogenic and methanogenic reactors, respectively. In the single-stage reactors, biomass determinations can be used to estimate microbial concentrations, and vice versa, as there is a high positive correlation between microorganism concentration and biomass. The syntrophic relationship between the bacteria involved in the anaerobic process is a possible explanation for the low values of viable population obtained in the reactors studied. Nevertheless, there is a high correlation between direct counts by epifluorescence microscopy and viable plate counts for the combined system studied.

Response of microbial biomass to air-drying and rewetting in soils and compost

The activity of microorganisms and the availability of composting substrates for decomposition are seriously affected by drying and rewetting cycles. We have measured microbial biomass C ( B C) and ninhydrin reactive N ( B NIN) of samples of composting material taken at different times from a pile of ligno-cellulosic wastes. Dynamics of these two parameters in control samples that were kept continuously moist were compared with those of samples of the same material that had been subjected to drying and rewetting. The study was also performed on soils to investigate analogies and differences of behaviour of microbial biomass in such different substrates. Moist samples from 10 soils with different organic C content (6.6–41.9 g kg −1 soil) were analyzed for their B C and B NIN content. The soils were then air-dried, rewetted and incubated at 25 °C for 10 days. On days 1, 3, 6 and 10 of incubation, samples were analyzed for B C and B NIN content. Compost samples from different composting stages of a mixture of cotton carding and yard wastes were air-dried, rewetted and incubated at 25 °C for 12 days, together with the corresponding moist samples (control). On days 1, 5 and 12 of incubation period, samples were analyzed for B C and B NIN content. The regression coefficient between B C and B NIN for all the compost data (continuously moist and rewetted) was 21.4 ( r=0.89 P<0.001), very close to values normally found in soils, confirming the reliability of microbial biomass measurements performed on composting substrates. Soil drying caused on average a decrease in the size of microbial biomass with respect to moist samples of 13% for B C and 30% for B NIN. B C and B NIN of moist and rewetted soil samples were always significantly correlated. Microbial biomass content of rewetted compost samples up to 19 days from the beginning of the process were significantly different, but highly correlated with moist controls. No significant differences between moist and rewetted samples (RW) were found in samples collected during the maturing and curing phases. The different response to drying of soils and compost could be related to the greater increase of new available substrate in compost with respect to soil following the drying–rewetting treatment. Dynamics of B C/ B NIN ratio in compost was coherent with the normal trend observed in the composition of microbial community during the process, from prevailing bacteria and actinomycetes to prevailing fungi.

Measurements and Modeling of Carbon and Nitrogen Cycling in Agroecosystems of Southern Wisconsin: Potential for SOC Sequestration during the Next 50 Years

Landmanagement practices such as no-tillage agriculture and tallgrass prairie restoration have been proposed as a possible means to sequester atmospheric carbon, helping to refurbish soil fertility and replenish organic matter lost as a result of previous agricultural management practices. However, the relationship between land-use changes and ecosystem structure and functioning is not yet understood. We studied soil and vegetation properties over a 4-year period (1995–98), and assembled measurements of microbial biomass, soil organic carbon (SOC) and nitrogen (N), N-mineralization, soil surface carbon dioxide (CO2) flux, and leached C and N in managed (maize; Zea mays L.) and natural (prairie) ecosystems near the University of Wisconsin Agricultural Research Station at Arlington. Field data show that different management practices (tillage and fertilization) and ecosystem type (prairie vs maize) have a profound influence on biogeochemistry and water budgets between sites. These measurements were used in conjunction with a dynamic terrestrial ecosystem model, called IBIS (the Integrated Biosphere Simulator), to examine the long-term effects of land-use changes on biogeochemical cycling. Field data and modeling suggest that agricultural land management near Arlington between 1860 and 1950 caused SOC to be depleted by as much as 63% (native SOC approximately 25.1 kg C m−2). Reductions in N-mineralization and microbial biomass were also observed. Although IBIS simulations depict SOC recovery in no-tillage maize since the 1950s and also in the Arlington prairie since its restoration was initiated in 1976, field data suggest otherwise for the prairie. This restoration appears to have done little to increase SOC over the past 24 years. Measurements show that this prairie contained between 28% and 42% less SOC (in the top 1 m) than the no-tillage maize plots and 40%–47% less than simulated potential SOC for the site in 1999. Because IBIS simulates competition between C3 and C4 grass species, we hypothesized that current restored prairies, which include many forbs not characterized by the model, could be less capable of sequestering C than agricultural land planted entirely in monocultural grass in this region. Model output and field measurements show a potential 0.4 kg C m−2 y−1 difference in prairie net primary production (NPP). This study indicates that high-productivity C4 grasslands (NPP = 0.63 kg C m−2 y−1) and high-yield maize agroecosystems (10 Mg ha−1) have the potential to sequester C at a rate of 74.5 g C m−2 y−1 and 86.3 g C m−2 y−1, respectively, during the next 50 years across southern Wisconsin.

Carbon Control on Nitrogen Dynamics in the Forest Floor of an N-Enriched Deciduous Forest Ecosystem

Recent evidence from nitrogen (N) saturation studies indicates that forest floors in moderately impacted forests are the primary sink for atmospheric N inputs. Some researchers have suggested that the sink capacity of organic horizons is dependent on the amount of available carbon (C), which can be used for microbial N assimilation. To test the hypothesis that C limitation in forest floors exposed to chronic N deposition leads to an enhanced N leaching, a field C input manipulation experiment is under way in a deciduous forest. Since September 1999 aboveground C input has been doubled (by doubling litter input or by amending glucose) or excluded in replicated plots. Here we report the short-term response of concentrations of dissolved inorganic N (DIN: NO3−-N and NH4+-N) in forest floor percolate to the C input manipulation. In autumn following the C input manipulation, DIN concentrations in forest floor percolate decreased in all plots except the No Litter plots compared to the pre-treatment summer concentrations. In contrast, the concentrations of DIN in the No Litter plots remained high. A different seasonal pattern of DIN leaching among treatments, along with measurements of microbial biomass C and potential nitrification rates of forest floor samples, indicates that seasonal N dynamics in the forest floor are largely regulated by C availability changes assoicated with litterfall C input.

The relationship between microbial carbon and the resource quality of soil carbon.

The biological health of soil is an important aspect of soil quality because of the many critical functions performed by organisms in soil. Various indicators of soil quality have been proposed, but measurements of microbial biomass are most commonly used. During decomposition of plant residues in soil the relative intensities of the O-alkyl-C signal decreases and the alkyl-C signal increases in nuclear magnetic resonance (NMR) spectra. This leads to the suggestion that the alkyl-C to O-alkyl-C ratio of a soil may indicate the degree of decomposition. Consequently, the overall resource quality of soil C as a substrate for heterotrophic microorganisms may be inversely related to the alkyl-C to O-alkyl-C ratio. Our hypothesis is that a relationship exists between the size of the soil microbial community (microbial biomass) and the quality of soil carbon as a resource for microorganisms. New data have been combined with previously published data to show that there was a significant, negative correlation between the biomass C to total C (Cmic, to Corg) ratio and the alkyl-C to O-alkyl-C ratio (p < 0.01), which supports our hypothesis.

Nitrogen Mineralization Pattern of an Oxisol of Guadeloupe, French West Indies

It is generally assumed that Oxisols have a low level of soil organic matter (SOM) and a high rate of SOM turnover due to the high temperatures in the tropics. This work was carried out to test this assumption by analyzing the kinetics and the temperature–moisture response of N mineralization and nitrification in a neutral (pH 6.2, organic N 1.5 g kg−1, bare for 10 yr) and an acid (pH 4.7, organic N 2.3 g kg−1, bare for 1 yr) Oxisol. Samples of these soils were incubated using a factorial design of temperature (20, 30, 40, and 50°C) by soil water (30, 200, and 1500 kPa). The mineralization pattern was related to measurements of microbial biomass, SOM light‐fraction, and the number of nitrifiers. Mineralization increased continuously with the increase of temperature and water content. No nitrification was observed at 50°C and at 1500 kPa. Although mineralization was well described by the double exponential model, nitrification showed a linear pattern with time. The size of the mineralizable N pool and N mineralization were different between the two soils, but microbial activity and microbial biomass were almost identical in both soils. Mineralization was very slow at 20 to 30°C (constant rate ≈ 10−4–10−5 d−1) and was limited by a relatively small (light‐fraction C = 8–10% of organic C) and low active mineralizable fraction. At &gt;40°C, the constant rate of mineralization (≈10−3 d−1) was equivalent to those found in temperate soils at 25 to 35°C. Our findings do not support the assumption of a fast SOM turnover at high temperatures. Nitrifiers constitute a small population (≈20 cells g−1) well adapted to changes in moisture and substrate availability. However, NH+4 was always present in soils, which was associated with the low number of nitrifiers. Microbial biomass decreased sharply with an increase in temperature and was not related to N mineralization. Thermal denaturation or changes in microbial population could cause this decrease.

Aerobic and Anaerobic Transformations of Pentachlorophenol in Wetland Soils

Strategies to enhance biotransformation of pentachlorophenol (PCP) in a spectrum of wetland soils were investigated under laboratory conditions, which included manipulations of electron acceptors and donors, and PCP concentrations. Maximum transformation rates were found at PCP concentrations &lt;10 μM (methanogenic conditions) and &gt;6 μM to &gt;23 μM (aerobic conditions). Differences in PCP toxicity and sorption among soils and treatments were largely governed by the activities of microbial groups. Within this concentration range, transformation was observed in soils under aerobic and methanogenic conditions, but was inhibited under denitrifying and SO2−4–reducing conditions. Aerobic PCP transformation initially produced small amounts of pentachloroanisole (PCA). However &gt;75% of both chemicals disappeared in 30 d from five soils. Measured soil properties were not significantly correlated to aerobic transformation rates. Under methanogenic conditions, PCP was reductively dechlorinated to yield a mixture of tetra‐, tri‐, and dichlorophenols in eight soils, with rates strongly correlated to measures of electron donor supply (total C, N, organic C mineralization rates) and microbial biomass. Addition of protein‐based electron donors enhanced reductive dechlorination in a soil low in organic matter and microbial biomass. Results demonstrated the widespread occurrence of PCP transforming microorganisms in soils, which may be promoted by manipulating environmental conditions.

Influence of petroleum hydrocarbon contamination on microalgae and microbial activities in a long-term contaminated soil.

Petroleum hydrocarbons are widespread environmental pollutants. Although biodegradation of petroleum hydrocarbons has been the subject of numerous investigations, information on their toxicity to microorganisms in soil is limited, with virtually no work conducted on soil algae. We carried out a screening experiment for total petroleum hydrocarbons (TPH) and their toxicity to soil algal populations, microbial biomass, and soil enzymes (dehydrogenase and urease) in a long-term TPH-polluted site with reference to an adjacent unpolluted site. Microbial biomass, soil enzyme activity, and microalgae declined in medium to high-level (5,200-21,430 mg kg(-1) soil) TPH-polluted soils, whereas low-level (<2,120 mg kg(-1) soil) pollution stimulated the algal populations and showed no effect on microbial biomass and enzymes. However, inhibition of all the tested parameters was more severe in soil considered to have medium-level pollution than in soils that were highly polluted. This result could not be explained by chemical analysis alone. Of particular interest was an observed shift in the species composition of algae in polluted soils with elimination of sensitive species in the medium to high polluted soils. Also, an algal growth inhibition test carried out using aqueous eluates prepared from polluted soils supported these results. Given the sensitivity of algae to synthetic pollutants, alteration in the algal species composition can serve as a useful bioindicator of pollution. The results of this experiment suggest that chemical analysis alone is not adequate for toxicological estimations and should be used in conjunction with bioassays. Furthermore, changes in species composition of algae proved to be more sensitive than microbial biomass and soil enzyme activity measurements.