Size Of Microbial Biomass Research Articles

Downsizing the Deep Biosphere

A recent study provides new constraints on the size and distribution of microbial biomass beneath the ocean floor, but key factors remain uncertain.

An overview of soil fertility management, maintenance, and productivity in Kenya

Soil fertility-related issues are a major concern to Kenya. For more than a decade, the country has experienced a declining trend in agricultural production as exemplified by low yields of major crops. Crop yields of no inputs declined by over 70% in 11 years. Continuous cropping without fertilizer caused declines in soil organic carbon and microbial biomass size and activity compared to farmyard manure alone or in combination with inorganic fertilizer. In fertilizer trials throughout the country, nitrogen (N) and phosphorus (P) were deficient in 57% and 26% of the sites, respectively. With declining productivity and soil nutrient depletion, many farmers have taken the initiative to improve the situation by use of both inorganic and organic nutrient sources such as manures, composts and traditional fallows where possible. There is strong evidence that application of fertilizer increases crop production, especially for maize. Among the bio-physical factors, a soil fertility decline remains the single most important constraint to food security in this region. After soil moisture stress, low soil fertility is the most important constraint limiting crop productivity in sub-Saharan Africa.

Carbon mineralization kinetics and soil biological characteristics as influenced by manure addition in soil incubated at a range of temperatures

This study was conducted to investigate the effects of incubation temperature on mineralization of native pools of C from a soil with a history of manure application, compared to a non-manured soil. Net C mineralization, microbial community structure, biomass size, and metabolic quotient ( qCO 2) were measured. Mineralization at cooler temperatures followed zero-order kinetics, indicating a non-limiting supply of substrate. First-order kinetics dominated at warmer temperatures as substrate supply increasingly limited microbial respiration. The soil with a history of manure application had a larger microbial biomass than the non-manured soil, and higher rates of C mineralization. There was a trend toward decreased biomass sizes with increasing incubation temperature. Bacterial DNA T-RFLP profiles were affected by incubation temperature and time with a significant difference in community structure detected after soils had been incubated for 120 days, as well as after incubation at 35 °C. Fungal DNA T-RFLP profiles indicated a distinct community in soils incubated at 35 °C, regardless of the length of the incubation. The key findings from the study were that C mineralization from native pools of organic matter does not follow Arrhenius kinetics at high temperatures, and that incubation of soils outside of their normal temperature range can alter soil biological characteristics which may impact estimates of mineralization parameters.

Glucose promotes the reduction of hexavalent chromium in soil

In this study we evaluated the dynamics of Cr(VI) reduction in contaminated soil amended or not with glucose together with investigation on its effects on soil microbial biomass size and activity. The influence of native or glucose-induced soil microbial biomass on chromate reduction was assessed. During the incubation period, the concentration of Cr(VI) in soil decreased over time whether glucose was added to soil or not, but with different reduction rates. Soil displayed a natural capacity towards chromate reduction, but the addition of glucose increased the reduction rate by both abiotic and biotic mechanisms. Cr(VI) negatively affected both the size and activity of soil microbial biomass. However, our data suggest that glucose is likely to have exerted an indirect role in the increased rate of Cr(VI) reduction, by promoting growth of indigenous microbial biomass.

SOIL BIOLOGICAL AND BIOCHEMICAL QUALITY OF WHEAT-MAIZE CROPPING SYSTEM IN LONG-TERM FERTILIZER EXPERIMENTS

SUMMARYTwo long-term field experiments, each consisting of three treatments (organic compost treatment, chemical fertilizer treatment and an untreated control) were established in 1993 and 1997, respectively. Soil samples were collected from each plot in June 2004 and 2005 after crop harvest and were used to determine soil physical-chemical properties, biological and biochemical activity, and the nematode community. Soil physicochemical parameters, microbial biomass, biological activities and nematode communities were significantly influenced by long-term application of organic compost. In general, soil total organic carbon, dissolved organic carbon, total nitrogen, alkaline-hydrolysable nitrogen, available phosphorus, and available potassium, microbial biomass, basal respiration, urease activities, total number of nematodes and bacterial-feeding nematodes were significantly higher in the compost plots than in the chemical fertilizer and control plots at two experimental sites and two sampling dates. Soil bulk density and pH values were significantly lower in the compost plots. We conclude that soil physical-chemical properties, size and activity of soil microbial biomass, metabolic quotient (qCO2), urease activity, total number of nematodes and bacteria-feeding nematodes could be used as indicators of soil quality.

Long-term exclusion of plant-inputs to soil reduces the functional capacity of microbial communities to mineralise recalcitrant root-derived carbon sources

Microbial communities in soil are highly species-rich, recognition of which has led to the view that functional redundancy within communities may buffer many impacts of altered community structure on soil functions. In this study we investigated the impact of long-term (>50 years) exclusion of plant-inputs (bare-fallow treatment) on soil microbial community structure and on the ability of the microbial biomass to mineralise tracer additions of 13C-labelled plant-derived C-substrates. Exclusion of plant-inputs resulted in depletion of soil organic matter (SOM) and a reduction in microbial biomass size. The microbial community structure was also strongly affected, as indicated by the distinct phospholipid fatty acid (PLFA) profiles in bare-fallow and grassland soils. Mineralisation of labile plant-derived substrates was not perturbed by the bare-fallow treatment. The incorporation of labile plant-derived C into PLFA biomarkers was found to differ between soils, reflecting the distinct community structures of the soils and indicating that these substrates were utilised by a broad range of microbial groups. In contrast, the mineralisation of recalcitrant plant-derived substrates was reduced in bare-fallow soil and the fate of substrate-derived C within PLFA biomarkers was, initially, similar between the soils. These results indicate that utilisation of these recalcitrant substrates was a function restricted to specific groups, and that exclusion of plant-derived inputs to soil had reduced the capacity of bare-fallow microbial communities to utilise this substrate type. Therefore, the study suggests that long-term selective pressure on microbial communities, resulting in altered community structure, may also result in altered functional attributes. This structure–function relationship was apparent for utilisation of recalcitrant plant-derived substrates, but not for the more widely distributed attribute of labile C-substrate utilisation.

Microbial community abundance and structure are determinants of soil organic matter mineralisation in the presence of labile carbon

Altered rates of native soil organic matter (SOM) mineralisation in the presence of labile C substrate (‘priming’), is increasingly recognised as central to the coupling of plant and soil-biological productivity and potentially as a key process mediating the C-balance of soils. However, the mechanisms and controls of SOM-priming are not well understood. In this study we manipulated microbial biomass size and composition (chloroform fumigation) and mineral nutrient availability to investigate controls of SOM-priming. Effects of applied substrate ( 13C-glucose) on mineralisation of native SOM were quantified by isotopic partitioning of soil respiration. In addition, the respective contributions of SOM-C and substrate-derived C to microbial biomass carbon (MBC) were quantified to account for pool-substitution effects (‘apparent priming’). Phospholipid fatty acid (PLFA) profiles of the soils were determined to establish treatment effects on microbial community structure, while the 13C-enrichment of PLFA biomarkers was used to establish pathways of substrate-derived C-flux through the microbial communities. The results indicated that glucose additions increased SOM-mineralisation in all treatments (positive priming). The magnitude of priming was reduced in fumigated soils, concurrent with reduced substrate-derived C-flux through putative SOM-mineralising organisms (fungi and actinomycetes). Nutrient additions reduced the magnitude of positive priming in non-fumigated soils, but did not affect the distribution of substrate-derived C in microbial communities. The results support the view that microbial community composition is a determinant of SOM-mineralisation, with evidence that utilisation of labile substrate by fungal and actinomycete (but not Gram-negative) populations promotes positive SOM-priming.

Amendment Placement Directs Soil Carbon and Nitrogen Cycling in Severely Disturbed Soils

The recovery of ecosystem processes in severely disturbed systems is often limited by biological resources in the soil. The objective of this study was to direct soil microbial biomass (SMB) size and activity with organic amendments. These amendments were applied to the soil at different amendment locations (incorporated versus surface-applied) and amounts (none, light, and heavy) in a 2 × 3 factorial design. The size and activity of SMB, soil nutrients, and aboveground biomass were monitored over 3 years to determine the rate and direction of change. Contrary to expectations that SMB and carbon mineralization potential (C-MIN) would be larger with amendment incorporation, SMB-carbon was greatest in the surface-heavy treatment and lowest in the incorporated-control treatment. SMB-nitrogen, C-MIN, and organic carbon were greater in the surface than in the incorporated treatments and in amended plots compared to controls. This departure from expectations suggests that other factors, such as microclimate or vegetation, are interacting with the amendment to affect SMB. The degree of contribution, however, is unclear. The treatments only affected planted aboveground biomass early in the experiment, with greater total biomass in the surface-light treatment in fall 2003. There was also a significant positive relationship between aboveground biomass and SMB in fall 2004. Inorganic nitrogen, total nitrogen, and the soil quality indicators qCO2 and Cmic/Corg did not vary systematically with amendment treatment. In general, amendment addition did enhance soil biotic properties and supported increased vegetation, but the complication of incorporating the amendment was not necessary for promoting biological development in disturbed soils.

Plant cover and epipedon SOM stability as factors affecting brown soil profile development and microbial activity

This study aimed to evaluate the role of different plant covers on brown soil profile development changing SOM stability in the epipedons. In particular, two brown soils (Eutric Cambisols), located in Monghidoro at the Italian Apennines, were analysed in order to test the hypothesis that plant cover affects the brown soil profile development under eutric qualifier changing SOM stability and soil pH. Also we verified if soil biochemical activities and microbial biomass size could be promoted by factors supporting Eutric Cambisols development. The two soil profiles were formed under coniferous (M con) and beech cover (M beech), respectively; both of them had similar lithology, morphology, exposition, slope and elevation. The development of soil profiles was evaluated by the determination of their physical, chemical and biochemical properties, while SOM stability of the epipedons was assessed through the determination of some properties of humic substances by thermal (TG-DSC) and spectroscopic (FTIR) analysis, humification indexes and eco-physiological indicators such as microbial quotient (C mic:C org) and metabolic quotient (qCO 2). The C org content decreased along both soil profiles. The epipedon of M beech soil was richer in SOM that in turn showed a higher level of humification than M con. The pH value, CEC and the concentrations of all earth-alkaline cations were also higher in M beech than in M con soil profile. Differences in soil reaction and SOM quality seemed to affect weathering process, since the content of all Al forms (amorphous—Alo, crystalline—Ald and linked to SOM—Alp) increased as the soil pH decreased, and the Fe forms increased as the humification level of SOM increased. Nevertheless, the active iron ratio (Feo/Fed) calculated in the two soil profiles did not highlight significant differences between them in terms of podzolitation intensity. The activity of hydrolytic enzymes (β-glucosidase, cellulase and chitinase) was higher in M beech soil than in M con soil profile. The activity of all enzymes, on mass basis, decreased along both soil profiles, while the specific activity (per unit of organic carbon), increased only in the endopedons of M beech. Although biochemical properties were not correlated with physical and chemical properties of soil profile, significant correlations were observed between enzyme activity and the soil organic carbon or nitrogen content. As far as the epipedons SOM quality is concerned, the content of humified C, the microbial biomass carbon (C mic), as well as the C mic:C org were higher in M beech than in M con, whereas the qCO 2 was lower in M beech than in M con. Also the FTIR and TG-DSC analysis showed differences on HAs and FAs hydrophilic character and thermal stability between epipedons of M beech and M con. The SOM quality of epipendon under coniferous resulted in a more recalcitrant and hydrophobic composition than that of epipedon under beech. In conclusion the type of plant cover (coniferous vs. beech-tree) affected the weathering of the pedogenetic substrate changing the SOM properties and soil pH. Even if the beech-tree cover promoted SOM accumulation in soil profile, which represented the organic substrates for microbial biochemical activities, the weathering of soil under coniferous appeared more marked than beech-trees covered soil. Consequently, soil biochemical activity of microbial biomass along soil profile was not promoted by factors supporting weathering processes.

Impact of Salinity on Soil Biological Activities: A Laboratory Experiment

In a laboratory study, the impact of sodium chloride (NaCl) on soil quality was examined through the monitoring of soil biological activity. Artificially salinized samples were prepared from the nonsaline soil by adding NaCl at electrical conductivities (EC) 2, 4, and 8 dS·m−1 in saturated extracts. The samples were kept at 25 °C and at 50% field capacity during an incubation period of 40 days. The ATP, soil basal respiration, protease, amylase, alkaline phosphatase, dehydrogenase, and catalase activities were monitored. The biological index of fertility (BIF), the enzyme activity number (EAN), and the metabolic potential (MP) were calculated. A regression analysis was used to calculate parameters from cumulative data of carbon dioxide (CO2) evolution. The size of microbial biomass, measured throughout the determination of ATP, was decreased by increasing salinity. Increasing concentrations of salt up to an EC of 4 dS·m−1 led to an increase of soil respiration. During incubation, protease and dehydrogenase were inhibited by NaCl; however, amylase, alkaline phosphatase, and catalase were not affected by the salt addition. Between indices, EAN confirmed the general depressive effect of NaCl on the biological properties of soil, while MP showed a pattern similar to that of soil respiration. Results of this study chiefly indicate that ATP, soil respiration, protease, dehydrogenase, EAN, and MP were able to put in evidence the effects of NaCl on soil biological activity and may be regarded as suitable tools to show the physiological reaction of soil microbial biomass under saline stress.

Relationships between soil organic matter and the soil microbial biomass (size, functional diversity, and community structure) in crop and pasture systems in a semi-arid environment

The quantity and/or quality of soil organic matter (SOM) and its fractions regulate microbial community composition and associated function. In this study an established, replicated agricultural systems trial in a semi-arid environment was used to test: (i) whether agricultural systems which have increased plant residue inputs increase the amount of labile SOM relative to total SOM, or change the quality of SOM fractions; and (ii) whether the size or quality of OM fractions is most strongly linked to the size, activity, functional diversity, and community structure of the soil microbial biomass. Soil (0–50 mm) was collected following 5 years of continuous wheat, crop rotation, crop–pasture rotation, annual pasture, or perennial pasture. Pastures were grazed by sheep. Direct drilling and non-inversion tillage techniques were compared in some cropping systems. Total carbon (C) increased with the proportion of pasture as a result of increased SOM inputs into these systems; land use also significantly affected SOM fractions and their chemical and physical nature. While the size, function, and structure of the soil microbial community were somewhat related to total soil C, they were better correlated with SOM fractions. The C : nitrogen (N) ratio of light fraction organic matter could be used to predict the amount of potentially mineralisable N in soil, while the C : N ratio of total SOM could not. Measurement of bacterial community structure (using denaturing gradient gel electrophoresis) significantly discriminated between land uses, while community-level physiological profiles revealed fewer differences. Overall, our findings support the premise that labile fractions of SOM are more strongly related to microbial community structure and function than is total SOM.

Soil Microbial Community Dynamics as Influenced by Composted Dairy Manure, Soil Properties, and Landscape Position

Manure applications can benefit crop productivity by adding required nutrients and organic matter to soil. There is a paucity of infor­ mation on how soil microbial community dynamics will be altered by the application of manure to different landscape positions. Thus, an in situ field study was conducted during the summer and winter months to eval­ uate microbiological properties of three soil types that have evolved be­ cause of different landscape positions in an agricultural field. The three Coastal Plain soils investigated were Bama (sandy loam), Lynchburg (loam), and Goldsboro (loam) representing the landscape position of a summit, drainageway, and sideslope, respectively. Composted dairy manure was incorporated into in situ soil cores at a rate of 350 kg N ha j1 and com­ pared with unamended controls. Soil microbial biomass N and dehydro­ genase enzyme activity were determined to evaluate changes in the microbial biomass size and activity, whereas phospholipid fatty acid analy­ sis was used as an indicator of the microbial community structure. Ad­ dition of composted dairy manure increased microbial activity and N immobilization, representing a shift in microbial response resulting from changes in substrate availability. This was most evident during summer months, with the composted dairy manure increasing dehydrogenase en­ zyme activity 21% and microbial activity 20% compared with without manure, suggesting that seasonal timing of application will influence mi­ crobial activity. Microbial properties were also impacted by landscape position. The drainageway landscape position soil, a loam, had the highest microbial biomass and microbial activity. Changes in microbial commu­ nity structure using phospholipid fatty acid profiles were evaluated with canonical discriminate analysis. This analysis indicated that a shift in microbial community structure occurred between season, manure appli­ cation, and landscape position. Findings from this study suggest that changes in soil variability from landscape positions and season can impact the growth and dynamics of the microbial community when manure is applied to agricultural fields.

Decomposition and stabilization of root litter in top- and subsoil horizons: what is the difference?

Mechanisms leading to high mean residence times of organic matter in subsoil horizons are poorly understood. In lower parts of the soil profile root material contributes greatly to soil organic matter (SOM). The objective of this study was to elucidate the decomposition dynamics of root-derived C and N in different soil depths during a 3 year field experiment and to examine the importance of different protection mechanisms as well as abiotic factors for the decomposition dynamics. Additionally, we assessed the effect of root litter addition on native SOM. Our conceptual approach included the exposure of litterbags with 13C and 15N labeled wheat root material mixed to loamy agricultural soil at three different soil depths (30, 60 and 90 cm). During the incubation period, we monitored soil temperature, humidity and the incorporation of root derived C and N into the soil microbial biomass and physical SOM fractions. Our results showed that abiotic decay conditions were better in subsurface horizons compared to the topsoil. Root litter addition significantly increased the size of microbial biomass in all three soil horizons. In the topsoil, root-derived C decomposition was significantly higher in the first 6 months of incubation compared to subsoil horizons. In 60 and 90 cm depths, a lag phase with development of soil microbial biomass seemed to be prevailing before decomposition was activated. For root-derived N, similar decomposition kinetics could be observed in top- and subsoil horizons. Despite of higher SOM contents, better soil structure and higher microbial activity in the topsoil horizon compared to subsoil horizons, the amounts of root-derived C and N remaining after 3 years were similar for all three depths. Most of the root-derived C and N was present as organo-mineral complexes or occluded in soil aggregates (oPOM), illustrating similar importance of these two protection mechanisms in all three soil depths. Addition of fresh root litter caused small losses of native soil C whereas native N was retained. We conclude that despite of similar SOM protection mechanisms, there are distinct differences in decomposition dynamics of root litter between top- and subsoil horizons. In the long run, the better abiotic decay conditions prevailing in subsoil horizons may compensate for their poorer physico-chemical characteristics.

The microbial biomass and activity in soil with shrub ( Caragana korshinskii K.) plantation in the semi-arid loess plateau in China

Caragana korshinskii K. is a shrub species which is adapted to arid and semi-arid environments and plays an important role in soil protection. The objective of this study was to determine the influence of this shrub plantation on the soil ecosystem functions driven by microorganisms in the long-term. The changes in the size and activity of soil microbial biomass and the relationship between soil microbial biomass and chemical properties were investigated under shrub plantations aged 6, 18 and 26 years. The results showed that the pH value in the soil decreased gradually, while soil organic carbon (OC) and total nitrogen (TN) significantly increased with the age of C. korshinskii. Although microbial biomass carbon (MBC) and MBC/OC ratio gradually increased, the ratio of basal respiration to MBC (qCO 2) decreased with the age of C. korshinskii. The microbial biomass nitrogen (MBN) and MBC had a positive relationship with soil TN and OC, respectively. The flux of CO 2 decreased with the age of C. korshinskii which had a significant negative relationship with soil OC, TN, MBN and MBC. The results indicate that C. korshinskii plantations may help to improve microbially driven ecosystem functioning through long-term creation of resource-island.

Size, activity and catabolic diversity of the soil microbial biomass in a wetland complex invaded by reed canary grass

Reed canary grass (Phalaris arundinacea, L.) invasion of wetlands is an ecological issue that has received attention, but its impact on soil microbial diversity is not well documented. The present study assessed the size (substrate-induced respiration), catabolic diversity (CLPP, community level physiological profiles) and composition (selective inhibition) of the soil microbial community in invaded (>95% P. arundinacea cover) and in non-invaded areas of a wetland occupied by native species grown either as a mixed assemblage (22 species) or as quasi-monotypic stands of Scirpus cyperinus (74% cover). The study also tested the hypothesis that decomposition of lignin- and phenolics-rich plant tissues would be fastest in soils exhibiting high catabolic diversity. Results showed that soil respiration, microbial biomass and diversity were significantly higher (P < 0.03; 1.5 to 3 fold) in P. arundinacea-invaded soils than in soils supporting native plant species. Fungal to bacterial ratios were also higher in invaded (0.6) than in non-invaded (0.4) plots. Further, canonical discriminant analysis of CLPP data showed distinct communities of soil decomposers associated with each plant community. However, these differences in microbial attributes had no effect on decomposition of plant biomass which was primarily controlled by its chemical composition. While P. arundinacea invasion has substantially reduced plant diversity, this study found no parallel decline in the size and diversity of the soil microbial community in the invaded areas.

Mineralization dynamics and biochemical properties during initial decomposition of plant and animal residues in soil

The use of organic residues as soil amendments or fertilisers may represent a valuable recycling strategy. In this study, a series of laboratory assays was performed to study the effects of the application of organic residues on C and N mineralization and biochemical properties in a Mediterranean agricultural soil. Two crop residues (straw and cotton) and two animal by-products (meat bone meal and blood meal) were added at three rates (5, 10 and 20 mg g −1 on dry weight basis) to a moist (40% water holding capacity) sandy soil and incubated at 20 °C for 28 days. Each residue underwent a different mineralization pattern depending on the nature and complexity of its chemical constituents. In all cases, the addition of the waste produced, after a short lag-phase, an exponential increase in the soil respiration rate, reflecting the growth of microbial biomass. The amount of total extra CO 2-C evolved after 28 days, expressed as % in respect to added C, differed significantly ( P < 0.005) among application doses: 5 > 10 > 20 mg g −1 and residue type: meat bone meal > blood meal > cotton cardings > wheat straw. Plant residues led to a rapid immobilisation of N that affected microbial size and activity and further mineralization. Animal by-products produced an immediate and remarkable increase of mineral N in the soil. However, the large amounts of NH 4 + released in the soil at high rates of animal residues led, in some cases, to temporary adverse effects on microbial biomass growth and nitrification. All residues produced a significant increase in soil microbial biomass size and activity, being the intensity of the response related to their chemical properties.

The effect of initial concentration, co-application and repeated applications on pesticide degradation in a biobed mixture

A 180 d laboratory experiment was conducted to investigate the degradation rates of chlorpyrifos (10 and 50 mg kg −1) and metalaxyl (100 mg kg −1) separately and co-applied in a biomix constituted by topsoil, vine-branches and urban-waste-garden compost. The effect of repeated application of metalaxyl was also investigated. Microbial biomass-C (MBC) content and metabolic quotient ( qCO 2) were measured to evaluate changes in microbial biomass size and activity induced by the presence of the two pesticides. Degradation rate decreased with increasing concentration of chlorpyrifos in all treatments. Metalaxyl half-life was significantly reduced in co-application with chlorpyrifos indicating a synergic interaction between the two pesticides in favour of enhanced degradation rate for metalaxyl but not for chlorpyrifos. Furthermore, repeated application resulted in a sharp reduction of metalaxyl half-life from 37 d after first application to 4 d after third application. MBC content was negatively influenced by the addition of pesticides but it started to recover immediately, in both separate and co-applied treatments, reaching the control value when pesticide residues were about 50% of the initial concentration. The qCO 2 reached a steady-state after about 20 d in separately applied and 40 d in co-applied treatments, indicating a tendency to arrive at a new metabolic equilibrium. In conclusion, the biomix tested has been shown to degrade pesticides relatively fast and to have a microbial community that is varied enough to allow selection of those microorganisms able to degrade metalaxyl and chlorpyrifos.

Effect of heat-induced disturbance on microbial biomass and activity in forest soil and the relationship between disturbance effects and microbial community structure

An important aspect of ecosystem sustainability is the ability to withstand and recover from disturbance or stress. In this study, we investigated the effect of a heat-disturbance on soil microbial biomass, microbial activity in response to the addition of organic acid (malate), and microbial community structure in a laboratory experiment. The soils investigated were from a jarrah ( Eucalyptus marginata) forest that had undergone rehabilitation following bauxite mining 12 years previously. Soils from a full factorial of two field treatments; contour ripping induced micro-topography (mound or furrow) and prior exposure to prescription fire (burnt or non-burnt), were sampled and found to exhibit treatment-dependent differences in soil biological and chemical properties. Exposure of soil microcosms from each field treatment to 90 °C and 120 °C for 20 min, resulted in a significant loss of microbial biomass C and malate-induced respiration in all soils, compared with non-disturbed control microcosms maintained at 15 °C. No significant changes were recorded following the exposure to 60 °C. There were no significant differences in the immediate loss of microbial biomass relative to control soils between field treatments (microbial biomass was approximately halved in size in all soils) and the microbial biomass size did not recover during 28 days of incubation. However, the resistance of malate-induced respiration following heat-disturbance was greater in the non-burnt soils, and in particular non-burnt mound soil. Multivariate canonical correlation analysis revealed a significant relationship between the resistance of malate-induced respiration and bacterial community structure (by PCR-DGGE and to a lesser extent by PLFA profiles). No relationship was found between the resistance of malate-induced respiration and other soil chemical and biological properties. The resilience of malate-induced respiration was significantly affected by the soil micro-topography and only non-burnt furrow soil recovered to the respiration rate of the non-disturbed control. Resilience was positively related to the size of the microbial biomass but not related to the microbial community structure.

Microbial indicators related to soil carbon in Mediterranean land use systems

Soil C mineralization activity and microbial indices were measured in agricultural, forest and grassland soils, characterized by different managements in two adjacent fields: agricultural (conventional vs. organic), forest (conifer vs. broadleaf), grassland (naturally grazed grass vs. alfalfa ungrazed). The aim of the study was to determine if the land use and the management practices modified C mineralization activity and kinetics, microbial biomass size, microbial and metabolic quotients ( C mic/ C org ratio and qCO 2). Land use induced significant changes in microbial biomass content, in most of the microbial indices, and in the cumulative CO 2 production which showed the highest values for agricultural soils (300 μg C–CO 2 g −1 28 d −1) and the lowest for grassland soils (120 μg C–CO 2 g −1 28 d −1). In agricultural soils, a large availability of potentially mineralizable C ( C 0) was determined. Forest soils mineralization activity was mainly dependent on environmental factors such as aboveground tree species and soil pH which probably induced changes in microbial community structure and/or functionality. This could also explain the significant differences found on chemical, biochemical and microbiological properties of the two forest soils under the two managements. Grassland soils were characterized by a high stability of soil organic matter (SOM) and consequently a low mineralization activity. Although total nitrogen and soil C/N ratio varied between the two soils, the management practices did not affect C mineralization activity. In conclusion the results show that microbially mediated processes can be largely affected by land use confirming its role as a significant driver of soil C changes.

Mineralization of native soil organic matter is not regulated by the size, activity or composition of the soil microbial biomass—a new perspective

Soil organic matter is extensively humified; some fractions existing for more than 1000 years. The soil microbial biomass is surrounded by about 50 times its mass of soil organic matter, but can only metabolize it very slowly. Paradoxically, even if more than 90% of the soil microbial biomass is killed, the mineralization of soil organic matter proceeds at the same rate as in an unperturbed soil. Here we show that soil organic matter mineralization is independent of microbial biomass size, community structure or specific activity. We suggest that the rate limiting step is governed by abiological processes (which we term the Regulatory Gate hypothesis), which convert non-bioavailable soil organic matter into bioavailable soil organic matter, and cannot be affected by the microbial population. This work challenges one of the long held theories in soil microbiology proposed by Winogradsky, of the existence of autochthonous and zymogenous microbial populations. This has significant implications for our understanding of carbon mineralization in soils and the role of soil micro-organisms in the global carbon cycle. Here we describe experiments designed to determine if the Regulatory Gate operates. We conclude that there is sufficient experimental evidence for it to be offered as a working hypothesis.