Microbial biomass and activity under oxic and anoxic conditions as affected by nitrate additions

Soil microbial activity, biomass, and community structure were examined during the transition from oxic to anoxic conditions after the addition of glucose and with or without nitrate addition. In two sets of treatments, samples were incubated for up to 35 d in closed ampoules either aerobically until oxygen was depleted or anoxically throughout the experiment. Heat‐flow rate was monitored to indicate microbial activity. Microbial biomass and community structure were measured by adenylate and phospholipid fatty acid (PFLA) content, and adenylate energy charge (AEC) was used to monitor the physiological status of the microbial biomass. Microbial activity was highest under oxic conditions and abruptly decreased under anoxic conditions. Activity peaks were observed after about 9 d of anoxic conditions probably triggered by increased nutrient availability from dying microbial biomass, but these peaks were smaller after initial oxic incubation or nitrate addition. Microbial biomass was unchanged under oxic conditions but decreased under anoxic conditions. Most surviving microbes switched into dormancy. Changes in the microbial‐population structure were small and occurred only after 9 d of anoxic incubation. The results show that the nutrient status and the availability of electron acceptors such as nitrate were important factors ruling the direction and the extent of shifts in the microbial activity and community structures due to anoxic conditions.     

Microbial responses to fluctuation of soil aeration status and redox conditions

 The response of the microbial community to changes in aeration status, from oxic to anoxic and from anoxic to oxic, was determined in arable soil incubated in a continuous flow incubation apparatus. Soil incubated in permanently oxic (air) and/or anoxic (O₂-free N₂) conditions was used as the control. Before experiments soil was preincubated for 6 days, then aeration status was changed and glucose added. Glucose concentration, extractable C, CO₂ production, microbial biomass, pH and redox potential were determined 0, 4, 8, 12, 16, 24, 36 and 48 h after change of aeration status. If oxic conditions were changed to anoxic, the amount of glucose consumed was reduced by about 60%, and CO₂ production was 10 times lower at the end of incubation compared to the control (permanently oxic conditions). Microbial biomass increased by 114% in glucose-amended soil but did not change in unamended soil. C immobilization prevailed over C mineralization. Redox potential decreased from +627 mV to –306 mV. If anoxic conditions were changed to oxic, consumption of glucose and CO₂ evolution significantly increased, compared to permanently anoxic conditions. Microbial biomass did not change in glucose-amended soil, but decreased by 78% in unamended soil. C mineralization was accelerated. Redox potential increased from +238 to +541 mV. The rate of glucose consumption was low in anoxic conditions if soil was incubated in pure N₂ but increased significantly when incubation was carried out in a CO₂/N₂ mixture. Received: 6 January 1999     

Short-term partitioning of <Superscript>14</Superscript>C-[U]-glucose in the soil microbial pool under varied aeration status

The effect of soil aeration status on carbon partitioning of a labelled organic substrate (¹⁴C-[U]-glucose) into CO₂, microbial biomass, and extra-cellular metabolites is described. The soil was incubated in a continuous flow incubation apparatus under four different aeration conditions: (1) permanently aerobic, (2) permanently anaerobic, (3) shifted from anaerobic to aerobic, and (4) shifted from aerobic to anaerobic. The soil was pre-incubated for 10 days either under aerobic or under anaerobic conditions. Afterwards, glucose was added (315 g C g^–1) and the soils were incubated for 72 h according to four treatments: aerobic or anaerobic conditions maintained, aerobic conditions shifted to anaerobic conditions and anaerobic conditions shifted to aerobic conditions. Carbon partitioning was measured 0, 8, 16, 24, 48 and 72 h after the glucose addition. In permanently aerobic conditions, the largest part of the consumed glucose was built into microbial biomass (72%), much less was mineralised to CO₂ (27%), and only a negligible portion was transformed to soluble extra-cellular metabolites. Microbial metabolism was strongly inhibited when aeration conditions were changed from aerobic to anaerobic, with only about 35% of the added glucose consumed during the incubation. The consumed glucose was transformed proportionally to microbial biomass and CO₂. In permanently anaerobic conditions, 42% of the consumed glucose was transformed into microbial biomass, 30% to CO₂, and 28% to extra-cellular metabolites. After a shift of anaerobic to aerobic conditions, microbial metabolism was not suppressed and the consumed glucose was transformed mainly to microbial biomass (75%) and CO₂ (23%). Concomitant mineralisation of soil organic carbon was always lower in anaerobic than in aerobic conditions.     

Experimental warming effects on the microbial community of a temperate mountain forest soil

Soil microbial communities mediate the decomposition of soil organic matter (SOM). The amount of carbon (C) that is respired leaves the soil as CO₂ (soil respiration) and causes one of the greatest fluxes in the global carbon cycle. How soil microbial communities will respond to global warming, however, is not well understood. To elucidate the effect of warming on the microbial community we analyzed soil from the soil warming experiment Achenkirch, Austria. Soil of a mature spruce forest was warmed by 4 °C during snow-free seasons since 2004. Repeated soil sampling from control and warmed plots took place from 2008 until 2010. We monitored microbial biomass C and nitrogen (N). Microbial community composition was assessed by phospholipid fatty acid analysis (PLFA) and by quantitative real time polymerase chain reaction (qPCR) of ribosomal RNA genes. Microbial metabolic activity was estimated by soil respiration to biomass ratios and RNA to DNA ratios. Soil warming did not affect microbial biomass, nor did warming affect the abundances of most microbial groups. Warming significantly enhanced microbial metabolic activity in terms of soil respiration per amount of microbial biomass C. Microbial stress biomarkers were elevated in warmed plots. In summary, the 4 °C increase in soil temperature during the snow-free season had no influence on microbial community composition and biomass but strongly increased microbial metabolic activity and hence reduced carbon use efficiency.     

Consequences of forest conversion to pasture and fallow on soil microbial biomass and activity in the eastern Amazon

The main change in soil use in Amazonia is, after slash and burn deforestation followed by annual crops, the establishment of pastures. This conversion of forest to pasture induces changes in the carbon cycle, modifies soil organic matter content and quality and affects biological activity responsible for numerous biochemical and biological processes essential to ecosystem functioning. The aim of this study was to assess changes in microbial biomass and activity in fallow and pasture soils after forest clearing. The study was performed in smallholder settlements of eastern Brazilian Amazonia. Soil samples from depths of 0–2, 2–5 and 5–10 cm were gathered in native forest, fallow land 8–10 yr old and pastures with ages of 1–2, 5–7 and 10–12 yr. Once fallow began, soil microbial biomass and its activity showed little change. In contrast, conversion to pasture modified soil microbial functioning significantly. Microbial biomass and its basal respiration decreased markedly after pasture establishment and continued to decrease with pasture age. The increase in metabolic quotient in the first years of pasture indicated a disturbance in soil functioning. Our study confirms that microbial biomass is a sensitive indicator of soil disturbance caused by land‐use change.     

Aggregate stability and microbial community dynamics under drying-wetting cycles in a silt loam soil

Aggregate stability often exhibits a large inter-annual and seasonal variability which occurs regardless of residue treatments and is often larger than the differences between soils or cropping systems. Variations in soil moisture and seasonal stimulation of microbial activity are frequently cited as the major causes. The goal of this paper was to evaluate the effects of drying-rewetting cycles on aggregate stability and on its main microbially mediated agents from a mechanistic point of view. The 3-5 mm aggregates of a silty soil were incubated at 20 °C for 63 days with the following treatments and their combinations: (i) with or without straw input and (ii) with or without exposure to four dry-wet cycles. Microbial activity was followed by measuring the soil respiration. We estimated the microbial agents of aggregate stability measuring hot-water extractable carbohydrate-C, microbial biomass carbon and ergosterol content. We measured the water drop penetration time to estimate the hydrophobicity and aggregate stability according to Le Bissonnais [1996. Aggregate stability and assessment of soil crustability and erodibility: I. Theory and methodology. European Journal of Soil Science 47, 425-437] to distinguish three breakdown mechanisms: slaking, mechanical breakdown and microcracking. The addition of straw stimulated microbial activity and increased the resistance to the three tests of aggregate stability, enhancing the internal cohesion and hydrophobicity of aggregates. All the estimated microbial agents of aggregate stability responded positively to the addition of organic matter and were highly correlated with aggregate stability. Fungal biomass correlated better with aggregate stability than total microbial biomass did, showing the prominent role of fungi by its triple contribution: physical entanglement, production of extracellular polysaccharides and of hydrophobic substances. Dry-wet cycles had less impact on aggregate stability than the addition of straw, but their effects were more pronounced when microbial activity was stimulated demonstrating a positive interaction.     

干旱区绿洲苜蓿地土壤微生物特性及其影响因子

研究了干旱区绿洲种植年限不同的3种苜蓿地0-10 cm,10-20 cm土层土壤微生物量碳、微生物量氮、微生物量商、呼吸强度和代谢商的变化特征。结果表明:在耕层0-20 cm范围内,苜蓿种植年限的长短对土壤微生物量碳、土壤微生物量氮含量有影响,相对来说微生物量碳、氮在四年生苜蓿地的含量最高,其次是一年生苜蓿地的。不同种植年限苜蓿地之间土壤微生物量商(qMB)、呼吸强度、代谢商(qCO2)差异显著,相对来说种植年限相差越大,差异越大。相关分析结果表明,土壤理化特性对土壤微生物活性有影响,其中土壤微生物量碳、微生物量氮与土壤有机质、全氮显著正相关,与土壤容重显著负相关。     

Methods for the investigation of metabolic activities and shifts in the microbial community in a soil treated with a fungicide

Samples from a sandy agricultural soil were treated with increasing amounts of a fungicide (Sportak). The effects on the soil microflora were investigated over several weeks by monitoring basal and substrate-induced respiration and basal and substrate-induced heat output. The microbial biomass, metabolic quotient (qCO₂), relative heat output (rqheat), lag phase of substrate use, and calorimetric: respirometric ratio were used as ecophysiological parametèrs. As structural and community-specific parameters, we recorded tryptophan contents and auxin metabolism, and calculated the ratios of fungal to bacterial respiration by antibiotic inhibition of substrate-induced respiration. Sportak either inhibited or stimulated the microbiota, depending on the length of exposure to the fungicide and the amount applied. Mineralization of dead biomass was reflected in increased soil tryptophan contents after the Sportak application. A shortened lag phase demonstrated inhibition and a prolonged lag phase stimulation of substrate use. This changed with the experimental phase. The rqheat and the calorimetric: respirometric ratio proved to be suitable parameters for the detection of stress metabolism (repair processes) in soil microbiota, because thermodynamic processes and catabolic and anabolic metabolism are taken into account at the same time. Following the application of Sportak, indole 3-acetic acid biosynthesis decreased while indole-3-ethanol biosynthesis increased, probably as a result of a transitional community shift from K-strategists towards r-strategists. It was not the fungicide but the formulation (mainly xylol) that damaged the organisms. A shift in the ratio of fungi to bacteria was also observed, suggesting that the bacteria were probably more sensitive to xylol than the fungi.     

Changes in soil acidity and the size and activity of the microbial biomass in response to the addition of sugar mill wastes

The effects of additions of three wastes from sugar mills on the properties of two acid soils were investigated. The wastes used were boiler ash and filter cake from a conventional mill and fly ash from a new mill, where filter cake is burnt. Additions of each of the wastes raised soil pH, reduced concentrations of exchangeable Al and total and monomeric Al in soil solution and increased maize yields in a pot experiment. Microbial biomass C and the percentage of organic C present as microbial biomass were decreased by additions of wastes but the effect was less marked at the higher rate of addition. Although basal respiration was decreased or unaffected by the lower rate of addition of wastes it was increased by the higher rate. The metabolic quotient increased in the order: control4⁺ and NO₃^- accumulated during incubation and arginine ammonification rate were all less at the higher rate of addition of each of the wastes. This was attributed to microbial immobilization of mineral N and arginine due to the wide C/N ratio of light fraction organic matter present in the study soil. It was concluded that all three waste materials were effective liming materials but their effect on soil microbial activity was complex.     

Surface and subsurface microbial biomass, community structure and metabolic activity as a function of soil depth and season

Microbial biomass, size and community structure along with an estimate of microbial activity and soil chemical parameters were determined at three depths in two soils (e.g. sandy loam Ultic Hapludalf and silt loam Mollic Hapludalf) replicated three times under one winter and summer season. Microbial biomass and community structure were estimated from phospholipid-PO₄ content and fatty acid methyl ester (FAME) measurements. Microbial activity and assimilative capacity were estimated using a ³H-acetate incorporation into phospholipids and by incubating the soil samples at the average winter and summer temperatures, 3 and 20 °C, respectively. We found that the size of the microbial biomass in both the surface and the subsurface soils was not significantly affected by the seasonal variation but activity increased by as much as 83% at the summer temperatures in the surface soil. We demonstrated using FAME analysis that for both soils seasonal changes in the subsurface microbial community occurred. These findings suggest that winter conditions will shift the population activity level in both the surface and subsurface systems and the biochemical structure of the community in the subsurface. In all cases, the inorganic chemical properties of the soil, as a function of season, remained constant. The greatly increased activity of microbial population at the higher temperature will favor the capacity of the system to utilize nutrients or organic materials that may enter soil. During low temperature seasons the capacity of either surface or subsurface soils to assimilate materials is generally diminished but the reduction reflects changes in metabolism and not a reduced biomass size.     

Carbon dioxide efflux and concentrations in two soils under temperate forests

The carbon dioxide efflux to the atmosphere and the concentrations at various depths in two soils were measured, for more than a year, under pure stands of silver fir ( Abies alba Mill.) and European beech ( Fagus sylvatica L.) in central Italy. Microbial biomass and activity at the monitored depths were determined in the laboratory and the CO₂ evolved from incubated samples was submitted to radiocarbon analysis to assess the mean residence time of the organic matter degraded by microorganisms. The CO₂ efflux showed similar trends in the two soils, with highest values in October and lowest in January. The efflux depended more on air and soil temperatures than soil moisture, and was related to these variables better under fir than under beech. In both soils, the CO₂ concentration increased with depth: in the top horizon it was low and similar to that of the atmosphere, while in the deeper horizons it often amounted to considerable values (up to more than 1% by volume in the BC horizon under fir). The subsoil of the fir stand generally showed much higher CO₂ concentrations than that of the beech. The basal respiration as determined in the laboratory was at a maximum in the topsoil and decreased sharply downwards. Therefore, the high CO₂ concentrations measured in the field at the bottom of the profiles--where roots were few, and microbial biomass and available C pool were at a minimum--appeared to be due more to slow diffusivity of the soil matrix rather than to heavy release of the gas by the biota. The organic matter respired by microorganisms in incubated soil samples showed positive values of j¹⁴C that revealed a recent synthesis. The estimated mean residence time increased with depth, suggesting a generally higher degree of stabilisation of the organic pool in the subsoil.     

Production of urease by microbial activity in soils under aerobic and anaerobic conditions

Summary Several workers have reported that O₂ has little, if any, effect on hydrolysis of urea by soil urease, but others have reported that it has a marked effect, hydrolysis being significantly faster in soils under aerobic conditions than in O₂-depleted soils. In studies to account for these divergent results, we found that whereas plant residues and other readily decomposable organic materials markedly stimulated microbial production of urease in soils under aerobic conditions, they did not greatly stimulate production of urease in soils under anaerobic conditions. We also found that although anaerobic conditions retarded production of urease by soil microorganisms, they did not inhibit hydrolysis of urea by soil urease. These observations suggest that the divergent findings concerning the effect of O₂ on hydrolysis of urea by soil urease may have resulted from differences in the amounts of readily decomposable organic materials in the soils studied.     

Enhanced Mobilization of Field Contaminated Soil-bound PAHs to the Aqueous Phase under Anaerobic Conditions

Although microbially-mediated redox environments can alter the characteristics of soil/sediment organic matter (SOM) and its interactions with persistent hydrophobic organic contaminants (HOCs) bound to soils and sediments, the nature of their effects has not been adequately addressed. In this study, a field soil collected from a manufacturer gas plant site and contaminated historically with creosotes was incubated under aerobic and anoxic/anaerobic conditions along with various amendments (extra carbon and enrichment minerals) for stimulating microbial activities. Anaerobic conditions stimulated significant fractions of bound polycyclic aromatic hydrocarbons (PAHs) encompassing naphthalene through benzo[g,h,i]perylene to be mobilized to the aqueous phase, leaving their aqueous phase concentrations far in excess of solubility (increases in their apparent aqueous phase concentrations by factors as high as 62.8 relative to their initial aqueous phase concentrations). Such effects became more evident for high molecular weight PAHs. Dissolved organic matter exhibiting a high affinity for PAHs was liberated from soils during the anaerobic soil incubations. Feasibility of this concept for field applications was evaluated with a lab-scale continuous flow system composed of an anaerobic soil column followed by an aerobic bioreactor inoculated with PAH-degrading microbes. High quantities of PAHs exceeding their aqueous solubilities were eluted from the anaerobic soil column and those mobilized PAHs were readily bioavailable in the secondary aerobic bioreactor. This study may offer a potential method for cost-effective and performance-efficient ex situ remediation technologies (or in situ if appropriate hydrological control available in the contaminated field site) and risk assessment for the HOC-contaminated soils/sediments.     

Microbial community structure varies in different soil zones of a potato field

Analyses of phosholipid fatty acids (PLFA) and phospholipid etherlipids (PLEL) revealed differences in size and structure of microbial communities in the three soil zones of a potato field: ridge (RS), uncompacted interrow (IS), and tractor‐compacted interrow soil (CS). The quantity of phosholipid biomarker concentrations (= microbial biomass) showed large differences among different zones, when lipid contents were related to fresh soil volume instead of soil dry matter. Compaction of interrow soil caused an increase in bacterial and eukaryotic biomass, expressed as total PLFA concentration, as well as an increase in total archaeal biomass, expressed as total PLEL concentration and caused a decrease in the fungi‐to‐bacteria ratio. Due to the higher waterfilled pore space (an indirect measure for reduced O₂ availability) in CS, a more pronounced anaerobic microbial community was estimated than in IS, which serves as an explanation for the elevated N₂O fluxes in this soil zone. Apart from the effect of O₂ availability, microbial communities, especially populations of aerobic bacteria, ascinomycetes, fungi, algae, protozoa, and aerobic archaea responded to organic matter composition in the individual zones. Only in RS PLEL derived cyclic isoprenoids were found, which presumably indicate root‐colonizing archaea. Following principal component analyses of specific biomarker profiles, the assumed substrate effect had the strongest influence on the differences in microbial community structure between the three soil zones.     

长期有机无机肥配施对褐土微生物生物量碳、氮量及酶活性的影响

通过对山西省寿阳长期定位试验田0―20 cm和20―40 cm的土壤测定和分析,探讨了长期有机无机肥配施下褐土微生物生物量碳、氮和酶活性的变化以及相关性。结果表明,褐土微生物生物量C、N变化基本一致。褐土微生物生物量碳、氮从0―20 cm到20―40 cm土层均呈减少趋势;长期单施高量有机肥、有机无机肥合理配施都能提高褐土微生物生物量碳、氮;不同用量的长期单施化肥处理不能使微生物生物量C、N显著增加。脲酶和碱性磷酸酶活性从0―20 cm到20―40 cm土层呈减少趋势;长期单施高量有机肥和有机无机肥合理配施可使褐土脲酶及碱性磷酸酶活性增加。脲酶活性随单施化肥量的增加有变大趋势,而碱性磷酸酶活性则呈变小趋势。土壤微生物量碳氮、土壤酶活性及土壤养分之间的显著相关性表明,微生物生物量C、N和土壤酶活性可以判断褐土土壤有机质和N素状况,可作为评价褐土土壤肥力水平和土壤培肥效果的生物学指标,同时也可为提高褐土土壤肥力水平和土壤培肥效果提供依据。     

Estimation of active soil microbial biomass by mathematical analysis of respiration curves: Calibration of the test procedure

The biokinetic properties of the active component of the soil biomass were estimated in a range of soils by mathematical simulation of the respiration curve obtained after the soil was amended with glucose. Microbial communities appeared to vary considerably with regard to their affinity for substrate, their maintenance coefficient and particularly the amount of active microbial biomass they contain. In soils from undisturbed natural ecosystems, the active biomasss component was on average about half that in arable agricultural soils. In a wheat field, the active but not the total microbial biomass was significantly stimulated by the growth of the crop. In the soils examined, the active biomass comprised 4% to 49% of the total soil biomass. A 10 h procedure is described for estimating the active soil biomass component, by monitoring either O₂ consumption or CO₂ production.     

Soil microbial biomass, dehydrogenase activity, bacterial community structure in response to long-term fertilizer management

This study describes the effects of balanced versus nutrient-deficiency fertilization on soil microbial biomass, activity, and bacterial community structure in a long-term (16 years) field experiment. Long-term fertilization greatly increased soil microbial biomass C and dehydrogenase activity, except that the P-deficiency fertilization had no significant effect. Organic manure had a significantly greater (P<0.05) impact on the biomass C and the activity, compared with mineral fertilizers. Microbial metabolic activity (dehydrogenase activity per microbial biomass C) was significantly higher (P<0.05) under balanced fertilization than under nutrient-deficiency fertilization. General bacterial community structure was analyzed by PCR-denaturing gradient gel electrophoresis (DGGE) targeting eubacterial 16S rRNA gene. Mineral fertilization did not affect the DGGE banding pattern, while specific DGGE band was observed in organic manure-fertilized soils. Phylogenetic analysis showed that the change of bacterial community in organic manure-fertilized soil might not be because of the direct influence of the bacteria in the compost, but because of the promoting effect of the compost on the growth of an indigenous Bacillus sp. in the soil. We emphasize the importance of balanced-fertilization, as well as the role of P, in maintaining soil organic matter, and promoting the biomass and activity of microorganisms.     

Microbial biomass and enzyme activities: Production and persistence

The production and persistence of microbial biomass and also urease, phosphate and casein-hydrolysing activities were investigated when either glucose or ryegrass were added, as energy sources, with ¹⁵NO₃^? to a clay-loam soil. Both direct count and fumigation methods were used to determine soil microbial biomass. Microbial biomass and enzyme activities increased after the addition of energy sources. Increases in phosphatase and urease activities coincided mainly with increases in bacterial biomass and with the rapid immobilization of labeled N. Conversely, the increase in casein-hydrolysing activities preceded the phase of net mineralization that occurred during the later period of incubation.Although microbial biomass and the biochemical activities tested increased in the soils treated with energy supplies, they eventually decreased to the level of the control soil. Even the increases in biomass and enzyme activities present at zero time, as a result of the addition to the soil of exogenous microorganisms and enzymes with the ryegrass, were not maintained after extensive incubation. The influence of homeostatie mechanisms which tend to maintain a stable biological composition in the soil microbial population is discussed. A possible relationship between the available “active” or “biological” space, total microbial biomass and total enzyme activity in soil is suggested.     

Mineralization of plant-incorporated residues of 14C-isoproturon in arable soils originating from different farming systems

¹⁴C-isoproturon residues were incorporated in wheat plants by growing seedlings for 18 days in quartz sand with nutrient solution which was treated with ring-labeled ¹⁴C-isoproturon, resulting in ¹⁴C-concentration equivalent to 15.4 nmol isoproturon per g dry shoot mass. The residues were characterized by extraction and HPLC-analysis, and were shown to consist of unchanged isoproturon, soluble metabolites (monodemethyl-isoproturon, didemethyl-isoproturon, 1-OH-isoproturon, 2-OH-isoproturon, 2-OH-monodemethyl-isoproturon, 2-OH-didemethyl-isoproturon, isopropenyl-isoproturon and unidentified metabolites), as well as nonextractable residues. Dried plant samples containing these residues were mixed with soil samples originating from different farming systems, and mineralization to ¹⁴CO₂ was determined in a closed aerated laboratory system. In addition, the microbial biomass and bioactivity of soils were estimated by determination of substrate-induced heat output, basal heat output, metabolic heat quotient, total adenylate content and adenylate energy charge. Significant positive correlations between ¹⁴CO₂ production or adenylate content and microbial biomass were observed in three soils; ¹⁴CO₂ production and total microbial biomass were highest in soil samples from organic farming. Soil samples from a former hops plantation contaminated with copper from previous fungicide applications did not fit this correlation, but exhibited a higher mineralization capacity per unit of microbial biomass. Our results indicate that general soil microbial parameters in many cases are insufficient to describe the influence of biotic factors on the fate of pesticides in soil.     

Pinus halepensis Mill. plantations did not restore organic carbon,microbial biomass and activity levels in a semi-arid Mediterranean soil

Maquis is a dense evergreen shrub layer which, in semi-arid Mediterranean lands, is commonly linked to the presence of well-conserved soils with large contents of mineralizable substrates. It was our aim to test whether: (i) maquis promotes soil microbial biomass and activity, and (ii) mature pine plantations without a shrubby understory support microbial biomass and activity levels comparable to those of stands with maquis. Surface soil samples were taken in four sites that sustain pine plantations (PP), maquis with pines (MP), maquis (MQ) and grasslands (GS). Microbial biomass was inferred from the C content in the soil microbiota. The ATP content in fresh samples and the CO₂–C production from incubated samples were used to assess microbial activity, as was the activity of β-glucosidase and alkaline phosphatase. Topsoils under maquis (MP and MQ) were the most fertile, both chemically (high organic carbon contents) and physically (low bulk density, high aggregate stability) and showed by far the largest levels of microbial biomass and activity. These levels in soils under PP, which sustained a successful plantation in terms of tree canopy density but lacked the shrubby understory, were significantly smaller than those of the adjacent shrubland with pines (MP). Redundancy analysis extracted a main axis explaining 67% of the variation of the microbiological soil properties, which was interpreted as an environmental gradient of soil fertility. Along this axis, the samples were separated according to the presence or absence of a maquis dominated by late-successional species; other factors such as soil type, slope position and aspect were less influential. The effects of afforestation practices on the detritus-based system should be considered in the design of future forest restoration strategies in desertification-threatened lands.