Substrate-induced Respiration Method Research Articles

Factors for Conversion of the Content of Double-Stranded DNA to Carbon of Soil Microbial Biomass

The relationship between the concentration of soil DNA and microbial biomass, determined by the method of substrate-induced respiration was studied in a wide range of soils differing in particle size distribution, acidity, organic carbon content, microbial biomass, and type of human activity in antiquity and present time. Calcaric Leptosols and Leptic Phaeozems of medieval agricultural lands in the Central Caucasus, Stratozems of agricultural terraces of the late Middle Ages – Modern times in the middle mountain zone of the Eastern Caucasus, as well as Kashtanozems and Solonetzes with different grazing intensity in the dry steppe zone (Rostov region) were chosen as the key sites. It has been shown that the determination of soil dsDNA concentration is a reliable and simple method for determining microbial biomass in soils with a loam texture, organic carbon content less 2%, and microbial biomass less 700 µg C/g of dry soil. The conversion factor FDNA in such soils varied in a narrow range from 5.24 to 5.41. In soils with a high content of organic carbon, an increase of FDNA (6.56 and 10.56) was observed due to the presence of recalcitrant e-xtracellular DNA. Agristratified soil of sandy loam texture was characterized by a lower degree of preservation of dsDNA, which resulted in a decrease in the determined microbial biomass (FDNA = 4.22). A reduced conversion factor FDNA (4.78) was also found in the soils of pastures in the dry steppe zone, which confirms the known limitations of using the substrate-induced respiration method in alkaline soils. Human activity does not significantly affect the relationship between the amount of soil DNA and microbial biomass.

Understanding the relative contributions of fungi and bacteria led nitrous oxide emissions in an acidic soil amended with industrial waste

Amendment of fertilized arable soil with alkaline industrial waste has the potential to ameliorate soil acidification whilst also improving crop yield. Another co-benefit is nitrous oxide (N2O) emission abatement but the contribution of fungi and bacteria involved in this process remains unclear. Two incubation experiments were conducted to: 1) examine how amendment of acidic soils with a mixture of phosphorus tailings mixture and insoluble potassium-containing rocks (PT) affect N2O emissions and 2) understand the microbial mechanisms and relative contributions of fungi and bacteria responsible for N2O emissions. In the first incubation experiment, the four treatments consisted of: i) the study control, ii) urea, iii) PT amendment and iv) PT amendment plus urea. Results showed that the PT amendment significantly increased soil pH from 4.8 to above 6.0, and reduced N2O emissions by 65.7%. PT-amended soils had higher N2 emissions and faster O2 consumption. The PT amendment significantly increased extracellular enzyme activities of leucine aminopeptidase and N-Acetyl-β-glucosaminidase, while it significantly decreased activities of β-1, 4-glucosidase and β-cellobiosidase. Two antibiotics (cycloheximide and streptomycin) combined with substrate-induced respiration method were used in the second incubation experiment. Compared to soil with urea, urea with PT amendment raised soil bacteria-related N2O from 9.2% to 18.8% while decreasing fungi-related N2O from 50.5% to 43.2%. These findings suggest that the N2O emissions from acidic soils can be considerably mitigated by the application of alkaline industrial wastes. The contribution of fungi should be considered when designing and applying N2O mitigation strategies in acidic soils. Data AvailabilityData will be made available on request.

Biochar reduced soil nitrous oxide emissions through suppressing fungal denitrification and affecting fungal community assembly in a subtropical tea plantation

Biochar amendment has been shown to reduce nitrous oxide (N2O) emissions from acidic soils in tea plantations. Given that both soil bacterial and fungal denitrifications can produce N2O, their relative contributions to the decrease in N2O emissions following biochar amendment remain unclear. Here, we examined soils sampled from a subtropical tea plantation that had received 2 years of biochar amendment. Measurements of the relative contributions of fungi and bacteria to N2O production were taken by the substrate-induced respiration method implemented with the addition of selective inhibitors. The abundances of total fungi, bacteria, and key N2O-related bacterial genes were quantified by q-PCR, and the composition of the fungal community was analyzed by 18S rRNA amplicon sequencing. The results showed that the contribution of fungi to N2O production (52%) was greater than that of bacteria (18%) for the N-applied acidic soils. Biochar amendment significantly decreased the fungal abundances and the fungal contribution to N2O production (by 28%). In contrast, biochar amendment significantly increased the abundances of N2O-related bacteria (e.g., ammonia-oxidizing bacteria (AOB), nirS, nosZ). Structural equation models (SEMs) revealed that biochar decreased the fungal contribution to N2O production through enhancing the soil pH and shifting the fungal community composition. Our results highlight that the decreased N2O emissions could be ascribed to the stimulated N2O consumption process (driven by N2O-consuming bacteria encoded by the nosZ gene) and suppressed fungal dominance in acidic soils from tea plantations. This study presents relatively comprehensive insights into the regulatory roles of biochar on soil microbe-mediated N2O production processes.

Microbiological Indicators of Heavy Metals and Carbon-Containing Preparations Applied to Agrosoddy-Podzolic Soils Differing in Humus Content

The response of the microbial community (microbial biomass carbon (Cmic), basal respiration (BR), and functional diversity (FD)) of agrosoddy-podzolic soil (Albic Glossic Retisols (Loamic, Aric Cutanic, Ochric)) to pollution by heavy metals (HMs: Cu 660, Zn 1100, Pb 650 mg/kg) and carbon-containing preparations (5% of biochar and 0.25% of lignohumate) was studied in model experiment (30 days). Soils with different organic carbon contents (Corg 3.86 and 1.30%) were sampled at two sites (Chashnikovo, Moscow oblast). We determined Cmic by the substrate-induced respiration method and FD by multisubstrate testing (47 substrates). It was found that HMs application reduced Cmic on average by 49–57%, BR by 23–52%, and FD by 45%, but, on the contrary, increased the microbial metabolic quotient (qCO2 = BR/Cmic) by 9–46%. The changes of these properties were most significant in the soil with low Corg content (1.30%). Carbon-containing preparations did not contribute to variations in Cmic, BR, and qCO2 in both soils with HMs, but increased their FD. It is concluded that the studied microbiological parameters may be used as indicators for optimal assessment of soil quality: FD and Cmic are the more sensitive to HMs than BR and qCO2.

Soil Microbial Respiration in Subtaiga and Forest-Steppe Ecosystems of European Russia: Field and Laboratory Approaches

Our study focuses on testing laboratory measurements of soil microbial respiration as a proxy that in the field conditions. The soil microbial respiration was measured in field (MRfield) and laboratory (MRlab) conditions monthly (from May to October) in subtaiga (mixed forest, meadow) and forest-steppe (broad-leaved forest, virgin steppe) ecosystems of the European Russia. The MRfield was determined through soil respiration partitioning by the conventional substrate-induced respiration method. The MRlab was measured as basal respiration of 10 cm topsoil at 22°C and 60% water holding capacity. The contribution of MRfield to total soil respiration varied during the growing season from 25 to 82% for subtaiga and from 41 to 88% for forest-steppe. The MRfield for studied ecosystems varied from 2.2 to 21.7 g СО2/(m2 d), while MRlab was from 3.5 to 18.6 g СО2/(m2 d). Similar results obtained by field and laboratory approaches were in 50% of measurements in the subtaiga ecosystems and in almost 20% of cases on the forest-steppe. The average MRfield and MRlab for growing season did not significantly differ for all studied ecosystems. These findings demonstrate possible prospects of using laboratory measurement of soil microbial respiration during the growing season to approximate and predict average MRfield for various ecosystems.

Microbial Biomass, Respiratory Activity and Nitrogen Fixation in Soddy-Podzolic Soils of the Pre-Urals Area under Various Agricultural Uses

Indicators of the microbiological activity of virgin (mixed forest and forb–grassy meadow) and arable (permanent black fallow, permanent barley crop, field crop rotation, perennial legume crops) soddy-podzolic soils (Eutric Albic Retisols (Abruptic, Loamic, Cutanic)) of Perm oblast were studied. Depending on the type of land use, the content of organic carbon (bichromate oxidation method) in the soil varied from 0.78 to 1.57%; the carbon of microbial biomass (substrate-induced respiration method), from 366 to 1236 μg/g; basal respiration rate, from 1.62 to 3.13 µg C–CO2/(g h); and nitrogen-fixing capacity (acetylene method), from 11.53 to 77.18 µg C2H4/(kg h). The maximum values of the studied parameters were noted in the soil under the mixed forest, and the minimum values were in the soil of the permanent (42 yr) fallow. In the soil under forest, a low specific respiration of microbial biomass—2.5 µg C–CO2/(mg Cmic h)—was determined, which indicates the stability and maturity of this ecosystem. The most stable functioning of the microbial community among the arable soils under consideration was observed in the soil of the plot with the long-term (30 yr) cultivation of perennial legume crop (eastern galega, Galega orientalis). The contents of the soil organic matter and the microbial biomass carbon and the specific microbial respiration under galega were close to those of the soil of the forest ecosystem. The content of microbial biomass carbon and basal respiration displayed strong and moderate correlation dependences with the total content of soil organic carbon and its labile fraction.

Changes in Functional Response of Soil Microbial Community along Chronosequence of Spontaneous Succession on Post Mining Forest Sites Evaluated by Biolog and SIR Methods

Soil formation in post-mining sites is crucial for restoring ecosystem function, and soil formation depend on the accumulation of soil organic matter and the development of an active microbial community. In this study, we used substrate-induced respiration (SIR) and Biolog plates to characterize microbial catabolic profiles in a chronosequence of soil samples from 15 unreclaimed post-mining sites in Sokolov, Czech Republic. The sites had been undergoing spontaneous succession for 3 to 45 years. Biolog ECO plates included 31 substrates. Of substrates used for SIR (glucose, chitin, cellulose, Tween 80, phenylethylamine, N-acetyl-D-glucosamine, L-asparagine, D-mannitol, D-galacturonic acid, α-cyclodextrin, and 4-hydroxy benzoic acid), eight were also used for the Biolog plates. Soil respiration, total bacteria number, and culturable bacteria number were also measured. The total and culturable number of bacteria increased with site age (p < 0.01 and p < 0.05, respectively). The percentage of culturable bacteria decreased with site age (p < 0.01). Biolog analysis indicated that average well-color development (AWCD), evenness, and richness increased with site age. SIR data indicated that only average activities tended to increase with site age (p = 0.06). According to redundancy analysis (RDA), the eight substrates, which were commonly used in both methods (SIR and BIOLOG) explained 74.4% of the variation of data from all Biolog substrates. Among the eight substrates common to both methods, only data for N-acetyl-D-glucosamine were positively correlated (p < 0.01) between Biolog and SIR. Both methods revealed microbial catabolic profile changed along the chronosequence. PCA indicated that site age, soil carbon, and pH were the most important drivers of microbial catabolic profiles.

Greenhouse study on the influence of natural biostimulators and fertilizers on improving bean plants growth and microbial activity in oil-polluted soil

Abstract The presence of oil hydrocarbons and salts in soils has as consequence the poor growth of plants due to the low availability of nutrients caused by unappropriate water regime, increased soil toxicity and the deterioration of soil structure. The present research has as main purpose the improvement of the bean plants (cultivar UNIDOR) growth and to stimulate microbial activity in rhizosphere using various natural stimulators and fertilizers (AMALGEROL,VERMIPLANT, POCO, IGUANA and FORMULEX) in greenhouse experiments on oil-polluted soil from Icoana farm, Olt county. The total counts of microorganisms (heterotrophic aerobic bacteria and filamentous fungi) were estimated by dilution plate method. The global microbial activity was measured as soil respiration by substrate-induced respiration method. Total bean plants biomass accumulation significantly increased under the influence of natural stimulators and fertilizers added (excepting IGUANA) when compared to untreated control. The best results were recorded for VERMIPLANT. Natural products induced activation of physiological activities of soil microbiota reflected in increased values of CO2 released by respiration, lower levels of colonization with phytopathogenic species, the domination of fluorescent pseudomonads, actinomycetes and cellulolytic fungi, too. The particular aspect of paper circular chromatograms reflected qualitative differences between rhizosphere soils determined by the effect of treatments with natural stimulators and fertilizers.

Impact of land use on soils microbial activity

AbstractThe article considered the influence the various types of land use on microbial activity of soils and thus using this parameter as a universal test of soil quality. Samples for soil respiration studies were taken from agricultural areas, meadows, forests and urban areas (estate cottages). All samples were subjected to the same analytical procedure and the method of measurement was followed by a Substrate-Induced Respiration (SIR) method. Since all the samples were from neighbouring regions and were characterized by similar soil parameters, the obtained results allowed to assess the quality of the soil environment of the surveyed area and to evaluate the total rating of whole area. The obtained results allowed to observe slight divergences between soil samples taken from areas of different use. And although statistically, in this case, these differences were not significant, the characteristics of the respiration curves clearly indicate that there is a dependency between the form of use and the size and rate of soil respiration. In addition, the results have shown that single family housing does not exhibit as much anthropopression to the soil environment as it might seem.

Assessment of Methods for Measuring Soil Microbial Biomass Carbon in Temperate Fruit Tree-Based Ecosystems

ABSTRACTWe investigated the potential of three methods of quantifying microbial biomass carbon (MBC), viz., chloroform fumigation-extraction (CFE) following organic C estimation through Vance method (CFE-V) and Snyder–Trofymow method (CFE-ST), and substrate-induced respiration (SIR) method in soils under various temperate fruit crops along with a control (no plantation) at 0–20 and 21–40 cm soil depths. CFE methods have shown significant (p < 0.05) increase in chloroform labile C in all orchards over the control in surface soil. The interaction between the fruit crops and methods, although significant (p < 0.01), indicated that CFE-ST and SIR methods were statistically at par with each other within the same fruit crop, except peach plantation (CEF-ST significantly lower than SIR) in 0–20 cm soil depth. The coefficient of variation recorded for chloroform labile organic C estimates by CFE-ST method makes it more precise than CFE-V method, especially in 0–20 cm soil depth. The very close agreement between the methods suggests that over this narrower range (i.e., smaller geographical area) all methods are appropriate for assessing MBC. However, SIR, being most sensitive to orchard plantations and strongly correlated with various soil chemical properties, could preferably be recommended for estimation of MBC in such soils. As an alternative to CFE-V method, CFE-ST may also be used for estimation of chloroform labile organic C in these soils.

基質誘導呼吸(SIR)法による微生物活性の測定

基質誘導呼吸(SIR)法による微生物活性の測定

Phospholipid fatty acid biomarkers show positive soil microbial community responses to conservation soil management of irrigated crop rotations

The increasing acreages of crops like potato and sugar beet, which return little C to the soil, and whose harvesting methods cause soil disturbance, led to the establishment of a 12-yr study to evaluate soil conservation (CONS) management systems in southern Alberta. The CONS management systems, applied to 3- to 6-yr crop rotations, were compared with conventional (CONV) management systems that included wheat monoculture. The CONS management was a suite of practices that included addition of cattle manure compost, reduced tillage, diverse crop rotations and use of cover crops that CONV management did not have. In the last two years of the study (2010 and 2011), soil microbial biomass was measured in bulk soil and wheat rhizosphere using the substrate-induced respiration method and phospholipid fatty acid (PLFA) biomarkers. β-glucosidase enzyme activity was measured to evaluate soil functioning (C cycling). Total soil microbial biomass, and that of its components (fungi and bacteria), was significantly greater under CONS management than under CONV management. The total PLFA contents of 3- and 4-yr CONS rotations (15.24–34.69nmolg−1 soil) were 84–193% greater than those in CONV management (33.45–63.66nmolg−1 soil) when differences were significant, and fungal PLFA was up to 382% greater. β-glucosidase activity was 50% greater under CONS management than CONV management. Principal component analysis confirmed that the soil microbial community structures in the different rotations were shaped by management practices. These positive responses of soil microbial communities to conservation management will enhance biological processes including nutrient cycling and biological pest control.

Impact of Wheat/Faba Bean Mixed Cropping or Rotation Systems on Soil Microbial Functionalities.

Cropping systems based on carefully designed species mixtures reveal many potential advantages in terms of enhancing crop productivity, reducing pest and diseases, and enhancing ecological services. Associating cereals and legume production either through intercropping or rotations might be a relevant strategy of producing both type of culture, while benefiting from combined nitrogen fixed by the legume through its symbiotic association with nitrogen-fixing bacteria, and from a better use of P and water through mycorrhizal associations. These practices also participate to the diversification of agricultural productions, enabling to secure the regularity of income returns across the seasonal and climatic uncertainties. In this context, we designed a field experiment aiming to estimate the 2 years impact of these practices on wheat yield and on soil microbial activities as estimated through Substrate Induced Respiration method and mycorrhizal soil infectivity (MSI) measurement. It is expected that understanding soil microbial functionalities in response to these agricultural practices might allows to target the best type of combination, in regard to crop productivity. We found that the tested cropping systems largely impacted soil microbial functionalities and MSI. Intercropping gave better results in terms of crop productivity than the rotation practice after two cropping seasons. Benefits resulting from intercrop should be highly linked with changes recorded on soil microbial functionalities.

Determination of carbon and nitrogen in microbial biomass of southern-Taiga soils by different methods

The results of methods for determining microbial biomass carbon vary in reproducibility among soils. The fumigation-extraction and substrate-induced respiration methods give similar results for Albic Luvisol and Gleyic Fluvisol, while the results of the rehydration method are reliably higher. In Histic Fluvisol, relatively similar results are obtained using the fumigation-extraction and rehydration methods, and the substrate-induced respiration method gives almost halved results. The seasonal dynamics of microbial biomass carbon also varies depending on the method used. The highest difference is typical for the warm period, when the concentrations found by the extraction and substrate-induced methods poorly agree between two out of three soils studied. The concentration of microbial biomass nitrogen is less sensitive to the analytical method: the differences between the results of the fumigation-extraction and rehydration methods are statistically insignificant in the all soils. To reveal stable relationships between the results of determining microbial carbon and the soil properties and analytical method, a large diversity of soils should be studied. This will allow for proposing of conversion factors for the recalculation of the obtained values to the concentrations of carbon and nitrogen in microbial biomass for different soils (or soil groups) and, hence, the more correct comparison of the results obtained by different methods.

Soil microbial component and its respiratory activity in coniferous forests of the Northwest Priladozhje area

The microbial biomass carbon (Cmic) by substrate-induced respiration method (SIR), the basal (microbial) respiration rate (BR) and the metabolic quotient (q CO2 = BR / Cmic) were calculated from samples of plant litter and mineral layers of forest soils (Podburs, Sod-podburs, Gleyzems, Litozems) of four catenas of northern and southern exposure of selgovy landscape in the Northwest Priladozhje area. Sample sites were located at different distances (300, 500 and 2000 m) from a source of aerotechnogenic pollution (a granite quarry). High levels of biogenecity were found in plant litter, low ones in corresponding mineral layers of the soil profile in the pine forests examined. Soils of accumulative parts of the landscape were characterized by higher Cmic and BR compared to transit and autonomic ones. The impact of aerotechnogenic pollution from the granite quarry on soil microbial component was found, it was expressed by decreased Cmic and BR mainly in forest plant litter. Refs 13. Fig. 1. Tables 6.

Preliminary Studies on Microbial Biomass and the Microarthropod Community as Soil Health and Quality Indicators in Urban Grasslands, Rīga as an Example / Sākotnējie Pētījumi Par Augsnes Mikrobiālo Biomasu Un Augsnes Sīkposmkāju Sabiedrībām Kā Augsnes Veselības Un Kvalitātes Indikatoriem Rīgas Pilsētas Urbānajos Zālājos

Abstract Anthropogenic pollution is considered as a one of the main factors that affects soil in urban areas. In 2014, soil quality of grasslands in Rīga was determined. The aim of the study was to determine microbial biomass and describe microarthropod communities in soils of different grasslands in Rīga city and to assess their use as indicators of soil health and quality of the urban environment. Six sampling plots with different building characteristics, density of street and rail network and transport load, and potential impact of pollution were selected for studies. The substrate-induced respiration method was used for determination of soil microbial biomass. Soil invertebrates were collected in sampling plots. Soil Mesostigmata mites, Oribatida and Collembola were extracted from samples and counted. In total over 9300 soil microarthropod individuals were recorded, of them 4500 Collembola, 3400 Oribatida and 1400 Mesostigmata mites. Statistically significant differences in soil microbial biomass among sampling sites were observed. Soil microbial carbon content significantly increased in most of sampling plots from July to October. To obtain more precise results in the future some modifications and optimisation of the standard method based on indicators will be necessary.

Changes in the cellulolytic activity of urban soils induced by the removal of plant litter (using Moscow as an example)

The parameters of the biological activity in the upper horizons of an urban soil in Tushino (the northwestern administrative district of Moscow) without the litter horizon because of the regular removal of the plant litter and an undisturbed background Albic Retisol with the litter horizon were compared. In the studied soils, the levels and dynamics of the cellulolytic activity were comparable and generally increased from the spring to the fall seasons. This increase was more pronounced in the urban soil during the leaf fall. The seasonal dynamic of the CO2 emission in the urban soil (upon its moistening) was similar to that in the background soil, though the absolute values were significantly lower in the summer and fall seasons. The method of substrate-induced respiration (SIR) showed that the rise in the CO2 emission in response to the addition of cellulose in the urban soil was 1.5–2.0 times lower than that in the background soil. The increase in the cellulolytic activity upon the application of lime leaves (Tilia cordata Mill.) in the uppermost 5 mm of the urban soil (immediately under the leaves) was also less pronounced than that in the natural background soil. At the same time, the increase in the CO2 emission upon the addition of cellulose into this layer was more pronounced and continued for a longer time in the urban soil.

Impact of multi-resistant transgenic Bt maize on straw decomposition and the involved microbial communities

A general concern associated with the use of transgenic Bt maize is its possible negative influence on non-target organisms and ecosystem functions such as organic matter transformation. Our study was carried out to assess the impact of the multi-resistant Bt maize hybrid MON 89034×MON 88017 on straw decomposition and the residue-colonizing microbial communities in comparison to the near-isogenic control and two conventionally bred varieties. Straw decomposition was analyzed by a substrate-induced respiration method and monitoring of the complete decomposition after 14C pulse labelling of the maize plants. Both approaches indicated significant differences in the decomposition rate between the conventional varieties, but the transgenic Bt hybrid and its near-isogenic control could not be distinguished. Potential effects on the residue-colonizing bacterial and fungal communities were analyzed by the quantification of metabolically active microbial groups using taxon-specific primers and T-RFLP analyses of the small-subunit ribosomal RNA gene. Results obtained over three years and three sampling dates did not reveal significant differences between the transgenic hybrid and the control. The abundance of the metabolically active microbial groups varied between the varieties only in some cases and temporarily restricted. Similarly, T-RFLP analysis did not show an impact of the plant genotype on the bacterial and fungal community structure. In contrast, seasonal effects given by different sampling dates as well as varying soil properties within the field trial were identified as drivers of the microbial communities colonizing the rotting maize straw. Conclusively, the multi-resistant Bt hybrid MON 89034×MON 88017 did not indicate an adverse impact on straw decomposition and the involved microbial communities.

Improving the MicroResp™ substrate-induced respiration method by a more complete description of CO2 behavior in closed incubation wells

The MicroResp™ method allows soil respiration and microbial community physiological profiles to be determined colorimetrically in microplates. This method, however, neglects CO2 storage in the agar gel carrying the colorimetric indicator, and calcite dissolution associated with CO2-induced change in soil solution pH. Our objective was to improve the method by taking into account CO2 in the gel in the calculation of microbial respiration, describing the effect of microbial CO2 on the pH of the soil solution and calcite dissolution, and checking whether CO2 distribution among calcite, soil solution, air and gel is near equilibrium after incubation. We propose a thermodynamic equilibrium model describing (a) distribution of CO2 among calcite, soil solution, gel and air, (b) dissociations of water, carbonic acid, cresol red, and substrates in the gel and soil solution, (c) exchange of adsorbed cations with H3O+ in the gel, and (d) calcite dissolution in soil. In-gel experiments were designed to calibrate the model, quantify the rate of CO2 exchange with air, and compare conservation procedures. On-soil experiments were designed to check whether calcite dissolution is near equilibrium and whether the model predicts the effect of CO2 on the pH of the solution. In-microplate experiments were designed to assess the effects of incubation period and soil quantity on estimated microbial respiration. The model can describe the distribution and speciation of CO2 in the gel, the soil solution and the air space of each microplate well. Initial properties of the gel vary with storage: soda lime partly extracts CO2 supplied as NaHCO3, and dries out the gel, which can skew the calibration. When incubation is over, the proportion of microbial CO2 in the gel is higher at lower microbial respiration. Incubations shorter than 4h underestimate microbial respiration due to the slow diffusion of CO2 in the gel. CO2 in the soil solution cannot be overlooked; it decreases the soil pH and may promote calcite dissolution in calcareous soil. It is important to precisely estimate initial CO2 air fraction and to control temperature, which affects both thermodynamic constants and microorganisms.

Evaluation of the Effects of Soil Depth on Microbial Activity in Three Agroecosystems in Venezuela

The objective of this study was to determine the total organic carbon (TOC), basal respiration (BR), microbial biomass carbon (MBC), the metabolic quotient (qCO2), and ratio of microbial biomass carbon to total organic carbon (MBC/TOC) at different soil depths in three agroecosystems in Fundo Zamorano Independencia, Cojedes State, Venezuela. The types of agroecosystems studied were short-cycle corn and bean cropland, pastureland, and gallery forest, which was used as the control soil. The BR was determined by measuring the carbon dioxide released in a closed system, and the MBC was analyzed by applying the substrate-induced respiration method. The BR and the MBC varied depending on depth and transect position, decreasing between 18% and 38% at a depth of >10 cm with respect to the measurements taken between 0 and 10 cm. Significantly greater BR and MBC values were found at the middle transect position, where the soils with the greatest moisture content are located. No significant differences for BR, MBC, and qCO2 values were found among the agroecosystems. This was due to the high spatial variability of the physical and chemical properties of the soils in the study site. However, with the application of the Kruskal–Wallis test, significant differences for the TOC and MBC/TOC values were found among the agroecosystems. These results highlight the importance of the type of plant cover over soil in different agroecosystems, mainly on pastureland, which maintains the potential of the inocular mycorrhizal of the soil.