Use of Rice Husk as Bulking Agent in Bioremediation of Automobile Gas Oil Impinged Agricultural Soil

Evaluation of rice husk (RH) as bulking agent in bioremediation of automobile gas oil (AGO) hydrocarbon polluted agricultural soil using renewal by enhanced natural attenuation (RENA) as control was the subject of the present investigation. The effect of different parameters such as total petroleum hydrocarbon (TPH), dehydrogenase activity (DHA), optical density and pH on bioremediation performance were evaluated. The studied parameters such as microbial dynamics, percentage degradation and DHA were found to be higher in RH-amended system and differed significantly with control at P < 0.05. RH resulted in high removal efficiency of 97.85 ± 0.93% under a two-month incubation period, while RENA had lesser removal efficiency of 53.15 ± 3.81%. Overall hydrocarbon biodegradation proceeded very slowly in the RENA particularly from week 0 to 4. Experimental data perfectly fitted into the first-order kinetic and generated high r² values (0.945), first-order degradation constant (0.47 day^?1), and shorter degradation half-life (1.50 d)—t_1/2 = Ln2/K and Ln2 numerically equals to 0.693 and hence written as 0.693/K. Micrococcus luteus and Rhizopus arrhizus were isolated in the present study, which displayed extreme AGO hydrocarbon biodegradative abilities. The use of RH in hydrocarbon-polluted soil significantly increased biodegradation rate and resulted in effective AGO cleanup within 2 months period. Therefore, RH provides an alternative source of bioremediation material in field application for abundant petroleum hydrocarbon soil pollution.     

Biostimulation and Phytoremediation Treatment Strategies of Gasoline-Nickel Co-Contaminated Soil

This study investigated the potential effect of poultry dung (biostimulation) and stubborn grass (Sporobolus pyramidalis) (phytoremediation) on microbial biodegradation of gasoline and nickel uptake in gasoline-nickel-impacted soil. In addition, the potential stimulatory effects of nickel on hydrocarbon utilization were investigated over a small range of nickel concentrations (2.5–12.5 mg/kg). The results showed that an increase in nickel concentration increased hydrocarbon degraders in soil by a range of 8.4–17.2% and resulted in a relative increase in gasoline biodegradation (57.5–62.4%). Also, under aerobic conditions, total petroleum hydrocarbons’ (TPH) removal was 62.4% in the natural gasoline-nickel microcosm (natural attenuation), and a maximum of 78.5%, 85.7%, and 95.8% TPH removal was obtained in phytoremediation, biostimulation, and a combination of biostimulation- and phytoremediation-treated microcosms, respectively. First-order kinetics described the biodegradation of gasoline and nickel uptake very well. Half-life times obtained were 28.88, 18.24, 14.44, and 8.56 days for gasoline degradation under natural attenuation, phytoremediation, biostimulation, and combined biostimulation and phytoremediation treatment methods, respectively. The results indicate that these remediation methods have promising potential for effective remediation of soils co-contaminated with petroleum hydrocarbons and heavy metals.     

Kinetic Modelling and Half-Life Study on Bioremediation of Soil Co-Contaminated with Lubricating Motor Oil and Lead Using Different Bioremediation Strategies

The focus of this study was to investigate the effect of nutrient supplement (urea fertilizer) and microbial species augmentation (mixed culture of Aeromonas, Micrococcus, and Serratia sp.) on biodegradation of lubricating motor oil (LMO) and lead uptake by the autochthonous microorganism in LMO and lead-impacted soil were investigated. The potential inhibitory effects of lead on hydrocarbon utilization were investigated over a wide range of lead concentrations (25–200 mg/kg) owing to the complex co-contamination problem frequently encountered in most sites. Under aerobic conditions, total petroleum hydrocarbons (TPH) removal was 45.3% in the natural attenuation microcosm while a maximum of 72% and 68.2% TPH removal was obtained in biostimulation and bioaugmentation microcosms, respectively. Lead addition, as lead nitrate, to soil samples reduced the number of hydrocarbon degraders in all samples by a wide range (11–52%) depending on concentration and similarly, the metabolic activities were affected as observed in mineralization of LMO (3–60%) in soils amended with various lead concentrations. Moreover, the uptake of lead by the autochthonous microorganisms in the soil reduced with increase in the initial lead concentration. First-order kinetics described the biodegradation of LMO very well. The biodegradation rate constants were 0.015, 0.033, and 0.030 day^?1 for LMO degradation in natural attenuation, biostimulation and bioaugmentation treatment microcosms, respectively. The presence of varying initial lead concentration reduced the biodegradation rate constant of LMO degradation in the biostimulation treatment microcosm. Half-life times were 46.2, 21, and 23 days for LMO degradation in natural attenuation, biostimulation and bioaugmentation treatment microcosms, respectively. The half-life time in the biostimulation treatment microcosm was increased with a range between 10.7 and 39.2 days by the presence of different initial lead concentration. The results have promising potential for effective remediation of soils co-contaminated with hydrocarbons and heavy metals.     

Bioremediation (natural attenuation and biostimulation) of diesel-oil-contaminated soil in an alpine glacier skiing area.

We investigated the feasibility of bioremediation as a treatment option for a chronically diesel-oil-polluted soil in an alpine glacier area at an altitude of 2,875 m above sea level. To examine the efficiencies of natural attenuation and biostimulation, we used field-incubated lysimeters (mesocosms) with unfertilized and fertilized (N-P-K) soil. For three summer seasons (July 1997 to September 1999), we monitored changes in hydrocarbon concentrations in soil and soil leachate and the accompanying changes in soil microbial counts and activity. A significant reduction in the diesel oil level could be achieved. At the end of the third summer season (after 780 days), the initial level of contamination (2,612 +/- 70 microg of hydrocarbons g [dry weight] of soil(-1)) was reduced by (50 +/- 4)% and (70 +/- 2)% in the unfertilized and fertilized soil, respectively. Nonetheless, the residual levels of contamination (1,296 +/- 110 and 774 +/- 52 microg of hydrocarbons g [dry weight] of soil(-1) in the unfertilized and fertilized soil, respectively) were still high. Most of the hydrocarbon loss occurred during the first summer season ([42 +/- 6]% loss) in the fertilized soil and during the second summer season ([41 +/- 4]% loss) in the unfertilized soil. In the fertilized soil, all biological parameters (microbial numbers, soil respiration, catalase and lipase activities) were significantly enhanced and correlated significantly with each other, as well as with the residual hydrocarbon concentration, pointing to the importance of biodegradation. The effect of biostimulation of the indigenous soil microorganisms declined with time. The microbial activities in the unfertilized soil fluctuated around background levels during the whole study.     

Microbial Activity and 13C/12C Ratio as Evidence of N-Hexadecane and N-Hexadecanoic Acid Biodegradation in Agricultural and Forest Soils

The dynamics of microbial degradation of exogenous contaminants, n-hexadecane and its primary microbial oxidized metabolite, n-hexadecanoic (palmitic) acid, was studied for topsoils, under agricultural management and beech forest on the basis the changes in O₂ uptake, CO₂ evolution and its associated carbon isotopic signature, the respiratory quotient (RQ) and the priming effect (PE) of substrates. Soil microbial communities in agricultural soil responded to the n-hexadecane addition more rapidly compared to those of forest soil, with lag-periods of about 23 ± 10 and 68 ± 13 hours, respectively. Insignificant difference in the lag-period duration was detected for agricultural (t_lag = 30 ± 13 h) and forest (t_lag = 30 ± 14 h) soils treated with n-hexadecanoic (palmitic) acid. These results demonstrate that the soil microbiota has different metabolic activities for using n-hexadecane as a reductive hydrocarbon and n-hexadecanoic acid as a partly oxidized hydrocarbon. The corresponding δ¹³C of respired CO₂ after the addition of the hydrocarbon contaminants to soils indicates a shift in microbial activity towards the consumption of exogenous substrates with a more complete degradation of n-hexadecane in the agricultural soil, for which some initial contents of hydrocarbons are inherent. It is supposed that the observed deviation of RQ from theoretically calculated value under microbial substrate mineralization is determined by difference in the time (Δt_i) of registration of CO₂ production and O₂ consumption. Positive priming effect (PE) of n-hexadecane and negative PE of n-hexadecanoic (palmitic) acid were detected in agricultural and forest soils. It is suggested that positive PE of n-hexadecane is conditioned by the induction of microbial enzymes that perform hydroxylation/oxygenation of stable SOM compounds mineralized by soil microbiota to CO₂. The microbial metabolism coupled with oxidative decarboxylation of n-hexadecanoic acid is considered as one of the most probable causes of the revealed negative PE value.     

Bioremediation (Natural Attenuation and Biostimulation) of Diesel-Oil-Contaminated Soil in an Alpine Glacier Skiing Area

We investigated the feasibility of bioremediation as a treatment option for a chronically diesel-oil-polluted soil in an alpine glacier area at an altitude of 2,875 m above sea level. To examine the efficiencies of natural attenuation and biostimulation, we used field-incubated lysimeters (mesocosms) with unfertilized and fertilized (N-P-K) soil. For three summer seasons (July 1997 to September 1999), we monitored changes in hydrocarbon concentrations in soil and soil leachate and the accompanying changes in soil microbial counts and activity. A significant reduction in the diesel oil level could be achieved. At the end of the third summer season (after 780 days), the initial level of contamination (2,612 ± 70 μg of hydrocarbons g [dry weight] of soil⁻¹) was reduced by (50 ± 4)% and (70 ± 2)% in the unfertilized and fertilized soil, respectively. Nonetheless, the residual levels of contamination (1,296 ± 110 and 774 ± 52 μg of hydrocarbons g [dry weight] of soil⁻¹ in the unfertilized and fertilized soil, respectively) were still high. Most of the hydrocarbon loss occurred during the first summer season ([42 ± 6]% loss) in the fertilized soil and during the second summer season ([41 ± 4]% loss) in the unfertilized soil. In the fertilized soil, all biological parameters (microbial numbers, soil respiration, catalase and lipase activities) were significantly enhanced and correlated significantly with each other, as well as with the residual hydrocarbon concentration, pointing to the importance of biodegradation. The effect of biostimulation of the indigenous soil microorganisms declined with time. The microbial activities in the unfertilized soil fluctuated around background levels during the whole study.     

Relationships of respiratory quotient to microbial biomass and hydrocarbon contaminant degradation during soil bioremediation

This work focused on monitoring respiratory quotient, RQ (defined as a ratio of CO₂ production to O₂ uptake rates), microbial growth and residual hydrocarbon concentration during bioremediation experiments performed on laboratory soil microcosms. The aim of the study was to determine if the time course biodegradation profile of the contaminant can be related to the RQ evolution and to investigate the effect of the water content on RQ measurements. A natural soil was artificially contaminated with hexadecane and adjusted with inorganic nutrients to stimulate biodegradation. Microbial growth, CO₂ production, O₂ uptake and residual hexadecane were periodically monitored at different soil water contents ranging from 0.15 to 0.35 g water g⁻¹ of dry soil. Results showed that microbial activity and contaminant degradation were strongly dependent on soil water content. Maximal growth and hexadecane depletion were obtained at a water content of 0.20 g water g⁻¹ of dry soil, which corresponded to 46.6% of the water holding capacity. Hexadecane degradation was considerably reduced with increasing soil water content. RQ values fluctuated as a function of the hexadecane biodegradation phases. The lowest RQs corresponded to the highest hexadecane depletion and microbial growth. The water content variation did not significantly affect the shape of the RQ evolution curves as a function of time. It only modified the magnitude of RQ values. This study indicates that additional biological and chemical analyses are needed to support RQ data when monitoring contaminant degradation to have an accurate understanding of all the biotic processes, which may occur simultaneously.     

Evaluation of microbial diversity of different soil layers at a contaminated diesel site

In this study, we evaluated the hydrocarbon removal efficiency and microbial diversity of different soil layers. The soil layers with high counts of recoverable hydrocarbon degrading bacteria had the highest hydrocarbon removal rate compared with soil layers with low counts of hydrocarbon degrading bacteria. Removal efficiency was 48% in the topsoil, compared with 31% and 11% at depths of 1.5 and 1 m, respectively. In the 1 and 1.5 m soil layers, there was no significant difference between total petroleum hydrocarbon (TPH) removal in nutrient amended treatments and controls. The respiration rate reflected the difference in the number of bacteria in each soil layer and the availability of nutrients. High O₂ consumption corresponded positively with high TPH removal. Analysis of the microbial diversity in the different soil layers using functional diversity (community-level physiological profile, via Biolog) and genetic diversity using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) of 16S rDNA revealed differences in, respectively, substrate utilisation patterns and DGGE profiles of 16S rDNA fragments. Microbial diversity as revealed by DNA fragments was lower in the highly contaminated soil layer (1.5 m) than in the topsoil and at 1 m.     

Controlling factors of temporal variation of soil respiration in a natural beech forest as revealed by natural incubation experiments

Microorganisms decompose organic substrates to obtain energy. This process releases carbon dioxide (CO₂) and is the main cause of soil carbon release in terrestrial ecosystems. Fluctuations in microbial composition significantly affect net CO₂ emissions in forests. Because factors are cross-correlated, addressing how they affect soil respiration (Rs), both directly and indirectly, is challenging. In this study, Rs-impacting soil properties, including soil organic carbon (SOC), soil organic nitrogen (SON), microbial diversity (Dsim, indicated by Simpson’s diversity index), and total microbial DNA concentration in natural beech forests were examined by structure equation modeling (SEM), which can explicitly evaluate the causal relationships among interacting variables. The results showed that decreasing Dsim, soil temperature, SOC, and SON clearly had direct and indirect effects on Rs under natural conditions. Increasing temperature was a primary factor and promoted a decrease in Rs during the growth season. Dsim was the only parameter with a direct positive effect on Rs, indicating that microbial diversity could accurately predict Rs. Soil nutrient factors indirectly affected Rs through Dsim, which was also affected by soil physical and chemical properties. Significant covariance between SON and Rs (0.42, p < 0.001) indicated multiple interacting variables affecting soil activity. Although the current study suggests that SEM can clarify complex functional processes related to Rs, future studies should consider additional impacting variables such as vegetation properties and enzyme dynamics.     

Empirical Models Estimating Carbon Dioxide Accumulation in Two Petroleum Hydrocarbon–Contaminated Soils

Bioremediation is a popular method in degrading diesel fuel contaminants from soil. Bioremediation can be enhanced by estimating the effect of important environmental parameters on microbial activity. Respirometry was used to develop empirical models describing the effects of temperature, moisture, nitrogen, and phosphorus concentration on microbial activity in a diesel-contaminated soil from Wyoming. Carbon dioxide (CO₂) data were analyzed using a base equation where its coefficient values were functions of each parameter. Two physiologically different groups of microorganisms were identified from the results under different operating temperatures. The empirical correlations were combined into one model and this model was tested against a hydrocarbon-contaminated soil collected from a site in Egypt with similar history of contamination. The predicted CO₂ evolution agreed well with the actual data obtained from the Egyptian soil samples, showing a sound predicting power of the empirical model for petroleum hydrocarbon biodegradation. Overall, the empirical correlations developed from the respirometric data provide a method to describe microbial activity in diesel-contaminated soils.     

Effects of elevated atmospheric CO2 on soil microbiota in calcareous grassland

Microbial responses to three years of CO₂ enrichment (600 μL L^–1) in the field were investigated in calcareous grassland. Microbial biomass carbon (C) and soil organic C and nitrogen (N) were not significantly influenced by elevated CO₂. Microbial C:N ratios significantly decreased under elevated CO₂ (– 15%, P = 0.01) and microbial N increased by + 18% (P = 0.04). Soil basal respiration was significantly increased on one out of 7 sampling dates (+ 14%, P = 0.03; December of the third year of treatment), whereas the metabolic quotient for CO₂ (qCO₂ = basal respiration/microbial C) did not exhibit any significant differences between CO₂ treatments. Also no responses of microbial activity and biomass were found in a complementary greenhouse study where intact grassland turfs taken from the field site were factorially treated with elevated CO₂ and phosphorus (P) fertilizer (1 g P m^–2 y^–1). Previously reported C balance calculations showed that in the ecosystem investigated growing season soil C inputs were strongly enhanced under elevated CO₂. It is hypothesized that the absence of microbial responses to these enhanced soil C fluxes originated from mineral nutrient limitations of microbial processes. Laboratory incubations showed that short-term microbial growth (one week) was strongly limited by N availability, whereas P was not limiting in this soil. The absence of large effects of elevated CO₂ on microbial activity or biomass in such nutrient-poor natural ecosystems is in marked contrast to previously published large and short-term microbial responses to CO₂ enrichment which were found in fertilized or disturbed systems. It is speculated that the absence of such responses in undisturbed natural ecosystems in which mineral nutrient cycles have equilibrated over longer periods of time is caused by mineral nutrient limitations which are ineffective in disturbed or fertilized systems and that therefore microbial responses to elevated CO₂ must be studied in natural, undisturbed systems.     

Functional adaptation of microbial communities from jet fuel-contaminated soil under bioremediation treatment: simulation of pollutant rebound

To investigate the link between the functionality and the diversity of microbial communities under strong selective pressure from pollutants, two types of mesocosms that simulate natural attenuation and phytoremediation were generated using soil from a site highly contaminated with jet fuel and under air-sparging treatment. An increase in the petroleum hydrocarbon concentration from 4900 to 18,500 mg kg(-1) dw soil simulated a pollutant rebound (postremediation pollutant reversal due to residual contamination). Analysis of soil bacterial communities by denaturing gradient gel electrophoresis of PCR-amplified 16S rRNA gene fragments showed stronger changes and selection for a phylogenetically diverse microbial population in the mesocosms with pollutant-tolerant willow trees. Enumeration of the main subfamilies of catabolic genes characteristic to the site detected a rapid increase in the degradation potential of both systems. A marked increase in the abundance of genes encoding extradiol dioxygenases with a high affinity towards various catecholic substrates was found in the planted mesocosms. The observed adaptive response to the simulated pollutant rebound, characterized by increased catabolic gene abundance, but with different changes in the microbial structure, can be explained by functional redundancy in biodegrading microbial communities.     

Bioremediation strategies for diesel and biodiesel in oxisol from southern Brazil

Leaks and spillages during the extraction, transport and storage of petroleum and its derivatives may result in environmental contamination. Biodiesel is an alternative energy source that can contribute to a reduction in environmental pollution. The aim of the present work was to evaluate biodegradation of diesel, biodiesel, and a 20% biodiesel-diesel mixture in oxisols from southern Brazil, using two bioremediation strategies: natural attenuation and bioaugmentation/biostimulation. Fuel biodegradation was monitored over 60 days by dehydrogenase activity, CO₂ evolution and gas chromatography. The bacterial inoculum employed for bioaugmentation/biostimulation consisted of Bacillus megaterium, Bacillus pumilus, Pseudomonas aeruginosa, and Stenotrophomonas maltophilia and PCR-DGGE using 16S RNAr primers showed that some members of this consortium survived in the soil after 60 days. The biodegradation of pure biodiesel was higher for bioaugmentation/biostimulation than for natural attenuation, suggesting that the addition of the microbial consortium, together with adjustment of the macronutrient ratio, increased biodiesel degradation. The results of dehydrogenase and respiratory activity, together with GC analysis, suggested that the presence of biodiesel may, by stimulating general microbial degradative metabolism, increase the biodegradation of petroleum diesel. The microbial community was altered by both treatments, with natural attenuation producing a lower diversity index than the amended soil. The bioaugmentation/biostimulation strategy was showed to have a high potential for cleaning up soils contaminated with diesel and biodiesel blends.     

In situ bioremediation of hydrocarbon in soil

Laboratory and field experiments were carried out for bioremediation of soils contaminated by fuel oil and motor oil. Bioventing was combined with the application of selected bacteria and dissolved nutrients. In the field experiments, soil gas was evacuated by air pumps from the permeable boreholes. The process was followed by both soil and gas analysis. Biodegradation of oil contamination and the microbial activity was measured by the oil and cell concentration in the soil. In 2 months, the oil content decreased considerably, and the cell number increased by one order of magnitude or more. The evacuated gas was tested for CO₂, O₂ and volatilized hydrocarbon content. The CO₂ level proves the presence of biodegradation: a permanent high value about ten times higher than normal, could be measured for 2 months, followed by a slow decrease in the third month. Volatilized hydrocarbon content was the highest in the first 2 d. After a continuous decrease, it dropped under the threshold of measurability for the third month. Selective biodegradation of hydrocarbon mixtures (oily wastes) was investigated as well: gas Chromatographic oil analysis showed the changes in the oil composition. The appropriate microflora was working in an ideal commensalism, and as a result, all of the hydrocarbon components were degraded nearly to the same extent.     

氮素添加和刈割对内蒙古弃耕草地土壤氮矿化的影响

以内蒙古多伦县恢复生态学试验示范研究站弃耕10余年的草地为研究对象,于2006年起分别设置对照、氮素添加、刈割和氮素添加+刈割4种处理,每种处理6次重复,研究弃耕草地氮素添加和刈割对土壤氮矿化的影响,结合土壤理化性质和植被地上生产力的动态变化,分析弃耕草地土壤氮矿化对植被恢复的响应,为当地草地恢复与重建提供理论依据和数据支持。实验结果表明:1氮素添加显著增加了植物地上净初级生产力(ANPP)和土壤无机氮库,与对照相比分别提高115%和196%,同时显著提高了土壤总硝化速率;但是氮素添加对总氨化速率、土壤微生物生物量碳(MBC)、微生物生物量氮(MBN)、微生物生物量碳氮比(MBC/MBN)、微生物呼吸(MR)以及呼吸熵(q CO2)均无显著影响;2总氨化速率和硝化速率对刈割处理的响应均不显著,但是刈割处理显著降低了土壤MR(P0.05);3氮素添加+刈割处理5—7a后,土壤总氨化和硝化速率均无显著变化;但是氮素添加+刈割处理显著增加了ANPP、土壤无机氮库和q CO2,同时显著降低了MBC和MBC/MBN。这说明在弃耕草地适应性管理中,氮素添加可以显著提高草地生产力,但是长期的氮添加对土壤微生物氮的转化是否有利还值得我们进一步研究。     

Decontamination of soil containing oil by natural attenuation,phytoremediation and chemical desorption

Abstract

An experiment was performed for 240 days to evaluate the oil removal through natural attenuation (NA) and phytoremediation (PH) combined with surfactant (SF), in soil up to 76,585?mg kg^?1 of total petroleum hydrocarbons (TPH). A completely randomized design was applied using a 4?×?6 factorial arrangement, with four concentrations of oil and six recovery technologies. The technologies were combinations of Leersia hexandra (Lh) grass, NA (native microorganisms), and doses of Tween^® 80. The results recorded treatment means with statistical differences (Tukey, p?≤?0.05 and 0.01). Oil in presence of 5% SF stimulated the formation of grass roots. The SF promoted a significant increase in the biomass of grass stems and leaves but did not contribute to oil removal or microbial density. Unexpectedly, the PH inhibited the removal of oil and induced a decrease in fungi, hydrocarbonoclastic bacteria, and heterotrophic fungi. NA combined with 2.5% SF removed 95% of 48,748?mg of TPH. The best technology for soil decontamination was bioremediation through hydrocarbonoclastic bacteria stimulated with 2.5% SF.     

增加降水对干旱河谷区云南松人工林土壤呼吸的影响

2013年5月至2014年6月,对干旱河谷区云南松(Pinus yunnanensis)人工林进行增加降水试验,试验设置对照(CK,0 mm m~(-2)a~(-1))、增水10%(A1,80 mm m~(-2)a~(-1))、增水20%(A2,160 mm m~(-2)a~(-1))和增水30%(A3,240 mm m~(-2)a~(-1))4个处理水平。采用LI-8100开路式土壤碳通量测量系统测定每月土壤呼吸速率。结果表明,4个处理云南松人工林土壤呼吸速率均呈明显的季节变化,7月最高,2月最低。与CK相比,A1年均土壤呼吸速率无显著性差异(P0.05),A2显著增加了12.88%(P0.05),而A3明显减少了17.71%(P0.05)。3个增水处理均提高了土壤呼吸的温度敏感性,减弱了土壤呼吸与土壤湿度的关系。与土壤温度相比,土壤湿度对土壤呼吸的影响相对较小。增水增加了湿季土壤微生物碳、氮含量,干季对微生物碳含量无影响,但明显降低了微生物氮含量。这说明,降水增加对干旱河谷区云南松人工林土壤呼吸的影响是不尽相同的,适当的增水会促进土壤呼吸,而过量的增水会抑制土壤呼吸。     

A comparison of soil qualities of different revegetation types in the Loess Plateau, China

Serious soil erosion has resulted in widespread land degradation in the Loess Plateau of China. In the past two decades, great efforts have been made to restore degraded soil such as reconverting croplands into forestlands or grasslands. A comparison of soil qualities of different revegetation types has important implications in soil reclamation. Our study investigated the effect of different revegetation types on the physicochemical and microbial soil properties in the Loess Plateau, with the aim of determining which revegetation type has the best capacity for soil recovery. The vegetation types included two shrublands (Caragana korshinskii and Hippophae rhamnoides), two grasslands (Astragalus adsurgens and Panicum virgatum), and two species from croplands that were abandoned for natural recovery (Artemisia capillaries and Heteropappus altaicus). Among the plants studied, H. altaicus and A. capillaries had the highest values of soil organic C, total N, total P, available N, available P, moisture content, microbial biomass C (MBC), substrate-induced respiration, saccharase, urease, catalase, and peroxidase. Soil sampled from the A. adsurgens plot had the highest bulk density and microbial biomass N, and soil from the H. rhamnoides plot had the highest metabolic quotient (basal respiration/MBC). The soil quality index, which was obtained based on the available N, metabolic quotient, MBC, urease, polyphenol oxidase, and bulk density, shows that the abandoned cropland for natural recovery had the highest soil quality, followed by grassland, and then shrubland. Vegetation types affect the physicochemical and microbial properties of soils in arid climatic conditions. Abandoned cropland for natural recovery has the best capacity for improving soil quality in the Loess Plateau among all studied revegetation types. Our study suggests that in the Loess Plateau, natural recovery is the best choice for soil revegetation of sloping croplands.     

帽儿山地区不同土地利用方式下土壤-微生物-矿化碳氮化学计量特征

土地利用方式的变化导致土壤碳氮含量及其化学计量关系的变化,然而土壤微生物化学计量及其驱动的碳氮矿化过程如何响应这种变化仍不明确。以帽儿山地区天然落叶阔叶林、人工红松林、草地和农田4种不同土地利用类型为对象,测定其土壤有机碳(C_(soil))、全氮(N_(soil))、微生物生物量碳和氮(C_(mic)和N_(mic))、土壤碳和氮矿化速率(C_(min)和N_(min)),旨在比较不同土地利用方式对土壤、微生物碳氮化学计量特征及矿化速率的影响,探索土壤-微生物-矿化之间碳氮化学计量特征的相关性,揭示微生物对土壤碳氮化学计量变化的响应和调控机制。结果显示:C_(soil)、N_(soil)、C_(mic)、N_(mic)和C_(min)均呈现天然落叶阔叶林人工红松林草地农田,而天然落叶阔叶林和草地的N_(min)显著高于人工红松林和农田。土地利用方式显著影响土壤和微生物碳氮比(C∶N_(soil)和C∶N_(mic)),均呈现农田最高。不同土地利用方式的数据综合分析发现:碳氮矿化速率比与C∶N_(mic)呈负相关,而和微生物与土壤碳氮化学计量不平衡性(C∶N_(imb))显著正相关。单位微生物生物量的碳矿化速率(qCO_2)随着C∶N_(mic)的增加而降低,而单位微生物生物量的氮矿化速率(qAN)随着C∶N_(mic)的增加而增加。C∶N_(imb)与qCO_2正相关,与qAN负相关。以上结果表明,微生物会通过改变自身碳氮化学计量、调整碳氮之间相对矿化速率,以适应土地利用变化导致的土壤碳氮及其化学计量的变异性,以满足自身生长和代谢的碳氮需求平衡。     

Increases in soil respiration following labile carbon additions linked to rapid shifts in soil microbial community composition

Organic matter decomposition and soil CO₂ efflux are both mediated by soil microorganisms, but the potential effects of temporal variations in microbial community composition are not considered in most analytical models of these two important processes. However, inconsistent relationships between rates of heterotrophic soil respiration and abiotic factors, including temperature and moisture, suggest that microbial community composition may be an important regulator of soil organic matter (SOM) decomposition and CO₂ efflux. We performed a short-term (12-h) laboratory incubation experiment using tropical rain forest soil amended with either water (as a control) or dissolved organic matter (DOM) leached from native plant litter, and analyzed the effects of the treatments on soil respiration and microbial community composition. The latter was determined by constructing clone libraries of small-subunit ribosomal RNA genes (SSU rRNA) extracted from the soil at the end of the incubation experiment. In contrast to the subtle effects of adding water alone, additions of DOM caused a rapid and large increase in soil CO₂ flux. DOM-stimulated CO₂ fluxes also coincided with profound shifts in the abundance of certain members of the soil microbial community. Our results suggest that natural DOM inputs may drive high rates of soil respiration by stimulating an opportunistic subset of the soil bacterial community, particularly members of the Gammaproteobacteria and Firmicutes groups. Our experiment indicates that variations in microbial community composition may influence SOM decomposition and soil respiration rates, and emphasizes the need for in situ studies of how natural variations in microbial community composition regulate soil biogeochemical processes.