Changes of microbial properties in (near-) rhizosphere soils after phytoextraction by Sedum alfredii H: A rhizobox approach with an artificial Cd-contaminated soil

The ultimate goal of soil remediation is to restore soil health. Soil microbial parameters are considered to be effective indicators of soil health. The aim of this study was to determine the effects of phytoextraction on microbial properties through the measurement of soil microbial biomass carbon, soil basal respiration and enzyme activities. For this purpose, a pre-stratified rhizobox experiment was conducted with the Cd hyperaccumulator Sedum alfredii H. for phytoextraction Cd from an artificial contaminated soil (15.81 mg kg⁻¹) under greenhouse conditions. The plant and soil samples were collected after growing the plant for three and six months with three replications. The results indicated that the ecotype of S. alfredii H. originating from an ancient silver mining site was a Cd-hyperaccumulator as it showed high tolerance to Cd stress, the shoot Cd concentration were as high as 922.6 mg kg⁻¹ and 581.9 mg kg⁻¹ at the two samplings, and it also showed high BF (58.4 and 36.8 after 3 and 6 months growth), and TF (5.8 and 5.1 after 3 and 6 months growth). The amounts of Cd accumulated in the shoots of S. alfredii reached to an average of 1206 μg plant⁻¹ after 6 months growth. Basal respiration, invertase and acid phosphatase activities of the rhizosphere soil separated by the shaking method were significantly higher (P < 0.01) than that of the near-rhizosphere soil and the unplanted soil after 3 months growth, so were microbial biomass carbon, urease, invertase and acid phosphatase activities of the rhizosphere soil after 6 months growth. Acid phosphatase activity of the 0–2 mm sub-layer rhizosphere soil collected by the pre-stratified method after 3 months growth was significantly higher (P < 0.05) than that of other sub-layer rhizosphere soils and bulk soil, and so were microbial biomass carbon, basal respiration, urease, invertase and acid phosphatase activities of the 0–2 mm sub-layer rhizosphere soil after 6 months growth. It was concluded that phytoextraction by S. alfredii could improve soil microbial properties, especially in rhizosphere, and this plant poses a great potential for the remediation of Cd contaminated soil.     

Stability of soil organic matter under long-term biosolids application

Little is know on the impact of biosolids application on soil organic matter (SOM) stability, which contributes to soil C sequestration. Soil samples were collected in 2006 at plow layer from fields that received liquid and dry municipal biosolids application from 1972 to 2004 at the cumulative rate of 1416 Mg ha⁻¹ in mined soil and 1072 Mg ha⁻¹ in nonmined soil and control fields that received chemical fertilizer at Fulton County, western Illinois. The biosolids application increased the soil microbial biomass C (SMBC) by 5-fold in mined soil and 4-fold in nonmined soil. The biosolids-amended soils showed a high amount of basal respiration and N mineralization, but low metabolic quotient, and low rate of organic C and organic N mineralization. There was a remarkable increase in mineral-associated organic C from 6.9 g kg⁻¹ (fertilizer control) to 26.6 g kg⁻¹ (biosolids-amended) in mined soil and from 8.9 g kg⁻¹ (fertilizer control) to 23.1 g kg⁻¹ (biosolids-amended) in nonmined soil. The amorphous Fe and Al, which can improve SOM stability, were increased by 2–7 folds by the long-term biosolids application. It is evident from this study that the biosolids-modified SOM resists to decomposition more than that in the fertilizer treatment, thus long-term biosolids application could increase SOM stability.     

Emission of carbon dioxide and dynamics of inorganic N in a gradient of alkaline saline soils of the former lake Texcoco

A plan was developed to apply biosolid to soil of the former lake Texcoco to fertilize the pioneer vegetation. Because, no information exists about how differences in electrolytic conductivity (EC) might affect mineralization of biosolid and dynamics of C and N in soil, 20 soil samples forming a gradient in EC ranging from 22 to 150 dS m⁻¹ were characterized, amended with 500 mg biosolid C kg⁻¹ dry soil and incubated aerobically at 22 ± 2 °C while production of CO₂, concentrations of ammonium (NH₄⁺), nitrite (NO₂⁻), and nitrate (NO₃⁻), and NH₃ volatilization were monitored at 22 ± 2 °C for 70 days. Soil characteristics showed large variations with maximum values often >10-times larger than minimum values. The production of CO₂ in the unamended soil ranged from 25 to 159 mg CO₂-C kg⁻¹ day⁻¹ and NH₃ volatilization from 0 to 189 μg NH₃-N kg⁻¹ day⁻¹_. Application of biosolid increased production of CO₂ significantly 1.4-fold and volatilization of NH₃ 11.5-fold. The EC explained most of the variation in production of CO₂, while particle size distribution explained most of the variation in volatilization of NH₃. The concentration of NH₄⁺ in the biosolid-amended soil decreased sharply in the first 14 days, with the EC explaining most of the variation found, and remained constant thereafter with a small increase at day 70. Significant increases in the concentration of NO₃⁻ were generally found in soil with EC < 64 dS m⁻¹. The EC explained most of the variation in production of CO₂, and dynamics of NH₄⁺ and NO₃⁻ while clay positively and sand content negatively affected NH₃ volatilization. It was found that increases in EC inhibited C and N mineralization in soil of the former lake Texcoco.     

Soil losses due to cassava and sweet potato harvesting: A case study from low input traditional agriculture

Soil loss due to crop harvesting (SLCH) has been established as an important soil erosion process that has significantly contributed to soil degradation in highly mechanised agriculture. This has stimulated the need to investigate the importance of this process of erosion under low input agriculture where, until now, only water and tillage erosion are known as important phenomena causing soil degradation. This study was conducted in Eastern Uganda with the following objectives: (1) to assess the amount of soil lost due to the harvesting of cassava roots and sweet potato tubers under low input agriculture, (2) to look into the factors that influence variations in these soil losses, and (3) to estimate the amount of plant nutrients lost due to SLCH for cassava and sweet potato. Soil sticking to roots and tubers was washed and the soil suspension oven dried to estimate the amount of soil lost after harvesting. Mean annual soil loss for cassava was 3.4 tonnes ha⁻¹ and for sweet potato was 0.2 tonnes ha⁻¹. Ammonium acetate lactate extractable soil nutrient losses for cassava were N = 1.71 kg ha⁻¹ harvest⁻¹, P = 0.16 kg ha⁻¹ harvest⁻¹, K = 1.08 kg ha⁻¹ harvest⁻¹ and for sweet potato were N = 0.14, P = 0.01 kg ha⁻¹ harvest⁻¹, K = 0.15 kg ha⁻¹ harvest⁻¹. Difference in soil loss due to crop harvesting for cassava and sweet potato could be due to: (1) smaller yields of sweet potato leading to smaller soil losses on an area basis, (2) smoother skin and less kinked morphology of sweet potato that allowed less soil to adhere, and (3) the fact that sweet potato is planted in mounds which dry out faster compared to the soil under cassava. Soil moisture content at harvesting time and crop age were significant factors that explained the variations in the soil lost at cassava harvesting. Soil loss under cassava justifies the need to conduct further investigations on this process of soil erosion under low input agriculture.     

Effects of azoxystrobin,chlorothalonil, and ethoprophos on the reproduction of three terrestrial invertebrates using a natural Mediterranean soil

The potential terrestrial toxicity of three pesticides, azoxystrobin, chlorothalonil, and ethoprophos was evaluated using reproduction ecotoxicological tests with different non-target species: the collembolan Folsomia candida, the earthworm Eisenia andrei, and the enchytraeid Enchytraeus crypticus. All reproduction tests were performed with natural soil from a Mediterranean agricultural area (with no pesticide residues) in order to improve the relevance of laboratory data to field conditions. Controls were performed with natural and standard artificial soil (OECD 10% OM). The fungicide azoxystrobin showed the highest toxicity to earthworms (EC₅₀ = 42.0 mg a.i. kg⁻¹ dw soil). Collembolans were the most sensitive taxa in terms of sublethal effects of chlorothalonil with an EC₅₀ of 31.1 mg a.i. kg⁻¹ dw soil followed by the earthworms with an EC₅₀ of 40.9 mg a.i. kg⁻¹ dw soil. The insecticide ethoprophos was the most toxic to collembolans affecting their reproduction with an EC₅₀ of 0.027 mg a.i. kg⁻¹ dw soil. Enchytraeids were generally the least sensitive of the three species tested for long-term effects. Earthworms were not always the most sensitive species, emphasizing the need to increase the number of mandatory assays with key non-target organisms in the environmental risk assessment of pesticides.     

Soil available phosphorus status determines indigenous mycorrhizal colonization of field and glasshouse-grown spring wheat from Argentina

Wheat production (Triticum aestivum L.) has increased across the world during last century with the intensification of agriculture. Phosphorus (P) fertilization is a common practice to improve wheat growth in Argentina. We investigate whether indigenous arbuscular mycorrhizal colonization (AMC) of hard red spring wheat is controlled by shoot P content (SPc) or by available soil P in an agricultural soil from the southeastern Argentine Pampas. In the field, AMC was monitored four times during two growing seasons of a conventional wheat crop. Treatments were: without P supply, annual supply of 11 and 22 kg P ha⁻¹ during the last 5 years, and 164 kg P ha⁻¹ applied once 5 years before the experiment. In the glasshouse, AMC was assessed three times in wheat growing in pots filled with the soil from unfertilized plots; treatments were: P (0 and 20 mg P pot⁻¹), and nitrogen (N) fertilization (0 and 150 mg N pot⁻¹). A range of soil P between 6 and 60 mg P kg⁻¹ was obtained and the AMC ranged from 1% to 67% of root length colonized under both field and glasshouse conditions. P supplied annually increased growth and SPc but decreased AMC. N fertilization did not affect growth or AMC. Variations in SPc did not account for AMC. Variability in AMC was best accounted for local current soil available P content (r² = 0.59). A linear-plateau relationship between soil P and indigenous AMC was established in wheat plants growing under contrasting environmental and experimental (field and glasshouse) conditions. Indigenous AMC was depressed by available soil P in the range 0–27 mg P kg⁻¹ (a decrease of 2.8% mg P⁻¹ kg⁻¹). Above 27 mg P kg soil⁻¹, AMC was stabilized at about 10%. Grain yield increased with fertilization and the highest relative shoot dry matter in field was obtained at 15.5 mg P kg soil⁻¹. The soil P range that ensures high wheat production without deterring indigenous AMC is discussed.     

Enhancement of arbuscular mycorrhizal fungus (Glomus versiforme) on the growth and Cd uptake by Cd-hyperaccumulator Solanum nigrum

Arbuscular mycorrhizal fungus (AMF) can enhance plant growth and resistance to toxicity produced by heavy metals (HMs), affect the bioavailability of HMs in soil and the uptake of HMs by plants, and thus has been emerged as the most prominent symbiotic fungus for contribution to phytoremediation. A greenhouse pot experiment was conducted to assess the effect of Glomus versiforme BGC GD01C (Gv) on the growth and Cd accumulation of Cd-hyperaccumulator Solanum nigrum in different Cd-added soils (0, 25, 50, 100 mg Cd kg⁻¹ soil). Mycorrhizal colonization rates were generally high (from 71% to 82%) in Gv-inoculated treatments at all Cd levels. Gv colonization enhanced soil acid phosphatase activity, and hence elevated P acquisition and growth of S. nigrum at all Cd levels. Moreover, the presence of Gv significantly increased DTPA-extractable (phytoavailable) Cd concentrations in 25 and 50 mg Cd kg⁻¹ soils, but did not affect phytoavailable Cd in 100 mg Cd kg⁻¹ soil. Similarly, inoculation with Gv significantly increased Cd concentrations of S. nigrum in 25 and 50 mg Cd kg⁻¹ soils, but decreased Cd concentrations of the plants in 100 mg Cd kg⁻¹ soil. Overall, inoculation with Gv greatly improved the total Cd uptakes in all plant tissues at all Cd levels. The present results indicated that S. nigrum associated with Gv effectively improved the Cd uptake by plant and would be a new strategy in microbe-assisted phytoremediation for Cd-contaminated soils.     

Characteristics of earthworms (Eisenia fetida) in PAHs contaminated soil amended with sewage sludge or vermicompost

Earthworms can be used to remove polycyclic aromatic hydrocarbons (PAHs) from soil, but this might affect their survival and they might accumulate the contaminants. Sterilized and unsterilized soil was contaminated with phenanthrene (Phen), anthracene (Anth) and benzo(a)pyrene (BaP), added with or without Eisenia fetida, sewage sludge or vermicompost. Survival, growth, cocoon formation and concentrations of PAHs in the earthworms were monitored for 70 days. Addition of sewage sludge to sterilized or unsterilized soil maintained the number of earthworms and their survival was 94%. The addition of sludge significantly increased the weight of earthworms 1.3 times compared to those kept in the unamended soil or in soil amended with vermicompost. The weight of earthworms was significantly lower in sterilized than in unsterilized soil. Cocoons were only detected when sewage sludge was added to unsterilized soil. A maximum concentration of 62.3 μg Phen kg⁻¹ was found in the earthworms kept in sterilized soil amended with vermicompost after 7 days and 22.3 μg Phen kg⁻¹ when kept in the unamended unsterilized soil after 14 days. Concentrations of Phen in the earthworms decreased thereafter and ≤2 μg kg⁻¹ after 28 days. A maximum Anth concentration of 82.5 μg kg⁻¹ was found in the earthworms kept in sterilized soil amended with vermicompost and 45.8 μg Anth kg⁻¹ when kept in the unamended unsterilized soil after 14 days. A maximum concentration of 316 μg BaP kg⁻¹ was found in the earthworms kept in sterilized soil amended with vermicompost after 56 days and 311 μg BaP kg⁻¹ when kept in the unsterilized soil amended with vermicompost after 28 days. The amount of BaP in the earthworm was generally largest after 28 days, but after 70 days still 60 μg kg⁻¹ was found in E. fetida when kept in the sterilized soil amended with sewage sludge. It was found that E. fetida survived in PAHs contaminated soil and accumulated only small amounts of the contaminants, but sewage sludge was required as food for its survival and cocoon production.     

Aggregate water-stability,particle-size and soil solution properties in conducive and suppressive soils to Fusarium wilt of banana from Canary Islands (Spain)

The influence of several soil properties on soil conduciveness or suppressiveness to disease caused by the soil fungus Fusarium oxysporum f. sp. cubense was studied in seven field plots of banana plantations, situated in Tenerife and Gran Canaria islands (Canary Islands, Spain). In each plot, soil samples were taken in conducive and suppressive areas to Fusarium wilt. Water-stable aggregates (WSA: 200–2000 μm diameter), soil particle size, and selected soil solution characteristics [pH, electric conductivity (EC) and soluble Na] were determined in the samples. Aggregate water-stability was higher in soils of conducive areas than in suppressive areas. The percentage of WSA in the conducive areas ranged from 460 to 330 g kg⁻¹, while in the suppressive areas the maximum value was 285 g kg⁻¹ and the minimum was 150 g kg⁻¹. The soils had high clay content and the EC and soluble Na tended to be higher in suppressive areas than in conducive areas. Soil solution pH was lower in conducive areas (except sites 1 and 9). Our data provide evidence that in different soil areas of the same plot, the structural stability of aggregates, presumably controlled in part by the clay fraction, soluble Na concentration and EC, is of great importance for the conduciveness or suppressiveness to banana wilt caused by Fusarium oxysporum f. sp. cubense of the soils studied. Finally, we hypothesize that a greater stability of the aggregates forming anaerobiosis could partly explain most of the available Fe found in soil areas where the disease was severe, at least in these types of soils.     

Crop growth,culturable bacteria,and degradation of petrol hydrocarbons (PHCs) in a long-term contaminated field soil

A greenhouse pot experiment was conducted for investigating the capability of a grass (annual ryegrass), a legume (summer vetch), and a crucifer (white mustard) to grow in a soil with portions from a former coal gasification site, influence the soil bacterial community, and promote the biodegradation of petrol hydrocarbons (PHCs). Soil concentrations of 1517 mg kg⁻¹ of total petrol hydrocarbons (TPHs), including 71.4 mg kg⁻¹ of total US EPA priority polycyclic aromatic hydrocarbons (TPAHs) have caused a significant (P < 0.05) reduction in shoot and root dry matter yields by more than 50%. Culturable bacteria and actinomycetes in soil were as much as 18-fold more abundant and the species composition was largely altered because of PHC contaminants and depending on crop species and age. After 95 days, 68.7% of initial TPH amounts and 59% of the TPAHs had disappeared from unplanted soil. Mustard and vetch fostered the removal of PHCs from soil reaching final TPH concentrations that were 15.6% and 12% lower than in unplanted soil. Both crops elicited the greatest degradative root activities and sustained particularly great populations of rhizosphere bacteria that are known hydrocarbon degraders. None of the crops aided the reduction of TPAHs in soil.     

Effects of nonylphenols on soil microbial activity and water retention

The main aim of this study is to analyze the influence of 4-nonylphenol (NP) on soil water retention and biological activity. Two doses of 4-nonylphenol (25 and 50 mg kg⁻¹) were tested in a loam soil with and without peat amendment. In general, one week after the start of the experiment, the soil water content retained at −0.75 MPa of soil suction was 18% higher in the soil amended and its basal respiration (BR) was 15% higher than soil without peat. In contrast, the microbial activity indices (CM: coefficient of mineralization or BR:total organic carbon (TOC) ratio; Cmic:Corg: microbial biomass carbon (MBC):TOC ratio; qCO₂: metabolic quotient or BR:MBC ratio) were higher in the soil without peat, compared to the soil amended with peat. On the other hand, the addition of NP to soil was able to modify soil biological but not physical (water retention, desorption) properties. When soil was amended with peat, MBC was reduced one week after applying NP. In contrast, no effects of NP on MBC were observed in the soil without peat. BR was reduced by 16% one week after applying 50 mg kg⁻¹ of NP to soil with peat, and was increased by 46% one week after applying 25 mg kg⁻¹ of NP to soil without peat. The effects of NP on MBC and BR could be associated more with the adsorption of NP by soil organic matter, while changes in CM or Cmic:Corg ratio were more closely related to changes in soil water retention. The potential toxic effects of NP (high qCO₂ values) were only observed in the absence of peat amendments. Peat addition reduced NP toxic effects on microorganisms.     

Climatic influences on active fractions of soil organic matter

Biologically active fractions of soil organic matter are important in understanding decomposition potential of organic materials, nutrient cycling dynamics, and biophysical manipulation of soil structure. We evaluated the quantitative relationships among potential C and net N mineralization, soil microbial biomass C (SMBC), and soil organic C (SOC) under four contrasting climatic conditions. Mean SOC values were 28±11 mg g⁻¹ (n=24) in a frigid–dry region (Alberta/British Columbia), 25±5 mg g⁻¹ (n=12) in a frigid–wet region (Maine), 11±4 mg g⁻¹ (n=117) in a thermic–dry region (Texas), and 12±5 mg g⁻¹ (n=131) in a thermic–wet region (Georgia). Higher mean annual temperature resulted in consistently greater basal soil respiration (1.7 vs 0.8 mg CO₂–C g⁻¹ SOC d⁻¹ in the thermic compared with the frigid regions, P<0.001), greater net N mineralization (2.8 vs 1.3 mg inorganic N g⁻¹ SOC 24 d⁻¹, P<0.001), and greater SMBC (53 vs 21 mg SMBC g⁻¹ SOC, P<0.001). Specific respiratory activity of SMBC was, however, consistently lower in the thermic than in the frigid regions (29 vs 34 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Higher mean annual precipitation resulted in consistently lower basal soil respiration (1.1 vs 1.3 mg CO₂–C g⁻¹ SOC d⁻¹ in the wet compared with the dry regions, P<0.01) and lower SMBC (31 vs 43 mg SMBC g⁻¹ SOC, P<0.001), but had inconsistent effects on net N mineralization that depended upon temperature regime. Specific respiratory activity of SMBC was consistently greater in the wet than the dry regions (≈33 vs 29 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Although the thermic regions were not able to retain as high a level of SOC as the frigid regions, due likely to high annual decomposition rates, biologically active soil fractions were as high per mass of soil and even 2–3-times greater per unit of SOC in the thermic compared with the frigid regions. These results suggest that macroclimate has a large impact on the portion of soil organic matter that is potentially active, but a relatively small impact on the specific respiratory activity of SMBC.     

Removal of benzo (a) pyrene from soil using an endogeic earthworm Pontoscolex corethrurus ()

The endogeic earthworm Pontoscolex corethrurus (Müller, 1857) was the most abundant species (75%) in soil contaminated with hydrocarbons, mostly benzo(a)pyrene (BaP), in the state of Tabasco (Mexico). The earthworm P. corethrurus was tested for its capacity to remove 100 mg BaP kg⁻¹ from an Anthrosol soil (sterilized or not) and amended with legume Mucuna pruriens (L.) DC. var. utilis (Wall. ex Wight) Baker ex Burck (3%) or the grass Brachiaria humidicola (L.) DC (3%) (recently renamed as Urochloa humidicola (Rendle) Morrone & Zuloaga) in an aerobic incubation experiment. P. corethrurus removed 26.6 mg BaP kg⁻¹ from the sterilized soil and application of B. humidicola as feed increased this to 35.7 mg BaP kg⁻¹ and M. pruriens to 34.2 mg BaP kg⁻¹ after 112 days. The autochthonous microorganisms removed 9.1 mg BaP kg⁻¹ from the unsterilized soil and application of B. humidicola increased this to 18.0 mg BaP kg⁻¹ and M. pruriens to 11.2 mg BaP kg⁻¹. Adding P. corethrurus to the unsterilized soil accelerated the removal of BaP and 36.1 mg kg⁻¹ was dissipated from soil. It was found that the autochthonous microorganisms removed BaP from soil, but addition of P. corethrurus increased the dissipation 4-fold. The endogeic earthworm P. corethrurus can thus be used to remediate hydrocarbon-contaminated soils in tropical regions.     

Rapid microbial phosphorus immobilization dominates gross phosphorus fluxes in a grassland soil with low inorganic phosphorus availability

Gross phosphorus (P) fluxes measured in isotopic dilution studies with ³³P labeled soils include the biological processes of microbial P immobilization, remineralization of immobilized P and mineralization of non-microbial soil organic P. In this approach, isotopic dilution due to physicochemical processes is taken into account. Our objectives were to assess the effect of inorganic P availability on gross P mineralization and immobilization in soil under permanent grassland, and to relate these fluxes to soil respiration, phosphatase activity and substrate availability as assessed by an enzyme addition method. We used soils from an 18-year-old grassland fertilization experiment near Zurich, Switzerland, that were collected in two treatments which differed only in the amount of mineral P applied (0 and 17 kg P ha⁻¹ yr⁻¹ in NK and NPK, respectively). Water-extractable phosphate was low (0.1 and 0.4 mg P kg⁻¹ soil in NK and NPK, while hexanol-labile (microbial) P was high (36 and 54 mg P kg⁻¹ soil in NK and NPK). Extremely fast microbial P uptake under P-limited conditions in NK necessitated the use of a microbial inhibitor when determining isotopic dilution due to physicochemical processes. At the higher inorganic P availability in NPK, however, isotopic exchange parameters were similar in the presence and absence of a microbial inhibitor. Phosphatase activity was higher in NK than in NPK, while soil respiration, water-extractable organic P and its enzyme-labile fraction were not affected by P status. Together, the results showed that inorganic P availability primarily affected microbial P immobilization which was the main component of gross P fluxes in both treatments. Gross P mineralization rates (8.2 and 3.1 mg P kg⁻¹ d⁻¹ for NK and NPK) during the first week were higher than reported in other studies on arable and forest soils and at least equal to isotopically exchangeable P due to physicochemical processes, confirming the importance of microbial processes in grassland soils.     

Application of biostimulants in benzo(a)pyrene polluted soils: Short-time effects on soil biochemical properties

The bioremediation effects of three biostimulants (BS): WCDS, wheat condensed distillers soluble; PAHE, hydrolyzed poultry feathers; and RB, rice bran extract in a soil polluted with two rates of benzo(a)pyrene (BaP) (50 or 100 mg kg⁻¹ soil, respectively) over 90 days were studied. Their effects on the soil biochemical properties (ATP and urease and phosphatase activities) and ergosterol were determined. Also, extractable BaP in soils was determined during the incubation period. An non-polluted and non-organic-amended soil was used as control. The results indicated that at the end of the incubation period and compared with the control soil, the ATP, ergosterol, urease and phosphatase activities decreased 29.4%, 24.8%, 44.7% and 42.9%, respectively in the non-organic amended soil polluted with polluted 100 mg BaP kg⁻¹ soil. The application of biostimulants to unpolluted soil increased the biochemical parameters. However, this stimulation was higher in the soil amended with PAHE, followed by RB and WCDS. The application of BaP in organic-amended soils decreased the biochemical properties. However, this decrease was lower than for the non-amended BaP polluted soil. Possibly the low molecular weight protein content easily assimilated by soil microorganisms is responsible for less inhibition of these soil biochemical parameters.     

Impact of high cadmium and nickel soil concentration on selected physiological parameters of Arundo donax L.

The purpose of this study was to investigate the effects of high cadmium and nickel soil concentrations on selected physiological parameters of Arundo donax L. A 2-year pot experiment was held in the field and the pots were irrigated with aqueous solutions of Cd and Ni in concentrations of 5, 50 and 100 ppm, against the control (tap water). At the end of the cultivation periods the pots soil was divided into three equal zones and total and NH₄OAc extractable Cd and Ni concentrations were determined. The top zone exhibited the highest metal content. Cadmium and nickel total concentrations at the end of the experiment were up to 973.8 mg kg⁻¹ and 2543.3 mg kg⁻¹ respectively, while NH₄OAc extractable Cd was up to 291.7 mg kg⁻¹ and Ni up to 510.3 mg kg⁻¹. Stomatal conductance ranged between 0.3 and 0.8 mol CO₂ m⁻² s⁻¹, intercellular CO₂ concentration ranged between 212.9 and 243.0 ppm CO₂, stomatal resistance between 0.6 and 1.3 s cm⁻¹, chlorophyll content (SPAD values) between 46.3 and 57.0 and chlorophyll fluorescence (F_v/F_m) ranged between 0.8 and 0.9. All studied physiological parameters did not show statistically significant differences among control and heavy metal treated plants for both years; therefore, high soil cadmium and nickel concentration did not inhibit stomatal opening and did not affect the function of the photosynthetic machine of A. donax plants.     

Microbial biomass in a semi arid soil of the central highlands of Mexico cultivated with maize or under natural vegetation

Microbial biomass (MB) is the key factor in nutrient dynamics in soil, but no information exists how clearing of vegetation to cultivate maize in the central highlands of Mexico might affect it. Soil MB was measured with the chloroform fumigation incubation (CFI) and fumigation extraction (CFE) techniques and the substrate-induced respiration (SIR) method in soil sampled under or outside the canopy of mesquite (Prosopis laevigata) and huisache (Acacia tortuoso), N₂ fixing shrubs, and from fields cultivated with maize. Microbial biomass C as measured with the CFI technique ranged from 122 mg C kg⁻¹ in agricultural soil to 373 mg C kg⁻¹ in soil sampled under mesquite shrubs. Microbial biomass N as measured with the CFI technique ranged from 11 mg N kg⁻¹ in agricultural soil to 116 mg N kg⁻¹ in soil sampled under mesquite shrub. The ratio of microbial biomass C as measured with CFI related to the ninhydrin-positive compounds (NPC) was 12.23 after 1 day and 8.43 after 10 days while the relationship with extractable C was 3.15 and 2.96, respectively. The metabolic quotient (qCO₂) decreased in the order OUTSIDE > MESQUITE > HUIZACHE > AGRICULTURE, and the microbial biomass:soil organic C ratio decreased in the order MESQUITE > HUIZACHE > OUTSIDE > AGRICULTURE using SIR to determine the microbial biomass. It was found that converting soil under natural vegetation to arable soil was not only detrimental for soil quality, but might be unsustainable as organic matter input is limited.     

Microbial properties and soil respiration in submontane forests of Venezuelian Guyana: characteristics and response to fertilizer treatments

The distribution of vegetation types in Venezuelan Guyana (in the ‘Canaima’ National Park) represents a transitional stage in a long term process of savannization, a process considered to be conditioned by a combined chemical and intermittent drought stress. All types of woody vegetation in this environment accumulate large amounts of litter and soil organic carbon (SOC). We hypothesized that this accumulation is caused by low microbial activity. During 1 year we measured microbial biomass carbon (Cmic), microbial respiration and soil respiration of stony Oxisols (Acrohumox) at a tall, a medium and a low forest and with three chemical modifications of site conditions by the addition of NO₃⁻, Ca²⁺ and PO₄³⁻ as possible limiting elements. Due to high SOC contents, mean Cmic was 1 mg g soil⁻¹ in the mineral topsoil and 3 mg g soil⁻¹ in the forest floor. Mean microbial respiration in the mineral topsoil and the forest floor were 165 and 192 μg CO₂-C g soil⁻¹ d⁻¹, respectively. We calculated high mean metabolic quotients (qCO₂) of 200 mg CO₂-C g Cmic⁻¹ d⁻¹ in the litter layer and 166 mg CO₂-C g Cmic⁻¹ d⁻¹ in the mineral topsoil, while the Cmic-to-SOC ratios were as low as 1.0% in the litter layer and 0.8% in the mineral topsoil. Annual soil respiration was 9, 12 and 10 Mg CO₂-C ha⁻¹ yr⁻¹ in the tall, medium and low forest, respectively. CO₂ production was significantly increased by CaHPO₄ fertilization, but no consistent effects were caused by Ca²⁺ and NO₃⁻, fertilization. Our findings indicate that Cmic and microbial respiration are reduced by low nutrient concentrations and low litter and SOC quality. Reduced microbial decomposition may have contributed to SOC accumulation in these forests.     

Estimating heterotrophic and autotrophic soil respiration in a semi-natural forest of Lombardy,Italy

We studied a semi-natural forest in Northern Italy that was set aside more than 50 years ago, in order to better understand the soil carbon cycle and in particular the partitioning of soil respiration between autotrophic and heterotrophic respiration. Here we report on soil organic carbon, root density, and estimates of annual fluxes of soil CO₂ as measured with a mobile chamber system at 16 permanent collars about monthly during the course of a year. We partitioned between autotrophic and heterotrophic respiration by the indirect regression method, which enabled us to obtain the seasonal pattern of single components.The soil pool of organic carbon, with 15.8 (±4.5) kg m^?2, was very high over the entire depth of 45 cm. The annual respiration rates ranged from 0.6 to 6.9 μmol CO₂ m^?2 s^?1 with an average value of 3.4 (±2.3) μmol CO₂ m^?2 s^?1, and a cumulative flux of 1.1 kg C m^?2 yr^?1. The heterotrophic component accounted for 66% of annual CO₂ efflux. Soil temperature largely controlled the heterotrophic respiration (R² = 0.93), while the autotrophic component followed irradiation, pointing to the role of photosynthesis in modulating the annual course of soil respiration.Most studies on soil respiration partitioning indicate autotrophic root respiration as a first control of the spatial variability of the overall respiration, which originates mainly from the uppermost soil layers. Instead, in our forest the spatial variability of soil respiration was mainly linked to soil carbon, and deeper layers seemed to provide a significant contribution to soil respiration, a feature that may be typical for an undisturbed, naturally maturing ecosystem with well developed pedobiological processes and high carbon stocks.     

Organic nitrogen stimulates the heterotrophic nitrification rate in an acidic forest soil

Many previous studies have demonstrated that heterotrophic nitrification processes play an important role in the production of NO₃⁻ in acidic soils. However, it is not clear whether a low concentration of nitrogenous organic compounds support heterotrophic nitrification processes in natural soils. In this study, we performed an ¹⁵N tracer experiment with a glycine concentration gradient (20, 40, 80, and 160 mg N kg⁻¹) to investigate the effect of the organic nitrogen concentration on the heterotrophic nitrification rate and its relative contribution to the total nitrification of the studied acidic forest soil. This experiment demonstrated that ¹⁵N–NO₃⁻ accumulated over time with all nitrogen treatments in the presence of acetylene, confirming that heterotrophic nitrification occurred even at a low organic nitrogen concentration (20 mg kg⁻¹) in the studied acidic forest soil. In the presence of acetylene, the ¹⁵N–NO₃⁻ concentration in the 20 and 40 mg kg⁻¹ glycine-N treatments was significantly lower than in the 80 and 160 mg kg⁻¹ glycine-N treatments (p < 0.05), indicating that a high organic nitrogen concentration stimulated the heterotrophic nitrification rate. There was no significant difference in the average contribution of heterotrophic nitrification to total nitrification among the different nitrogen treatments, suggesting that the organic nitrogen concentration did not affect the relative contribution of heterotrophic nitrification to total nitrification in the studied acidic soil. Our results confirmed that a low concentration of organic N (20 mg kg⁻¹) supported heterotrophic nitrification in the studied soil. The organic nitrogen concentration stimulates the heterotrophic nitrification rate, but does not affect the relative contribution of heterotrophic nitrification to total nitrification in the studied acidic soil.