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.     

Improved soil structure and citrus growth after inoculation with three arbuscular mycorrhizal fungi under drought stress

In a controlled potted experiment, citrus (Poncirus trifoliata) seedlings were inoculated with three species of arbuscular mycorrhizal (AM) fungi, Glomus mosseae, G. versiforme or G. diaphanum. Two soil-water levels (ample water, −0.10 MPa; drought stress, −0.44 MPa) were applied to the pots 4 months after transplantation. Eighty days after water treatments, the soils and the citrus seedlings were well colonized by the three AM fungi. Mycorrhizal fungus inoculation improved plant biomass regardless of soil-water status but decreased the concentrations of hot water-extractable and hydrolyzable carbohydrates of soils. Mycorrhizal soils exhibited higher Bradford-reactive soil protein concentrations than non-mycorrhizal soils. Mycorrhizas enhanced >2 mm, 1–2 mm and >0.25 mm water-stable aggregate fractions but reduced 0.25–0.5 mm water-stable aggregates. Peroxidase activity was higher in AM than in non-AM soils whether drought stressed or not, whereas catalase activity was lower in AM than non-AM soils. Drought stress and AM fungus inoculation did not affect polyphenol oxidase activity of soils. A positive correlation between the Bradford-reactive soil protein concentrations, soil hyphal length densities, and water-stable aggregates (only >2 mm, 1–2 mm and >0.25 mm) suggests beneficial effects of the AM symbiosis on soil structure. It concluded that AM fungus colonization enhanced plant growth under drought stress indirectly through affecting the soil moisture retention via glomalin's effect on soil water-stable aggregates, although direct mineral nutritional effects could not be excluded.     

Enzyme activities as affected by soil properties and land use in a tropical watershed

Enzyme activities play key roles in the biochemical functioning of soils, including soil organic matter formation and degradation, nutrient cycling, and decomposition of xenobiotics. Knowledge of enzyme activities can be used to describe changes in soil quality due to land use management and for understanding soil ecosystem functioning. In this study, we report the activities of the glycosidases (β-glucosidase, α-galactosidase, and β-glucosaminidase), acid phosphatase, and arylsulfatase, involved in C (C and N for β-glucosaminidase), P, and S cycling, respectively, as affected by soil order and land use within a watershed in north-central Puerto Rico (Caribbean). Representative surface soil (0–15 cm) samples were taken from 84.6% of the total land area (45,067 ha) of the watershed using a completely randomized design. The activity of α-galactosidase was greater in soils classified as Oxisols than in soils classified as Ultisols and Inceptisols, and it was not affected by land use. The activity of β-glucosidase was greater in Oxisols compared to the Inceptisols and Ultisols, and it showed this response according to land use: pasture > forest > agriculture. The activity of β-glucosaminidase was higher in Oxisols than the other soil orders, and it was higher under pasture compared to forest and agriculture. Acid phosphatase and arylsulfatase activities were greater in Oxisols and Ultisols than in Inceptisols, and they decreased in this order due to land use: forest = pasture > agriculture. As a group, β-glucosaminidase, β-glucosidase, and acid phosphatase activities separated the sites under forest and pasture from those under agriculture in a three-dimensional plot. Thus, enzyme activities in Inceptisols under agriculture could be increased to levels comparable to other soil orders with conservative practices similar to those under pasture and secondary forest growth. Our findings demonstrate that within this watershed, acid and low fertility soils such as Oxisols and Ultisols have in general higher enzyme activities than less weathered tropical soils of the order Inceptisols, probably due to their higher organic matter content and finer texture; and that the activities of these enzymes respond to management with agricultural practices decreasing key soil biochemical reactions of soil functioning.     

Population size and specific nitrification potential of soil ammonia-oxidizing bacteria under long-term fertilizer management

Population size of soil ammonia-oxidizing bacteria (AOB) was quantified by real-time PCR in a long-term (16 years) field experiment under different fertilizer managements. AOB population sizes in mineral nitrogen-fertilized soils and organic manure-fertilized soil were 10.3 and 3.1 times, respectively, that of the control, while phosphorus and potassium fertilization had no significant effect. On the other hand, the AOB specific nitrification potential (soil nitrification potential per AOB cell) was significantly higher (P < 0.05) in organic manure-fertilized soil than in mineral-fertilized soils and the control, indicating that AOB was likely more metabolically active in organic manure-fertilized soils than in mineral nitrogen-fertilized soils after long-term application.     

Impact of bovine urine deposition on soil microbial activity,biomass, and community structure

Nitrogen (N) from urine excreted by grazing animals can be transformed into N compounds that have detrimental effects on the environment. These include nitrate, which can cause eutrophication of waterways, and nitrous oxide, which is a greenhouse gas. Soil microbes mediate all of these N transformations, but the impact of urine on microbes and how initial soil conditions and urine chemical composition alter their responses to urine are not well understood. This study aimed to determine how soil inorganic N pools, nitrous oxide fluxes, soil microbial activity, biomass, and the community structure of bacteria containing amoA (nitrifiers), nirK, and nirS (denitrifiers) genes responded to the addition of urine over time. Bovine urine containing either a high (15.0 g K⁺ l^?1) or low salt content (10.4 g K⁺ l^?1) was added to soil cores at either low or high moisture content (hereafter termed dry and wet soil respectively; 35% or 70% water-filled pore space after the addition of urine). Changes in soil conditions, inorganic N pools, nitrous oxide fluxes, and the soil microbial community were then measured 1, 3, 8, 15, 29 and 44 days after urine addition. Urine addition increased soil ammonium concentrations by up to 2 mg g d.w.^?1, soil pH by up to 2.7 units, and electrical conductivity (EC) by 1.0 and 1.6 dS m^?1 in the low and high salt urine treatments respectively. In response, nitrate accumulation and nitrous oxide fluxes were lower in dry compared to wet urine-amended soils and slightly lower in high compared to low salt urine-amended soils. Nitrite concentrations were elevated (>3 μg g d.w.^?1) for at least 15 days after urine addition in wet urine-amended soils, but were only this high in the dry urine-amended soils for 1 day after the addition of urine. Microbial biomass was reduced by up to half in the wet urine-amended soils, but was largely unaffected in the dry urine-amended soils. Urine addition affected the community structure of ammonia-oxidising and nitrite-reducing bacteria; this response was also stronger and more persistent in wet than in dry urine-amended soils. Overall, the changes in soil conditions caused by the addition of urine interacted to influence microbial responses, indicating that the effect of urine on soil microbes is likely to be context-dependent.     

Groundwater level controls CO2, N2O and CH4 fluxes of three different hydromorphic soil types of a temperate forest ecosystem

Hydromorphic soils should exhibit higher climate change feedback potentials than well aerated soils since soil organic matter (SOM) losses in them are predicted to be much larger than those of well aerated soils. To evaluate a combined feedback relationship between groundwater level (GWL) and total greenhouse gas (GHG) emission, a greenhouse microcosm experiment was performed by exposing three hydromorphic forest soil types that differed in carbon content to three water levels (?40, ?20 and ?5 cm) while plants were excluded. Net GHG fluxes were measured continuously. GHG concentrations plus oxygen were measured in soil air and soil water at different depths. In this study, soil type hardly affected GHG emissions but GWL did. CO₂ emissions peaked at GWL of ?40 cm and declined on average to 65 and 33% during GWL at ?20 and ?5 cm, respectively. CH₄ emissions showed the opposite pattern having the highest emission rates at GWL of ?5 cm and compared to that on average only ?3 and ?8% during GWL at ?20 and ?40 cm, respectively. The highest mean N₂O emissions were detected at the intermediate GWL of ?20 cm, whereas it is reduced on average to 18% for GWL at ?40 cm and at ?5 cm. The highest greenhouse gas emissions (in CO₂ equivalents) were calculated for GWL at ?20 cm. During GWL at ?40 cm, CO₂ equivalent fluxes were only insignificantly lower. CO₂ equivalent fluxes reduced explicitly in mean to 35% with GWL at ?5 cm. The outcome emphasizes that anaerobic SOM decomposition apparently produces a lower warming potential than aerobic SOM decomposition. Undoubtedly, hydromorphic soils have to be considered for climate–carbon feedback scenarios.     

Effects of two soil reclamation techniques on the distribution of the organic N compounds in a 15N labelled burnt soil

The evolution of the soil organic-N forms and their bio-availability was studied in a ¹⁵N labelled and burnt soil (BLS) after two successive reclamation steps under greenhouse conditions: a 3-month growing period of Lolium, without (BLS-L) or with poultry manure addition (4 and 8 Mg ha^− 1: BLS + PM4-L and BLS + PM8-L), followed by a 12-month growing phase of pine seedlings (BLS-P, BLS + PM4-P and BLS + PM8-P). The results were compared with those obtained for the homologous labelled unburnt soil (LS, LS-L and LS-P) to evaluate the efficacy of these reclamation techniques in the mitigation of the drastic post-fire changes exhibited by the major biologically available N pool in terrestrial ecosystems: the soil organic N. The significant and steady decrease of the ¹⁵N enrichment observed in the unburnt soil during the successive plant growth cycles (LS > LS-L > LS-P) contrasts with the lack of significant changes, in both the content of total organic ¹⁵N and the atom % ¹⁵N in excess, among the treatments with the burnt soil (BLS ≥ BLS-L ≥ BLS-P). These results showed that: a) in LS, N mineralization proceeds faster for the recently incorporated N (¹⁵N enriched) than for the native N, supplying the growing vegetation with inorganic N more ¹⁵N enriched than the bulk soil N; and b) in BLS, soil combustion has reduced the usually higher biological availability of the recently added N to levels similar to those of the endogenous N.The re-vegetation with Lolium and Pinus and the addition of poultry manure mitigated the high differences observed in the size of the amino acid and the organic derived NH₄⁺–N pools due to the combustion process, which are usual between burnt and unburnt soils. Conversely, these burnt soil reclamation techniques (re-vegetation and poultry manure addition), even jointly used, were unable to reduce the huge differences observed between the burnt and the unburnt soils for the other N fractions considered (amides, amino sugars, hydrolysable unidentified-N, hydrolysable organic N and un-hydrolysable N) that accounted for more than 80% of the soil organic N. Consequently, it seems that without the introduction of N₂-fixing microorganisms or plants in the burnt soils the recovery of the natural soil organic N composition will take place slowly.     

Respiratory substrate availability plays a crucial role in the response of soil respiration to environmental factors

We examined the response of the temperature coefficient (Q₁₀) for soil respiration to changes in soil temperature and soil moisture through a laboratory incubation experiment. Two types of soils differing in vegetation and moisture status were collected and incubated under two temperatures (10 and 30 °C) and two soil moisture regimes (35 and 75% of water holding capacity, WHC) for 5 weeks. Before and after the incubation experiment, the temperature coefficient of soil respiration was measured using soda-lime method by changing temperature in a water bath. For both soils, the mean Q₁₀ values of the respiration rate were 2.0 in the 30 °C and 2.3 in the 10 °C soil treatments. Higher temperature with lower soil moisture treatment significantly decreased the Q₁₀ value, whereas lower temperature with higher soil moisture treatment significantly enhanced the Q₁₀ value (ANOVA, p < 0.05). These results indicate that soils became less sensitive to temperature when incubated under higher temperature with higher moisture conditions, and more sensitive in lower temperature with higher moisture conditions.There was a significant correlation (r² = 0.67, p < 0.05) between water-soluble carbon (WSC) and soil respiration rate. However, the correlation between soil respiration rate and microbial biomass carbon (MBC) was weak (r² = 0.27, p > 0.05). Although incubation temperature and moisture accounted for 40 and 29% (as r² × 100%), respectively, of variations in Q₁₀, soil water-soluble carbon content alone could have explained 79% of the variation, indicating that the availability of respiratory substrate, rather than the pool of soil microorganisms, played a crucial role in the response of the temperature coefficient to environmental factors. These results suggest that biotic factors should also be taken into consideration when using the Q₁₀ function to predict the response of soil respiration to global warming.     

The relationships between erodibility and erosion in a soil treated with two organic amendments

The influence of two organic wastes, cotton gin crushed compost (CC) and beet vinasse (BV) applied for 5 years on a Typic Xerofluvent under dryland conditions near to Sevilla city (Guadalquivir River Valley, Andalusia, Spain) on soil erodibility (K factor of the USLE and RUSLE) and soil loss was studied. CC and BV were applied at rates of 1780, 5340, and 10,680 kg ha⁻¹ (expressed as organic matter content). When CC was applied to the soil, erodibility factor (K) is correlated with soil loss, highlighting a decrease in K and soil loss when increased the dose of CC applied to the soil. In this respect, K decreased 17% in CC-amended soils respect to control soil at the end of the experiment, and soil loss decreased 36% in CC-amended soils respect to control soil at the end of the experiment and for 45 min and 60 mm h⁻¹. However, when BV was applied, soil physical and biological properties decreased. K decreased 6.4% in BV-amended soils respect to control soil at the end of the experiment, and soil loss increased 59.7% in BV-amended soils respect to control soil at the end of the experimental period and for 45 min and 60 mm h⁻¹. We think that this is because the higher level of Na⁺ (and possibly of fulvic acids) in BV increased the exchangeable sodium percentage (ESP) and reduced structural stability of BV-amended soil, leading to higher soil loss. This explains the relatively higher soil loss in BV-amended soils. These results contradict many previous reports in which soil organic matter prevented soil loss. For this reason, the equation of soil erodibility (K factor of USLE and RUSLE) must have in consideration other aspects such as the chemical composition of the soil organic matter as well as the soil structural stability.     

Responses of soil dissolved organic matter to long-term plantations of three coniferous tree species

Tree species have significant effects on the availability and dynamics of soil organic matter. In the present study, the pool sizes of soil dissolved organic matter (DOM), potential mineralizable N (PMN) and bio-available carbon (C) (measured as cumulative CO₂ evolution over 63 days) were compared in soils under three coniferous species — 73 year old slash (Pinus elliottii), hoop (Araucaria cunninghamii) and kauri (Agathis robusta) pines. Results have shown that dissolved organic N (DON) in hot water extracts was 1.5–1.7 times lower in soils under slash pine than under hoop and kauri pines, while soil dissolved organic C (DOC) in hot water extracts tended to be higher under slash pine than hoop and kauri pines but this was not statistically significant. This has led to the higher DOC:DON ratio in soils under slash pine (32) than under hoop and kauri pines (17). Soil DOC and DON in 2 M KCl extracts were not significantly different among the three tree species. The DOC:DON ratio (hot water extracts) was positively and significantly correlated with soil C:N (R² = 0.886, P < 0.01) and surface litter C:N ratios (R² = 0.768, P < 0.01), indicating that DOM was mainly derived from litter materials and soil organic matter through dissolution and decomposition. Soil pH was lower under slash pine (4.5) than under hoop (6.0) and kauri (6.2) pines, and negatively correlated with soil total C, C:N ratio, DOC and DOC:DON ratio (hot water extracts), indicating the soil acidity under slash pine favored the accumulation of soil C. Moreover, the amounts of dissolved inorganic N, PMN and bio-available C were also significantly lower in soils under slash pine than under hoop and kauri pines. It is concluded that changes in the quantity and quality of surface litters and soil pH induced by different tree species largely determined the size and quality of soil DOM, and plantations of hoop and kauri pine trees may be better in maintaining long-term soil N fertility than slash pine plantations.     

Long-term effect on soil biochemical status of a Vertisol under conservation tillage system in semi-arid Mediterranean conditions

Long-term field experiments are expected to provide important information regarding soil properties affected by conservation management practices. Several studies have shown that soil enzyme activities are sensitive in discriminating among soil management effects. In this study we evaluated the long-term effect of direct drilling (DD) under a crop rotation system (cereals–sunflower–legumes), on the stratification of soil organic matter content and on biochemical properties in a dryland in southwest Spain. The results were compared to those obtained under conventional tillage (CT). Soil biochemical status was evaluated by measuring the enzymatic activities (dehydrogenase, β-glucosidase, alkaline phosphatase and arylsulphatase) during the flowering period of a pea crop. Soil samples were collected in May 2007 at three depths (0–5, 5–10 and 10–20 cm).Total organic carbon (TOC) contents and values of soil enzyme activities were higher in soils subjected to DD than to CT, specifically at 0–5 cm depth. Although a slight decrease of TOC and enzymatic activities with increasing soil depth was observed, no significant differences were found among different depths of the same treatment. This could be related to the high clay content of the soil, a Vertisol. Enzyme activities values showed high correlation coefficients (from r = 0.799 to r = 0.870, p < 0.01) with TOC. Values of activity of the different enzymes were also correlated (p < 0.01).Values of stratification ratios did not show significant differences between tillage practices. The high clay content of the soil is responsible for this lack of differences because of the protection by clay mineral of TOC and soil enzymes activities.Long-term soil conservation management by direct drilling in a dryland farming system improved the quality of a clay soil, especially at the surface, by enhancing its organic matter content and its biological status.     

Temporal variation of soil compaction and deterioration of soil quality in pasture areas of Colombian Amazonia

In the Amazon basin, tropical rainforest is being slashed and burned at accelerated rates for annual crops over a couple of years, followed by forage grasses. Because of poor management, the productivity of established pastures declines in a few years so that grazing plots are abandoned and new areas are deforested. Previous studies in the region report higher bulk density in soils under pasture than in similar soils under forest. The objective of this study was to detect changes in the physical quality of the topsoil of nutrient-poor Typic Paleudults in the colonisation area of Guaviare, Colombian Amazonia, and analyse the effect of soil deterioration on pasture performance. Temporal variation of soil compaction under pasture was analysed by comparing natural forest taken as control and pasture plots of Brachiaria decumbens (Stapf) grouped into three age ranges (<3, 3–9, >9 years). Evidence of soil compaction through cattle trampling, after clearing the primary forest, included the formation of an Ap horizon with platy structure and dominant greyish or olive colours, reflecting impaired surface drainage, the increase of bulk density and penetration resistance, and the decrease of porosity and infiltration rate. From primary forest to pastures older than 9 years, bulk density of the 5–10 cm layer increase was 42% in fine-textured soils and 30% in coarse-textured soils. Penetration resistance ranged from 0.45 MPa under forest to 4.25 MPa in old pastures, with maximum values occurring at 3–12 cm depth in pastures older than 9 years. Average total soil porosity was 58–62% under forest and 46–49% under pasture. Basic infiltration dropped from 15 cm h⁻¹ in the original forest conditions to less than 1 cm h⁻¹ in old pastures. Crude protein content and dry matter yield of the forage grass steadily decreased over time. No clear relationship between declining protein content as a function of pasture age and changes in chemical soil properties was found, but there was a high negative correlation (r=−0.81) between protein content and bulk density, reflecting the effect of soil compaction on pasture performance. After about 9–10 years of use, established grass did no longer compete successfully with invading weeds and grazing plots were abandoned. As land is not yet a scarcity in this colonisation area, degraded pastures are seldom rehabilitated.     

Soil amendments with organic wastes reduce the toxicity of nickel to soil enzyme activities

The effects of adding a crushed cotton gin compost (CCGC) and poultry manure (PM) on the biological properties of a Typic Xerofluvent soil contaminated with Ni were studied in the laboratory. Urease, BBA-protease, alkaline phosphatase, β-glucosidase and arylsulfatase activities were measured in soils containing seven concentrations (100, 250, 500, 1000, 2500, 5000 and 8000 mg kg⁻¹ soil) of Ni after four incubation times (1 day, 7 days, 15 days and 45 days). The resulting inhibition was compared with that of the enzymatic activities in the same soil containing similar concentrations of the Ni but amended with crushed cotton gin compost and poultry manure. The 50% ecological dose (ED₅₀) values were calculated by the two kinetic models used by Speir et al. [T.W. Speir, H.A. Kettles, A. Parshotam, P.L. Searle, L.N.C. Vlaar, A simple kinetic approach to derive the ecological dose value, ED50, for the assessment of Cr(VI) toxicity to soil biological properties, Soil Biol. Biochem. 27 (1995) 801–810] and by the sigmoidal dose–response model used by Haanstra et al. [L. Haanstra, P. Doelman, J.H. Oude Voshaar, The use of sigmoidal dose response curves in soil ecotoxicological research, Plant Soil 84 (1985) 293–297]. The urease, BBA-protease, β-glucosidase, alkaline phosphatase and arylsulfatase activities were higher in the organic amended soils (76%, >99.7%, >95.7%, >27.6% and >87.2%, respectively) than in the control soil. Also, the enzymatic activities were higher in CCGC-amended soils than in the PM-amended soils (51%, 20%, 11.2%, and 11.3% increase for urease, BBA-protease, β-glucosidase and alkaline phosphatase, respectively). For all soil enzymatic activities and at the end of the period of incubation, the ED₅₀ values were lowest in control soil, followed by PM and CGCC-amended soils. This may have been due to the adsorption capacity of Ni being higher in the humic acid (CGCC) than in the fulvic acid-amended soil (PM).     

Hydrological properties of a Mediterranean soil burned with different fire intensities

The influence of vegetation cover on soil hydrological properties and its response to the impact of different fire intensities, in a Mediterranean forest environment, has been evaluated. The study was carried out in the Permanent Experimental Field Station of La Concordia (Llíria–Valencia, Spain), on a set of nine erosion plots (4 × 20 m²). The Station is located on a calcareous hillside S–SE oriented, with soils of Rendzic Leptosol type and supporting Mediterranean shrubland vegetation. All runoff generated and sediment produced in every rain event was collected from each plot. The set up includes a system of sensors for the continuous monitoring of climatic parameters (air temperature and humidity, rain volume, intensity, etc.).In June 1995, a set of experimental fires was carried out to the Station. Three of the plots were burned with high intensity fire, three with moderate intensity and the remaining were left unaltered. Soil water content and water retention capacity (WRC) were measured in the different plots and in two different vegetation covers: under canopy (UC) and in bare soil (BS). The pF curves were also obtained for each fire treatment.A year after the fires (June 1995–June 1996), great differences, reaching 77.15%, in runoff generation between fire treatments and the control plots were observed.No significant differences were detected on water retention capacity between soils UC and BS in the burned plots. However, these differences appeared in the control plots, giving UC and BS values of 13% and 18%, respectively. Plots corresponding to the high intensity fire treatment showed values of WRC significantly higher than those of the moderate intensity and of the control treatments.The pF curves show that the values of water volume, at the different pressure points studied, were slightly greater on UC soil. Values obtained for BS samples are higher in the fire treatments, showing significant differences in respect to the control plots at pF 1 and 2. These differences were also observed for UC soil, but in this case at pF 2, 2.5 and 4.2.     

Effects of inorganic fertilizer and manure on soil archaeal abundance at two experimental farms during three consecutive rotation-cropping seasons

Soil archaeal population dynamics at two experimental sites of the same clay-loam type in Ottawa and Woodslee, Ontario, were investigated to determine fertilizer and manure effects following their different long-term crop rotation and fertilization schemes. Phylogenetic analysis of cloned soil archaeal 16S rRNA gene libraries of both sites identified them with group 1.1b of Thaumarchaeota. The gene population dynamics subtly varied in the order of 10⁷ copies g⁻¹ soil when monitored by quantitative real-time PCR during three growing seasons (2007–2009). In Ottawa, where plots were amended with dairy-farm manure, soil thaumarchaeal gene abundance was double of the unamended plots. At the Woodslee N-P-K-fertilized plots, it remained at least 30% fewer than that of the unfertilized ones. These cultivated plots showed soil carbon limitation while the fertilized ones were low in soil pH (ca. 5.5). Surface soils from an unfertilized sod plot and an adjacent deciduous forest had higher total carbon content (C:N ratio of 9 and 11, respectively). Their thaumarchaeal gene abundance varied up to 4.8 × 10⁷ and 7.0 × 10⁷ copies g⁻¹ soil, respectively. The former value was also attained at the manure-amended plots in Ottawa, where the C:N ratio was just below 10. Where soil pH was above 6.0, there was a weak and positive correlation between soil total C and the estimated gene abundance. Such gene population dynamics consistently demonstrated the stimulating and suppressive effects of dairy-farm manure (Ottawa site) and inorganic fertilizers (Woodslee site), respectively, on soil thaumarchaea. At both sites archaeal amoA and 16S rRNA gene abundance were similarly affected. Archaeal amoA gene abundance also outnumbered bacterial amoA abundance, suggesting that ammonia-oxidizing archaea might be dominant in these soils. Only minor crop effects on gene population dynamics were detected.     

Impact of allelochemical exuded from allelopathic rice on soil microbial community

Allelopathic rice releases allelochemicals from its roots to paddy soils at early growth stages to inhibit neighboring weeds. However, little is currently known about the effects of allelochemicals on soil microbes. In this study, we show that allelopathic rice can have great impact on the population and community structure of soil microbes. Allelopathic rice PI312777 seedlings reduced the culturable microbial population and total PLFA when compared to non-allelopathic rice Liaojing-9. Similar results were observed when, instead of growing seedlings, soils were incubated with plant root exudates. This result demonstrates that the composition of root exudates from the rice varieties tested contributes to the soil microbial community. Further experiments showed that the microbial community was affected by the allelochemical 5,4′-dihydroxy-3′,5′-dimethoxy-7-O-β-glucopyranosylflavone exuded from allelopathic rice roots, through immediately hydrolyzing glucose with stimulation on soil bacteria and aglycone (5,7,4′-trihydroxy-3′,5′-dimethoxyflavone) with inhibition on soil fungi. This result indicates that the flavone O-glycoside can provide carbon and interact with soil microbes. PC analysis of the fatty acid data clearly separated the allelopathic PI312777 and the non-allelopathic Liaojing-9 variety (PC1 = 46.4%, PC2 = 20.3%). Similarly, the first principal component (PC1 = 37.4%) together with the second principal component (PC2 = 17.3%) explained 54.7% of the variation between the allelopathic and non-allelopathic root exudates. Furthermore, the canonical correlation between allelopathic root exudates and the flavone O-glycoside was statistically significant (Canonical R = 0.889, χ² (25) = 69.72, p = 0.0041). Although the data generated in this study were not completely consistent between culturable microbes and PLFA profile, it is a fact that variation in soil microbial populations and community structures could be distinguished by the allelopathic and non-allelopathic rice varieties tested. Our results suggest that individual components of rice root exudates, such as allelochemicals from allelopathic rice, can modify the soil microbial community.     

Influence of carbon amendments on soil denitrifier abundance in soil microcosms

It is known that carbon (C) amendments increase microbial activity in anoxic soil microcosm studies, however the effects on abundance of total and denitrifier bacterial communities is uncertain. Quantitative PCR was used to target the 16S rRNA gene for the total bacterial community, the nosZ functional gene to reflect a broad denitrifier community, and functional genes from narrow denitrifier communities represented by Pseudomonas mandelii and related species (cnorB_P) and Bosea/Bradyrhizobium/Ensifer spp. (cnorB_B). Repacked soil cores were amended with varying amounts of glucose and red clover plant tissue (0–1000 mg C kg^? 1 of soil) and incubated for 96 h. Carbon amendment significantly increased respiration as measured by cumulative CO₂ emissions. Inputs of red clover or glucose at 1000 mg C kg^? 1 of soil caused increased abundance in the total bacteria under the conditions used. There was about an approximate 2-fold increase in the abundance of bacteria bearing the nosZ gene, but only in treatments receiving 500 or 1000 mg C kg^? 1 of soil of glucose or red clover, respectively. Additions of ≥ 500 mg C kg^? 1 soil of red clover and ≥ 250 mg C kg^? 1 of glucose increased cnorB_P-gene bearing denitrifiers. Changes in abundance of the targeted communities were related to C availability in soil, as indicated by soil respiration, regardless of C source. Applications of C amendments at rates that would occur in agricultural soils not only increase microbial activity, but can also induce changes in abundance of total bacterial and denitrifier communities in studies of anoxic soil microcosms.     

Effects of trifluralin on soil carbon mineralization at different temperature conditions

Trifluralin is a herbicide intensively used in Turkish cotton agriculture. The recommended field dose [(RFD), 480 g active ingredient l^?1], 2 × RFD, 4 × RFD and 6 × RFD of this herbicide were added to virgin (previously no trifluralin applied) and cotton field soils (previously trifluralin applied) from a district (Yumurtal?k, Adana) under Mediterranean climate conditions in order to determine their effects on soil microbial activity as measured by carbon mineralization at the different temperature conditions (20 °C, 25 °C and 30 °C). C mineralization of all samples was determined by the CO₂ respiration method over 30 days (20 °C, 25 °C and 30 °C at constant moist). The ratio (%) of carbon mineralization at all doses of cotton field soil at 30 °C was significantly higher than all other field dose–temperature combinations (P < 0.001). Based on these results, trifluralin is used as a carbon source by soil microorganisms. The herbicide trifluralin was degraded completely in the cotton field but a small fraction remained in the virgin field. This result can be explained by the cotton field soil having both more active microbial populations and more microorganisms adapted to the trifluralin applications than the virgin field.     

The ecological dose value (ED50) for assessing Cd toxicity on ATP content and dehydrogenase and urease activities of soil

Two soils of contrasting texture, organic matter content and pH were treated with CdSO₄ solutions to give a Cd concentration range of 0–4000 mg kg⁻¹ soil. The content of ATP and dehydrogenase and urease activities of soils were assayed after 3 h, and 7 and 28 days of Cd contamination. The relative ED₅₀ values were calculated by two kinetic models (model 1 and model 2) used by Speir et al. (1995) and by the sigmoidal dose–response model (model 3) employed by Haanstra et al. (1985). Model 1 was the most successful in calculating the ED₅₀ values for the ATP content, urease and dehydrogenase activities when both soils were contaminated by Cd. Similar ED₅₀ values were predicted by model 1 (describing the full inhibition) and model 3 only when the correlation coefficients r² were higher than 0.9. The ED₅₀ values of ATP calculated by model 1 were markedly higher than those calculated by model 2 (describing partial inhibition) when both models gave correlation coefficients higher than 0.9. This behavior was due to the high asymptote values obtained using model 2. According to model 2, some of the enzyme activities responsible for the ATP synthesis were probably not inhibited at the highest Cd concentrations. The inhibitory effect of Cd on the ATP content and both enzymatic activities was lower in the Castelporziano soil, which had the highest total organic carbon content.