A strategy for marginal semiarid degraded soil restoration: A sole addition of compost at a high rate. A five-year field experiment

This work evaluates the mid-term impact of the addition of large amounts of an organic amendment on the recovery of the physical, chemical and, particularly, the microbiological properties of a marginal semiarid degraded soil and on increasing the soil organic C pool. In order to perform this study, a semiarid degraded soil was treated with composted urban waste at doses equivalent to the addition of 1% (S + CCD1) and 3% (S + CCD2) of organic C (C_org). Changes in soil characteristics in the amended soils were evaluated with respect to a control soil without organic amendment for a period of 5 years after the organic amendment was applied. A spontaneous vegetal cover developed on both amended and unamended soils 3–4 months after the organic amendments were added, yet the level of vegetal biodiversity was lower in the amended plots. Compost-amended soils showed higher concentrations of C_org, water-soluble C and water-soluble carbohydrates than the control soil throughout the experimental period. Furthermore, all of these C fractions were significantly higher (p ≤ 0.05) in S + CCD2 than in S + CCD1 and the control soil. However, compost addition also increased soil electrical conductivity and nitrate content, particularly at the higher dose. Likewise, compost addition produced a 4- to 10-fold increase in soil heavy metal concentrations, although the levels of heavy metal were under the limits allowed in soils. Five years after the organic amendment was added, the soil water holding capacity, stable aggregate percentage, porosity and nutrient and humic substance and humic acid content were greater in amended soils than in control soil, and the higher dose produced greater increases than the lower dose. Soils receiving the highest dose of compost also showed the highest values of basal respiration, dehydrogenase activity and β-glucosidase and phosphatase activity, as well as a greater abundance of total PLFAs, bacterial and fungal PLFAs, and saturated and monounsaturated fatty acids. A greater level of functional diversity was also observed in amended soils, particularly in the soil receiving the higher dose of compost. It can be concluded that the addition of high doses of compost can be a suitable strategy for restoring semiarid degraded soils and for fixing C in these soils, provided that the organic material is of high quality and has a low concentration of heavy metals.     

Response of soil microbial communities to the herbicide mesotrione: A dose-effect microcosm approach

Mesotrione is a new selective herbicide used for maize crops. The responses of microbial communities of a chernozem soil (Limagne basin, France) to pure or formulated (Callisto^®) mesotrione, applied at three different doses [one fold field rate (1 × FR), 10 × FR and 100 × FR], were studied using a laboratory microcosm approach. The effects were assessed on the prokaryotic cell abundance, the overall microbial activities (substrate-induced respiration (SIR) and dehydrogenase activity (DHA)) and the genetic structure of the bacterial and fungal communities (temporal temperature/denaturing gradient gel electrophoresis (TT/DGGE)). Mesotrione dissipation was similar whatever the formulation applied and the amounts dissipated were positively correlated to application rates. Several biodegradation products including the metabolites 4-methylsulfonyl-2-nitrobenzoic acid (MNBA) and 2-amino-4-methylsulfonylbenzoic acid (AMBA) were detected from day 42 post-treatment, in 10 × FR and 100 × FR treated soils. No response of the soil microbial communities was detected in soil spread with both the 1 × FR applications. Overall soil microbial activity was stimulated from day 6 by 10 × FR of Callisto^® and more strongly by 100 × FR of pure mesotrione and Callisto^®, whereas prokaryote abundance did not increase before day 95 in both the 100 × FR treatments. Genetic structural shifts recorded from day 42 in the bacterial and fungal communities were small and mainly attributable to variations in band intensity. Maximum dissimilarity of the bacterial and fungal genetic structures between control and 100 × FR treated soils did not exceed 12% and 28%, respectively. The general pattern was that more consistent effects occurred with increasing exposure times, especially in both the 100 × FR treated soils. These microbial responses could be due to the stimulation of (i) adapted mesotrione-degrading microorganisms and (ii) the activity of resistant heterotrophic microbial groups promoted by dead biomass from sensitive organisms. In addition, at 100 × FR doses, pure mesotrione seemed to induce stronger microbial responses than Callisto^®, formulation which contains adjuvants with potential side-effects on some microbial populations. This experimental approach indicated that pure mesotrione and Callisto^® affected soil microbial communities, but the effects were only detected at doses far exceeding the recommended field rates.     

A microcosm experiment to evaluate the influence of location and quality of plant residues on residue decomposition and genetic structure of soil microbial communities

The effects of location (soil surface vs. incorporated in soil) and nature of plant residues on degradation processes and indigenous microbial communities were studied by means of soil microcosms incubation in which the different soil zones influenced by decomposition i.e. residues, soil adjacent to residues (detritusphere) and distant soil unaffected by decomposition (bulk soil) were considered. Plant material decomposition, organic carbon assimilation by the soil microbial biomass and soil inorganic N dynamics were studied with ¹³C labelled wheat straw and young rye. The genetic structure of the community in each soil zone were compared between residue locations and type by applying B- and F-ARISA (for bacterial- and fungal-automated ribosomal intergenic spacer analysis) directly to DNA extracts from these different zones at 50% decomposition of each residue. Both location and biochemical quality affected residue decomposition in soil: 21% of incorporated ¹³C wheat straw and 23% left at the soil surface remained undecomposed at the end of incubation, the corresponding values for ¹³C rye being 1% and 8%. Residue decomposition induced a gradient of microbial activity with more labelled C incorporated into the microbial biomass of the detritusphere. The sphere of influence of the decomposing residues on the dynamics of soluble organic C and inorganic N in the different soil zones showed particular patterns which were influenced by both residue location and quality. Residue degradation stimulated particular genetic structure of microbial community with a gradient from residue to bulk soil, and more pronounced spatial heterogeneity for fungal than for bacterial communities. The initial residue quality strongly affected the resulting spatial heterogeneity of bacteria, with a significance between-zone discrimination for rye but weak discrimination between the detritusphere and bulk soil, for wheat straw. Comparison of the different detrituspheres and residue zones (corresponding to different residue type and location), indicated that the genetic structure of the bacterial and fungal communities were specific to a residue type for detritusphere and to its location for residue, leading to conclude that the detritusphere and residue corresponded to distinct trophic and functional niches for microorganisms.     

A sediment exchange experiment to assess the limiting factors of microbial sulfate reduction in acidic mine pit lakes

Purpose

Microbial sulfate reduction is an alkalinity-producing process and potentially supports the neutralization of acidic mine pit lakes. In many acidic lakes the process does not occur. Sulfate-reducing bacteria are known to be pH sensitive. There are, however, several reports of sulfate reduction occurring in the sediment of acidic lakes. To find out why sulfate reduction occurs in some acidic lakes but not in others, we conducted a field experiment.

Materials and methods

Surface sediment from lake ML111 (pH?2.6, no sulfate reduction), in the Koyne-Plessa lignite mining district of Lusatia in Germany, was incubated in the less-acidic lake ML117 (pH?3.4, sulfate reduction) and vice versa. After 19?weeks of incubation, the sediments were sampled and analyzed for microbial sulfate reduction rates, bacterial numbers, and geochemical composition.

Results and discussion

Incubation of ML117 sediment in ML111 resulted in a partial inhibition of sulfate reduction while incubation of ML111 sediment in ML117 did not initiate sulfate reduction. We observed a linear relationship between sediment pH and sulfate reduction, while there was no relation with sedimentary iron content. Sulfate reduction was not only affected by the water quality but also by the experimental treatment. Homogenization of the sediment prior to incubation stimulated microbial sulfate and iron reduction. Due to the low pH, incubation in ML111 resulted in the dissolution of reduced inorganic sulfur.

Conclusions

We conclude that the water pH is the major regulator of sulfate reduction in the surface sediment of acidic lakes. The rate of sulfate reduction in the sediment of acidic lakes depends on a fragile equilibrium between proton flux between water and sediment, and buffering reactions in the sediment.     

The effect of feeding behavior on Hg accumulation in the ecophysiologically different earthworms Lumbricus terrestris and Octolaseon cyaneum: A microcosm experiment

 A soil microcosm experiment was performed to assess the uptake of Hg from various Hg-spiked food sources (soil, leaf litter and root litter of Trifolium alexandrinum) by two earthworm species, Lumbricus terrestris (anecic) and Octolaseon cyaneum (endogeic). Treatments were applied in which one of the three food sources was Hg spiked and the other two were not. Additional treatments in which all or none of the food sources were Hg spiked were used as controls. Uptake of Hg from soil into tissues of both earthworm species was significantly higher than uptake of Hg from leaf litter or root litter, indicating that soil may be the most important pool for the uptake of Hg into earthworms. In addition, the anecic L. terrestris significantly accumulated Hg from all Hg-spiked food sources (leaf litter, root litter and soil), whereas the endogeic O. cyaneum took up Hg mainly from soil particles. Interestingly, there was no further increase in Hg in L. terrestris when all food sources were Hg spiked compared to the single Hg-spiked sources. This may be attributed to the relatively high Hg content in the soil, which may have influenced the feeding behavior of the earthworms, although their biomass did not significantly decline. We suggest that, in addition to the physiological differences, feeding behavior may also play a role in the contrasting uptake of Hg by the two earthworm species.     

Plant extracts from the Mediterranean zone of Chile potentially affect soil microbial activity related to N transformations: A laboratory experiment

Abstract

Available soil N can be lost through ammonia volatilization, nitrate leaching to surface and ground waters, and nitrous oxide emission under intensive systems of agricultural production. Research has shown that phenolic compounds isolated from certain plants influence N cycling in soils, which results in a decrease of N losses and represents a key issue for environmental protection worldwide. In this study, water-soluble and ethanol-soluble compounds were extracted from the leaves, bark and roots of certain tree and shrub species in the Mediterranean zone of Chile, with the aim of measuring their effects on N transformation, soil respiration, soil microbial biomass and urease activity. The studied species were Acacia caven Mol, Quillaja saponaria Mol., Bacharis linearis (Ruiz & Pav.) Pers. and Pinus radiata D. Don. Plant extracts were applied to soil (Haploxeralf) fertilized with urea. The extracts obtained from the roots of A. caven in water, bark of A. caven in ethanol, bark of P. radiata in ethanol, leaves of P. radiata in water and roots of B. linearis in water showed a reduction in the soil nitrification rate. The bark of A. caven in ethanol and bark of P. radiata in ethanol extracts showed a reduction in urease activity. In conclusion, only the bark of P. radiata in ethanol extract affected all the biological parameters considered, reducing soil nitrification, urease activity, microbial biomass and carbon dioxide emissions from the soil, these being attributed to high levels of phenolic content in the extract. However, further research is necessary to explain the mechanisms involved.     

The Use of Goats Grazing to Restore Pastures Invaded by Shrubs and Avoid Desertification: A Preliminary Case Study in the Spanish Cantabrian Mountains

Spanish mountains have been affected by the expansion of shrubs and forests since the mid‐20th century. This secondary succession in vegetation has some positive effects, but also drawbacks, such as an increase in fire risk, loss of diversity in land use, a reduction in landscape and cultural value, less water available in river channels and reservoirs, constraints on livestock farming, a reduced number of local species and loss of biodiversity. This paper analyses the potential for grazing domestic goats to help control the spread of several species of shrubs such as the common broom (Cytisus scoparius), red raspberry (Rubus idaeus) and roses (Rosa sp.) that are commonly found in degraded pastures in the Cantabrian Mountains of northern Spain. Using experimental plots, the effects of two levels of stocking density (4·5 and 9 goats ha⁻¹ y⁻¹) are compared with other land management systems used in the region: burning, mechanical clearing and trimming. The combined use of goats with support from burning, clearing and trimming controls the spread of shrubs. The most efficient treatment was found with nine goats ha⁻¹ y⁻¹. Goat grazing also changes the distribution of shrubs, transforming a dense and continuous coverage into separate clumps and thereby enabling livestock to graze more easily. Maintaining a mixed structure of shrubs and pastures is the best treatment due to the low population density of the Cantabrian Mountains, as this enhances the biodiversity, controls fire risk and enriches the landscape; it also allows extensive livestock grazing as a main economic resource. Copyright © 2013 John Wiley & Sons, Ltd.     

New metabolites in the degradation of alpha- and gamma-hexachlorocyclohexane (HCH): pentachlorocyclohexenes are hydroxylated to cyclohexenols and cyclohexenediols by the haloalkane dehalogenase LinB from Sphingobium indicum B90A

Technical hexachlorocyclohexane (HCH) and lindane are obsolete pesticides whose former production and use led to widespread contaminations posing serious and lasting health and environmental risks. Out of nine possible stereoisomers, alpha-, beta-, gamma-, and delta-HCH are usually present at contaminated sites, and research for a better understanding of their biodegradation has become essential for the development of appropriate remediation technologies. Because haloalkane dehalogenase LinB was recently found responsible for the hydroxylation of beta-HCH, delta-HCH, and delta-pentachlorocyclohexene (delta-PCCH), we decided to examine whether beta- and gamma-PCCH, which can be formed by LinA from alpha- and gamma-HCH, respectively, were also converted by LinB. Incubation of such substrates with Escherichia coli BL21 expressing functional LinB originating from Sphingobium indicum B90A showed that both beta-PCCH and gamma-PCCH were direct substrates of LinB. Furthermore, we identified the main metabolites as 3,4,5,6-tetrachloro-2-cyclohexene-1-ols and 2,5,6-trichloro-2-cyclohexene-1,4-diols by nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry. In contrast to alpha-HCH, gamma-HCH was not a substrate for LinB. On the basis of our data, we propose a modified gamma-HCH degradation pathway in which gamma-PCCH is converted to 2,5-cyclohexadiene-1,4-diol via 3,4,5,6-tetrachloro-2-cyclohexene-1-ol and 2,5,6-trichloro-2-cyclohexene-1,4-diol.