Assessing changes in physical and biological properties in a soil contaminated by oil sludges under semiarid Mediterranean conditions

9 pages, 2 figures, 4 tables.We have studied the changes in biological and physical properties of a soil, in a semiarid area, contaminated by hydrocarbons in order to predict the potential for bioremediation of this soil. The microbial biomass C, the basal respiration and the metabolic quotient (qCO2) of contaminated soil were significantly higher than that of the control soil, which points to a markedly reduced efficiency of substrate use and to a possible toxic effect of hydrocarbons in the soil. The low global rate of mineralisation in the contaminated soil also indicates the presence of hydrocarbons resistant to biodegradation. The dehydrogenase activity and the activities of hydrolases involved in the N and P cycles (urease, protease and phosphatase) were stimulated by the contamination with hydrocarbons. Soil total porosity was increased by the contamination, about 15-fold in comparison with the control soil. The contamination by hydrocarbons led to an increase in cracks in the 100–200 μm size classes, which can be regarded as a reservoir to hold water for plants and microorganisms. The biodegradation of hydrocarbons in this soil would require long periods of time, and it would be advisable to apply methods of bioremediation to this contaminated soil.F. Caravaca acknowledges a grant from the European Commission (HPMF-CT-2000-00822).Peer reviewe

Effect of Eisenia foetida earthworms on mineralization kinetics, microbial biomass, enzyme activities, respiration and labile C fractions of three soils treated with a composted organic residue

7 pages, 6 tables, 1 figure.The aim of this work was to assess and compare the influence of Eisenia foetida Savigny earthworms on C mineralization rate, labile C fractions (water-soluble C and water-soluble carbohydrates), microbial biomass C, and enzyme activities (dehydrogenase, urease, phosphatase and ß-glucosidase) in three soils of varying texture treated with a composted organic residue and cropped with Avena sativa L. Mineralization decreased with the addition of earthworms to the sandy and clay-loam soils, especially in sandy soil (by about 4 µg CO 2-C g -1 day -1). There were no significant effects on the amount of CO 2 evolved from clay soil due to the addition of E. foetida. The addition of E. foetida to sandy soil significantly decreased microbial biomass C and increased microbial metabolic quotient the qCO 2 (CO 2-C to biomass C ratio). The addition of E. foetida did not affect the microbial biomass or the qCO 2 of the clay-loam and clay soils.F. Caravaca acknowledges a grant from the European Commission (HPMF-CT-2000-00822).Peer reviewe

Land use in relation to soil chemical and biochemical properties in a semiarid Mediterranean environment

Low organic matter levels and fragile structure of coarse textured soils from arid and semiarid environments can lead to progressive degradation of their quality and productivity. Our objective was to evaluate the effects of land use on soil organic matter quality and quantity and biochemical properties related to soil microbial activity that are involved in the cycling of nutrients and carbon. Four sandy soils under different agricultural practices and four adjacent soils under grass cover in a Mediterranean environment comprising the humid-temperature climate of centralwest Italy were characterised chemically and biochemically. A sensitivity index was calculated for each of the measured properties by dividing the values for uncultivated soils with their cultivated counterparts. A metabolic potential index was calculated as the ratio between dehydrogenase activity (DHase), and water-soluble carbon (WSC). There was a general decline of total organic carbon (TOC), extractable humic substances (HS), WSC, water-soluble carbohydrates (WSCH), DHase, urease, protease, phosphatase and β-glucosidase activities following the different agricultural practices with respect to soil under grass cover. DHase, urease and phosphatase activities were more sensitive and responded more frequently to land use than TOC and WSC and WSCH. DHase/WSC ratio confirmed the reduction of cultivated soils metabolism as consequence of management practices.Peer reviewe

Soil aggregate stability and organic matter in clay and fine silt fractions in urban refuse-amended semiarid soils

Division S-6 — NotesOrganic refuse application to soil may improve soil structure and thus soil quality and productivity. Two semiarid, structureless soils with <10 g kg-1 organic C (OC), differing in clay mineralogy, were amended annually at rates of 30 g kg-1 fresh or composted refuse to a total of 60 g kg-1 in a greenhouse experiment lasting 2 yr. Fresh refuse additions significantly increased the soil aggregate stability in water by 17%. Composted refuse application only increased aggregate stability in the beidellitic clay-dominated soil (13%). In both soils, the greatest increase in OC and humin following refuse additions occurred in the fine silt fraction, which contained less decomposed organic matter than the clay fraction. In the beidellitic clay soil, this increase in OC was larger than in the illitic clay soil. The aggregate stability of the beidellitic clay soil was significantly correlated with OC and humin of fine silt fraction.F. Caravaca acknowledges a grant from Caja de Ahorros del Mediterráneo (CAM).Peer reviewe

Aggregate stability and carbon characteristics of particle-size fractions in cultivated and forested soils of semiarid Spain

8 pages, 1 figure, 2 tables.Organo-mineral particle-size fractions play a major role in soil structure. However, our knowledge of these size fractions must increase if we are to optimise the physical regeneration of degraded soils in semiarid areas. Twenty soils (sieved <2 mm) used for two contrasting purposes (agriculture and forestry) in southeastern Spain were fractionated into <2 and 2–20 μm particle-size fractions by ultrasonic dispersion in water followed by sedimentation. The organic matter from these size fractions was separated by extraction with 0.1 M Na4P2O7 (pH 9.8) and by oxidation with H2O2. The different size fractions were analysed for C concentration before and after treatment with pyrophosphate and peroxide. For both cultivated and forested soils, the highest proportion of organic C was associated with 2–20 μm particle-size fraction. The <2 and 2–20 μm particle-size fractions were characterised by a high proportion of organic matter non-extractable with sodium pyrophosphate, which was partly resistant to peroxidation. The aggregate stability of cultivated soils was significantly lower (mean 40%) than that of forested soils (mean 82%). Soil aggregate stability was not correlated with the organic constituents associated with the <2 and 2–20 μm particle-size fractions, either for cultivated or for forested soils. In conclusion, the organic matter associated with the <20 μm particle-size fractions does not appear to contribute greatly to the stability of macroaggregates in semiarid soils.F. Caravaca acknowledges a grant from Caja de Ahorros del Mediterráneo (CAM).Peer reviewe

Stability of desiccated rhizosphere soil aggregates of mycorrhizal Juniperus oxycedrus grown in a desertified soil amended with a composted organic residue

9 pages, 4 tables, 1 figure.Adequate soil structural stability favours the establishment and viability of a stable plant cover, protecting the soil against water erosion in desertified Mediterranean environments. We studied the effect of soil drying–rewetting, inoculation with a mixture of three exotic arbuscular mycorrhizal (AM) fungi (Glomus intraradices Schenck & Smith, Glomus deserticola (Trappe, Bloss. & Menge) and Glomus mosseae (Nicol & Gerd.) Gerd. & Trappe) and addition of a composted organic residue on aggregate stabilisation of the rhizosphere soil of Juniperus oxycedrus. The AM fungi and composted residue produced similar increases in plant growth, independently of the water conditions. Under well-watered conditions, the highest percentages of stable aggregates were recorded in the amended soil, followed by the soil inoculated with AM fungi. Excepting microbial biomass C, the soil drying increased labile C fractions (water soluble C, water soluble and total carbohydrates), whereas the rewetting decreased significantly such C fractions. Desiccation caused a significant increase in aggregate stability of the rhizosphere soil of all plants, particularly in the amended and inoculated plants. In all treatments, the aggregates formed after soil drying were unstable, since, in the rewetting, they disappear, reaching the initial levels before soil drying. Our results suggest that the aggregation mechanisms developed by rhizosphere microbial community of the amended and inoculated plants under water stress can be particularly relevant in desertified soils exposed to long desiccation periods.This research was supported by CICYT.Peer reviewe

Changes in rhizosphere microbial activity mediated by native or allochthonous AM fungi in the reafforestation of a Mediterranean degraded environment

10 pages, 3 figures, 2 tables.This study was carried out in a semiarid degraded area to assess the effectiveness of mycorrhizal inoculation with a mixture of native arbuscular mycorrhizal (AM) fungi or an allochthonous AM fungus (Glomus claroideum), on the establishment of Olea europaea subsp. sylvestris L. and Retama sphaerocarpa (L.) Boissier in this area. Associated changes in the soil microbiological properties and aggregate stability related to these AM inocula were also recorded. Eighteen months after planting, G. claroideum had increased available P in the rhizosphere of both shrub species. In general, both inoculation treatments increased water-soluble C and water-soluble and total carbohydrates, G. claroideum being the most effective inoculum, particularly in R. sphaerocarpa. The mixture of native AM fungi was the most effective treatment for increasing the aggregate stability of R. sphaerocarpa soil, while that of O. europaea was increased only by G. claroideum. Increased (dehydrogenase, urease, protease-BAA, acid phosphatase and -glucosidase) enzyme activities, in particular of dehydrogenase and acid phosphatase, were recorded in the rhizosphere of both mycorrhizal shrub species. The mixture of native AM fungi was the most effective treatment for stimulating the growth of O. europaea and R. sphaerocarpa (11.6-fold and 3.3-fold, respectively, greater than control plants). The establishment of mycorrhizal shrub species favoured the reactivation of soil microbial activity, which was linked to an increase in aggregate stability.This research was supported by the EC + CICYT co-financed FEDER programme (1FD97-0507 FOREST).Peer reviewe

Synergistic influence of an arbuscular mycorrhizal fungus and organic amendment on Pistacia lentiscus L. seedlings afforested in a degraded semiarid soil

7 pages, 5 tables.A field experiment was undertaken to evaluate the effect of mycorrhizal inoculation with Glomus intraradices and added composted residue on the establishment of Pistacia lentiscus L. seedlings in a semiarid area. Composted residue greatly increased macronutrient (NPK) content, soil microbial activity and enzymatic activities, and decreased soil bulk density. There was a significant correlation between soil bulk density and both enzyme activities and labile C fractions (water-soluble C and water-soluble carbohydrates), which are also related to soil microbial activity. The most suitable methodology for revegetating with P. lentiscus seedlings was addition of composted residue to soil in conjunction with a mycorrhizal inoculation pretreatment of seedlings in a nursery, to increase available P uptake from composted residue. One year after planting, such a combined treatment had increased the plant height of P. lentiscus seedlings by 106% with respect to the control.This research was supported by the EU+CICYT co-financed FEDER programme. We acknowledge the technical support of Paisajes del Sur and TRAGSA. F. Caravaca acknowledges a grant from the European Commission.Peer reviewe

Synergistic influence of an arbuscular mycorrhizal fungus and organic amendment on Pistacia lentiscus L. seedlings afforested in a degraded semiarid soil

7 pages, 5 tables.A field experiment was undertaken to evaluate the effect of mycorrhizal inoculation with Glomus intraradices and added composted residue on the establishment of Pistacia lentiscus L. seedlings in a semiarid area. Composted residue greatly increased macronutrient (NPK) content, soil microbial activity and enzymatic activities, and decreased soil bulk density. There was a significant correlation between soil bulk density and both enzyme activities and labile C fractions (water-soluble C and water-soluble carbohydrates), which are also related to soil microbial activity. The most suitable methodology for revegetating with P. lentiscus seedlings was addition of composted residue to soil in conjunction with a mycorrhizal inoculation pretreatment of seedlings in a nursery, to increase available P uptake from composted residue. One year after planting, such a combined treatment had increased the plant height of P. lentiscus seedlings by 106% with respect to the control.This research was supported by the EU+CICYT co-financed FEDER programme. We acknowledge the technical support of Paisajes del Sur and TRAGSA. F. Caravaca acknowledges a grant from the European Commission.Peer reviewe

Elevated functional versatility of the soil microbial community associated with the invader Carpobrotus edulis across a broad geographical scale

Exotic invasive plants may shape their own rhizosphere microbial community during global invasions. Nevertheless, the impacts of such plant invasions on the functional capacities of soil microbial communities remain poorly explored. We used an approach at a broad geographical scale to estimate the composition and abundance of the fungal functional groups, as well as the bacterial metabolic functions, associated with the rhizospheres of Carpobrotus edulis (L.) L. Bolus and the predominant native plants in coastal ecosystems located in different geographical regions. We used the ASV method to infer the potential functions of the soil microbial community with the PICRUSt2 and FUNGuild tools. The predictive functional profiling of the bacterial communities differed between the rhizospheres of the invasive and native plants, regardless of the biogeographic location of the invaded soil. Some predicted pathways related to the biosynthesis of nucleotides such as ppGpp and pppGpp, lipids, carbohydrates and secondary metabolites and the degradation of organic matter were enriched in the C. edulis rhizosphere. Moreover, the invasive microbiota was characterised by a greater richness and diversity of catabolic enzymes involved in nutrients cycling and higher relative abundances of saprotrophs and pathotrophs. Invasion by C. edulis promoted a shift in the potential functional versatility of the soil microbial communities, which can cope with nutrient limitations and biotic stress, and can favour the establishment of the invasive plant, but also alter the functioning and stability of the invaded ecosystems.This investigation was funded by the Spanish Plan Nacional-FEDER Project RTI2018-094731-B-I0