Adaptive attributes of tropical forage species to acid soils I. Differences in plant growth,nutrient acquisition and nutrient utilization among C4 grasses and C3 legumes

Low supply of nutrients is a major limitation of forage adaptation and production in acid soils of the tropics. A glasshouse study was conducted to find differences in plant growth, nutrient acquisition and use, among species of tropical forage grasses (with C₄ pathway of photosynthesis) and legumes (with C₃), when grown in two acid soils of contrasting texture and fertility. Twelve tropical forage legumes and seven tropical forage grasses were grown in sandy loam and clay loam Oxisols at low and high levels of soil fertility. After 83 days of growth, dry matter distribution among plant leaves, stems, and roots, leaf area production, shoot and root nutrient composition, shoot nutrient uptake, and nutrient use efficiency were measured. Soil type and fertility affected biomass production and dry matter partitioning between roots and shoots. The allocation of dry matter to root production was greater with low soil fertility, particularly in sandy loam. The grasses responded more than the legumes to increased soil fertility in both shoot and root biomass production. Leaf area production and the use of leaf biomass for leaf expansion (specific leaf area) were greater in legumes than in grasses, irrespective of soil type and fertility. But soil type affected shoot biomass production and nutrient uptake of the grasses more than those of the legumes. There were significant interspecific differences in terms of shoot nutrient uptake. The grasses were more efficient than legumes in nutrient use (grams of shoot biomass produced per gram of total nutrient uptake) particularly for nitrogen (N) and calcium (Ca).     

Legume–grass species influence plant productivity and soil nitrogen during grassland succession in the eastern Tibet Plateau

In recent years, numerous studies have evaluated the effect of plant function diversity on ecosystem functions such as productivity and soil nutrient status. We performed a redundancy analysis (RDA) to examine the relationship between plant functional diversity, productivity and soil nitrogen in a chronosequence of abandoned fields in sub-alpine meadow in the eastern part of the Tibet Plateau, China. We found that along the secondary succession sequence, legume richness and aboveground biomass significantly increased and both were positively correlated with total species richness (S) and aboveground biomass (T-bio). This pattern suggests that legume richness increases community productivity. In addition, we found that total aboveground biomass, legume and grass richness were positively correlated with soil microbial nitrogen (MBN), the ratio of microbial nitrogen to soil total nitrogen (MBN/TN) and the ratio of soil organic carbon and soil total nitrogen (C/N), whereas they were negatively correlated with soil total nitrogen (TN), organic carbon (C_org), and microbial carbon (MBC). Contrary to our predictions grasses such as Stipa grandis, Scirpus tripueter, Koeleria cristata were more closely associated with MBN, MBN/TN than legumes such as Oxytropis ochrocephala, Thermopsis lanceolate and Astragalus polycladus. The late-successional grass Kobresio humilis had a stronger positive correlation with NH₄-N as compared to the legumes and NO₃-N was not associated with any legume species. This suggests that the grasses and legumes have a synergetic positive influence on the ecosystem properties, especially nitrogen. Therefore, in this N-limited, plant community diversity of both legumes and grasses has a strong influence on ecosystem changes during succession.     

Subsurface earthworm casts can be important soil microsites specifically influencing the growth of grassland plants

Earthworms (Annelida: Oligochaeta) deposit several tons per hectare of casts enriched in nutrients and/or arbuscular mycorrhizal fungi (AMF) and create a spatial and temporal soil heterogeneity that can play a role in structuring plant communities. However, while we begin to understand the role of surface casts, it is still unclear to what extent plants utilize subsurface casts. We conducted a greenhouse experiment using large mesocosms (volume 45 l) to test whether (1) soil microsites consisting of earthworm casts with or without AMF (four Glomus taxa) affect the biomass production of 11 grassland plant species comprising the three functional groups grasses, forbs, and legumes, (2) different ecological groups of earthworms (soil dwellers—Aporrectodea caliginosa vs. vertical burrowers—Lumbricus terrestris) alter potential influences of soil microsites (i.e., four earthworms × two subsurface microsites × two AMF treatments). Soil microsites were artificially inserted in a 25-cm depth, and afterwards, plant species were sown in a regular pattern; the experiment ran for 6 months. Our results show that minute amounts of subsurface casts (0.89 g kg^?1 soil) decreased the shoot and root production of forbs and legumes, but not that of grasses. The presence of earthworms reduced root biomass of grasses only. Our data also suggest that subsurface casts provide microsites from which root AMF colonization can start. Ecological groups of earthworms did not differ in their effects on plant production or AMF distribution. Taken together, these findings suggest that subsurface earthworm casts might play a role in structuring plant communities by specifically affecting the growth of certain functional groups of plants.     

Effect of Biosolids and Cd/Pb Interaction on the Growth and Cd Accumulation of Brassica rapa Grown in Cd-Contaminated Soils

Mixed metals in the cropped lands in central Taiwan contaminated about 230 ha. According to the Soil and Groundwater Protection Remediation Act (SGWR Act) of Taiwan, these lands were restored. However, some grains of paddy rice grown in these remediated soils still contained more than 0.5 mg Cd kg^?1, which the Department of Health of Taiwan notified as the maximum allowable Cd content in rice. The suitability of planting edible crops in these soils is now in doubt. Brassica rapa is the crop most often sold in Taiwan's market and is planted in the interval between the first and second stages of planting of paddy rice, especially in central Taiwan where this experiment was conducted. A pot experiment was conducted using soils contaminated artificially with Cd or both Cd and Pb. The soil was then amended with 5% of biosolid and followed by planting of B. rapa. The objectives were to study the effect of biosolid amendment on the soil and the interaction between Cd and Pb on the growth of and Cd accumulation in B. rapa. Experimental result showed that the biomass and the accumulation of Cd by B. rapa were significantly increased in the biosolid-amended soils compared with the control. Lead has a synergistic effect on enhancing the accumulation of Cd by B. rapa grown in artificially Cd-contaminated soils.     

Regulation of macro,micro, and toxic element uptake by <Emphasis Type="Italic">Salix</Emphasis> × <Emphasis Type="Italic">smithiana</Emphasis> using liming of heavily contaminated soils

Purpose

Willow cultivation in soils heavily contaminated by risk elements is a challenging issue due to phytotoxic effects that restrict plant growth. Liming reduces the mobility of some risk elements in contaminated soils and therefore can be a suitable measure for contaminated soils but can also affect availability of nutrients for planted willows. We investigate how liming affects concentrations of macro, micro, and toxic elements in the organs of willows planted in contaminated soils.

Materials and methods

We established a 3-year pot experiment with Salix × smithiana planted in weakly acid and alkaline soils anthropogenically seriously contaminated by As, Cd, Pb, and Zn. Soils were both untreated and treated with two doses of lime and dolomite in the first year before planting. We determined biomass production, mortality, and the concentration of macro- and micronutrients and toxic elements in the willows’ aboveground organs.

Results and discussion

Lime application increased biomass production in both soils; dose of lime played an important role for its increase only in alkaline soil. Lime in a higher dose was incompatible with the vitality of just-planted willows in both soils. Doses of dolomite significantly affected the biomass production and mortality of willows, where lower doses caused a permanent decrease of biomass production and mortality in weakly acid soil. The toxicity of Cd and Zn in leaves was recorded in both untreated soils; the latent deficiency of P and deficiency of Fe in leaves was only recorded in weakly acid untreated soil.

Conclusions

Lime application irrespective of dose with foliar Fe application seemed to be the most suitable measure for increasing biomass production and decreasing toxic elements, especially Cd and Zn, without decreasing the macro- and micronutrients in the aboveground organs of willows in weakly acid soil. In alkaline soil, only higher doses of lime had a positive effect on the studied parameters. Dolomite application is not a suitable measure for planting willows in both contaminated soils. Dolomite in a lower dose impairs the growth of willows in weakly acid soil.

     

Increasing the Knowledge on the Management of Cu‐Contaminated Agricultural Soils by Cropping Tomato (Solanum Lycopersicum L.)

This paper increases the knowledge on the potential use of Cu‐contaminated agricultural soils with tomato (Solanum lycopersicum L.). The effect of Cu and its interaction with soil properties on plant biomass production and on the accumulation of this metal in plant tissues were evaluated by conducting biomass assays in four representative Mediterranean agricultural soils contaminated by Cu. Copper toxicity on plant biomass production, evaluated through the effective concentrations of Cu added to soil that reduce the biomass production by 50% (EC₅₀) and by 10% (EC₁₀), was higher in soils having less soil organic matter and clay content and even in soils with favourable properties but having salinity. For the cases in which tomato was collected, Cu concentrations in them were similar for all soils and doses and never exceed the maximum Cu concentration allowed by the Codex Alimentarius Commission Regulation (10 mg_Cu kg⁻¹ in fresh weight basis). According to our results, tomato could be cropped in Cu‐contaminated Mediterranean agricultural soils when concentrations of Cu determined in them rely between their respective EC₅₀ and EC₁₀, because production and quality of fruits, the latter understood as the Cu concentration in them, would not be compromised. For the soils assayed, these values would range between 32·9 and 1696·5 mg kg⁻¹, depending on soil properties. Because the baseline value considered is similar to those established in other parts of the European Mediterranean region, these results can be used as guidance for this region to establish adequate phytoremediation strategies and prevent land degradation processes. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of multiple heavy metal contamination and repeated phytoextraction by Sedum plumbizincicola on soil microbial properties

The threat of heavy metal contamination to food and human health in south and east China has become a public concern as industrial development continues. The aims of this study were to investigate the influence of repeated phytoextraction over a two-year period by successive crops of the Zn and Cd hyperaccumulator Sedum plumbizincicola on multiple metal contaminated soils and to assess recovery of soil quality. Total and NH₄OAc-extractable Zn and Cd concentrations were significantly reduced in planted soils compared to unplanted soils. Microbial biomass C (C_mic), basal respiration and microbial quotient (qM) were significantly and positively correlated and soil metabolic quotient (qCO₂) was negatively correlated with heavy metal concentrations in unplanted soils (P < 0.05). However, C_mic, basal respiration and qM values increased significantly after phytoremediation by five crops over two years compared to unplanted soil. Urease, β-glucosidase, neutral phosphatase and arylsulfatase activities also increased significantly with decreasing heavy metal contents and hydrolase activity was enhanced in planted soil (P < 0.05) compared to the unplanted control. The data indicate the capacity of S. plumbizincicola to extract Zn and Cd from contaminated soil and also that phytoremediation had beneficial effects on soil microbial and hydrolase activities, with the metal phytoextraction procedure restoring soil quality.     

Environmental fate of polycyclic aromatic hydrocarbons under different plant traits in urban soil as affected by nitrogen deposition

Nitrogen deposition and contamination with anthropogenic substances such as polycyclic aromatic hydrocarbons (PAHs) increasingly threaten ecosystems, in particular, urban systems. We planted different plant communities (Holcus lanatus, Lotus corniculatus, Picea abies, Calluna vulgaris) in N-deficient urban soil to test whether the divergent plant–soil systems differ in their ability to affect the fate of phenanthrene and pyrene in the soil, and whether the ability of the system to sequester external N would differ between the plant and soil systems. Importantly, the influence of the N input as affecting PAH dynamics under the plant communities was explored. The effects of plants on soil microbial biomass (PLFA) and microbial activity (basal respiration) were studied. Although plant type did not affect the dynamics of phenanthrene and pyrene in the soil, N addition had a positive effect on phenanthrene retention in each plant–soil treatment. The content of soil extractable phenanthrene decreased due to N addition especially in soils growing P. abies. This conifer also sequestered the added N to its shoots most efficiently suggesting restoration success of urban soils to be plant trait dependent.     

Effect of ammonium and phosphorus supply on h+ production in gel by two tropical forage grasses

The effect of the supply of ammonium (NH₄ ⁺) and phosphorus (P) in gel on the amounts of hydrogen ion (H⁺) excreted from plant roots was studied with Brachiaria humidicola (a highly acid‐soil tolerant tropical grass) and B. brizantha (less acid‐soil tolerant) grown in soil in a glasshouse. The H⁺ production was measured over 24 h in agar gel containing full nutrient solution with a range of NH/‐N levels (0, 0.25, 0.5, and 5.0 mM NH₄ ⁺‐N). Highly soluble P, K₂HPO₄, or relatively insoluble P, rock P, was supplied at four concentrations (0, 11.5, 34.5, or 115 μM p) in the gel. Increasing NH₄ ⁺ concentration in the gel increased H⁺ production for both grasses, but there was some inhibition of growth for B. brizantha at the highest N concentration. For B. humidicola, but not B. brizantha H⁺ production was greater with 34.5 μM K₂HPO₄ than 11.5 μM K₂HPO₄. At 34.5 μM P for both grasses there was no difference in H⁺ production when P was supplied as rock P or K₂HPO₄. With 11.5 μM P both grasses produced less acid in the gel with the rock P compared with K₂HPO₄. The reduced H⁺ production is probably due to a lower availability of P in the rock P compared with K₂HPO₄. This effect was greater with B. brizantha than B. humidicola, implying that 11.5 μM rock P was not able to supply sufficient P for the growth of B. brizantha. Brachiaria humidicola was able to dissolve more rock P than B. brizantha or alternatively, the growth of B. humidicola was less adversely affected by the low P supply from rock P than B. brizantha. Plant‐induced acidity does not seem to occur as a response to a lack of available P, but rather these grasses only produce acid if there are enough nutrients for growth, i.e., both NH₄ ⁺ and P. If either N or P is limiting, growth is limited as is NH₄ ⁺ uptake, so that H⁺ production is curtailed.     

Evaluation of the characteristics of 14 cover crops used in a soil rehabilitation trial

Many of the soils in Santa Cruz Department, Bolivia, are degraded by serious weed infestation, subsoil compaction, and low organic matter and nitrogen contents. Fallow periods with cover crops are frequently used to recuperate the fertility of degraded soils, but little information exists on the desired characteristics of cover crops for this purpose. The aim of this study was to describe those characteristics, believed to be most relevant to the rehabilitation of degraded soils in Santa Cruz, of 14 cover crops, which included three grasses, nine legumes, and two winter cover crops sown after summer soybean (Glycine max). The cover crop characteristics, evaluated over two years, were ease of establishment, competitiveness against weeds, tolerance to drought, dry matter production and nutrient contents of the above-and below-ground residues, nodulation, rooting density and root diameters. The three grasses Tobiata (Panicum maximum var. Tobiatd), Centenario (Panicum maximum var. Centenario) and Brizantha (Brachiaria brizantha) appeared to be the most promising for increasing soil organic matter contents, and the three grasses and groundnuts (Arachis hypogaea)/pigeon pea (Cajanus cajan) for the recuperation of subsoil structure. Mucuna deeringiana and Lablab (Dolichos lablab) were the most promising for increasing soil N status. All of these cover crops competed successfully with weeds.     

Selection of Leguminous Trees Associated with Symbiont Microorganisms for Phytoremediation of Petroleum-Contaminated Soil

Leguminous trees have a potential for phytoremediation of oil-contaminated areas for its symbiotic association with nitrogen-fixing bacteria and arbuscular mycorrhizal fungi (AMF). This study selects leguminous tree associated with symbiotic microorganisms that have the potential to remediate petroleum-contaminated soil. Seven species of trees were tested: Acacia angustissima, Acacia auriculiformis, Acacia holosericea, Acacia mangium, Mimosa artemisiana, Mimosa caesalpiniifolia, and Samanea saman. They were inoculated with AMF mix and nitrogen-fixing bacteria mix and cultivated over five oil levels in soils, with five replicates. The decreasing of total petroleum hydrocarbons (TPH) values occurred especially with S. saman and its symbiotic microorganisms on highest oil soil contamination. Despite the large growth of A. angustissima and M. caesalpiniifolia on the highest level of oil, these species and its inoculated microorganisms did not reduce the soil TPH. Both plants were hydrocarbon tolerant but not able to remediate the polluted soil. In contrast were significative hydrocarbon decrease with M. artemisiana under high oil concentrations, but plant growth was severely affected. Results suggest that the ability of the plants to decrease the soil concentration of TPH is not directly related to its growth and adaptation to conditions of contamination, but the success of the association between plants and its symbionts that seem to play a critical role on remediation efficiency.     

Soil properties and plant production after short-term fallows in Senegal

Abstract. Field trials of vegetation modification were carried out to test the hypothesis that the management of key plant groups such as trees, perennial grasses and legumes would improve soil chemical properties in short-term fallows. Soil properties and plant production during a 4-year fallow period and millet yields after clearing were recorded at two sites representing dry and subhumid climates in Senegal, West Africa. During the four years of fallow, soil organic matter did not vary significantly at either site. A decrease in amounts of Olsen P, calcium and potassium in soil (0–10 cm depth) by 42–50% occurred at one site due to an effect of perennial grasses. The highest millet yields were measured in plots on which the largest amounts of biomass had been burnt after clear-cutting. These results confirmed that short-term fallows do not replenish soil organic matter and nutrient contents. The introduction of planted species did not arrest the decline in soil quality.     

Combined amendment of immobilizers and the plant growth-promoting strain Burkholderia phytofirmans PsJN favours plant growth and reduces heavy metal uptake

In the surroundings of a former Pb/Zn smelter in Arnoldstein (Austria) heavy metal concentrations in planted crops exceed thresholds for usage as food and feed. The aim was to study the effects of a plant growth-promoting bacterial strain in combination with immobilizing soil amendments on plant growth, heavy metal uptake and on microbial community structure. Pot experiments were performed whereby two maize cultivars were grown in different contaminated soils and treatments consisted of Burkholderia phytofirmans strain PsJN with and without addition of gravel sludge and siderite bearing material. Inoculation with strain PsJN significantly improved root and shoot biomass of maize independent of immobilizer addition. Analysis of heavy metal content of the rhizosphere and leaves indicated that immobilizing amendments had significant reducing effects on NH₄NO₃ extractable Zn and Pb in soil and in plants grown in treated soils. Microbiomes were analysed by cultivation-independent pyrosequencing analysis of 16S rRNA genes. The results showed clear effects on community composition in response to the immobilizer amendments, whereas inoculation with B. phytofirmans affected microbiome diversity only to a minor extent.     

Use of green manures in increasing inputs of biologically fixed nitrogen to sugar cane

In this study the contribution of biological N₂ fixation (BNF) to several green manure crops used at planting of sugar cane, or in the inter-rows of the ratoon crop, was evaluated. The subsequent effects of these legumes on the N supply and yield of the sugar cane crop was also investigated. The experiment was conducted in an extremely sandy N-deficient soil over two consecutive years. In the first year the legumes were planted on the same day as the sugar cane and in the second year they were planted immediately after the harvesting of the cane. The treatments consisted of four legume species (Crotalaria juncea, C. spectabilis, Canavalia ensiformis and Mucuna deeringiana), 100 kg N as urea fertiliser in a split addition, and no fertiliser N addition. The legumes were planted as two rows spaced 40 cm apart between each row of cane. Sorghum (Sorghum vulgare) and maize (Zea mays) were planted along the side of the experiment to act as non-N₂-fixing reference plants for the application of the ¹⁵N natural abundance technique to quantify the BNF input to the legumes and the sugar cane. The soil cover, monitored using an overhead digital camera, showed that Crotalaria juncea was the quickest to establish and cover the soil. At 31 days after planting (DAP) it reached a height of over 60 cm, more than twice that of any of the other green manures, and covered 55% of the soil, 16% more than that of the second most vigorous legume, Canavalia ensiformis. Using the natural abundance ¹⁵N technique it was deduced that all of the legumes had obtained over 60% of their N from BNF. BNF inputs to some of the green-manure legumes were quite significant, ranging from 35 to 55 kg N ha^?1 for Canavalia ensiformis, Crotalaria juncea and Mucuna deeringiana at plant crop, whereas the effect of the presence of these legumes on final cane yield and dry matter and N accumulation was not significant. There was also considerable evidence of negative (allelopathic) effects on cane growth induced by the presence of some of the legumes, especially Crotalaria juncea.     

Plant monocultures produce more antagonistic soil Streptomyces communities than high-diversity plant communities

Plant–soil feedbacks are important to productivity and plant community dynamics in both natural and managed ecosystems. Among soil bacteria, the Streptomyces possess particularly strong antagonistic activities and inhibit diverse plant pathogens, offering a clear pathway to involvement in plant–soil feedbacks. We hypothesized that feedback effects and the ability of individual host plant species to foster antagonistic Streptomyces populations may be modified by the richness of the surrounding plant community. To test this, we collected soil associated with four different plant species (two C4 grasses: Andropogon gerardii, Schizachyrium scoparium; and two legumes: Lespedeza capitata, Lupinus perennis), grown in communities that spanned a gradient of plant species richness (1, 4, 8, 16, or 32 species). For each of these soils, we characterized the potential of soil Streptomyces to antagonize plant pathogens, using an in vitro plate assay with indicator strains to reveal inhibition. We cultivated each plant species in each conditioned soil to assess feedback effects on subsequent plant growth performance. Surrounding plant richness modified the impacts of particular plant species on Streptomyces antagonistic activity; A. gerardii supported a higher proportion of antagonistic Streptomyces when grown in monoculture than when grown in 32-spp plant communities, and L. capitata supported more strongly antagonistic Streptomyces when grown in 4- or 32-spp plant communities than in 8-spp plant communities. Similarly, the feedback effects of particular plant species sometimes varied with surrounding plant richness; aboveground biomass production varied with plant species richness for A. gerardii in L. perennis-trained soil, for L. capitata in A. gerardii-trained soil, and for L. perennis in L. capitata-trained soil. Streptomyces antagonist density increased with overall Streptomyces density under low but not under high plant richness, suggesting that plant diversity modifies selection for antagonistic phenotypes among soil Streptomyces. This work highlights the complexity of feedback dynamics among plant species, and of plant–microbiome interactions in soil.     

Nitrogen nutrition and changes in the chemical attributes of the soil for cultivars of Brachiaria brizantha intercropped with Stylosanthes in different forage systems

ABSTRACT

Nitrogen in the soil-plant system is extremely complex, and any change in soil management or cultivated species can modify its dynamics. To minimize environmental problems and sustainably optimize agricultural production, it is necessary to understand the dynamics of nitrogen in the soil-plant system. Thus, the objective of this study was to evaluate plant nitrogen nutrition and changes in the chemical attributes of the soil for cultivars of Brachiaria brizantha intercropped with Stylosanthes in different forage systems. The experimental design consisted of randomized block experimental design was used, with three replicates. The treatments consisted of forage systems: Piata palisadegrass, Paiaguas palisadegrass, Stylosanthes cv. Campo Grande, row intercropping of Piata palisadegrass with Stylosanthes, mixed intercropping of Piata palisadegrass and Stylosanthes, row intercropping of Paiaguas palisadegrass and Stylosanthes, and mixed intercropping of Paiaguas palisadegrass and Stylosanthes. Evaluations were performed in winter, spring, summer, and autumn in the same plots and over a 2-year period. The intercropping of grasses and legumes positively influences the chemical characteristics of the soil and the nutritional status of the forage by improving the concentration of nutrients. Overall the row seeding method provided better nutrient concentrations. The second year provided higher nitrogen absorption but resulted in greater soil acidification.     

Effect of earthworm on growth of late succession plant species in postmining sites under laboratory and field conditions

Earlier studies of postmining heaps near Sokolov, Czech Republic (0–46 years old) showed that massive changes in plant community composition occur around 23 year of succession when the heaps are colonized by the earthworms Lumbricus rubellus (Hoffm.) and Aporrectodea caliginosa (Savigny). The aim of the current study was to test the hypothesis that the introduction of earthworms into a postmining soil enhances growth of late succession plant species. In a laboratory experiment, earthworms significantly increased biomass of Festuca rubra and Trifolium hybridum grown in soil from a 17-year-old site. The biomass increase corresponded to a significant decrease in pH and an increase in oxidable C, total N, and exchangeable P, K, and Ca content. A second laboratory experiment showed higher biomass production of late successional plant community (Arrhenatherum elatius, Agrostis capillaris, Centaurea jacea, Plantago lanceolata, Lotus corniculatus, and Trifolium medium) in soil from late successional stage (46 years old); the introduction of earthworms into soil from an early successional stage (17 years old) increased biomass production. In a field experiment, introduction of L. rubellus to enclosures containing a 17-year-old soil not colonized by earthworms significantly increased the biomass of grasses after 1 year. The results support the hypothesis that colonization of postmining areas by earthworms can substantially modify soil properties and plant growth.     

Surfactant-Enhanced Soil Washing for Removal of Petroleum Hydrocarbons from Contaminated Soils: A Review

An increase in energy demand leads to further exploration, transportation, and utilization of petroleum, which creates severe soil contamination because of recurrent accidents and oil spills. Remediation of these contaminated soils is challenging. Among many treatment methods practiced for remediation of petroleum-contaminated soils, surfactant-enhanced soil washing has been widely practiced as a preferred treatment option, as it is a fast and environmentally accepted method. In this paper, we review research undertaken on various anionic, nonionic, cationic, biological, and mixed surfactants for the remediation of petroleum hydrocarbon-contaminated soils. Upcoming surfactants like gemini and switchable surfactants are summarized. We assess the challenges and opportunities of in-situ and ex-situ soil washing, the mechanisms of surfactant-enhanced soil washing, and the criteria to follow for surfactant selection. Furthermore, we briefly discuss the operational and environmental factors affecting soil washing efficiency and soil and surfactant properties affecting surfactant adsorption. We also describe the advantages of coupling soil washing with effluent treatment and surfactant reuse challenges and opportunities. Moreover, challenges and possible new directions for future research on surfactant-enhanced soil washing are proposed.     

Interaction between legume and arbuscular mycorrhizal fungi identity alters the competitive ability of warm-season grass species in a grassland community

Arbuscular mycorrhizal fungi (AMF) and N₂-fixing legumes can alter the community structure of grasses. However, the effect of AMF, N₂-fixing legumes, and their interaction on the dynamics of prairie grass communities remains unclear. The aim of this study was to clarify the influence of two AMF (Glomus cubense and Glomus sp.) and two legumes (Medicago sativa and Dalea purpurea) on the competitive relationship between three native cool-season (Elymus canadensis, Elymus trachycaulus ssp. subsecundus, and Elymus lanceolatus ssp. lanceolatus) and two native warm-season species of grasses (Schizachyrium scoparium and Bouteloua gracilis). Results show that AMF and legumes altered the community structure of the grasses. G. cubense favoured the productivity of warm-season B. gracilis when growing with M. sativa. This might be related to a negative impact of G. cubense on the nitrogen-fixing activity of M. sativa and to a lower N-use efficiency of E. canadensis and E. lanceolatus ssp. lanceolatus under competition. This suggested an increased ability of B. gracilis to use the available N resource as affected by more competitive species, whereas Glomus sp. reduced the competitive ability of this grass when associated with M. sativa. The decrease in B. gracilis biomass was thus likely caused by enhancement of P uptake by M. sativa over this grass. Glomus sp. was beneficial to S. scoparium, another warm-season species, in the absence of legumes, and this may be attributed to improved P-use efficiency of this grass under competition with cool season-grasses. In contrast, AMF and legumes were not beneficial for the cool season grasses. G. cubense depressed the growth of E. trachycaulus ssp. subsecundus, and M. sativa decreased nutrient uptake by cool-season native grasses. This study shows that beneficial effect of the arbuscular mycorrhizal symbiosis on the coexistence of warm-season grasses with more competitive cool-season grasses depends on the identity of the AMF symbiont, the presence of legume species, and nitrogen resource availability that was affected by the most competitive species or P-use efficiency of warm season species.     

Internal Mechanisms of Plant Adaptation to Aluminum Toxicity and Phosphorus Starvation in Three Tropical Forages

Many tropical forage grasses and legumes grow well in acid soils, adapting to excess aluminum (Al) and phosphorus (P) starvation stresses by using mechanisms that are still unclear. To determine these mechanisms, responses to Al toxicity and P starvation in three tropical forages were studied: two grasses, Brachiaria hybrid cv. ‘Mulato’ (B. ruziziensis clone 44-06 × B. brizantha cv. ‘Marandú’) and Andropogon gayanus, and one legume, Arachis pintoi. The tropical grasses tolerated high levels of Al toxicity and P starvation, with the Brachiaria hybrid maintaining very low levels of Al concentration in shoots. ²⁷Al Nuclear Magnetic Resonance spectroscopy (NMR) analysis revealed that, in the Brachiaria hybrid, Al makes complexes with some ligands such as organic-acid anions in the root symplast. The forages probably adapted to P starvation through high P-use efficiency. These experiments provide the first direct evidence we know of that organic acid anions within root tissue help detoxify Al in non-accumulator species such as the Brachiaria hybrid.