Soil Compartments Research Articles

How to Outgrow Your Native Neighbour? Belowground Changes under Native Shrubs at an Early Stage of Invasion

AbstractWhile it is acknowledged that invasive species are a global driver of land degradation, their effects are often only noticed when the invasion has been going on for a while. However, early stage processes must play a fundamental role in plant establishment until invasive plants are able to outgrow the native vegetation. In ten plots of 100 m2 each, we tested the hypothesis that belowground properties are associated with early invasion processes aboveground. We examined the early stage of invasion by a woody legume (Acacia longifolia), growing in the canopy of native dune shrubs (Corema album) as a model system in oligotrophic primary dunes in southern Portugal. Biomass under canopies of invaded and non‐invaded C. album shrubs as well as organic matter (OM) distribution in various soil fractions was measured. In accordance with our hypothesis, A. longifolia presence was related to increased C. album foliar δ15N, a proxy for nitrogen derived from the invasive legume. Under invaded canopies, root and rhizosphere biomass were higher, as was OM in the silt‐clay fraction. Also, δ15N of the OM in the silt‐clay fraction under invaded canopies was enriched, while δ13C was depleted. Finally, we found that the ratio between OM in the biotic versus soil compartment could be a good early indicator for invasion. These findings suggest that even when aboveground invasion pressure on the system is low, it is imperative for ecosystem conservation to remove young plants, as they might alter soil functioning already at an early stage of invasion. Copyright © 2017 John Wiley & Sons, Ltd.

Micropredator niche differentiation between bulk soil and rhizosphere of an agricultural soil depends on bacterial prey.

Predation is a fundamental mechanism of all food webs, but its drivers and organismic connectivities, especially at microbial level, are still poorly understood. Specifically, competitive carbon flows in the presence of multiple micropredators, as well as trophic links within and between microbial kingdoms have rarely been resolved. Here, using maize-planted agricultural soil as a model system, we have investigated the predation of amended bacterial prey by both prokaryotic and eukaryotic micropredators. We have queried how soil compartment (rhizosphere vs bulk soil) and nature of prey (Gram-positive vs Gram-negative) influence predation outcomes. We added 13C-labelled biomass of Pseudomonas putida and Arthrobacter globiformis to soil microcosms and found that P. putida was consumed much more rapidly. Bacteria and microeukaryotes specifically responsive to the biomass amendments were identified by RNA-stable isotope probing. Amongst the bacteria, only a few myxobacteria sequestered C from A. globiformis, whereas a considerable diversity of predatory bacteria incorporated C derived from P. putida. Diverse groups of heterotrophic protists, especially amoeba including Glaeseria, Hartmanella and Vahlkampfia spp., were observed to incorporate 13C from both amendments, but with pronounced niche differentiation between rhizosphere and bulk soil. This provides novel insights into niche partitioning between bacterial and eukaryotic micropredators in soil, driven not only by the nature of bacterial prey itself, but also by soil compartments.

Fungal community assemblage of different soil compartments in mangrove ecosystem

The fungal communities of different soil compartments in mangrove ecosystem are poorly studied. We sequenced the internal transcribed spacer (ITS) regions to characterize the fungal communities in Avicennia marina root-associated soils (rhizosphere and pneumatophore) and bulk soil compartments. The rhizosphere but not pneumatophore soil compartment had significantly lower fungal species richness than bulk soil. However, bulk soil fungal diversity (Shannon diversity index) was significantly higher than both pneumatophore and rhizosphere soil compartments. The different soil compartments significantly affected the fungal community composition. Pairwise sample analyses showed that bulk soil microbial community composition significantly different from rhizosphere and pneumatophore soil compartments. There was, however no significant difference observed between rhizosphere and pneumatophore soil fungal community composition and they shared relatively more OTUs between them. Further, there was a significant correlation observed between fungal community compositional changes and carbon or nitrogen availability of different soil compartments. These results suggest that few characteristics such as fungal richness and taxa abundance of rhizosphere and pneumatophore soil compartments were significantly different in mangrove ecosystem.

Effect of different crop management practices on soil Collembola assemblages: A 4-year follow-up

Soil biota are a major component of agroecosystems, playing a decisive role in ecosystem services with synergistic effects on crop production. The conservation of their diversity has become a key component of a strategy towards agricultural sustainability. Over four years (2010–2014), under the “SOil Functional diversity as an Indicator of sustainable management of Agroecosystems” (SOFIA) project, we followed soil Collembola assemblages in response to the set-up of 5 cropping systems differing in crop rotations (annual or perennial), rate of N fertilization, and in tillage intensity (annual ploughing vs. shallow). Our results demonstrated that shifting from a conventional to conservation cropping system had a strong positive effect upon species richness and density of Collembola. Specifically, all treatments with a reduction in intensity of soil tillage fostered Collembola assemblages. At the end of our study, density and species richness were 3 to 4 times higher in reduced tillage (RT) than in conventional tillage (CT). Nevertheless, differentiation between the assemblages only occurred after 2 years but steadily increased until 4 years. At the last sampling date, all treatments contained significantly different Collembola assemblages (Anosim with Bray-Curtis distance). In parallel, we noticed shifts in the functional structure of the assemblages, even if globally, all life-forms were promoted under reduced tillage. However, contrary to our expectations, euedaphic Collembola were not promoted by restitution of crop residues. Our study over several years under field conditions showed that Collembola assemblages were more sensitive to tillage intensity than to either residue management or N fertilization. Clearly, conservation agriculture can foster one of the numerous services provided by the soil compartment, namely the soil biodiversity and therefore improve soil quality and health.

A key role for arbuscular mycorrhiza in plant acquisition of P from sewage sludge recycled to soil

Dried and incinerated sewage sludge (SS) have the potential to be used as phosphorus (P) fertilisers. Arbuscular mycorrhizal fungi (AMF) contribute to plant P uptake; however, their role in P uptake from SS has yet to be fully explored. A compartmented pot system with an isotope pool dilution approach was used to investigate wheat (Triticum aestivum L.) P uptake from soluble P, dried SS and incinerated SS, via roots and/or AMF hyphae. Wheat was sown into an inner compartment containing a 33P label with/without AMF (Rhizophagus irregularis) inoculum. An outer soil compartment contained the P source. Compartments were separated by mesh of different sizes to allow root and/or AMF hyphal access. Plants obtained P from dried SS via AMF but there was limited evidence that plants obtained P from incinerated SS via AMF. Phosphorus uptake was significantly greater when both roots and hyphae could access dried SS than when only roots could access dried SS. We discuss the results in terms of availability of P to roots and hyphae. We conclude that AMF play an important role in wheat P acquisition from dried SS and therefore can assist in the recycling of P in waste.

Intercropping affects genetic potential for inorganic nitrogen cycling by root-associated microorganisms in Medicago sativa and Dactylis glomerata

The effect of intercropping on the genetic potential for N-cycle pathways promoting either retention or loss of N by the microbial communities associated to the roots or present in the bulk soil for two forage crops was studied in a greenhouse experiment. Medicago sativa (‘lucerne’) and Dactylis glomerata (‘cocksfoot’) were grown as sole crops or intercropped in soils with or without addition of biogas digestate to also evaluate fertilizer effects. Eight genes involved in inorganic N-cycling were quantified using real-time PCR to determine the genetic potential for different N-cycling pathways. Both plant species and intercropping affected the abundance of root-associated microbial-derived genes involved in N-cycling processes, while there was no effect of amendment with biogas digestate. The genetic potential for the various pathways differed between bulk soil and roots, as well as between the roots of the two plant species, suggesting that organisms involved in different N-cycling processes were favored in different compartments in the soil-root environment. Ammonia oxidizers involved in nitrification, a pathway resulting in N-leaching, dominated in soil whereas those related to N2 fixation and gaseous N losses (denitrification) were more abundant on roots. We also observed niche differentiation between the two major groups of organisms with the capacity to reduce N2O in the root- and soil compartment. Legume-grass intercropping resulted in a decreasing trend for several root-associated functional communities. More specifically, the legume exerted an effect on the N-cycling communities on the roots of both the legume and grass species, which suggests altered plant-microbial or microbial-microbial interactions during intercropping.

Rhizosphere-associated bacterial network structure and spatial distribution differ significantly from bulk soil in wheat crop fields

The spatial distribution of bacteria in bulk soil has been well studied, but little is known about the bacterial biogeography in the rhizosphere of crops. Here, we investigated bacterial distribution in bulk soil, loosely- and tightly-bound soils, from wheat fields distributed across 800,000 km2 of the North China Plain. Bacterial community composition differed dramatically among bulk and rhizospheric soils, and bacterial diversity decreased with the root proximity. Soil pH correlated with bacterial community composition and diversity in three compartments. Bacterial community in tightly bound soil formed a hub-based network topology with higher transitivity and greater number of central nodes compared with loosely bound and bulk soils, potentially as a result of more direct ecological interactions between the members of the tightly bound soil compartment. Bulk and rhizospheric soils maintained similar compositional distance decay patterns (with equal decay rates), but distinct phylogenetic distance decay patterns (with steeper slope of tightly bound soil). Geographical distance described a relatively greater proportion of bacterial spatial distribution in tightly bound soil, compared with loosely bound soil and bulk soil. Deterministic processes dominated the assemblage of bacterial communities in all soil compartments, while phylogenetic clustering was weaker in tightly bound soil. Taken together, our results suggest distinct bacterial network structure and distribution patterns among bulk soil, loosely bound soil and tightly bound rhizospheric soil, which could possibly result in potential functional differentiation.

Review of the ecotoxicological effects of emerging contaminants to soil biota

ABSTRACTIn recent years, emerging contaminants (e.g. pesticides and their metabolites, pharmaceuticals, personal and house care products, life-style compounds, food additives, industrial products and wastes, as well as nanomaterials) have become a problem to the environment. In fact, the cumulative use of a panoply of chemical substances in agriculture, industrial activities, in our homes and in health care services has led to their recent appearance in detectable levels in soils, surface, and groundwater resources, with unpredictable consequences for these ecosystems. Few data exist regarding the toxicity and potential for bioaccumulation in biota. When available, data were obtained only for some representatives of the main groups of chemical substances, and for a limited number of species, following non-standard protocols. This makes difficult the calculation of predicted no effect concentrations (PNEC) and the existence of sufficient data to set limits for their release into the environment. This is particularly concerning for the soil compartment, since only recently the scientific community, regulators, and the public have realised the importance of protecting this natural resource and its services to guarantee the sustainability of terrestrial ecosystems and human well-being. In this context, this review paper aims to identify the major groups of soil emerging contaminants, their sources, pathways and receptors, and in parallel to analyse existing ecotoxicological data for soil biota.

N fertilization in a Mediterranean ecosystem alters N and P turnover in soil, roots and the ectomycorrhizal community

Increased anthropogenic nitrogen (N) deposition is a major contributor to alteration of soil nutrient cycles particularly in nutrient poor ecosystems, such as the Mediterranean basin, where co-limitation of N and phosphorus (P) occurs and N addition might thus lead to an exacerbation of P limitation. Here, we measured the effect of medium term (6 years) N fertilization in different forms and doses (40 kg N ha−1 yr−1 as 1:1 NH4Cl and (NH4)2SO4; 40 and 80 kg N ha−1 yr−1 as NH4NO3) on nutrient stoichiometry, potential turnover rates and abundance of roots, ectomycorrhizal (ECM) root tips and adjacent soil in Cistus ladanifer L. In order to assess the impact of N addition at its most extreme point, we sampled roots and topsoil (10 cm) with and without plant influence in the summer months. We analysed N and P concentrations in soil and roots and determined the abundance of the most dominant mycorrhizal root tip morphotypes. We also assessed nutrient turnover in soil, roots and mycorrhizal root tips by measuring their N and C related enzyme activities (EAs) as well as acid phosphatase (AP) activity. Resultsshowed decreased soil Pinorg and increased soil N:Pinorg in the treatment plots. Also, a decline in Cenococcum geophilum in N addition plots was found and a general reduction in ECM colonization in the treatment receiving ammonium without nitrate. We also detected a decrease of C. geophilum absolute EA and AP, as well as N related EA in the whole soil compartment. Furthermore, we observed lower root AP activity and found a loss of correlation between N related EA and AP activity in all treatments, while a high correlation between N related EA and C related EA persisted in all plots. EA was also generally negatively related with root P/soil P, which we used as a measure for plant P status.The negative effect of ammonium on the ECM community of C. ladanifer and a putative loss of short distance exploration morphotypes, such as C. geophilum, together with decreased AP activity in the plant roots, might be connected with low Pinorg availability in soil with plant influence, thus being in line with the hypothesis of P depletion due to N addition. Furthermore, the decrease of N related EA in the soil compartment, as well as the decoupling of N and P cycles, might be signs of altered soil microbial communities. This decoupling, together with the strong dependence of EA on plant P status, could point to a shift from N and P scavenging ECM communities to more copiotrophic saprophytic fungi that rely on C and N acquisition from soil organic matter rather than plant C inputs. We posit that a decline in root colonization by ECM fungi and changes in N:P cycling could be detrimental to ecosystem development, as C. ladanifer is a common ECM species in early successional stages, providing a host for ECM fungi that also colonize late-successional plants.

Fate of pharmaceuticals in soil after application of STPs products: Influence of physicochemical properties and modelling approach

Depurated effluents can be employed as reclaimed water for irrigation in regions with low precipitations, while nutrients-rich biosolids are usually applied as fertilizer for agricultural purposes. However, both practices may result in the accumulation of micropollutants in the soil compartment, the contamination of groundwater, and/or their biotransfer to different living organisms until they finally may reach human receptors. The fate and transport of seven pharmaceutical and personal care products (PPCPs) with different physicochemical properties was analysed for these scenarios employing two different models: the buckets model and the HYDRUS-1D software package. The results indicated that these compounds have a low potential to contaminate groundwater in the type of soil under study (silty loam), although triclosan tended to accumulate in the top layers of the soil. Similar conclusions were reached through both models, although they predicted different solute plant uptake and accumulation patterns of the evaluated compounds. Therefore, the buckets model can be considered as an adequate option to perform a screening level assessment of these compounds. However, HYDRUS-1D could provide more information on the fate of pharmaceuticals in soil, thus contributing to reduce the uncertainty already associated to PPCPs. Additionally, the significant difference of the Hazard Index (HI) obtained from a human health risk assessment performed using the estimated soil concentrations from both models also contributed to recommend the use of a model that considers all the relevant mechanisms of mass transfer to reduce overestimation of risk.

Altitude affects the quality of the water-extractable organic matter (WEOM) from rhizosphere and bulk soil in European beech forests

Water-extractable organic matter (WEOM) is the most dynamic and bioavailable fraction of the soil organic matter pool. Although the litter floor is considered the main source of WEOM, roots also release a great amount of labile organic compounds through rhizodeposition processes. This makes the rhizosphere, the small soil volume in proximity to the roots, a soil compartment relatively enriched in WEOM. Since both the rhizosphere and the labile organic C pool are highly sensitive to the environmental conditions we evaluated the characteristics of WEOM from rhizosphere and bulk soil collected from the A horizons of European beech (Fagus sylvatica L.) forest soils of Apennines mountains (central Italy) at two altitudes (800 and 1000m), using elevation as a proxy for temperature change. Specifically, we tested if i) the rhizosphere contains higher amounts of WEOM with a greater diversity of compounds with respect to the bulk soil, and ii) this effect is more pronounced at higher altitude. At both 800m and 1000m above sea level, the main distinction between WEOM from rhizosphere and bulk soil was the larger amounts of sugars in the soil close to the roots. Further, our results indicated an influence of altitude on rhizospheric processes as suggested by the larger concentrations of organic C and soluble phenols, and richness of tannins in the rhizosphere WEOM than in the bulk soil at 1000m. We attributed this influence to environmental constraints which enhanced the release of labile organics and secondary metabolites by rhizodeposition and humification processes in the rhizosphere. As a whole, our data draw a picture where the roots are able to affect the characteristics of WEOM and environmental constraints enhance the differentiation between rhizosphere and bulk soil. This view confirms the influence of the rhizosphere on the soil C cycle, and the importance of the rhizospheric processes when environmental conditions become harsher.

Reactive mineral removal relative to soil organic matter heterogeneity and implications for organic contaminant sorption

Soil organic matter (SOM) is generally treated as a static compartment of soil in pollutant fate studies. However, SOM might be altered or fractionated in soil systems, and the details of SOM property/composition changes when coupled with contaminant behavior are unknown. In this study, a mild acid treatment was adopted to remove reactive minerals and partially remove SOM components. After acid treatment, biomarker signatures showed that lignin-derived phenols were released and black carbon (as suggested by benzene-polycarboxylic acids) and lipids were enriched. The biomarker information was consistent with common bulk chemical characterization. The sorption coefficient Kd for PHE was two times higher after acid treatment, whereas Kd for OFL was three times lower. The organic carbon normalized sorption coefficient KOC values for PHE were higher for soils after acid treatment, indicating stronger interactions between PHE and SOM. The linear regression line between Kd and fOC for OFL showed lower intercepts and slopes after reactive mineral removal, suggesting a decreased contribution of minerals and reduced dependence on SOM. These results were attributed to the release of polar compositions in SOM accompanied by reactive mineral removal. Our results suggest that the mobility of ionic organic contaminants increases, whereas that of hydrophobic organic contaminants decreases after acid treatment with respect to reactive mineral depletion. This study emphasized that new insights into the coupling of SOM dynamics should be incorporated into organic contaminant behavior studies. SOM molecular biomarkers offer a useful technique for correlating SOM composition and sorption property changes.

Ecotoxicity of boric acid in standard laboratory tests with plants and soil organisms.

To verify the continuous sensitivity of ecotoxicological tests (mainly the test organisms), reference substances with known toxicity are regularly tested. Ideally, this substance(s) would lack specificity in its mode action, be bioavailable and readily attainable with cost-effective means of chemical characterization. Boric acid has satisfied these criteria, but has most recently been characterized as a substance of very high concern, due to reproductive effects in humans, thus limiting its recommendation as an ideal reference toxicant. However, there is probably no other chemical for which ecotoxicity in soil has been so intensively studied; an extensive literature review yielded lethal (including avoidance) and sublethal data for 38 taxa. The ecotoxicity data were evaluated using species sensitivity distributions, collectively across all taxa, and separately according to species type, endpoints, soil type and duration. The lack of specificity in the mode of action yielded broad toxicity among soil taxa and soil types, and provided a collective approach to assessing species sensitivity, while taking into consideration differences in test methodologies and exposure durations. Toxicity was species-specific with Folsomia candida and enchytraied species demonstrating the most sensitivity; among plants, the following trend occurred: dicotyledonous (more sensitive) ≫ monocotyledonous ≫ gymnosperm species. Sensitivity was also time and endpoint specific, with endpoints such as lethality and avoidance being less sensitive than reproduction effects. Furthermore, given the breadth of data and toxicity demonstrated by boric acid, lessons learned from its evaluation are discussed to recommend the properties required by an ideal reference substance for the soil compartment.

Adsorption behavior of Sudan I-IV on a coastal soil and their forecasted biogeochemical cycles.

Sudan I-IV as synthetic azo dyes have been concerned worldwide and ever caused a panic on food safety because of illegal addition into foodstuffs. In the past decades, various methods are being developed to identify and determine Sudan dyes in foodstuffs. However, relevant studies about their biogeochemical behaviors and potential environmental effects are rarely reported, although it is of great importance and necessity accounting for their potential environmental contamination from various sources. In this work, the experimental studies on adsorption behavior of Sudan I-IV acting on soil (10, 25, 50, 75, and 100mg/L) were carried out, and their transport in soil compartments and between soil-water, and air-soil interfaces were discussed. Results showed that the amount of Sudan I-IV adsorbed on soil increased accordingly with the increasing concentration of Sudan dyes in aqueous solution, and Sudan II and IV were more likely adsorbed on the tested soils than Sudan I and III based on their maximum adsorption amount. However, for Sudan I, III, and IV, in some high concentrations (under the treatment of 75mg/L for Sudan III, 100mg/L for Sudan I and IV), the adsorption was significantly increased, and then came back to the "normal" level (under the treatment of 100mg/L for Sudan III). It is expected that relevant researches on their biogeochemical behaviors in soil compartments, and between soil-water and air-soil interfaces would be concerned and addressed.

Novel Experimental-Modeling Approach for Characterizing Perfluorinated Surfactants in Soils.

Soil contamination is still poorly understood and modeled in part because of the difficulties of looking inside the "black box" constituted by soils. Here, we investigated the application of a recently developed 1H NMR technique to 19F NMR relaxometry experiments and utilized the results as inputs for an existing model. This novel approach yields 19F T2 NMR relaxation values of any fluorinated contaminant, which are among the most dangerous contaminants, allowing us to noninvasively and directly monitor their fate in soils. Using this protocol, we quantified the amount of a fluorinated xenobiotic (heptafluorobutyric acid, HFBA) in three different environments in soil aggregate packings and monitored contaminant exchange dynamics between these compartments. A model computing HFBA partition dynamics between different soil compartments showed that these three environments corresponded to HFBA in solution (i) between and (ii) inside the soil aggregates and (iii) to HFBA adsorbed to (or strongly interacting with) the soil constituents. In addition to providing a straightforward way of determining the sorption kinetics of any fluorinated contaminant, this work also highlights the strengths of a combined experimental-modeling approach to unambiguously understand experimental data and more generally to study contaminant fate in soils.

Arbuscular mycorrhiza enhance the rate of litter decomposition while inhibiting soil microbial community development

Although there is a growing amount of evidence that arbuscular mycorrhizal fungi (AMF) influence the decomposition process, the extent of their involvement remains unclear. Therefore, given this knowledge gap, our aim was to test how AMF influence the soil decomposer communities. Dual compartment microcosms, where AMF (Glomus mosseae) were either allowed access (AM+) to or excluded (AM−) from forest soil compartments containing litterbags (leaf litter from Calophyllum polyanthum) were used. The experiment ran for six months, with destructive harvests at 0, 90, 120, 150, and 180 days. For each harvest we measured AMF colonization, soil nutrients, litter mass loss, and microbial biomass (using phospholipid fatty acid analysis (PLFA)). AMF significantly enhanced litter decomposition in the first 5 months, whilst delaying the development of total microbial biomass (represented by total PLFA) from T150 to T180. A significant decline in soil available N was observed through the course of the experiment for both treatments. This study shows that AMF have the capacity to interact with soil microbial communities and inhibit the development of fungal and bacterial groups in the soil at the later stage of the litter decomposition (180 days), whilst enhancing the rates of decomposition.

Influence of cover cropping on water uptake dynamics in an irrigated Mediterranean vineyard

AbstractAs a result of its physiological characteristics, vine's yield and quality responses are greatly affected by water availability in the soil. Knowledge about water uptake of grapevines in pedoclimatic conditions of high water availability is essential when considering the increasing use of irrigation in a previously rainfed crop. We studied the water uptake of irrigated grapevines under different soil covers. The study was carried out over 2 years in southern Portugal, a Mediterranean climate region, in ‘Aragonez’ grapevines planted in Vertisols. A cover crop was sown in half the area while maintaining the permanent resident vegetation in the remaining area. Five levels of annual irrigation supply were applied: 200, 150, 100 and 50 mm, and no irrigation. Soil moisture was monitored every 2 or 3 weeks, and weekly after the start of irrigation. Results show that water uptake in the interrows did not cease after the beginning of irrigation. Water uptake by the vines occurred to depths of approximately 3 m, hence 7.5 times greater than the cover crop root system depth. Results suggest that over time, the presence of the cover crop forces the vine root system, mainly its thinner roots, to seek water in increasingly deeper soil compartments. Copyright © 2017 John Wiley & Sons, Ltd.

Legacy of contaminant N sources to the NO3\u2212 signature in rivers: a combined isotopic (\u03b415N-NO3\u2212, \u03b418O-NO3\u2212, \u03b411B) and microbiological investigation

Nitrate content of surface waters results from complex mixing of multiple sources, whose signatures can be modified through N reactions occurring within the different compartments of the whole catchment. Despite this complexity, the determination of nitrate origin is the first and crucial step for water resource preservation. Here, for the first time, we combined at the catchment scale stable isotopic tracers (δ15N and δ18O of nitrate and δ11B) and fecal indicators to trace nitrate sources and pathways to the stream. We tested this approach on two rivers in an agricultural region of SW France. Boron isotopic ratios evidenced inflow from anthropogenic waters, microbiological markers revealed organic contaminations from both human and animal wastes. Nitrate δ15N and δ18O traced inputs from the surface leaching during high flow events and from the subsurface drainage in base flow regime. They also showed that denitrification occurred within the soils before reaching the rivers. Furthermore, this study highlighted the determinant role of the soil compartment in nitrate formation and recycling with important spatial heterogeneity and temporal variability.

Phosphorus retention and fractionation in an eutrophic wetland: A one-year mesocosms experiment under fluctuating flooding conditions

This study aimed to evaluate the response of salt marshes to pulses of PO43--enriched water, with and without the presence of Phragmites australis. A one-year mesocosms experiment was performed in simulated soil profiles (fine-textured surface layers and sandy subsurface layers) from a coastal salt marsh of the Mar Menor lagoon under alternating flooding-drying conditions with eutrophic water, under low (1.95 mg L−1 P-PO43-) and high (19.5 mg L−1 P-PO43-) P load, and with the presence/absence of Phragmites. The PO43- concentrations in soil porewater and drainage water were regularly measured, and P accumulated in soils (including a fractionation procedure) and plants (roots, rhizomes, stems and leaves) were analyzed. The experimental mesocosms were highly effective in the removal of P from the eutrophic flooding water (>90% reduction of the P added to the system both in the soil pore water and drainage water), regardless of the nutrient load, the season of the year and the presence/absence of Phragmites. The soil was the main sink of the P added to the system, while Phragmites had a minor role in P removal. The biomass of Phragmites accumulated ∼27% of the P added with the flooding water in the treatment with water of low P load while ∼12% of P in that of high P load; the rhizomes were the organs that contributed the most (∼67–72% of the total P retained by the plants). Ca/Mg compounds were the main contributors to the retention of P in the soil compartment, especially in the fine-textured surface soil layers (∼34–53% of the total P in the soil was present in this fraction). Phragmites favored the retention of P onto metal oxides (∼12% increase of the P retained in the metal oxides fraction in the treatment with water of high P load). Hence, the use of constructed wetlands to ameliorate the negative impacts of P-enriched waters in the Mar Menor lagoon and similar areas is recommended. We propose the incorporation of fine-textured carbonated materials, with high content of Ca/Mg compounds, and the use of Phragmites to favor the retention of P by these systems.

Role of Rhizosphere of Ricinus communis L. (Castor) on Nickel Phytoextraction

Investigation on the dynamics of the heavy metals in the rhizosphere has been hampered by the lack of reliable methods for sampling rhizosphere soil. The distribution of Ni in the rhizosphere was examined by using rhizobox system, composed of two components namely, central and bulk soil compartments. The rhizobox allows a clear separation of rhizosphere and bulk soil. Ricinus communis wild type plants were grown in the central compartment of the rhizobox system containing Alfisols added with 0, 50, 100, 250 and 500 mg Ni kg-1 soil. The results showed that the roots decreased the soil pH across the rhizosphere by a factor of around 0.36 units, which cause tremendous increase in DTPA extractable Ni content in the rhizosphere region compared to the bulk soil region. Nickel accumulation by Ricinus communis was increased with increasing levels of Ni in the soil.