To mitigate excess of nitrate-N (NO3−-N) derived from agricultural activity, constructed wetlands (CWs) are created to simulate natural removal mechanisms. Irrigated agricultural drainage water is ...commonly characterized by an organic carbon/nitrogen (C/N) imbalance, thus, C limitation constrains heterotrophic denitrification, the main biotic process implicated in NO3−-N removal in wetlands. We studied a pilot plant with three series (169 m2) of hybrid CWs over the first two years of functioning to examine: i) the effect of adding different C-rich substrates (natural soil vs. biochar) to gravel on NO3−-N removal in a subsurface flow (Phase I), ii) the role of a second phase with a horizontal surface flow (Phase II) as a source of dissolved organic C (DOC), and its effect in a consecutive horizontal subsurface flow (Phase III) on NO3−-N removal, and iii) the contribution of each phase to global NO3−-N removal. Our results showed that the addition of a C-rich substrate to gravel had a positive effect on NO3−-N removal in Phase I, with mean efficiencies of 40% and 17% for soil and biochar addition, respectively, compared to only gravel (0.75%). In Phase II, the algae growth turned into a DOC concentration increase, but it did not enhance NO3−-N removal in Phase III. In series with C-rich substrate addition, the largest contribution to NO3−-N removal was found in Phase I. However, in series with only gravel, Phase II was the most effective on NO3−-N removal. Contribution of Phase III to NO3−-N removal was almost negligible.
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•Gravel was not a suitable substrate to treat C/N imbalanced agricultural water.•Both soil and biochar addition to gravel increased NO3−-N removal from water.•A HSSF CW with a gravel + soil mix was the most effective design for NO3−-N removal.•Inclusion of a HSF phase in a hybrid CW increased DOC availability by algae growth.
Leachates from intensive agriculture containing high nitrate have been identified as a major cause of the severe eutrophication crisis that impacts Mar Menor (SE Spain), the largest hypersaline ...coastal lagoon in the Mediterranean basin. A best management practice for removing NO3−-N is denitrifying bioreactors. This is the first study to assess the efficiency of citrus woodchips bioreactors in treating agricultural leachates that flow to the Mar Menor via surface discharges. Denitrification capacity, woodchip degradation (by weight loss), formation of potentially harmful compounds, and greenhouse gas (GHG) emissions were assessed. Three bioreactors (6 m × 0.98 m x 1.2 m) filled with citrus woodchips (3 m3 d−1 per bioreactor) through which the untreated ditch water over 1.5 years. Bioreactors were operated at 8 h, 16 h, and 24 h hydraulic residence time respectively, in each bioreactor. The main characteristics of the ditch water were: pH ≈ 7.5–8.0, electrical conductivity ≈ 5–8 dS m−1, dissolved organic carbon ≈6–10 mg L−1, and NO3−-N ≈ 22–45 mg L−1. Bioreactors were highly efficient in reducing NO3−-N. The average RNO3 in effluents was for the complete experimental period 8 g N m−3 d−1, 10.9 g N m−3 d−1, and 12.6 g N m−3 d−1 for 8, 16 and 24 h residence time, respectively. Nitrate reduction efficiency was modulated by seasonal changes in temperature, with an increasing efficiency in warmer periods (maximum ≈ 85–90% for all hydraulic residence time) and decreasing in colder ones (minimum ≈ 12%, 23% and 41% for hydraulic residence time 8, 16 and 24 h respectively). Woodchips degradation was greatest during the first six months (average ≈ 29% weight loss) in the material above the water level, attributable to aerobic mineralization of the organic carbon, while weight loss was ≈11% in woodchip media continuously below the water level. Dissolved organic carbon, sulfide, ammonium, and soluble phosphorus concentrations in the effluents were mostly low, although some peaks in concentrations occurred. Design consideration must be taken to avoid environmental impacts due to the occasional presence of harmful compounds in the effluents.
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•Citrus woodchip bioreactors denitrified saline leachates with NO3−-N = 22–45 mg L−1•Woodchip aging negatively affected nitrate removal particularly below ≈20 °C.•Sulfide, ammonium, and soluble phosphorus may appear in bioreactor effluents.•Higher CO2 emissions and woodchips degradation occurred above the water level.•Woodchips continuously flooded had a higher production of N2O emissions.
Woodchip bioreactors are widely known as a best management practice to reduce excess nitrate loads that are discharged with agricultural leachates. The aim of this study was to evaluate the ...performance of citrus woodchip bioreactors for denitrification of brine (electrical conductivity ≈ 17 mS cm−1) from groundwater desalination plants with high nitrate content (NO3−-N ≈ 48 mg L−1) in the Campo de Cartagena agricultural watershed, one of the main providers of horticultural products in Europe. The performance was evaluated relative to seasonal changes in temperature, dissolved organic carbon (DOC) provided by woodchips, hydraulic residence time (HRT) and woodchip aging. Bioreactors (capacity 1 m3) operated for 2.5 years (121 weeks) in batch mode (24 h HRT) with three batches per week. Denitrification efficiency was modulated by DOC concentration, temperature, hydraulic residence time and the drying-rewetting cycles. High salinity of brine did not prevent nitrate removal from occurring. The high DOC availability (>25 mg C L−1) during the first ≈48 weeks resulted in high nitrate removal rate (>75%) and nitrate removal efficiency (until ≈ 25 g N m−3 d−1) regardless of temperature. Moreover, the high DOC contents in the effluents during this period may present environmental drawbacks. Denitrification was still high after 2.5 years (reaching ≈9.3 g N m−3 d−1 in week 121), but dependence on warm temperature became more apparent with woodchips aging from week ≈49 onwards. Nitrate removal efficiency was highest on the first weekly batch, immediately after woodchips had been unsaturated for four days. It was attributable to a flush of DOC produced by aerobic microbial metabolism during drying that stimulated denitrification following re-saturation. Hence, alternance of drying-rewetting cycles is an operation practice that increase bioreactors nitrate removal performance.
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•Citrus woodchips effectively denitrify high-nitrate concentration brine.•Citrus woodchip bioreactors provide enough organic carbon for the denitrification.•Drying-rewetting cycles improve denitrification due to aerobic conditions.•DOC, temperature and HRT had an effect on denitrification efficiency.•Evolution woodchips from a two-year-old denitrifying bioreactor were evaluated.
Increasing knowledge of nitrate removal using denitrifying bioreactors has illustrated the usefulness of this management practice for treating discharge water from agricultural land uses. The ...objective of this study was to assess the viability of almond shell, chopped carob, olive bone, and citrus woodchip as carbon media for denitrification of brine with high nitrate load (EC ≈ 20 dS m
−1
, NO
3
−
-N concentration ≈ 65–80 mg NO
3
−
-N L
−1
) in bioreactors. To the authors’ knowledge, this is the first test of denitrifying brine using organic wastes as the carbon substrate, and the first use of these carbon media for that purpose. Nitrate removal efficiency and efficiency:cost ratio were considered. The results indicated that the best removal efficiency and cheapest cost were provided by citrus woodchip (3.02 ± 0.15 mg NO
3
−
-N m
−3
d
−1
) at a cost of ≈ 6€ m
−3
, followed by almond shell (1.54 ± 0.20 mg NO
3
−
-N m
−3
d
−1
) at a cost of ≈ 19€ m
−3
. Chopped carob and olive bone showed negligible nitrate removal in the brine; chopped carob generated acidic leachate with extremely high dissolved organic carbon, and olive bone resulted in a highly saline leachate. Of the four media tested, the results of this study indicated that citrus woodchip was the most suitable media for denitrification of the brine.
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•Soil metal(loid)s did not hinder spontaneous vegetation colonization of mine tailings.•Spontaneously vegetated metal(loid) mine tailings showed improved soil function.•Similar soil ...biological activity in vegetated mine tailing sites than in forests.•Vegetated metal(loid) mine tailing can be even less toxic than surrounding forests.•Passive restoration is valuable for phytomanagement of mine tailings.
This work studies a set of soil indicators (physical, physico-chemical and biological), evaluated under field and laboratory conditions, in different stages of spontaneous vegetation colonization in abandoned metal(loid) mine tailings from Mediterranean semiarid areas. The results provide evidence about the interest of spontaneous colonization by native vegetation for the phytomanagement of abandoned metal(loid) mine tailings in terms of providing ecosystem functions. Bare soils (B), small groups of pine trees (P), scattered pine trees with shrubs and herbs under the canopy (P + S), and dense patches of pine trees with shrubs and herbs under the canopy (DP + S) were studied inside mine tailings abandoned ≈40 years ago. Besides, pine forests next (FN) and away (FA) from the tailings were also studied. Pioneer and nurse plants were mainly found inside the tailings, although ecological indexes in P + S and DP + S were similar to FN and FA. Pedogenesis evidences such as structure development and increase in cation exchange capacity, organic C and N were found in tailing soils from B to DP + S. However, soil metal(loid)s did not follow the same variation pattern. For example (in mg kg−1): P showed the maximum total Cu (≈277) and Zn (≈17,860), while P + S of As (≈1250) and Pb (≈14,570). B had the maximum water extractable Pb (≈4) and Zn (≈207), while FA of As (≈0.192) and Cu (≈0.149). Soil microbial biomass C, enzyme activity, CO2 emission, organic matter decomposition and feeding activity of soil dwelling organisms indicated similar, or even higher, biological activity in P + S and DP + S than in FN and FA. In fact, FA showed the highest soil ecotoxicity risk (reduced enchytraeid reproduction). Therefore, mine tailing soils can be effectively modified following spontaneous vegetation colonization, achieving conditions with capacity to provide certain ecosystem functions. Hence, phytomanagement of these tailings should be preceded by a detailed knowledge of the existing spontaneously colonized sites, which should be preserved to take advantage of their potentiality.
Coastal marine lagoons are environments highly vulnerable to anthropogenic pressures such as agriculture nutrient loading or runoff from metalliferous mining. Sediment microorganisms, which are key ...components in the biogeochemical cycles, can help attenuate these impacts by accumulating nutrients and pollutants. The Mar Menor, located in the southeast of Spain, is an example of a coastal lagoon strongly altered by anthropic pressures, but the microbial community inhabiting its sediments remains unknown. Here, we describe the sediment prokaryotic communities along a wide range of environmental conditions in the lagoon, revealing that microbial communities were highly heterogeneous among stations, although a core microbiome was detected. The microbiota was dominated by
Delta
- and
Gammaproteobacteria
and members of the
Bacteroidia
class. Additionally, several uncultured groups such as
Asgardarchaeota
were detected in relatively high proportions. Sediment texture, the presence of
Caulerpa
or
Cymodocea
, depth, and geographic location were among the most important factors structuring microbial assemblages. Furthermore, microbial communities in the stations with the highest concentrations of potentially toxic elements (Fe, Pb, As, Zn, and Cd) were less stable than those in the non-contaminated stations. This finding suggests that bacteria colonizing heavily contaminated stations are specialists sensitive to change.
Spontaneous growth of native vegetation in abandoned metal(loid) mine tailings can be valuable for phytomanagement restoration projects. This study aimed to assess the degree to which spontaneous ...plant colonization of abandoned metal(loid) mine tailings from Mediterranean semiarid areas led to functional soil improvement, and to identify, if possible, a critical level indicating that this functionality was moving towards that of the vegetated soils from the surroundings. Vegetation ecological indexes, plant life forms and species functional roles, together with physicochemical and functional soils parameters, were studied in metal(loid) mine tailings abandoned ~40 years ago and surrounding forests in SE Spain. Vegetation patches showed only small differences in physicochemical parameters related to soil abiotic stress conditions (pH, salinity and metals), regardless of the vegetation. However, vegetation patches with greater species diversity and richness and presence of plants with contrasted life forms and functional traits that facilitate the growth of less stress-tolerant species showed an increase of the soil microbial functionality (higher microbial biomass C, β-glucosidase activity, bacterial metabolic activity and functional diversity). Moreover, these vegetation patches showed a functional soil status comparable to that of the forests outside the mine tailings. In this sense, the present study showed the value of preserving these vegetation patches since they may act as nucleation spots favoring positive plant-soil feedbacks that may help to accelerate the functional recovery of these degraded areas. Furthermore, strategies to promote the creation of new vegetation patches including a variety of species with contrasted life forms and functional traits should be considered in phytomanagement restoration projects for abandoned metal(loid) mine tailings.
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•Soil microbial functionality evaluated by CLPP technique.•Similar soil abiotic stress parameters in tailing vegetation patches.•Higher plant diversity-richness, higher scores of functional-related soil parameters.•More diverse and rich vegetation patches had closer soil functional status to forests.•Spontaneously colonized niches are valuable spots for phytomanagement.
This study aimed to assess the effectiveness of biochar from sewage sludge -BSS- and from pruning trees -BPT- (addition dose of 6% d.w.) to immobilise metals in acidic (pH ∼ 4.7) and basic (pH ∼ 7.4) ...mine soils under variable flooding conditions, and to determine biochar influence on plant (Sarcocornia fruticosa -Sf-) growth and metal uptake. BSS had lower pH (∼8.2 vs. ∼ 9.8), CaCO3 (∼71 vs. ∼ 85 g kg−1), total organic carbon (∼354 vs. ∼ 656 g kg−1) and higher water soluble organic carbon (WSOC ∼ 0.15 vs ∼ 0.06 mg kg−1) than BPT. PVC columns (15 × 30 cm) were prepared with the following treatments (n = 4): 1) no Biochar-no Sf; 2) no Biochar-Sf; 3) BSS-no Sf; 4) BSS-Sf; 5) BPT-no Sf; 6) BPT-Sf. Changes in water table level (WL) were simulated for 303 days with tap water (upper 0–15 cm alternating flooding-drying conditions, lower 15–30 cm always underwater). The pH, redox potential (Eh), temperature and porewater WSOC, Zn, Cd and Pb concentrations were regularly measured, and plants were removed at the end and length, fresh weight and metal concentrations in tissues measured. In the basic soil, there were no consistent evidences that BSS and BPT were effective decreasing porewater metal concentrations and reducing metal uptake in plants. Sf contributed to mobilise Zn, and in less extent Cd, in the upper soil layer, regardless of the type of biochar, and this effect increased with aging. In the acidic soil, BSS and BPT were effective increasing the pH and decreasing porewater metals. BSS increased its efficiency with aging, which can be mainly attributable to the more reduced conditions that induced (lower Eh values) due to its higher WSOC content. Biochar was effective hindering metal mobilisation by Sf and reducing plant's metal uptake (e.g. reduction in roots: ∼7 fold for Zn, ∼19-fold for Cd and ∼ 2-fold for Pb). BSS was more effective promoting Sf growth (fresh weight) than BPT. Therefore, in relation with the use of biochar from sewage sludge and from pruning trees as soil amendments under flooding-non flooding conditions, we can conclude that it can be a useful option in acidic mine soils for decreasing water soluble metals and improving plant growth. However, in basic mine soils, we have no evidences to support the advantages of using these two types of biochar as amendments. Hence, the use of biochar in metal-polluted wetlands has environmental implications that must be planned for each specific case in order to optimize the positive aspects (wetland as sinks of pollutants) and reduce the drawbacks (wetland as source of pollutants).
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•Hydric mine soils were amended with biochar from sewage sludge and pruning trees.•Biochar did not modify the pH and porewater metals in hydric basic mine soils.•Biochar increased the pH and decreased porewater metals in hydric acidic mine soils.•Flooding regime, temperature and plants modulated biochar effectiveness.•Biochar reduced plant's metal uptake in acidic but not in basic hydric mine soils.
Abandoned metal(loid) mine tailings show inhospitable conditions for the establishment of above- and below-ground communities (e.g., high metal(loid) levels, organic matter and nutrient deficiency). ...This worsens in semiarid areas due to the harsh climate conditions. Fertility islands (vegetation patches formed by plants that spontaneously colonize the tailings) can serve as potential nucleation spots fostering beneficial plant-microbial interactions. However, less attention has been paid to the soil invertebrates living beneath these patches and their functional role. Here, we studied whether the spontaneous plant colonization of abandoned metal(loid) mine tailings led to a greater presence of soil microarthropod communities and whether this could contribute to improving ecosystem functionality. Microarthropods were extracted, taxonomically identified and subsequently assigned to different functional groups (saphrophages, omnivores, predators) in bare soils and differently vegetated patches within metalliferous mine tailings and surrounding forests in southeast Spain. Microarthropod communities were significantly different in bare soils compared with vegetated patches in mine tailings and surrounding forests. Plant colonization led to an increase in microarthropod abundance in tailing soils, especially of mites and springtails. Moreover, saprophages and omnivores, but not predators, were favored in vegetated patches. The initial microarthropod colonization was mainly linked to higher organic matter accumulation and greater microbial activity in the vegetated patches within mine tailings. Moreover, soil formation processes already initiated in the tailings were beneficial for soil biota establishment. Thus, below-ground communities created an anchorage point for plant communities by primarily starting heterotrophic activities in the vegetated patches, thereby contributing to recover ecosystem functionality.
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•Microarthropods spontaneously colonized vegetated metal(loid) mine tailings.•Microarthropod communities in tailing plant patches ruled by mites and springtails.•Saprophages and omnivores but not predators increased in tailing plant patches.•Linked development of soil conditions and microarthropod communities in plant patches.•Forests near tailings showed poorer microarthropod communities than distant forests.
