Biochars are biological residues combusted under low oxygen conditions, resulting in a porous, low density carbon rich material. Their large surface areas and cation exchange capacities, determined ...to a large extent by source materials and pyrolysis temperatures, enables enhanced sorption of both organic and inorganic contaminants to their surfaces, reducing pollutant mobility when amending contaminated soils. Liming effects or release of carbon into soil solution may increase arsenic mobility, whilst low capital but enhanced retention of plant nutrients can restrict revegetation on degraded soils amended only with biochars; the combination of composts, manures and other amendments with biochars could be their most effective deployment to soils requiring stabilisation by revegetation. Specific mechanisms of contaminant-biochar retention and release over time and the environmental impact of biochar amendments on soil organisms remain somewhat unclear but must be investigated to ensure that the management of environmental pollution coincides with ecological sustainability.
► Biochars can reduce mobilities of some organic and inorganic pollutants in soil. ► Source material and production conditions influence pollutant retention. ► Highly alkaline pH and water soluble carbon can undesirably mobilise some elements. ► Large surface area may be toxic to soil fauna but create microbial niches. ► Efficacy of biochar may depend on other organic materials applied in combination.
Biochars can reduce the mobility and impact of some soil pollutants but, if applied alone, may fail to support soil restoration, revegetation and hence ecologically circumspect remediation.
Purpose
To build a more holistic understanding of soil pH change we assessed the synchronised effects of a contrived soil pH change on soil chemistry, vegetation growth and nutrition, and soil faunal ...abundance and diversity.
Methods
We established a fifteen year old field experiment with a contrived pH gradient (pH 4.3 to 6.3) and measured the effect on soil chemistry, plant biomass and elemental composition and the impact of these changes on soil fauna (earthworms, nematodes, rotifers and tardigrades) and biological indices (based on ecological group structures of earthworms and nematodes). A single 20 × 20 × 20 cm soil block was excavated from each sample site to directly attribute biotic parameters in the block to the abiotic (soil) conditions.
Results
Acidification affected the extractable concentrations of Al, Ca, Mn and P and the C:N ratio of the soil and caused a reduction in plant Ca (r
s
for pH vs Ca = 0.804
p
< 0.01), an increase in plant Mn (r
s
= −0.450
p
= 0.019), along with significant decrease in root:shoot ratio (r
s
= 0.638,
p
< 0.01). There was a significant positive correlation between pH and earthworm index (r
s
= 0.606,
p
< 0.01), and a negative correlation between pH and nematode index (r
s
= −0.515,
p
< 0.01).
Conclusion
Soil pH influenced the mobility of Ca, Al, Mn and P, which in turn has impacted on plant tissue chemistry and plant biomass ratios. Linked changes in soil chemistry and vegetation had a corresponding effect on the abundance and diversity of nematodes and earthworms in the soil blocks.
Polycyclic aromatic hydrocarbons (PAHs) and potentially toxic elements (PTEs) were monitored over 56 days in calcareous contaminated-soil amended with either or both biochar and Eisenia fetida. ...Biochar reduced total (449 to 306mgkg−1) and bioavailable (cyclodextrin extractable) (276 to 182mgkg−1) PAHs, PAH concentrations in E. fetida (up to 45%) but also earthworm weight. Earthworms increased PAH bioavailability by >40%. Combined treatment results were similar to the biochar-only treatment. Earthworms increased water soluble Co (3.4 to 29.2mgkg−1), Cu (60.0 to 120.1mgkg−1) and Ni (31.7 to 83.0mgkg−1) but not As, Cd, Pb or Zn; biochar reduced water soluble Cu (60 to 37mgkg−1). Combined treatment results were similar to the biochar-only treatment but gave a greater reduction in As and Cd mobility. Biochar has contaminated land remediation potential, but its long-term impact on contaminants and soil biota needs to be assessed.
► Biochar reduced total and bioavailable PAH concentrations. ► Biochar was less effective at immobilising PTEs, due to its low cation exchange capacity. ► E. fetida increased PAH bioavailability and PTE mobility. ► When used in combination biochar reduced the concentration of PTEs mobilised by E. fetida. ► Biochar had a negative effect on E. fetida in terms of weight loss.
Biochar decreased PAH biovailability but was less effective at reducing PTE mobility, whilst E. fetida increased both PAH and PTE bioavailability.
The bioaccumulation of Potentially Toxic Elements (PTEs) by benthic invertebrates in estuarine sediments is poorly understood. We sampled and analysed PTEs in sediments and benthic invertebrates from ...five sites in the Skeena Estuary (British Columbia, Canada), including sites adjacent to an abandoned cannery and a decommissioned papermill. Our aim was to elucidate baseline levels of PTE concentrations at sites that may be recovering from disturbance associated with prior industrial development and identify organisms that could be used to biomonitor the impact of future industrial developments. There was no indication that sediments of the salmon cannery were polluted, but acidic sediments adjacent to the papermill contained elevated concentrations of Cd, Cr, Hg and Pb. Benthic invertebrate community assemblages confirm that sediments have mostly recovered from prior industrial development associated with discharge of papermill sludge. Overall, we did not observe any relationship between PTE concentrations in the sediment and PTE concentrations in invertebrate tissues. However, we did observe a negative relationship between sediment pH and the Biota-Sediment Accumulation Factor (BSAF) of most PTEs for Oregon pill bugs (Gnorimosphaeroma oregonensis). G. oregonensis, observed at all sites, feeds on the fibers associated with the papermill discharge. Thus, G. oregonensis is a useful biomonitors for quantifying the impact of the decommissioned papermill, and are candidate biomonitors for assessing the impact of similar industrial development projects on intertidal ecosystems.
Biochar produced from the pyrolysis of plant-based feedstock has been advocated as an alternative soil amendment for landfill cover. Previous literature indicated that the pyrolysis temperature ...influences the intra-pore distribution and surface functional groups (especially hydroxyl groups), resulting in “love-hate relationship” of the biochar-amended soil (BAS) with water. From the purview of geotechnical engineering, the effects of pyrolysis temperature on geotechnical properties are rarely investigated. In total, three biochar rates (0, 5, and 10%) were considered for a set of geotechnical experiments in sand clay mixture soil with biochar produced at 350°C and 550°C. Test results show that biochar addition in soil, in general regardless of pyrolysis temperature, increased the optimum moisture content (OMC), plasticity index, and soil water retention characteristics (SWRC) and decreased the maximum dry density (MDD), shear strength parameters (cohesion, friction), and erosion rates. Whilst comparing the pyrolysis temperature effects on two biochar-amended soils, only marginal effects (in terms of magnitude) on SWRC were observed. The most significant decrease of MDD (or increase of OMC) for 5% (w/w) and 10% (w/w) biochar additions occurred at pyrolysis temperatures of 550 °C and 350°C, respectively. In addition, biochar produced at lower pyrolysis temperature (350 °C) was more effective in reducing cracks and enhancing shrinkage area ratio. Ten percent of biochar addition with pyrolysis temperature of 350 °C was the optimum combination in resisting soil erosion. The study provides evidence that the geotechnical properties of biochar-amended soils for landfill cover soil applications could be tailor made by controlling the pyrolysis temperature.
Graphical abstract
We hereby show that root systems adapt to a spatially discontinuous pattern of water availability even when the gradients of water potential across them are vanishingly small. A paper microfluidic ...approach allowed us to expose the entire root system of
plants to a square array of water sources, separated by dry areas. Gradients in the concentration of water vapor across the root system were as small as 10
⋅mM⋅m
(∼4 orders of magnitude smaller than in conventional hydrotropism assays). Despite such minuscule gradients (which greatly limit the possible influence of the well-understood gradient-driven hydrotropic response), our results show that 1) individual roots as well as the root system as a whole adapt to the pattern of water availability to maximize access to water, and that 2) this adaptation increases as water sources become more rare. These results suggest that either plant roots are more sensitive to water gradients than humanmade water sensors by 3-5 orders of magnitude, or they might have developed, like other organisms, mechanisms for water foraging that allow them to find water in the absence of an external gradient in water potential.
Earthworms are ecosystem engineers, capable of modifying the soil environment they inhabit. Recent evidence indicates that they increase the mobility and availability of potentially toxic elements in ...soils, but a quantitative synthesis of the evidence required to understand mechanisms and identify soils most susceptible to earthworm-induced potentially toxic element mobilisation is lacking. We undertook a meta-analysis of 42 peer reviewed journal studies, comprising 1185 pairwise comparisons between earthworm-inhabited and earthworm-free soils to quantify the impact of earthworms on potentially toxic element mobility in bulk earthworm-inhabited soil and earthworm casts, and on plant uptake and concentration. We find that endogeic and epigeic earthworms increase the mobility of potentially toxic elements in the bulk soil, and earthworms from all ecological groups mobilise potentially toxic elements during passage of soil through the earthworm gut. We also observe an increase in the concentration and uptake of potentially toxic elements by plants growing on soils inhabited by epigeic (mostly Eisenia fetida) earthworms. Earthworms mobilise potentially toxic elements in geogenic soils to a greater extent than anthropogenically contaminated soils. Soils with very low (<2%) soil organic matter content are most susceptible to earthworm-induced potentially toxic element mobilisation. These findings have important implications for the ability of exotic earthworms to alter soil biogeochemical cycles when introduced to new environments. Mixing amendments with contaminated soils with the intention of reducing the mobility of potentially toxic elements may be aided by the activity of earthworms that accelerate the mixing processes. Furthermore, our findings also highlight a promising phenomenon that, if harnessed, may help to alleviate micronutrient deficiencies in degraded soils.
Display omitted
•Earthworms increase the mobility and plant uptake of potentially toxic elements.•The primary mechanism is due to passage of soil through the earthworm gut.•Soils with low (<2%) SOM are most susceptible to mobilisation.•The magnitude of mobilisation is greater in geogenic than contaminated soils.•Our findings have implications for micronutrient availability and exotic invasions.
Anaerobic digestate is a nutrient rich slurry by-product derived from biogas production, often used as a fertiliser due to its high nitrogen content. However, nitrogen losses from its application can ...lead to environmental pollution. In a laboratory experiment, the addition of high organic carbon materials to digestate-amended soil as a potential means to stimulate microbial immobilisation of digestate supplied nitrogen was investigated. Soil was incubated in pots for 5 months with digestate (equivalent to 250 kgN ha −1 ). The impact of adding carbon into the digestate (equivalent to 540 kgC ha −1 ) as either glycerol, straw, woodchip, or biochar on soil microbial and chemical parameters was quantified. Glycerol amended soils had significantly higher microbial biomass compared to digestate alone during the first month and at 30 days after application had a 4x higher on average microbial N. The digestate + straw treatment resulted in a 2.5x significantly greater nitrogen immobilisation compared to digestate alone after 3 months of incubation. The digestate + woodchip had a 2× higher mean microbial N after 5 months, whilst the biochar amendment did not stimulate significant nitrogen immobilisation at any time. These results suggest that mixing a labile to moderately labile organic carbon amendment, such as straw, with digestate has the greatest potential to reduce nitrogen losses following digestate application through microbial immobilisation.
Zero Budget Natural Farming (ZBNF) is a grassroot agrarian movement and a state backed extension in Andhra Pradesh, and has been claimed to potentially meet the twin goals of global food security and ...environmental conservation. However, there is a lack of statistically evaluated data to support assertions of yield benefits of ZBNF compared to organic or conventional alternatives, or to mechanistically account for them. In order to fill this gap, controlled field experiments were established in twenty-eight farms across six districts, spanning over 800 km, over three cropping seasons. In these experiments, we compared ZBNF (no synthetic pesticides or fertilisers, home-made inputs comprising
desi
cow dung and urine with mulch) to conventional (synthetic fertilisers and pesticides) and organic (no synthetic pesticides or fertilisers, no mulch, purchased organic inputs, e.g. farmyard manure and vermicompost) treatments, all with no tillage. Comparisons were made in terms of yield, soil pH, temperature, moisture content, nutrient content and earthworm abundance. Our data shows that yield was significantly higher in the ZBNF treatment (
z
score = 0.58 ± 0.08), than the organic (
z
= −0.34 ± 0.06) or conventional (−0.24 ± 0.07) treatment when all farm experiments were analysed together. However, the efficacy of the ZBNF treatment was context specific and varied according to district and the crop in question. The ZBNF yield benefit is likely attributed to mulching, generating a cooler soil, with a higher moisture content and a larger earthworm population. There were no significant differences between ZBNF and the conventional treatment in the majority of nutrients. This is a particularly important observation, as intensive use of synthetic pesticides and fertilisers comes with a number of associated risks to farmers’ finances, human health, greenhouse gas emissions, biodiversity loss and environmental pollution. However, long-term field and landscape scale trials are needed to corroborate these initial observations.
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