The frequency and duration of flooding events is increasing due to land-use changes increasing run-off of precipitation, and climate change causing more intense rainfall events. Floodplain soils ...situated downstream of urban or industrial catchments, which were traditionally considered a sink of potentially toxic elements (PTEs) arriving from the river reach, may now become a source of legacy pollution to the surrounding environment, if PTEs are mobilised by unprecedented flooding events.
When a soil floods, the mobility of PTEs can increase or decrease due to the net effect of five key processes; (i) the soil redox potential decreases which can directly alter the speciation, and hence mobility, of redox sensitive PTEs (e.g. Cr, As), (ii) pH increases which usually decreases the mobility of metal cations (e.g. Cd2+, Cu2+, Ni2+, Pb2+, Zn2+), (iii) dissolved organic matter (DOM) increases, which chelates and mobilises PTEs, (iv) Fe and Mn hydroxides undergo reductive dissolution, releasing adsorbed and co-precipitated PTEs, and (v) sulphate is reduced and PTEs are immobilised due to precipitation of metal sulphides. These factors may be independent mechanisms, but they interact with one another to affect the mobility of PTEs, meaning the effect of flooding on PTE mobility is not easy to predict. Many of the processes involved in mobilising PTEs are microbially mediated, temperature dependent and the kinetics are poorly understood.
Soil mineralogy and texture are properties that change spatially and will affect how the mobility of PTEs in a specific soil may be impacted by flooding. As a result, knowledge based on one river catchment may not be particularly useful for predicting the impacts of flooding at another site. This review provides a critical discussion of the mechanisms controlling the mobility of PTEs in floodplain soils. It summarises current understanding, identifies limitations to existing knowledge, and highlights requirements for further research.
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•Floodplains may switch from being sinks to becoming sources of legacy pollution.•Flooding influences pH, DOM and the mobility of potentially toxic elements (PTEs).•Reductive dissolution of Fe and Mn oxides mobilises PTEs.•Precipitation of metal sulphides reduces PTE mobility.•Field observations are required to further advance our understanding.
This study aimed to evaluate the influence of Eisenia fetida (Savigny), added to an acidic soil contaminated with potentially toxic elements (PTEs; As, Sb, Cd, Pb, Zn) and amended with a ...softwood-derived biochar (2 and 5% w/w), on the mobility of PTEs and soil health (i.e. nutrient availability, enzyme activity and soil basal respiration). The PTEs bioaccumulation by E. fetida and the acute ecotoxicity effects of the amended soils were also evaluated. The interaction between earthworms and biochar led to a significant increase in soil pH, organic matter, dissolved organic carbon content, cation exchange capacity, and exchangeable Ca compared to the untreated soil. Moreover, the water-soluble and readily exchangeable PTE fraction decreased (with the exception of Sb) between 1.2- and 3.0-fold in the presence of biochar and earthworms. Earthworms, biochar, and their combination, led to a reduction of phosphomonoesterase activity which in soils amended with biochar and earthworms decreased between 2.2- and 2.5-fold with respect to the untreated soil. On the other hand, biochar and earthworms also enhanced soil basal respiration and protease activity. Although the survival rate and the weight loss of E. fetida did not change significantly with the addition of 2% biochar, adding the highest biochar percentage (5%) resulted in a survival rate that was ~2-fold lower and a weight loss that was 2.5-fold higher than the other treatments. The PTE bioaccumulation factors for E. fetida, which were less than 1 for all elements (except Cd), followed the order Cd > As>Zn > Cu > Pb > Sb and were further decreased by biochar addition. Overall, these results highlight that E. fetida and biochar, especially at 2% rate, could be used for the restoration of soil functionality in PTE-polluted environments, reducing at the same time the environmental risks posed by PTEs, at least in the short time.
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•Soil labile PTE fractions decreased in the presence of biochar and E. fetida.•The E. fetida-biochar interaction decreased acid and alkaline phosphomonoesterases.•Biochar and earthworms enhanced soil basal respiration and protease activity.•Biochar addition at 5% (w/w) decreased the survival rate and weight of E. fetida.•The PTE bioaccumulation factors by E. fetida decreased by biochar addition.
Plant legacy effects observed in plant-soil feedback experiments have largely been attributed to the root or litter material of the previous plant. The legacy effects of rhizodeposits are defined as ...changes in the soil microbiome that remain after a plant has died or been removed from the soil and caused by the release of substances of various compositions by living plants (rhizodeposits). Rhizodeposit-mediated legacy effects have been largely ignored mainly due to the high spatial and temporal variability of rhizodeposits and difficulties quantifying and tracking them in the rhizosphere. In this perspective article, we discuss what is known about the legacy effects of rhizodeposits and provide ideas for future experiments to improve understanding of this phenomenon. Only a few studies separate rhizodeposit-mediated plant legacy effects from legacy effects of root decomposition. Results from these experiments indicate that rhizodeposit-mediated legacy effects on soil microbial communities may persist for several months to several years, especially if the same crop is cultivated persistently for several years in a ‘conditioning’ phase. Rhizodeposit-mediated legacy effects on fungal communities usually last longer than those on bacterial communities due to fungal life-cycle strategies (spore formation) and slower reproduction rates, compared to bacterial communities. We highlight the need for further experimentation to investigate the influence that the length of a conditioning phase has on the persistence of the legacy effect, differentiate the effect of root exudates from the effects of sloughed root cells, separate the influence of simple sugars from that of high molecular-weight exudates and plant derived compounds with antimicrobial properties, and explore whether plant species diversity influences the nature of the legacy. To address these questions, we propose the use of contemporary tools such as stable isotope probing, plant genetics, and reverse microdialysis. We think that harnessing rhizodeposit-mediated plant legacy effects could be a promising approach to improve sustainable crop production by creating disease-suppressive soils and simulating plant growth-promoting micro-organisms within soil systems.
•Legacy effect of rhizodeposits represent effects after plant death or removal.•Negative legacy effects on soil-borne pathogens are mitigated by crop rotations.•Beneficial legacies of rhizodeposits on the following plant are underexplored.•Legacies are potentially persistent, particularly on fungal communities.•Further research using SIP, plant genomics, and reverse microdialysis is needed.
This study aimed to investigate the effect of conversion from natural forest to cinnamon plantation on the top 1 m soil carbon stocks and soil characteristics. The project was conducted on Andosols ...of Kerinci Regency, Sumatera, Indonesia, sampling the soil profile under natural forests and a chronosequence of cinnamon plantations of different ages (1, 5 and 10 years). SOC stocks were quantified alongside physical properties (bulk density) and chemical properties (carbon, nitrogen, C/N ratio) to investigate the impact of land conversion. SOC stocks increased 1 year after conversion to cinnamon plantations, but then tended to decrease as the plantations got older. The initial increase was observed alongside decreasing bulk density 1 year after forest conversion to cinnamon plantation, likely as a result of the fresh input of (less dense) pyrogenic soil organic matter as a result of slash and burn practices and transport down the soil profile owing to leaching. In older plantations SOC stocks were lower, probably because organic matter had been decomposed or leached out of the profile. The free particulate organic matter (fPOM) was isolated from selected topsoil and subsoil layers and analysed for carbon, nitrogen, and FTIR analysis. FTIR analysis revealed that topsoil fPOM contained more aromatic functional groups than subsoils and had a higher degree of decomposition. Aromatic and carbohydrate functional groups were initially lower in recently converted cinnamon plantation, but the trend was reversed 10 years after conversion. The initial flush of fresh organic matter into soils after slash and burn provides fPOM with a lower degree of decomposition but is short‐lived and fPOM becomes more microbially processed as the cinnamon plantation ages. We conclude that, after a short term increase brought about by slash and burn, forest conversion to cinnamon plantation in Kerinci Regency depletes SOC stocks both in topsoils and subsoils.
The impact of anaerobic digestate on soil life: A review van Midden, Christina; Harris, Jim; Shaw, Liz ...
Applied soil ecology : a section of Agriculture, ecosystems & environment,
November 2023, 2023-11-00, Volume:
191
Journal Article
Peer reviewed
Open access
Using organic amendments to fertilise crops is a crucial part in the sustainability of agricultural systems. The residual slurry remaining after biogas production (anaerobic digestate) contains a ...rich source of plant nutrients that provides an alternative to mineral fertilisers. The delivery of many nutrients to plants is facilitated by a healthy soil biota: free-living and symbiotic microflora (e.g. archaea, bacteria and fungi) mineralize, solubilize and facilitate plant uptake of nutrients and the soil fauna (e.g. protozoa, microarthopods and earthworms) influence nutrient cycling processes as higher-level consumers and litter transformers. The delivery of nutrients to plants via the activity of this soil food web is influenced by fertiliser inputs. Here we review the impact of anaerobic digestate on soil biota. The quantity and composition of the carbon in digestate has a large influence on soil heterotrophic microbial dynamics and their subsequent influence on nutrient bioavailability. The main points are (1) digestate low in carbon has little effect on soil microorganisms, whereas digestate higher in carbon increases soil microbial abundance and diversity; (2) labile carbon stimulates fast-growing bacteria, whereas recalcitrant carbon shifts the microbial community in favour of slower-growing fungi and Gram-positive bacteria; and (3) earthworms, springtails and nematodes dwelling in the soil surface layer can be negatively affected by digestate application due to toxicity when compounds such as ammonia are present in high concentrations. Generalized understanding of the effect by digestates on soil biota is made difficult by differences in digestate properties caused by varying feedstock and production methods and the inherent heterogeneity of soil. There is a lack of research investigating the impact of repeated digestate application on soil biota and subsequently soil health. This information would give end users more confidence to substitute mineral fertilisers with digestate.
•We review the impacts of anaerobic digestate application on soil biota.•Positive effects of digestate on soil microbiota depend on digestate carbon and nutrient contents.•Digestate affects surface-dwelling meso- and macro- fauna more negatively than soil-dwellers.•There is a critical need to understand long term impacts of digestate application on soil biota.
The importance of earthworms to ecosystem functioning has led to many studies on the impacts of metals on earthworms. Far less attention has been paid to the impact that earthworms have on soil ...metals both in terms of metal mobility and availability. In this review we consider which earthworms have been used in such studies, which soil components have been investigated, which types of soil have been used and what measures of mobility and availability applied. We proceed to review proposed reasons for effects: changes in microbial populations, pH, dissolved organic carbon and metal speciation. The balance of evidence suggests that earthworms increase metal mobility and availability but more studies are required to determine the precise mechanism for this.
We review the impact of earthworms on metal mobility and availability and suggest areas for further investigation.
Aims
The home-field advantage (HFA) hypothesis predicts faster decomposition of plant residues in
home
soil compared to soils with different plants (
away
), and has been demonstrated in forest and ...grassland ecosystems. It remains unclear if this legacy effect applies to crop residue decomposition in arable crop rotations. Such knowledge could improve our understanding of decomposition dynamics in arable soils and may allow optimisation of crop residue amendments in arable systems by cleverly combining crop-residue rotations with crop rotations to increase the amount of residue-derived C persisting in soil.
Methods
We tested the HFA hypothesis in a reciprocal transplant experiment with mesh bags containing wheat and oilseed rape residues in soils at three stages of a short-rotation cropping system. Subsets of mesh bags were retrieved monthly for six months to determine residue decomposition rates, concomitantly measuring soil available N, microbial community structure (phospholipid fatty acid analysis), and microbial activity (Tea Bag Index protocol) to assess how plants may influence litter decomposition rates via alterations to soil biochemical properties and microbial communities.
Results
The residues decomposed at similar rates at all rotational stages. Thorough data investigation using several statistical approaches revealed no HFA within the crop rotation. Soil microbial community structures were similar at all rotational stages.
Conclusions
We attribute the absence of an HFA to the shortness of the rotation and soil disturbance involved in intensive agricultural practices. It is therefore unlikely that appreciable benefits could be obtained in short conventionally managed arable rotations by introducing a crop-residue rotation.
Biochar is a soil amendment capable of influencing plant growth and potentially toxic elements (PTEs) bioavailability in soils. At the same time Eisenia fetida (Savigny) is able to interact with ...biochar influencing its performance. As such they could constitute a resource for assisted phytostabilisation of PTE-polluted soils. To this end, a softwood-derived biochar was added at 2 and 5 % (w/w) rate, with and without E. fetida, to a soil contaminated with Cd, Pb, Zn, As, and Sb, to evaluate the PTE phytostabilisation potentials when combined with Sorghum vulgare. The combination of sorghum, 5 % biochar, and earthworms reduced the mobility of most PTEs in soil (e.g., up to 65 % and 60 % for Pb and Zn), while sorghum biomass was greatly increased (i.e., ~3- and 2-fold for roots and shoots, respectively).
Biochar addition alone reduced the PTE uptake by plants, while the presence of earthworms slightly increased it. Overall, the joint action of biochar and earthworms increased the PTE removal efficiency by S. vulgare compared to control plants (e.g., the amount of root As, Pb and Sb was ~5-, 4- and 3-fold higher, respectively). Although 2 % biochar didn't affect E. fetida fitness, the highest biochar rate (5 %) exhibited toxic effects (the survival rate reduced by ~2-fold; weight loss increased by ~3-fold). Taken together, these results indicated that S. vulgare, in combination with softwood biochar and E. fetida, could be used for the assisted phytostabilisation of PTEs contaminated soils.
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•Sorghum biomass was positively affected by the amendment rate.•Earthworms (E. fetida) addition led to a further increase in plant growth.•Biochar alone reduced the PTEs concentration in plant tissues.•Biochar and earthworms enhanced the PTEs removal efficiency of S. bicolor in roots.•Biochar addition at 5 % rate decreased the survival and weight of E. fetida.
Inundation of river water during flooding deposits contaminated sediments onto floodplain topsoil. Historically, floodplains were considered an important sink for potentially toxic elements (PTEs). ...With increasing flood frequency and duration, due to climate change and land use change, it is important to understand the impact that further flooding may have on this legacy contamination. In this study a field-based approach was taken, extracting soil pore waters by centrifugation of soils sampled on multiple occasions from multiple locations across a floodplain site, which lies adjacent to the River Loddon in southeast England. Flooding generally decreased pore water PTE concentrations and significantly lower pore water concentrations of Cd, Cu, and Cr were found post-flood compared to pre-flood. The dominant process responsible for this observation was precipitation with sulphides resulting in PTE removal from the pore water post-flood. The changes in pH were found to be associated with the decreased pore water concentration of Cu, which suggests the pH rise may have aided adsorption mechanisms or precipitation with phosphates. The impact of flooding on the release and retention of PTEs in floodplain soils is the net effect of several key processes occurring concurrently. It is important to understand the dominant processes that drive mobility of individual PTEs on specific floodplains so that site-specific predictions can determine the impact of future floods on the environmental fate of legacy contaminants.
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•PTE mobility measured on floodplain soil before, during, and after a flood event•Flooding decreased pore water concentrations of Cd, Cu and Cr.•No evidence that floodplains become a source for PTEs due to flooding.•Impact of flooding on PTE mobility is the net effect of 5 key concurrent processes.•Precipitation with sulphides was the dominant process identified to reduce mobility.
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