Mitigating Contaminant-Driven Risks for the Safe Expansion of the Agricultural-Sanitation Circular Economy in an Urbanizing World.

The widespread adoption of an agricultural circular economy requires the recovery of resources such as water, organic matter, and nutrients from livestock manure and sanitation. While this approach offers many benefits, we argue this is not without potential risks to human and environmental health that largely stem from the presence of contaminants in the recycled resources (e.g., pharmaceuticals, pathogens). We discuss context specific challenges and solutions across the three themes: (1) contaminant monitoring; (2) collection transport and treatment; and (3) regulation and policy. We advocate for the redesign of sanitary and agricultural management practices to enable safe resource reuse in a proportionate and effective way. In populous urban regions with access to sanitation provision, processes can be optimized using emergent technologies to maximize removal of contaminant from excreta prior to reuse. Comparatively, in regions with limited existing capacity for conveyance of excreta to centralized treatment facilities, we suggest efforts should focus on creation of collection facilities (e.g., pit latrines) and decentralized treatment options such as composting systems. Overall, circular economy approaches to sanitation and resource management offer a potential solution to a pressing challenge; however, to ensure this is done in a safe manner, contaminant risks must be mitigated.

Enhanced weathering in the US Corn Belt delivers carbon removal with agronomic benefits.

Terrestrial enhanced weathering (EW) of silicate rocks, such as crushed basalt, on farmlands is a promising scalable atmospheric carbon dioxide removal (CDR) strategy that urgently requires performance assessment with commercial farming practices. We report findings from a large-scale replicated EW field trial across a typical maize-soybean rotation on an experimental farm in the heart of the United Sates Corn Belt over 4 y (2016 to 2020). We show an average combined loss of major cations (Ca2+ and Mg2+) from crushed basalt applied each fall over 4 y (50 t ha-1 y-1) gave a conservative time-integrated cumulative CDR potential of 10.5 ± 3.8 t CO2 ha-1. Maize and soybean yields increased significantly (P < 0.05) by 12 to 16% with EW following improved soil fertility, decreased soil acidification, and upregulation of root nutrient transport genes. Yield enhancements with EW were achieved with significantly (P < 0.05) increased key micro- and macronutrient concentrations (including potassium, magnesium, manganese, phosphorus, and zinc), thus improving or maintaining crop nutritional status. We observed no significant increase in the content of trace metals in grains of maize or soybean or soil exchangeable pools relative to controls. Our findings suggest that widespread adoption of EW across farming sectors has the potential to contribute significantly to net-zero greenhouse gas emissions goals while simultaneously improving food and soil security.

Behavior and Fate of Chromium and Carbon during Fe(II)-Induced Transformation of Ferrihydrite Organominerals.

The mobility of chromium (Cr) is controlled by minerals, especially iron (oxyhydr)oxides. The influence of organic carbon (OC) on the mobility and fate of Cr(VI) during Fe(II)-induced transformation of iron (oxyhydr)oxide, however, is still unclear. We investigate how low-weight carboxyl-rich OC influences the transformation of ferrihydrite (Fh) and controls the mobility of Cr(VI/III) in reducing environments and how Cr influences the formation of secondary Fe minerals and the stabilization of OC. With respect to the transformation of Fe minerals, the presence of low-weight carboxyl-rich OC retards the growth of goethite crystals and stabilizes lepidocrocite for a longer time. With respect to the mobility of Cr, low-weight carboxyl-rich OC suppresses the Cr(III)non-extractable associated with Fe minerals, and this suppression is enhanced with increasing carboxyl-richness of OC and decreasing pH. The presence of Cr(III) mitigates the decrease in total C associated with Fe minerals and increases the Cnon-extractable especially for Fh organominerals made with carboxyl-rich OC. Our study sheds new light on the mobility and fate of Cr in reducing environments and suggests that there is a potential synergy between Cr(VI) remediation and OC stabilization.

Assessing soil system changes under climate-smart agriculture via farmers' observations and conventional soil testing.

Soil degradation remains a challenge in African highlands, where land management lacks a strong context-specific evidence base. We investigated the impacts of recently implemented soil and water conservation (SWC) practices-farmyard manure addition, incorporation of crop residues in soil and fanya juu terracing under an agroforestry system on soil health indicators in the East Usambara Mountains of Tanzania. Farmers' observations of soil changes were combined with conventional soil testing to assess the initial impacts of SWC practices relative to conventional non-SWC practice. Majority of farmers (66%-83%) reported that combining fanya juu terracing with organic amendments led to soil colour change from red to black and an increase in crop yield. Despite the observed darkening of the soil, there was no significant increase in soil organic carbon stock and the contents of N, P, K. There were important changes in soil physical properties, including greater aggregate stability (mean weight diameter of 1.51-1.71 mm) in the SWC plots, a greater volume of transmission pores (>60 µm) and coarse storage pores (10-60 µm) in the surface soil layer (0-15 cm), and greater volume of fine storage pores (0.2-10 µm) and residual pores (0.2 µm) in the sub-surface layer (15-30 cm) of the SWC plots compared with the conventional plots. These changes indicate that SWC rapidly enhances infiltration and retention of water within the root zone, which are important for increasing crop yields and improving the resilience of the agro-ecosystem to environmental stress. Combining SWC with effective soil fertility management is needed for sustainable highland agriculture.

Farmers' indicators of soil health in the African highlands.

Improving soil health is necessary for increasing agricultural productivity and providing multiple ecosystem services. In the African Highlands (AH) where conversion of forests to cultivation on steep slopes is leading to soil degradation, sustainable land management practices are vital. Farmers' awareness of soil health indicators (SHI) influences their choice of land management and needs to be better understood to improve communication between land managers and other stakeholders in agricultural systems. This study aims to collate and evaluate case study analyses of farmers' awareness and use of soil health indicators in African Highlands. This is achieved by using a multi-method approach that combines a meta-summary analysis of AH's SHI data from 24 published studies together with farmer interviews in the East Usambara Mountain region of Tanzania (EUM). Our findings show that farmers across the AH use observable attributes of the landscape as SHI. Out of 16 SHI reported by the farmers, vegetation performance/crop yield and soil colour were most frequently used across the AH. These were also the only two SHI that influenced farmers' land management decisions in the EUM, where organic manure addition was the only land management option resulting from observed changes in SHI. Farmers' use of only one or two SHI in land management decisions, as is the case in the EUM, seems to limit their choice and/or adoption of sustainable land management options, highlighting the need to increase awareness and use of more relevant SHI. This could be achieved by sharing SHI knowledge through learning alliances and agricultural extension service. Integration of farmers' observation techniques and conventional soil testing in a hybrid approach is recommended for a more targeted assessment of soil health to inform appropriate and sustainable land management practices.

The Role of Extracellular DNA in Microbial Attachment to Oxidized Silicon Surfaces in the Presence of Ca2+ and Na.

Attachment assays of a Pseudomonas isolate to fused silica slides showed that treatment with DNaseI significantly inhibited cellular adsorption, which was restored upon DNA treatment. These assays confirmed the important role of extracellular DNA (eDNA) adsorption to a surface. To investigate the eDNA adsorption mechanism, single-molecule force spectroscopy (SMFS) was used to measure the adsorption of eDNA to silicon surfaces in the presence of different concentrations of sodium and calcium ions. SMFS reveals that the work of adhesion required to remove calcium-bound eDNA from the silicon oxide surface is substantially greater than that for sodium. Molecular dynamics simulations were also performed, and here, it was shown that the energy gain in eDNA adsorption to a silicon oxide surface in the presence of calcium ions is small and much less than that in the presence of sodium. The simulations show that the length scales involved in eDNA adsorption are less in the presence of sodium ions than those in the presence of calcium. In the presence of calcium, eDNA is pushed above the surface cations, whereas in the presence of sodium ions, short-range interactions with the surface dominate. Moreover, SMFS data show that increasing [Ca2+] from 1 to 10 mM increases the adsorption of the cations to the silicon oxide surface and consequently enhances the Stern layer, which in turn increases the length scale associated with eDNA adsorption.

A novel permeable reactive biobarrier for ortho-nitrochlorobenzene pollution control in groundwater: Experimental evaluation and kinetic modelling.

Three novel permeable reactive barrier (PRB) materials composed of Cu/Fe with 0.24% and 0.43% (w/w) Cu loadings or Fe0 supported on wheat straw were prepared (termed materials E, F and G). These materials exhibited excellent pollutant removal efficiency and physical stability as well as the ongoing release of organic carbon and iron. Column experiments showed that materials E, F and G removed almost 100% of ortho-nitrochlorobenzene (o-NCB) from water. The rates of iron release from the E and F columns exceeded those from column G but this had no significant effect on o-NCB removal. The bacteria that degraded o-NCB in E and F were also different from those in G. The levels of these bacteria in the columns were higher than those in the initial materials, with the highest level in column E. The simultaneous reduction and microbial degradation of o-NCB was observed, with the latter being dominant. A kinetic model was established to simulate the dynamic interactions and accurately predicted the experimental results. Organic carbon from the wheat straw supported the majority of the biomass in each column, which was essential for the bioremediation process. The findings of this study suggest an economically viable approach to mitigating o-NCB pollution.

Insights into the mechanism of the interference of sulfadiazine on soil microbial community and function.

The accumulation of sulfonamides in the soil environment possessed the potential to change soil microbial community and function. Metabolomics is capable of providing insights into the carbon metabolic pool and molecular mechanisms associated with external stressors. Here we evaluated alternations in soil bacterial community and soil metabolites profiles under sulfadiazine (SDZ) exposure and proposed a potential mechanism that SDZ accumulation in soil affected soil organic matter (SOM) cycling. Sequencing analysis showed that the relative abundance of bacterial species associated with carbon cycling significantly decreased under high concentrations of SDZ exposure. Untargeted metabolomics analysis showed that 78 metabolites were significantly changed with the presence of SDZ in soil. The combination of functional predictions and pathway analysis both demonstrated that high concentrations of SDZ exposure could cause disturbance in anabolism and catabolism. Moreover, the noticeable decline in the relative content of carbohydrates under high concentrations of SDZ exposure might weaken physical separation and provide more chances for microbes to degrade SOM. The above results provided evidence that SDZ accumulation in soil held the potential to disturb SOM cycling. These findings spread our understanding about the environmental risk of antibiotic in the soil environment beyond the dissemination of antibiotic resistance.

Technologies to deliver food and climate security through agriculture.

Agriculture is a major contributor to environmental degradation and climate change. At the same time, a growing human population with changing dietary preferences is driving ever increasing demand for food. The need for urgent reform of agriculture is widely recognized and has resulted in a number of ambitious plans. However, there is credible evidence to suggest that these are unlikely to meet the twin objectives of keeping the increase in global temperature within the target of 2.0 °C above preindustrial levels set out in the Paris Agreement and delivering global food security. Here, we discuss a series of technological options to bring about change in agriculture for delivering food security and providing multiple routes to the removal of CO2 from the atmosphere. These technologies include the use of silicate amendment of soils to sequester atmospheric CO2, agronomy technologies to increase soil organic carbon, and high-yielding resource-efficient crops to deliver increased agricultural yield, thus freeing land that is less suited for intensive cropping for land use practices that will further increase carbon storage. Such alternatives include less intensive regenerative agriculture, afforestation and bioenergy crops coupled with carbon capture and storage technologies.

Response of soil bacterial communities to sulfadiazine present in manure: Protection and adaptation mechanisms of extracellular polymeric substances.

Extracellular polymeric substances (EPS) play a dominant role in protective biofilms. However, studies exploring the underlying protective mechanism of EPS have mainly focused on activated sludge, whereas their positive roles in protecting soil microbes from environmental stress have not been elucidated. In this study, we revealed the response of soil bacterial communities to various dosages of sulfadiazine (SDZ) present in manure, with a special emphasis on the role of EPS. Sequencing analysis showed that the bacterial community demonstrated stronger symbiotic relationships and weaker competitive interaction patterns to cope with disturbance induced by SDZ. EPS was mainly composed of tyrosine-like and tryptophan-like substances, and moreover, carboxyl, hydroxyl and ether groups were the main functional groups. An adaptation mechanism, namely the enhanced secretion of tryptophan-like substances, could help alleviate the SDZ stress effectively in the biofilms occurring in soil that experienced long-term manure application. Furthermore, the existence of EPS weakened the accumulation of antibiotic resistance genes (ARGs) in soil. Our results for the first time systematically uncover the joint action of biofilm tolerance and ARGs in resisting SDZ stress, which enhances understanding of the protective role of EPS and the underlying mechanisms governing biofilm functions in soil environments.

Elevated CO2 concentration modifies the effects of organic fertilizer substitution on rice yield and soil ARGs.

Antibiotic resistance and rising CO2 levels are considered among the most significant challenges we will face in terms of global development over the following decades. However, the impact of elevated CO2 on soil antibiotic resistance has rarely been investigated. We used a free-air CO2 enrichment system to investigate the potential risks posed by applying mineral and organic fertilizers to paddy soil at current CO2 concentration (370 ppm) and future elevated CO2 (eCO2, 570 ppm predicted for 2100). Organic fertilizer substitution (substituting the mineral fertilizer by 50% N) alone increased the plant uptake and soil residue of sulfamethazine, and enriched sulfonamide resistance genes (sul1, sul2), tetracycline resistance genes (tetG, tetM) and class 1 integron (intl1). But it decreased the rice grain yield (by 7.6%). Comparatively, eCO2 decreased the sul2, tetG and intl1 gene abundances by organic fertilizer substitution, and meanwhile increased grain yield (by 8.4%). Proteobacteria and Nitrospirae were potential hosts of antibiotic resistance genes (ARGs). Horizontal gene transfer via intl1 may play an important role in ARGs spread under eCO2. Results indicated that future elevated CO2 concentration could modify the effects of organic fertilizer substitution on rice yield and soil ARGs, with unknown implications for future medicine and human health.

Impacts of conservation agriculture on soil structure and hydraulic properties of Malawian agricultural systems.

Sub-Saharan Africa (SSA) faces climate change and food insecurity challenges, which require action to create resilient farming systems. Conservation agriculture (CA) is widely promoted across SSA but the impacts on key soil physical properties and functions such as soil structure and hydraulic properties that govern water storage and transmission are not well understood. The aim of this study was to assess the impacts of long term (10-12 years) maize-based CA on soil hydraulic conductivity, water retention and pore size distribution. Root zone (0-30 cm depth) soil total porosity, pore size distribution, saturated hydraulic conductivity (Ksat) and plant available water capacity (PAWC) of conventional maize monocrop farming systems (CP) are compared with those of adjacent CA trials with either sole maize or maize intercrop/rotation with cowpea (Vigna unguiculata L.), pigeon pea (Cajanus cajan L.) or velvet bean (Mucuna pruriens L) in trial locations across central and southern Malawi. Results show that maize-based CA systems result in significant changes to soil hydraulic properties that correlate with improved soil structure. Results demonstrate increases of 5-15 % in total porosity, 0.06-0.22 cm/min in Ksat, 3-7 % in fine pores for water storage and 3-6 % in PAWC. Maize monocrop CA had similar effect on the hydraulic properties as the maize-legume associations. The values of Ksat for CA systems were within optimum levels (0.03-0.3 cm/min) whereas PAWC was below optimum (<20 %). There was no significant build-up in soil organic matter (OM) in the CA systems. The results lead to a recommendation that crop residue management should be more pro-actively pursued in CA guidance from agricultural extension staff to increase soil OM levels, increase yields and enhance climate resilience of sub-Saharan African farming systems.

Potential for large-scale CO2 removal via enhanced rock weathering with croplands.

Enhanced silicate rock weathering (ERW), deployable with croplands, has potential use for atmospheric carbon dioxide (CO2) removal (CDR), which is now necessary to mitigate anthropogenic climate change1. ERW also has possible co-benefits for improved food and soil security, and reduced ocean acidification2-4. Here we use an integrated performance modelling approach to make an initial techno-economic assessment for 2050, quantifying how CDR potential and costs vary among nations in relation to business-as-usual energy policies and policies consistent with limiting future warming to 2 degrees Celsius5. China, India, the USA and Brazil have great potential to help achieve average global CDR goals of 0.5 to 2 gigatonnes of carbon dioxide (CO2) per year with extraction costs of approximately US$80-180 per tonne of CO2. These goals and costs are robust, regardless of future energy policies. Deployment within existing croplands offers opportunities to align agriculture and climate policy. However, success will depend upon overcoming political and social inertia to develop regulatory and incentive frameworks. We discuss the challenges and opportunities of ERW deployment, including the potential for excess industrial silicate materials (basalt mine overburden, concrete, and iron and steel slag) to obviate the need for new mining, as well as uncertainties in soil weathering rates and land-ocean transfer of weathered products.

Increased yield and CO2 sequestration potential with the C4 cereal Sorghum bicolor cultivated in basaltic rock dust-amended agricultural soil.

Land-based enhanced rock weathering (ERW) is a biogeochemical carbon dioxide removal (CDR) strategy aiming to accelerate natural geological processes of carbon sequestration through application of crushed silicate rocks, such as basalt, to croplands and forested landscapes. However, the efficacy of the approach when undertaken with basalt, and its potential co-benefits for agriculture, require experimental and field evaluation. Here we report that amending a UK clay-loam agricultural soil with a high loading (10 kg/m2 ) of relatively coarse-grained crushed basalt significantly increased the yield (21 ± 9.4%, SE) of the important C4 cereal Sorghum bicolor under controlled environmental conditions, without accumulation of potentially toxic trace elements in the seeds. Yield increases resulted from the basalt treatment after 120 days without P- and K-fertilizer addition. Shoot silicon concentrations also increased significantly (26 ± 5.4%, SE), with potential benefits for crop resistance to biotic and abiotic stress. Elemental budgets indicate substantial release of base cations important for inorganic carbon removal and their accumulation mainly in the soil exchangeable pools. Geochemical reactive transport modelling, constrained by elemental budgets, indicated CO2 sequestration rates of 2-4 t CO2 /ha, 1-5 years after a single application of basaltic rock dust, including via newly formed soil carbonate minerals whose long-term fate requires assessment through field trials. This represents an approximately fourfold increase in carbon capture compared to control plant-soil systems without basalt. Our results build support for ERW deployment as a CDR technique compatible with spreading basalt powder on acidic loamy soils common across millions of hectares of western European and North American agriculture.

Evaluating a novel permeable reactive bio-barrier to remediate PAH-contaminated groundwater.

Permeable reactive barriers (PRBs) are an environmentally-friendly, cost-effective in-situ technology that can be used to remediate polycyclic aromatic hydrocarbons (PAHs)-contaminated groundwater. In this study, PRBs of two different materials (A and B) that relied on microbes self-domestication mechanism were designed and tested. The materials A and B were the same apart from their carbon source: A was based on wheat straw and B was based on coconut shell biochar. We used laboratory batch experiments followed by long-term column tests to assess the capacity of these two materials to remediate PAHs. The results showed that both A and B removed almost 100% of the phenanthrene. More carbon was released from A (80-500 mg/L) than from B (72-195 mg/L), and slightly more oxygen was released from B (7.31-10.31 mg/L) than A (7.15-9.64 mg/L). The release of organic carbon from material B was more stable than that from material A. The bacterial communities of both columns comprised members of the Mycobacterium, Pseudomonas, and Sphingomonas genera that are known to degrade phenanthrene, and Pseudomonas and Sphingomonas were 7 times more abundant in column B than in column A. Material B is more promising for treating PAH-contaminated groundwater than material A.

Simulating carbon capture by enhanced weathering with croplands: an overview of key processes highlighting areas of future model development.

Enhanced weathering (EW) aims to amplify a natural sink for CO2 by incorporating powdered silicate rock with high reactive surface area into agricultural soils. The goal is to achieve rapid dissolution of minerals and release of alkalinity with accompanying dissolution of CO2 into soils and drainage waters. EW could counteract phosphorus limitation and greenhouse gas (GHG) emissions in tropical soils, and soil acidification, a common agricultural problem studied with numerical process models over several decades. Here, we review the processes leading to soil acidification in croplands and how the soil weathering CO2 sink is represented in models. Mathematical models capturing the dominant processes and human interventions governing cropland soil chemistry and GHG emissions neglect weathering, while most weathering models neglect agricultural processes. We discuss current approaches to modelling EW and highlight several classes of model having the potential to simulate EW in croplands. Finally, we argue for further integration of process knowledge in mathematical models to capture feedbacks affecting both longer-term CO2 consumption and crop growth and yields.

Adsorption of poly acrylic acid onto the surface of calcite: an experimental and simulation study.

Macromolecular binding to minerals is of great importance in the formation of biofilms, and carboxylate functional groups have been found to play a pivotal role in the functioning of these macromolecules. Here we present both fluorescence time-resolved anisotropy measurements and simulation data on the conformational behaviour and binding of a poly acrylic acid polymer. In solution the polymer exhibits a pH dependent behaviour, with a coiled conformation at a low pH and extended conformation at higher pH values. The polymer is readily adsorbed on the surface of calcite, preferring to bind in an extended conformation, with the strength of the adsorption dependent on the pH and presence of counter ions. We discuss the reasons why the calculated adsorption free energy differs from that obtained from a Langmuir isotherm analysis, showing that they refer to different quantities. The enhanced binding of the extended conformations shows the importance of flexibility in the binding of macromolecules.

Effect of extracellular polymeric substances on the mechanical properties of Rhodococcus.

The mechanical properties of Rhodococcus RC291 were measured using force spectroscopy equipped with a bacterial cell probe. Rhodococcal cells in the late growth stage of development were found to have greater adhesion to a silicon oxide surface than those in the early growth stage. This is because there are more extracellular polymeric substances (EPS) that contain nonspecific binding sites available on the cells of late growth stage. It is found that EPS in the late exponential phase are less densely bound but consist of chains able to extend further into their local environment, while the denser EPS at the late stationary phase act more to sheath the cell. Contraction and extension of the EPS could change the density of the binding sites, and therefore affect the magnitude of the adhesion force between the EPS and the silicon oxide surface. By treating rhodococcal EPS as a surface-grafted polyelectrolyte layer and using scaling theory, the interaction between EPS and a solid substrate was modelled for the cell approaching the surface which revealed that EPS possess a large capacity to store charge. Changing the pH of the surrounding medium acts to change the conformation of EPS chains.