Using highly stabilized digestate and digestate-derived ammonium sulphate to replace synthetic fertilizers: The effects on soil, environment, and crop production

Enhancing soil fertility and organic carbon stability with high-nitrogen biogas slurry: Benefits and environmental risks

Large-scale livestock farming has increased the amount of excrement, leading to ecological and environmental issues. To address this, sustainable solutions like biogas slurry (BS) are needed to enhance soil fertility and soil organic carbon (SOC). This study investigated the effects of conventional nitrogen fertilizer (NPK), standard BS (BS1), and high-nitrogen input BS treatments (BS5, BS7, and BS11) on soil quality, nutrient dynamics, SOC fractions, and environmental risks across soil depths (0-20, 20-40, and 40-60 cm). Field experiments revealed that BS application significantly enhanced the soil quality index (SQI) by 32.5 %-61.6 % in the 0-20 cm soil layer and ecosystem multifunctionality (EMF) by 42.3 %-169.0 % across all layers compared to NPK. Mineral-associated organic carbon (MOC) dominated SOC stabilization (57.2 %-91.3 % of total SOC), with BS11 increasing MOC content by 76.3 % in the 40-60 cm soil layer. High-nitrogen BS treatments increased microbial biomass carbon (MBC) by 318.1 % (BS11) and β-glucosidase activity by 144 % (BS7) in 0-20 cm soil, indicating enhanced soil enzyme activities. However, excessive nitrogen inputs induced nitrate accumulation in the 40-60 cm soil layer, where NO3--N concentrations surged to 97.2 %-200.6 % above NPK levels. Vertical nutrient migration also triggered subsoil acidification, with the pH decreasing by 5.0 % under the BS11 treatment. These results underscore the dual role of BS: while it enhances SOC stability (via MOC dominance) and multifunctionality, nitrogen inputs must be capped below 300 kg N ha-1 yr-1 to mitigate leaching risks. The findings provide actionable insights for optimizing BS applications in sustainable agriculture, balancing soil health improvements with environmental safeguards.

Application of a practical methodology for the selection of suitable value chains to produce circular fertilisers from secondary raw materials

Background

The growing demand for food products, driven by a growing world population, has increased Europe's dependence on conventional fertilisers, which have a high impact on the environment. In the last decade, new circular fertiliser value chains have appeared as promising alternatives to conventional fertilisers.

Methods

Because of the huge number of alternatives, this study aimed to develop a practical methodology that facilitates the analysis of data related to each value chain to identify and select the most promising circular fertiliser value chains to promote their wide-scale production and use in agriculture, replacing the dependence on conventional fertilisers in Europe. This methodology is based on two stages (funnelling process and scoring system) and considers the 16 criteria (e.g. technical viability, nutrient content, among others) defined in the study. The methodology was tested for 48 value chains identified during the mapping of secondary raw materials in Europe with the potential to be used as circular fertilisers, classifying them into seven different raw materials: urban wastewater (UWW), industrial wastewater (IWW), sewage sludge (SS), biowaste (BW), biological by-products (BBP), treated manure (TM), and digestate (DIG). The funnelling process is based on a GO/NO-GO approach that meets six criteria and allows the discarding of 18 value chains, from 30 to the second stage. The scoring system was a more complete analysis, including ten new scoring criteria.

Results

This system allowed the identification of the potential of the value chains analysed, concluding that struvite from UWW, struvite from IWW, stabilized sludge from SS, composted biowaste from BW, feather meal from BBP, solid fraction from DIG, and spent mushroom substrate from TM are the most promising options for agriculture.

Conclusions

The develop methodology was used to evaluate 48 different value chains with the potential to generate promising circular fertlizers. Seven value chains were finally selected.

Application of digestate from low-tech digesters for degraded soil restoration: Effects on soil fertility and carbon sequestration

The application of digestate to soil represents a common practice for its recycling, but its application to degraded lands to achieve their restoration and sequester organic C into the soil is still almost unexplored. In this context, this study describes a first attempt to use digestate from a low-tech digester for degraded soil restoration in Colombia. An experimental site (2700 m2) previously subjected to intensive mono-cultivation was treated with digestate application for 4 months (40 Mg ha-1 dry weight). Soil samples were collected (0, 4, 8, and 12 months after digestate application) to evaluate chemical and biochemical parameters, as well as total soil organic C stocks and their fractionation among different pools. Results showed that soil pH (from 5.3 to 6), total organic C (from 1.9 to 3 %), total N (from 0.17 to 0.27 %), available P (from 10 to 68 mg kg-1), exchangeable nutrients content (K, Mg, Ca, Fe), respiration rate, microbial biomass C and N, and metabolic activities exhibited an increasing trend after digestate application, leading to a recovery of the soil biological fertility (i.e. biological fertility index increased from 8 to 19 in a range from 1 to 20). Digestate promoted C sequestration in the more stable and recalcitrant pools. Soil application of digestate from low-tech digesters may thus represent a win-win resource recovery strategy to enhance degraded land recovery, contribute to climate change mitigation and support rural communities. In the circular bioeconomy context, afforestation appears as the most promising strategy to take advantage of the restored land.

Biogas potential of organosolv pretreated wheat straw as mono and co-substrate: substrate synergy and microbial dynamics

Anaerobic digestion (AD) technology can potentially address the gap between energy demand and supply playing a crucial role in the production of sustainable energy from utilization of biogenic waste materials as feedstock. The biogas production from anaerobic digestion is primarily influenced by the chemical compositions and biodegradability of the feedstock. Organosolv-steam explosion offers a constructive approach as a promising pretreatment method for the fractionation of lignocellulosic biomasses delivering high cellulose content.This study showed how synergetic co-digestion serves to overcome the challenges of mono-digestion's low efficiency. Particularly, the study evaluated the digestibility of organosolv-steam pretreated wheat straw (WSOSOL) in mono as well as co-digesting substrate with cheese whey (CW) and brewery spent grains (BSG). The highest methane yield was attained with co-digestion of WSOSOL + CW (338 mL/gVS) representing an enhanced biogas output of 1-1.15 times greater than its mono digestion. An ammonium production was favored under co-digestion strategy accounting for 921 mg/L from WSOSOL + BSG. Metagenomic study was conducted to determine the predominant bacteria and archaea, as well as its variations in their populations and their functional contributions during the AD process. The Firmicutes have been identified as playing a significant role in the hydrolysis process and the initial stages of AD. An enrichment of the most prevalent archaea genera enriched were Methanobacterium, Methanothrix, and Methanosarsina. Reactors digesting simpler substrate CW followed the acetoclastic, while digesting more complex substrates like BSG and WSOSOL followed the hydrogenotrophic pathway for biomethane production. To regulate the process for an enhanced AD process to maximize CH4, a comprehensive understanding of microbial communities is beneficial.

Unraveling the relationship between soil carbon-degrading enzyme activity and carbon fraction under biogas slurry topdressing

Biogas slurry, a by-product of the anaerobic digestion of biomass waste, predominantly consisting of livestock and poultry manure, is widely acclaimed as a sustainable organic fertilizer owing to its abundant reserves of essential nutrients. Its distinctive liquid composition, when tactfully integrated with a drip irrigation system, unveils immense potential, offering unparalleled convenience in application. In this study, we investigated the impact of biogas slurry topdressing as a replacement for chemical fertilizer (BSTR) on soil total organic carbon (TOC) fractions and carbon (C)-degrading enzyme activities across different soil depths (surface, sub-surface, and deep) during the tasseling (VT) and full maturity stage (R6) of maize. BSTR increased the TOC content within each soil layer during both VT and R6 periods, inducing alterations in the content and proportion of individual C component, particularly in the topsoil. Notably, the pure biogas slurry topdressing treatment (100%BS) compared with the pure chemical fertilizer topdressing treatment (CF), exhibited a 38.9% increase in the labile organic carbon of the topsoil during VT, and a 30.3% increase in the recalcitrant organic carbon during R6, facilitating microbial nutrient utilization and post-harvest C storage during the vigorous growth period of maize. Furthermore, BSTR treatment stimulated the activity of oxidative and hydrolytic C-degrading enzymes, with the 100%BS treatment showcasing the most significant enhancements, with its average geometric enzyme activity surpassing that of CF treatment by 27.9% and 27.4%, respectively. This enhancement facilitated ongoing and efficient degradation and transformation of C. Additionally, we screened for C components and C-degrading enzymes that are relatively sensitive to BSTR. The study highlight the advantages of employing pure biogas slurry topdressing, which enhances C component and C-degrading enzyme activity, thereby reducing the risk of soil degradation. This research lays a solid theoretical foundation for the rational recycling of biogas slurry.

Organic amendment in climate change mitigation: Challenges in an era of micro- and nanoplastics

As a global strategy for mitigating climate change, organic amendments play critical roles in restoring stocks in carbon (C) depleted soils, preserving existing stocks to prevent further soil organic carbon (SOC) loss, and enhancing C sequestration. However, recent emerging evidence of a significant proportion of micro- and nanoplastics (M/NPs) occurrence in most organic substrates (e.g., compost manure, farmyard manure, and sewage sludge) compromises its role in climate change mitigation. Given the predicted surge of soil M/NPs proliferation in the coming years, we argued whether organic amendment remains a reliable climate change mitigation strategy. Toxicity effects of M/NPs influx within the soil matrix disrupt plants and their associated key microbial taxa responsible for crucial biogeochemical processes and restructuring of SOC, leading to increasing emissions of potent greenhouse gases (GHGs, e.g., CO2, CH4, and N2O) that feedback to aggravate the rapidly changing climate. Here, we summarize evidence based on literature that the discovery of M/NPs in organic substrates compromises its role in the climate change mitigation strategy. We briefly discuss the overview of synthetic fertilizers and their impact on SOC and atmospheric emissions. We discuss the role of organic amends in climate change mitigation and the emergence of M/NPs in it. We discuss M/NPs-induced damages to SOC and subsequent emissions of GHGs. We briefly highlight management approaches to clean organic substrates of M/NPs to improve their use in agrosystems and provide recommendations for future research studies. We found that organic amendment plays pivotal role in modulating the biotic and abiotic drivers responsible for climate mitigation. However, M/NPs in organic amendments weaken the regulatory mechanisms of organic amendments in plant-soil systems. We conclude that organic amendments of soils are critical for restoring SOC and mitigating the rapidly changing climate; yet, the discovery of M/NPs in organic substrates put their usage in a dilemma.

Synergistic effect on soil health from combined application of biogas slurry and biochar

Biogas slurry and biochar, as typical by-products and derivatives of organic waste, have been applied in agricultural production to improve the soil carbon (C) pool. However, whether the combined application of biogas slurry and biochar produces synergistic effects on the soil C pool and soil health requires quantitative clarification. In this study, we performed a pot experiment to analyze the changes of soil organic carbon (SOC), potassium permanganate-oxidized carbon (POXC), mineralizable carbon (MC), soil β-glucosidase (S-β-GC), and soil protein (SP) in different treatments at the flowering and fruit-setting stages, and full fruit stage of tomato by establishing two base fertilizer modes (base fertilizer N and base biogas slurry N), three topdressing modes (topdressing chemical fertilizer N, topdressing 50% biogas slurry N + 50% chemical fertilizer N, and topdressing biogas slurry N), and two biochar levels (no addition and 3% biochar addition). During the full fruit period, the SOC content of bottom applications of biogas slurry and topdressings of biogas slurry significantly increased by 9.92-15.52% and 13.02-18.26%, respectively (P < 0.05), when compared to chemical fertilizer bottom applications and topdressings of chemical fertilizer. When compared to non-biochar treatment, the SOC content of the biochar considerably increased by 52.56-58.94% (P < 0.05). Moreover, biogas slurry treatment increased the MC, steady-state C, and C pool index, and decreased the S-β-GC, C pool efficiency, C pool activity, and C pool activity index. Application of biogas slurry initially reduced POXC, SP, the C pool management index, and the soil quality index; nonetheless, these indicators eventually recovered or even exceeded the result of single application chemical fertilizer. Overall, the combined application of biogas slurry and biochar strongly increases the soil C pool, improves soil health, and reduces the short-term negative effects of using only biogas slurry.

Application of high-intensity static magnetic field as a strategy to enhance the fertilizing potential of sewage sludge digestate

Anaerobic digestion (AD) is a sustainable and well-established option to handle sewage sludge (SS), as it generates a methane-rich biogas and a digestate with potential fertilizing properties. In the past, different strategies have been proposed to enhance the valorization of SS. Among these, the application of a static magnetic field (SMF) has been poorly evaluated. This study aims to determine the effects of a high-intensity SMF (1.5 and 2 T) on the chemical composition of SS anaerobic digestate. Several strategies (i.e., number of magnetization cycles, addition of different sources and quantities of magnesium, and digestate aeration) have been applied to evaluate the possible formation of compounds with valuable fertilizing properties in the digestate. Experimental results showed that by combining different strategies promoting digestate exposure to the magnetic field it is possible to favour the reduction in the liquid phase of NH4+, NO3-, PO43-, SO42- and Mg2+ concentrations up to 28%, 38%, 34%, 39% and 31%, respectively. The XRD analyses conducted on the solid phase of the same magnetized digestate samples showed an increase in crystalline and amorphous phases of nitrogen and phosphorus compounds with fertilizing value, such as struvite. These results highlight that SMF application can increase the fertilizing potential of sewage sludge digestate and promote its valorization in a sustainable and circular perspective.

Organic contaminants in bio-based fertilizer treated soil: Target and suspect screening approaches

Using bio-based fertilizer (BBF) in agricultural soil can reduce the dependency on chemical fertilizer and increase sustainability by recycling nutrient-rich side-streams. However, organic contaminants in BBFs may lead to residues in the treated soil. This study assessed the presence of organic contaminants in BBF treated soils, which is essential for evaluating sustainability/risks of BBF use. Soil samples from two field studies amended with 15 BBFs from various sources (agricultural, poultry, veterinary, and sludge) were analyzed. A combination of QuEChERS-based extraction, liquid chromatography quadrupole time of flight mass spectrometry-based (LC-QTOF-MS) quantitative analysis, and an advanced, automated data interpretation workflow was optimized to extract and analyze organic contaminants in BBF-treated agricultural soil. The comprehensive screening of organic contaminants was performed using target analysis and suspect screening. Of the 35 target contaminants, only three contaminants were detected in the BBF-treated soil with concentrations ranging from 0.4 ng g-1 to 28.7 ng g-1; out of these three detected contaminants, two were also present in the control soil sample. Suspect screening using patRoon (an R-based open-source software platform) workflows and the NORMAN Priority List resulted in tentative identification of 20 compounds (at level 2 and level 3 confidence level), primarily pharmaceuticals and industrial chemicals, with only one overlapping compound in two experimental sites. The contamination profiles of the soil treated with BBFs sourced from veterinary and sludge were similar, with common pharmaceutical features identified. The suspect screening results suggest that the contaminants found in BBF-treated soil might come from alternative sources other than BBFs.

Pig Slurry Management Producing N Mineral Concentrates: A Full-Scale Case Study

Manure treatment to recover nutrients presents a great challenge to delocalize nutrients from overloaded areas to those needing such nutrients. To do this, approaches for the treatment of manure have been proposed, and currently, they are mostly under investigation before being upgraded to full scale. There are very few fully operating plants recovering nutrients and, therefore, very few data on which to base environmental and economic studies. In this work, a treatment plant carrying out full-scale membrane technology to treat manure to reduce its total volume and produce a nutrient-rich fraction, i.e., the concentrate, was studied. The concentrate fraction allowed the recovery of 46% of total N and 43% of total P. The high mineral N content, i.e., N-NH₄/total-N > 91%, allowed matching the REcovered Nitrogen from manURE (RENURE) criteria proposed by the European Commission to allow the potential substitution of synthetic chemical fertilizers in vulnerable areas characterized by nutrient overloading. Life cycle assessment (LCA) performed by using full-scale data indicated that nutrient recovery by the process studied, when compared with the production of synthetic mineral fertilizers, had a lower impact for the 12 categories studied. LCA also suggested precautions which might reduce environmental impacts even more, i.e., covering the slurry to reduce NH₃, N₂O, and CH₄ emissions and reducing energy consumption by promoting renewable production. The system studied presented a total cost of 4.3 € tons^-1 of slurry treated, which is relatively low compared to other similar technologies.

Nitrogen dynamics in soils fertilized with digestate and mineral fertilizers: A full field approach

Highly stabilized digestate from sewage sludge and digestate-derived ammonium sulphate (RFs), were used in a comparison with synthetic mineral fertilizers (SF) to crop maize in a three-year plot trial in open fields. RFs and SF were dosed to ensure the same amount of mineral N (ammonia-N). In doing so, plots fertilized with digestate received much more N (+185 kg ha^-1 of organic N) because digestate also contained organic N. The fate of nitrogen was studied by measuring mineral and organic N in soil at different depths, ammonia and N₂O emissions, and N uptake in crops. Soil analyses indicated that at one-meter depth there was no significant difference in nitrate content between RF, SF and Unfertilized plots during crop season indicating that more N dosed with digestate did not lead to extra nitrate leaching. Ammonia emissions and N content in plants and grains measured were also similar for both RF and SF. Measuring denitrification activity by using gene makers resulted in a higher denitrification activity for RF than SF. Nevertheless, N₂O measurements showed that SF emitted more N₂O than RF (although it was not statistically different) (7.59 ± 3.2 kgN ha^-1 for RF and 10.3 ± 6.8 kgN ha^-1 for SF), suggesting that probably the addition of organic matter with digestate to RF, increased the denitrification efficiency so that N₂ production was favoured. Soil analyses, although were not able detecting N differences between SF and Rf after three years of cropping, revealed a statistical increasing of total carbon, suggesting that dosing digestate lead to carbon (and maybe N) accumulation in soil. Data seem to suggest that N₂O/N₂ emission and organic N accumulation in soil can explain the fate of the extra N dosed (organic-N) in RF plots.

Management of biological sewage sludge: Fertilizer nitrogen recovery as the solution to fertilizer crisis

In the current situation of a serious raw material crisis related to the disruption of supply chains, the bioeconomy is of particular significance. Rising prices and the problem with the availability of natural gas have made N fertilizers production very expensive. It is expected that due to natural gas shortages, conventional production of nitrogen fertilizers by chemical synthesis will be hindered in the coming season. An important alternative and an opportunity to solve the problems of fertilizer nitrogen availability are biological wastewater treatment plants, which can be treated as a renewable biological nitrogen mines. Sewage sludge (including activated sludge) contains up to 6-8% DM. N. Considering the quantity of sewage sludge generated in wastewater treatment plants, it can become an important raw material for the sustainable production of organic-mineral fertilizers from renewable resources available locally, with a low carbon footprint. Furthermore, the sewage sludge management method should take nitrogen retention into account and should not allow the emission of greenhouse gases containing nitrogen. This article analyzes the technological solutions of nitrogen recovery for fertilization purposes from biological wastewater treatment plants in the context of a new and difficult resource situation. Conventional and new nitrogen recovery methods were analyzed from the perspective of the current legal situation. An attempt was made to evaluate the possibility of implementing the assumptions of the circular economy through the recovery of renewable nitrogen resources from municipal wastewater treatment plants.

Effects of electrokinetic and ultrasonication pre-treatment and two-step anaerobic digestion of biowastes on the nitrogen fertiliser value by injection or surface banding to cereal crops

Biogas production from anaerobic digestion (AD) of biowastes is restricted by the recalcitrant nature of many substrates, and this may also reduce the fertiliser value of the produced digestate. The degradability of substrates can potentially be enhanced by physico-chemical pre-treatments before AD, and/or the degradation can be increased by a longer digestion time. In this study, we evaluated the effects of electrokinetic (high voltage) and ultrasonication pre-treatments of biowastes in a two-step AD process on nitrogen fertiliser replacement value (NFRV) of digestates obtained from two biogas plants with contrasting hydraulic retention time (HRT) in the primary AD step. The fertiliser value was tested by direct injection to spring barley and surface-banding to winter wheat, and the ammonium N was ¹⁵N-labelled to evaluate ammonia losses. The electrokinetic pre-treatment step significantly (p < 0.05) increased the NH₄⁺-N/total N in the digestates before the second AD step but had an insignificant effect on the fertiliser value in winter wheat and spring barley. Ultrasonication pre-treatment had also no significant effect on the fertiliser value. The two-step AD significantly (p < 0.001) increased ¹⁵N recoveries and mineral fertiliser equivalence of labelled ammonium-N in winter wheat and reduced ammonia losses, with a significant effect (p < 0.001) observed in digestates sourced from a shorter HRT biogas reactor. The fertiliser equivalence of labelled ammonium-N in the digestates was 80-88% after injection, indicating relatively low N immobilisation with all the digestates. NFRV in the crops was mainly explained by the NH₄⁺-N/total N ratio, C/N ratio and dry matter content of the digestates. The findings suggest that electrokinetic and ultrasonication pre-treatments combined with a second AD step have no considerable impact on the fertiliser value of digestates, whereas a second AD step significantly reduced ammonia losses after application by surface-banding in winter wheat.

Nitrogen dynamics and carbon sequestration in soil following application of digestates from one- and two-step anaerobic digestion

Anaerobic digestion (AD) is an important tool for reducing greenhouse gas emissions from agricultural production. A prolonged retention time by adding an extra anaerobic digestion step can be utilized to further degrade the digestates, contributing to increased nitrogen mineralisation and reducing decomposable organic matter. These modifications could influence the potential N fertiliser value of the digestate and soil carbon sequestration after field application. This study investigated the effects of prolonging retention time by implementing an additional anaerobic digestion step on carbon and nitrogen dynamics in the soil and soil carbon sequestration. Two digestates obtained from two biogas plants operating at contrasting hydraulic retention times, with and without an additional digestion step, were applied to a loamy sand soil. N mineralisation dynamics were measured during 80 days and C mineralisation during 212 days. After 80 days of incubation, the net inorganic N release from digestates obtained from a secondary AD step increased by 9-17 % (% of the N input) compared to corresponding digestates obtained from a primary AD step. A kinetic four-pool carbon model was used to fit C mineralisation data to estimate carbon sequestration in the soil. After 212 days of incubation, the net C mineralisation was highest in undigested solid biomass (68 %) and digestates obtained from the primary AD step (59-65 %). The model predicted that 26-54 % of C applied is sequestered in the soil in the long-term. The long-term soil C retention related to the C present before digestion was similar for one- and two-step AD at 12-16 %. We conclude that optimizing the anaerobic digestion configurations by including a secondary AD step could potentially replace more mineral N fertiliser due to an improved N fertiliser value of the resultant digestate without affecting carbon sequestration negatively.

Perspectives on Converting Keratin-Containing Wastes Into Biofertilizers for Sustainable Agriculture

Keratin-containing wastes become pollution to the environment if they are not treated properly. On the other hand, these wastes can be converted into value-added products applicable to many fields. Organic fertilizers and biofertilizers are important for sustainable agriculture by providing nutrients to enhance the growth speed of the plant and production. Keratin-containing wastes, therefore, will be an important resource to produce organic fertilizers. Many microorganisms exhibit capabilities to degrade keratins making them attractive to convert keratin-containing wastes into valuable products. In this review, the progress in microbial degradation of keratins is summarized. In addition, perspectives in converting keratin into bio- and organic fertilizers for agriculture are described. With proper treatment, feather wastes which are rich in keratin can be converted into high-value fertilizers to serve as nutrients for plants, reduce environmental pressure and improve the quality of the soil for sustainable agriculture.