Nutrient recovery from anaerobic digestion systems provides several side streams that are useful as biobased fertilisers (BBFs). A microcosm approach was employed to assess the short-term greenhouse gas emissions from a sandy-loam soil enriched with 18 BBFs in comparison with mineral fertilisers (urea and calcium ammonium nitrate). In total, 20 different fertilisers were homogeneously incorporated into an arable sandy loam soil at a rate of 170 kg nitrogen (N) ha -1 and incubated at 80% water-filled pore space. Over 18 days, the fluxes of nitrous oxide (N 2 O), methane (CH 4 ), and carbon dioxide (CO 2 ) released in the headspace of the microcosms were measured using a Gasera One Multi-gas analyser. Cumulative N 2 O emissions from the BBF treatments were either comparable or lower (0.04 – 0.09 %N applied) than the mineral fertilisers (0.10 – 0.14 %N applied). Nitrification of the initial ammonium-N present in the BBFs was likely the dominant biological process driving N 2 O production. The application of digestate and evaporator concentrates led to an increase in CO 2 emissions (8–51% of applied carbon (C)), mostly in the first days of the incubation. Meanwhile, the solid fraction of digestate exhibited slow mineralisation patterns (3–7 % of applied C). The variability in CO 2 respiration was strongly influenced by the availability of labile C. Fertilisation had no effect on soil-borne CH 4 emissions. Estimation of global warming potential, with respect to added N, suggests that BBFs obtained from the post-digestion treatment of digestate have a lower environmental impact compared to the unprocessed digestate due to lower N 2 O emissions.

The information compiled in this report is divided over 5 chapters.Introduction is given in Chapter 1.Chapter 2 focuses on characterisation of products collected and analysed from five SYSTEMIC demo plants.The characterisation complies information on macronutrients, micronutrients and heavy metals.In 2020, the monitoring was extended with analysis of organic pollutants (residues from pesticides, herbicides and pharmaceuticals).The content of nutrients in digestate varied between different biogas installations, according to the type and composition of processed feedstock, and biogas process conditions (e.g.organic loading rate, hydraulic retention time, temperature).Moreover, product characterisation has confirmed that application of NRR technology changes the initial composition of the treated digestate and upconcentrates nutrients in the recovered products.The number of compounds (herbicides, pesticides, and pharmaceuticals) detected varied between the plants and can be related to the feedstock of the plant.Residues of pharmaceutically active compounds were detected in digestate of AD plants where animal manure is part of the ration.However, no residues of pharmaceuticals were detected in digestate obtained by thermophilic digestion of sewage sludge.Finally, no residues were detected in purified water or ammonium sulphate solution.Chapter 3 covers nine experiments which are reported in the form of extended abstracts: as certain experiments have been published in peer-review scientific journals or are currently under review or under preparation for submission to peer-review scientific journal.The first two experiments deal with assessment of nitrogen (N) and carbon (C) mineralisation rates of raw digestate (section 3.1) and solid fraction (SF) of digestates (section 3.2) from SYSTEMIC demo plants.For raw digestates, results showed that N release and mineralisation were significantly positively correlated with the initial NH 4 + -N:total N ratio of the products and negatively correlated with total C:total N. The N mineralisation was observed for all products and humifiable fraction of C ranged from 50 to 81% for raw digestates, suggesting that these materials could be suitable candidates to increase C storage in agricultural soils.For SFs, the various SFs showed similar patterns of C mineralisation and it was concluded that the nature of the organic matter (OM) was the main factor controlling C mineralisation in the different treatments.In terms of N, some SFs may cause temporary N immobilisation.

Phosphorus (P) is a finite resource and its reuse in organic fertilisers made from biowaste and manure should therefore be encouraged. The composition of solid organic fertilisers (SOFs) depends on the type of feedstock and processing conditions, and this may affect P speciation and hence P availability. Phosphorus speciation was assessed in eighteen different SOFs produced from biowaste and digestate. Available P was determined in 10 mM CaCl2 extracts at a fixed pH of 5.5 and at a fixed total P concentration in the suspension. P was dominantly present as inorganic P (>80% of total P). There was a strong variation in the Fe content of the SOFs and hence in the fraction of P bound to reactive Fe/Al-oxides (PFe). The fraction of total P soluble at pH 5.5 correlated negatively with PFe pointing to fixation of P by metal salts added during processing, or by soil mineral particles in case garden waste was processed. Therefore, the use of iron salts in processing plants should be avoided. In addition, the presence of P in poorly soluble precipitates lowered the fraction of easily available P. Overall, this study shows that Pt alone is not a good indicator for the agronomic efficiency of SOFs due to large differences in P speciation among SOFs.

Complete separation of mixed plastic and biomass waste is a technically difficult, laborious, expensive and timeconsuming process. Hence, co-pyrolysis of these agricultural waste streams with low levels of plastic contamination presents a novel approach for the management of these plastic containing wastes, producing stable forms of carbon with potential use in environmental, agricultural and industrial applications. In this study, spent growing medium along with plastic growing bags, and bean crop residues along with mulching sheets were selected to assess how the presence of plastics would affect the characteristics of the biochars produced. These feedstocks were combined in mass ratios (of plastic in the biomass-plastic mixture) of 0, 0.25, 2.5, 5 and 10 %. The resulting feedstock underwent slow pyrolysis in a fixed bed pyrolysis reactor at a temperature of 550 degrees C to ensure complete conversion of the plastic components of the feedstock. From the results obtained from pyrolysis, low ratios of plastic were found to have a positive impact on biochar yield, while high plastic ratios were found to have negative effect. Higher level of plastic in the feedstock have resulted peculiar functional groups in the biochar, including carboxylate anions, amides and aromatic groups. Biochars produced from spent growing medium along with plastic grow bags (GM biochars) showed no phytotoxic effect, irrespective of the concentration of plastic contamination in the feedstock. Biochars produced from bean crop residues along with mulching sheets (BM biochars) on the other hand showed high level of phytotoxicity (zero germination), irrespective of level of plastic contamination. After washing all BM biochar, very low phytotoxicity levels with no statistically significant effect of plastic contamination were observed, with the exception of 10BM that showed somewhat a reduced germination rate (93 %). The results of this study will be beneficial for determining the tolerable level of plastic contamination in managing mixed agricultural waste biomass and to produce biochars suitable for environmental, agricultural and/or industrial applications.

Purpose This study investigated the C and N mineralisation potential of solid fractions (SFs) from co-digestated pig manure after P-stripping (P-POOR SF) in comparison with P-rich SFs, as a means to estimate their organic matter stability in soil. Compost (COMP) and biochar (BCHR) (made from P-POOR SF) were also included in the study as reference biosolids. Methods The SFs were incubated in a sandy-loam soil under moist conditions to determine production of CO 2 and mineral N. At specified intervals, CO 2 evolution in the mixtures was measured via the alkali trap method and titration over a period of 81 days, while mineral N was measured using a flow analyser after KCl extraction over a period of 112 days. Results The various SFs showed similar patterns of C mineralisation (15–26% of added total C in 81 days) that were clearly higher than for COMP and BCHR (6% and 7%, respectively). Temporary N immobilisation was observed in biosolids with a high C/N ratio. The effective organic matter (EOM) of the SFs was calculated based on the C mineralisation data and varied between 130 and 369 kg Mg −1 . Conclusions The SF with a reduced P content had a high EOM/P ratio which is beneficial in areas where P status of the soil is already high. Moreover, the N mineralisation patterns confirm that a high C/N ratio may also reduce risks for N leaching due to temporary N immobilisation.

Biochar prepared from waste biomass was evaluated as a soil amendment to immobilize metals in two contaminated soils. A 60-day incubation experiment was set up on a French technosol which was heavily contaminated with Pb due to former mining activities. Grass biochar, cow manure biochar (CMB) and two lightwood biochars differing in particle size distribution (LWB1 and LWB2) were amended to the soil at a rate of 2% (by mass). Rhizon soil moisture samplers were employed to assess the Pb concentrations in the soil solution at regular times. After 30 days of incubation, soil solution concentrations in the CMB-amended soil decreased by more than 99% compared to the control. CMB was also applied to a moderately contaminated Flemish soil and resulted in lowered soil solution Cd and Zn concentrations. While the application of 4% CMB resulted in 90% and 80% reductions in soil solution concentrations of Cd and Zn, respectively, the solid fraction of digestate (as a reference) reduced the soil pore water concentrations by only 63% for Cd and 73% for Zn, compared to the concentrations in the control. These results emphasize the potential of biochar to immobilize metals in soil and water systems, thus reducing their phytotoxicity.

This chapter focuses on the potential application of biochar in the context of risk-based management of land contaminated with moderate concentrations of potentially toxic metals and metalloids. It starts by providing a brief introduction on the chemistry and behavior of trace elements in soil, and moves on to the issues related to diffuse contamination, where relatively low contamination over large areas of land render the use of conventional remediation techniques prohibitively expensive and warrants risk-based land management. After elaborating on the effects of pyrolysis conditions on biochar-sorption properties, interactions between biochar and soil properties are explored. Next, the potential for using biochar in phytomanagement and to control uptake of hazardous trace elements in agriculture is discussed. A final section focuses on the relatively limited field experience that is available today.

To determine the long-term impact of organic amendments on metal (Cd and Zn) immobilization, soil from the Campine region was amended with holm oak-derived biochar, compost, and peat, and monitored over a 3-year period. Pot experiments were conducted by mixing the amendments independently at 2% and 4% (g/g) with the soil. The mobility and solubility of metals in the treatments were assessed by means of rhizon soil moisture samplers, sequential BCR extractions, and diffusive gradient in thin films (DGT). Over the three-year period, the 2% biochar addition resulted in an average decrease in pore water concentration of 40% for Cd and 48% for Zn whereas the 4% addition led to an average decrease of 66% for Cd and 77% for Zn. The immobilization effect in the biochar treatments was attributed to the consistently higher pH and lower concentrations of dissolved organic carbon (DOC) in the soil. The latter may have been caused by sorption of DOC onto the surface of biochar thereby increasing its negatively charged functional groups that are able to sorb cations. On the other hand, compost and peat had the unwanted effect of significantly increasing the concentrations of Cd and Zn in the soil pore water. This was partly due to the formation of soluble organo-metallic complexes as significantly higher DOC concentrations were found in the compost and peat treatments. Results from the DGT measurements, after a 24 h deployment time, revealed a low resupply (R <= 0.4) of Cd and Zn from the solid phase to the soil solution in both amended and unamended soil. This suggests a case of slow metal desorption kinetics in the soil that was relatively unchanged by the presence of organic amendments. (C) 2018 Elsevier B.V. All rights reserved.

Soil in the vicinity of smelter factories are to some extent significantly contaminated by heavy metals. The incorporation of organic amendments to stabilize metal concentrations in such soils may provide a long-term and cost-effective remediation solution. In a three-year study, the potential of holm oak-derived biochar (pyrolysed at 650°C) for the immobilization of Cd was assessed in a smelter impacted sandy soil. Pot experiments were prepared with the soil-biochar mixture and monitored by means of rhizon soil moisture samplers (SMS) and diffusive gradient in thin films (DGT). Analyses of pore water samples obtained by rhizon SMS showed that biochar significantly reduced the concentration of soluble Cd to 10 µg/l, compared to 120 µg/l at the start of the experiment. The Cd immobilization effect was relatively stable throughout the three years of the study, suggesting that aging did not significantly affect the sorption capacity of biochar. This observation was attributed to the consistently alkaline pH and lower concentration of dissolved organic carbon (DOC) in the biochar treatments. The latter may have been caused by sorption of DOC onto the surface of biochar, thereby increasing its negatively charged functional groups that are able to sorb cations. For comparison, compost and peat significantly increased the Cd concentrations in the soil pore water. This was due, in part, to the high concentrations of DOC in the compost and peat treatments, which may have caused the formation of soluble organo-metallic complexes. DGT measurements were taken in the third year to gain insight into the kinetics of metal resupply from the solid phase to soil solution. The results revealed that the addition of biochar, compost, or peat had no significant effect on the inherently slow metal desorption kinetics of the contaminated soil. In conclusion, these results highlight the superiority of biochar over other organic amendments for metal immobilization, however, it also raises questions about influence of biochar on the solid phase buffering of metals in soil solution. Further studies are required to better understand the kinetics of metal resupply from the solid phase in biochar amended soils.

Soil degradation due to metal contamination is a major environmental concerns threatening the sustainability of agricultural production. The use of biochar has emerged as a potentially viable and affordable strategy for the long-term stabilization of metal concentrations in such soils. The potential of holm-oak derived biochar to immobilize Cd in a diffusely contaminated soil in the Campine region of Belgium was investigated. Incubation experiments were prepared with the soil-biochar mixture and Cd concentration was monitored by means of rhizon soil moisture samplers (SMS) over a period of three years. Over the three years of incubation, concentrations in the soil solution tended to continue to decrease, although to a limited extent. Biochar strongly reduced the concentration of soluble Cd from 120 µg/l at the start of the experiment to 10 µg/l. The mechanisms responsible for observation were suggested to be the formation of metal precipitates due to increase in pH and the sorption of DOC onto the surface of biochar which increases its negatively charged functional groups that are able to sorb cations. For comparison, addition of compost or peat to the contaminated soil led to a significant increase in the soluble Cd concentration. This was likely due to a negative or a complete absence of a pH effect from the addition of the amendments. Furthermore, the high concentrations of DOC in the compost and peat treatments may have caused the formation of soluble organo-metallic complexes. The results from this study highlight advantages of biochar over other organic amendments for metal immobilization in soils. However, there are still unanswered questions about the long-term stability of biochar’s metal immobilization effect. For example, what are the impacts of changing soil physico-chemical conditions on metal mobility? Further studies are required to explore the interaction mechanisms among biochar, soil and metals over more extended periods of time.