Struvite is emerging as a circular, slow-release phosphorus (P) fertiliser. However, its effectiveness for crops and its impact on soil P tests still remain largely unclear. We analysed the effectiveness of struvite as a row fertiliser in a pot experiment with maize in a sandy soil and a loamy soil, both exhibiting a low agronomic soil P status. Maize was seeded in both soils, amended with either granular struvite or water-soluble di-ammonium phosphate (DAP) as row fertilisers. A positive control received an excess of sodium phosphate, mixed homogeneously through both soils; a negative control did not receive additional P. We measured the agronomic maize performance at the end of the vegetative growth stage (V10) and assessed the suitability of two established agronomic soil P tests (i.e. P-CaCl2 and P-AL) to predict plant-available P in the soils amended with struvite. Furthermore, we performed an additional batch experiment to better understand the dissolution dynamics of struvite during a prolonged period of extraction of soil in 0.01 M CaCl2. Placed struvite application led to a lower performance of maize in comparison to DAP in terms of biomass production as well as P uptake (53 and 71 % lower, respectively). Yet the agronomic performance of struvite in general was higher than the performance of the negative control, confirming the potential of struvite as an emerging P fertiliser. Surprisingly, both soils fertilised with struvite showed a significantly higher soil P test than any other P treatment in our pot experiment. This was an artefact due to the dissolution of residual struvite granules during the soil extraction procedures. These results call for a reconsideration of how to interpret P-CaCl2 and P-AL as a basis for P fertiliser recommendations for soils receiving struvite as a P fertiliser.

This report describes the results of a three-year field trial on sandy soil in the Achterhoek region in the Netherlands, testing the effects of an organic soil improver on soil quality and leaching of nutrients. Results show that yearly application of additional organic matter had no effect on soil organic matter content, nitrate leaching or nitrogen content of the soil.

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 application of animal manure on agricultural land in the Netherlands is bound by legal limits to prevent the leaching of nitrogen (N) and phosphorus (P) to ground and surface waters. The surplus of animal manure is transported abroad at high costs. In this study, a full-scale cascaded membrane filtration system (GENIUS) comprising two decanter centrifuges, microfiltration (MF) reverse osmosis (RO) and an ion exchanger was monitored. The system processed agricultural digestate from anaerobically co-digested animal manure into two solid fractions (SFs), RO concentrate, MF concentrate and purified water. The goal was to separate P and ammoniacal nitrogen (NH4-N) and remove water from the digestate. From the initial digestate, 66% of P was recovered in the first SF, which constituted 15% of the total mass, without the addition of iron or aluminium salts or polymer flocculants. Another 29% of P was recovered in the MF concentrate and used as a liquid organic fertiliser. Of the P in the initial digestate, 98% was removed before RO. For N, 34% ended up in the RO concentrate and this product can be regarded as an alternative for synthetic N fertiliser as it contains N solely in mineral form. Overall, around 18% of the total mass of initial digestate was discharged as purified water and 31% was locally applied in the form of RO concentrate. We found that aqua regia digestion before chemical analysis can decrease the measured S content of processed digestate. Compared to the transport of raw (unprocessed) digestate, the implementation of the GENIUS system led to a 53% reduction in the mass-weighted average transport distance.

The research was undertaken as part of the project called SYSTEMIC: 'Systemic large scale

In the Netherlands, the application of phosphorus (P) from fertilising products is regulated through application rate limits targeting equilibrium fertilisation and a neutral P status of the soil meaning that a higher P application rate limit is assigned to soils with a low P status and vice versa. Until 2021, the soil P status was derived based on one single soil P indicator namely P-AL-value for grassland and Pw-value for arable land. From 2021 onwards, the soil P status will be derived from the combination of P-AL-value(indicator for the P capacity) and P-CaCl2 –value (indicator for P intensity) enabling to differentiate based on the P buffering capacity of the soil. Data from long term field trials supply valuable data on trends in soil P indicators in relation to crop uptake and P application rate. This report assesses trends in the single P indicator (Pw-value or P-AL-value) and combined indicator (P-AL-value and P-CaCl2-value) in relation to P fertilisation rate and P uptake rate using data from long term field experiments on grassland and arable land.

Compost and other organic fertilizers contain phosphorus and their application rates should therefore comply with the P application rate standards for agricultural soil. Phosphorus in compost is exempted from this regulation for 50% (with a maximum of 3.5 kg P2O5/kg dry matter) due to the fact that compost consist partly of soil originating from garden waste. This project assessed phosphorus speciation in composts and several other organic fertilisers and relates the results to the potential availability of phosphorus for crop uptake.

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.

Humic substances are increasingly used as biostimulants in agriculture because of their supposed effects on nutrient uptake and crop yield. The objective of this study was to assess the effect of fulvic acid (FA) and humic acid (HA) addition with and without fertilization (N, P, K, S, and Ca) on (i) grass yield (Poa trivialis) and nutrient uptake and (ii) composition of soil porewater and 0.01 M CaCl2 soil extracts. Therefore, a pot experiment was performed using a loamy soil and a non-calcareous sandy soil. Fertilization increased N and P uptake and grass yield but there was no effect of FA or HA application (200 mg C kg(-1) soil) regardless of whether N or P was the growth limiting nutrient. Nutrient availability was assessed in soil pore water samples extracted by centrifugating moist soil and by 0.01 M CaCl2 extracts of ovendried soil. HA and FA had no effect on the availability of NO3 or ortho-P, neither in soil porewater nor in 0.01 M CaCl2 soil extracts. Fertilization led to an increase in 0.01 M CaCl2-extractable ortho-P but, remarkably, to a decrease in ortho-P and Dissolved Organic Carbon (DOC) concentrations in soil pore water samples. This discrepancy was explained by the higher ionic strength of pore water in the fertilized soils, which caused the pH to drop by 0.2 to 0.4 units and thereby stimulated adsorption of ortho-P and DOC to reactive soil minerals. Such salt-induced effects do not occur in 0.01 M CaCl2 soil extracts where the ionic strength is nearly constant.

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.

This final report of the four-year PPS (public-private collaboration) project ‘Meerwaarde Mest en Mineralen 2 (More Value from Manure and Minerals 2): nutrient recovery from manure’ contains the evaluation of five large-scale installations for processing of animal manure or co-digested animal manure (digestate). The different processing techniques of the installations include hygienisation, solid-liquid separation, drying and pressing of the solid fraction and production of mineral concentrate and dischargeable water by means of membrane filtration and ion exchangers or by means of membrane filtration and biological treatment. Based on a performed monitoring of the installations the achieved separation efficiencies, (nutrient) mass balances, processing costs and composition, agronomic and environmental quality of the end products, have been evaluated. Also the environmental gains of processing were calculated via a simplified life cycle assessment (LCA). Moreover, the construction, monitoring and evaluation of an innovative installation which separates the solid fraction of co-digested animal manure into a phosphate (P) fertiliser and an organic low-P soil improver was a central part of the project. Finally, recommendations are given for environmentally beneficial adaptations of manure processing installations in the Netherlands.

The research was undertaken as part of the project called SYSTEMIC: ‘Systemic large scale eco-innovation to advance circular economy and mineral recovery from organic waste in Europe’. This project has received funding from the European Union’s H2020 research and innovation programme under the grant agreement No: 730400. SYSTEMIC started on 1 June 2017 and continued for 4 years. A full list of all end products is available at www.systemicproject.eu. The SYSTEMIC project was coordinated by Oscar Schoumans (oscar.schoumans@wur.nl) and Inge Regelink (inge.regelink@wur.nl) from Wageningen Environmental Research. The NRR process consist of two independent NRR systems. In the GENIUS system, digestate is first separated into a solid (SF) and a liquid fraction (LF) of digestate by a decanter centrifuge. The SF of digestate is subsequently processed by the RePeat system. The RePeat system separates the P from the organic matter through leaching with water and sulphuric acid. Two sequential leaching steps remove in total 70–90% of the P present in the ingoing digestate, thereby producing a low-P soil improver. The dissolved P subsequently precipitates through addition of lime (Ca(OH) 2 ) or magnesium hydroxide (Mg(OH) 2 ), thereby producing precipitated P salts . Part of the sulphate, which was added as sulphuric acid, precipitates with calcium as gypsum. The gypsum partly ends up in precipitated P salts and partly in a separated organic gypsum-rich sludge which can be used as fertiliser. Water is continuously reused within the process, thereby preventing the creation of a waste stream. The LF of digestate is further processed by the rest of the GENIUS system: a second decanter centrifuge, a microfiltration (MF) unit, two RO units placed in series and ion exchangers. The following end products are thereby produced: RO concentrate, rich in N and potassium (K), purified water and a blend of the SF of the second decanter centrifuge and MF concentrate.

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.