## 2 Materials and Methods
### Sheltered Flux Chamber Experiment  
Ammonia emissions from fertilized grassland were studied using transparent cylindrical chambers (inside \(\approx\) 24 cm, height 32.6 cm), filled with grass sods in each chamber, located in a plastic film tunnel greenhouse that was open on both sides. The cumulative ammonia emissions of multiple products from APF were determined using a method in which 2 L of air above each grass sod was drawn at a flow rate of 2 L per minute per chamber through acid trap solutions consisting of 100 mL of 0.05 M H\({}_{2}\)SO\({}_{4}\), using methods described earlier [21]. Approximately two weeks before the start of the 1 st and 2 nd experiment on 23 November and 7 December 2023, respectively, the grass sods were taken from an unfertilized patch of grassland, near to the location of the field demonstration with grass on sand in 2021 and 2022 (Table A1), and placed in standard pots (outside \(\approx\) 24 cm, height 14 cm), filled up to a height of 13 cm (excluding grass), that were placed inside transparent cylindrical chambers (\(\approx\) 25 cm, wall thickness 5 mm, height 25 cm). Due to a lot of rain falling during the month of November 2023, the amount of water in the freshly sampled soil, including grass sods, was at field capacity.  
The tested products from the APF were: untreated cattle slurry (CS) and AS resulting from stripping the liquid fraction of CS, sampled on 27 October 2023 (composition, Table A6). The compositions of the AS and CS were determined at LUFA Nord West in Hameln, Germany (for methods, see the footer of Table A2). An amount of 8 mL of CS or 7 mL of AS, both equal to 100 kg of N ha\({}^{-1}\) (453 mg of N per chamber of 0.032 m\({}^{2}\)) were added to three chambers on day 0 (23 November 2023). Two reference chambers were used without a fertilizer. The emissions from all chambers were determined before the fertilization after 4 days, resulting in limit of quantification (LOQ) for this method when assuming ten times the standard deviation of these blank measurements: 0.05 mg of N. The acid trap solutions of 0.05 M H\({}_{2}\)SO\({}_{4}\) were sampled, weighed, and replaced by fresh solutions after 1, 4, 8 and 15 days. Each acid solution was analyzed for NH\({}_{4}\) using automated segmented flow spectroscopy (SFA) (ISO/TS-14256-1, 2003) [19] without dilution. The amount of NH\({}_{4}\) in the acid solution was multiplied with the weighted volume of around 100 mL, this gave the emissions for the amount of time that it was exposed to the acid. The sum of the sampled solutions gives the cumulative NH\({}_{3}\) emissions per product per 15 days (total time measured). The cumulative emissions were calculated per total ammoniacal nitrogen (TAN): the NH\({}_{4}\) content of cattle slurry (CS) and AS, as given in Table 10. The 1 st experiment was discontinued after 5 days when the temperature fell below freezing during the night, for which the glassware had to be emptied. The 2 nd experiment started a week later (7 December 2023). Due to time limitations, the flux chamber experiment does not exactly match the greenhouse and field experiments since the experiment solely included a grass crop on sandy soil. Moreover, NH\({}_{3}\) emissions were tested with CS as a reference and not CAN because NH\({}_{3}\) emissions from CAN are often very low [22] while the NH\({}_{3}\) emissions from cattle manure are high and often used as a benchmark [23; 24].