## 2 Materials and Methods
### Greenhouse Experiment  
The greenhouse experiment was set up to determine the NFRV of AS (6% N, 7% S) for maize (_Zea mays_ L.) and grass (_Lollium perenne_ L.) on two soil types (sand and clay, both from Wageningen, The Netherlands): the soil types, coordinates and analysis are given in Table 14. Furthermore, 5 rates of fertilizer were used based on recommendations determined by routine soil analysis [14], including fertilization advice to farmers [15]. The routine laboratory (Eurofins Agro, Wageningen, The Netherlands) recommended a N fertilization rate of 120 kg of N ha\({}^{-1}\) for grass (first grass cut) and 165 kg of N ha\({}^{-1}\) for maize. Five fertilization rates were used in the greenhouse experiment: 0%, 50%, 75%, 100%, and 125% of Eurofins advice. CAN (26% N) was tested at all rates of fertilizer, AS was tested at 50% and 75% to focus on the fertilizer levels at which a fertilizer response is expected. Gypsum (24% S) was added to the CAN treatments to make the S dosage equal to that of the AS treatments (Table 15). The trial was performed in triplicate, which resulted in 84 pots. Both fertilizers were applied via low-emission methods respective to their type: CAN pellets via broadcast application and AS via injection. The soils were established and maintained gravimetrically at 60% water holding capacity by daily watering. The samples of AS used for all the experiments were freshly obtained from APF and stored at 4 \({}^{\circ}\)C before use. The samples were analyzed by LUFA Nord West in Hameln, Germany, using methods given below in Table 12.  
The grass was cut at 5 cm above the soil surface 23, 63 and 113 days after the fertilization. The maize was cut at 3 cm above the soil 41 days after fertilization. The fresh weight of the maize and each grass cut was determined, and the dry matter was determined after drying for 48 h at 70 \({}^{\circ}\)C. After the cuts, the plant samples were dried at 70 \({}^{\circ}\)C, and the N content of the dry matter was determined after digestion with H\({}_{2}\)SO\({}_{4}\)/H\({}_{2}\)O\({}_{2}\)/Se via SFA analysis [16], and the soil mineral N content (for methods see the footnote of Table 13) was determined after removing the plant roots. The CH\({}_{4}\) and N\({}_{2}\)O measurements during the greenhouse experiments were executed using a photo-acoustic infrared gas analyzer (Innova 1512, (LumaSense Technologies A/S, Ballerup, Denmark). CH\({}_{4}\) and N\({}_{2}\)O were measured 10 times during the first 31 and 38 days after fertilization (for grass and maize, respectively). The pots were covered with a polyvinyl chloride (PVC) flux chamber, to create a closed headspace, 30 min before the measurement [17]: a flux chamber of 8 L and a height of 20 cm was used to completely enclose the grass and small maize plants. Cumulative emissions (over 31 (grass) or 38 (maize) days; in \(\mu\)g m\({}^{-2}\)) of N\({}_{2}\)O and CH\({}_{4}\) were calculated according to the method [17], assuming linear emissions during the closure time and a soil surface area of 0.0314 m\({}^{2}\) in the pot. For the NH\({}_{3}\) emission measurements, we used acid traps during the greenhouse experiment. To do so, extra pots were added to completely replicate all treatments. After a closure time of 10 days, these pots were discarded because of the potential damage to the plants resulting from the long closure period. Closed-chamber acid traps [18] were used to measure emissions of NH\({}_{3}\) in the first 10 days after fertilization. The acid traps were changed twice, resulting in two measurements per pot over 10 days, and the total emissions across the two measurements were further analyzed for the results. For the measurement,a 100 mL vessel with 50 mL of 0.5 M H\({}_{2}\)SO\({}_{4}\) was placed on the soil, and a PVC chamber (8 L) was placed over each pot. Each acid solution was analyzed after 1:10 dilution using automated segmented flow spectroscopy (SFA) [19]. The total NH\({}_{3}\) emissions (g pot\({}^{-1}\)) were calculated by adding up the emissions from each of the measurements.