## 4 Discussion  
The NFRV of the AS from stripped manure was similar to the CAN in this study on sandy and clay soil, for maize and grass. Only in one of the six combinations of the greenhouse experiment (clay and maize) was the NFRV significantly lower than 1. The NFRV of the AS from the processed animal manure, reported by others, was significantly lower (30-94%) [12] compared to our results, which might be explained by NH\({}_{3}\) losses due to higher temperatures, the application technique (spoke wheel and injection) or the higher pH values of the stripped AS or soil. Usually, solid ammonium sulfates have relevant NH\({}_{3}\) emissions (84 g of NH\({}_{3}\)/kg of N) [22], 11.3% (%N of applied N) [29], while lower emissions from are assumed the liquid effluent from air scrubbers (1.8% N of applied N) [29]. In the current study, the emissions from AS were near to zero, probably because the target pH for the AS at the production site was <5.5. The NH\({}_{3}\) emissions from the cattle slurry (CS), the reference for the NH\({}_{3}\) measurements, was 14% (% of TAN) (Figure 5) after 15 days at 8.5 \({}^{\circ}\)C, which corresponds well to the standard emission factor for ammonia in The Netherlands from dairy slurry applied with low-emission techniques as used in this study (applying manure in vertical slots in soil): 17% of TAN [29].  
Figure 5: Cumulative ammonia emissions (% TAN) from cattle slurry (CS) and ammonium sulfate from animal manure stripping (AS) from 1 st (started on 23 November 2023) and 2 nd experiment (started on 7 December 2023) (means are given, with the standard deviation of three repetitions). The average temperature in the 1 st period was 5.1 \({}^{\circ}\)C and in the 2 nd period it was 8.5 \({}^{\circ}\)C.  
S levels in animal feed of 3 to 4 g kg\({}^{-1}\) DM can cause toxic effects in cattle and sheep but can also have indirect effects such as copper deficiencies [30; 31; 32; 33]. S in crops is related to S fertilization at deficient nutrient levels [34; 35; 36], while it is unknown if S levels have a plateau or increase at surplus S levels (luxury consumption) [37]. With 2.1 g of S kg\({}^{-1}\) DM in the unfertilized grass in the sandy and clay soil, the fertilized grass had 3.6-5.2 g of S kg\({}^{-1}\) DM, showing the risk of AS fertilization at 298 and 326 kg of SO\({}_{3}\) ha\({}^{-1}\) for the first grass cut and 247 and 262 kg of SO\({}_{3}\) ha\({}^{-1}\) for the second grass cut (Table A3). This was far above the S fertilization advice, namely 0 and 35 kg of SO\({}_{3}\) ha\({}^{-1}\), respectively, for the clay and the sandy soil. A fertilization recommendation of \(<\)35 kg of SO\({}_{3}\) ha\({}^{-1}\), in combination with fertilization advice of 200 kg of N ha\({}^{-1}\), can only be met by using small amounts of AS in comparison to other nitrogen fertilizers. Note that the S contents in grass may already be close to the optimum. In The Netherlands, the average S levels in silage grass increased from 3 g of S kg\({}^{-1}\) DM in the period 1996-2009 [38] to 3.3 g of S kg\({}^{-1}\) in the period 2015-2019, according to routine analysis by Eurofins Agro, probably due to targeted S fertilization due to increased awareness by farmers of a decrease in S deposition [38].