In order to study the effects of solid manure application on several variables, we combined lysimeter experiments with field plot experiments and laboratory soil incubations. The lysimeter experiments were included to study nutrient leaching and the field experiments to study ammonia emissions, grain yield, and soil mineral nitrogen profiles. In the soil incubations we studied the course of net N mineralisation for a greater number of manures. In all experiments, the same two soils representing clay soil and sandy soil (Table 1) and the two manure types representing a high and low C/N-ratio (Table 2) were included. Manure A, which represented a solid farmyard manure with a relatively low C/N-ratio of 10, originated from a sow stable with moderate amounts of straw bedding compared to a deep litter. Manure B represented a solid manure with a relatively high C/N-ratio of 18 and was acquired from a deep litter for dairy heifers. The lysimeter facility and one of the field experiments were located at Lanna Research Station (58°20 N, 13°7 E) and another field experiment was located at Fotegården (58°28 N, 13°14 E), 17 km north from Lanna (Figure 1). Weather data was only recorded at Lanna (Figure 2), but data from a weather station in close to Fotegården confirms that temperatures are very similar and that precipitation may be a bit higher at Fotegården. The hydrological year (July-June) 2014/2015 had unusually high precipitation in August and a milder winter than usual (Figure 2).

Two incubation experiments were performed for this study. In the first, the relations between mineral N content in soil after manure addition and the C/N-ratio of the manure was studied using thirteen different manures from various animals (cattle, pigs, sheep, horses and minks) and the loamy sand soil from Fotegården (Table 1). In the second incubation, the effects of soil type on mineral N content in soil was studied by repeating the incubation with two manures (Table 2) on the silty clay soil from Lanna (Table 1). The incubations were performed in metal cylinders that were 5 cm high and 7 cm wide. Manure corresponding to 6-ton dry matter ha^–1 was sandwiched between two layers of soil that was watered to 50 or 60% of water holding capacity on the silty clay and loamy sand, respectively. The cylinders with loamy sand were prepared by adding 125 g soil, then the manure and finally another 125 g of soil. Due to the soil structure of the silty clay, it could not be prepared in the same way. Instead, those cylinders were taken out from the field. When arriving in the lab the soil was gently pushed so that half the column entered a second cylinder, and it was then divided into two parts with a knife. After adding the manure to one of the half-filled cylinders, the other half could be returned. To obtain equal amounts of soil in all the cylinders, some soil was removed from each one until the soil weight reached 300 g. The reason for adding the manure as a layer in the middle, rather than mixing it with the soil, was to imitate when manure is incorporated by ploughing and to retain a field like structure of the silty clay.

The cylinders were placed in 10°C and sampled after 0, 7, 28 and 56 days. To prevent the soil from drying out and to also maintain an aerobic environment, the cylinders were placed on grids in closed boxes that were aerated at least twice a week. A sufficient amount of cylinders were prepared to sample three replicates at each sampling date. At sampling, the whole cylinders were poured into plastic bags and put in the freezer. The samples were milled in a frozen state and subsamples of 40 g were extracted with 100 mL 2 M KCl (Mulvaney, 1996). The ammonium (NH₄₊) and nitrate (NO₃₋) analyses were then carried out using colorimetric methods on a Seal AA3 auto-analyzer (SEAL Analytical Inc., Mequon, Wisconsin, United States).

From the results the amount of mineral N supplied from manure addition to soil was calculated by subtracting the min N content from the unmanured control. This is hereafter referred to as Net mineral N release, expressed as % of added total N. It provides a measure of potentially plant-available N from manure, and was calculated according to Equation 2: Net  mineral  N   release = Mineral   N   in   soil   with   slurry   –   Mineral   N   in   control   soil Total   N   added   with   slurry (2)

To ensure the results from these experiments were in line with models that describe nitrogen leaching after manure application, nitrogen leakage was calculated using the NLeCCS system (Johnsson et al., 2022), which includes the SOILNDB simulation tools (Johnsson et al., 2002). This system is used for calculations of leakage to report to different commissions for water protection (HELCOM, OSPAR and EEA). The model setup from the PLC6-calculations was used (Johnsson et al., 2016). The NLeCCS method calculates nitrogen leaching from arable land, using a 30-year climate data series assumed to represent normal climate. Climate conditions and agricultural management represent leaching regions. The climate series was run a total of 500 times with a crop sequence that included the crops grown in the leaching region in the proportion reported by Statistics Sweden (SCB, 2014). Also, the fertilizer regimes (Only mineral fertilization and Manure fertilization with supplementary mineral fertilization) were distributed in the crop sequence according to Statistics Sweden (SCB, 2014). The calculated leaching region was lr 5a (Johnsson et al., 2016).

Calculations of nitrogen leaching after application of manure at five different points between 30 September and 14 March, before sowing of spring barley, was made for a crop sequence where manure application occurred a sufficient number of times. Ploughing was carried out the day after manure application. Calculations were made for similar soil types (clay and sandy loam) and for manure types with similar C/N-ratios and ammonium contents as in our experiments, as well as the leaching region where the experiments were performed.

The measured increase in mineral N content in soil directly after manure addition correlated well (r² _adj = 0.92***) but was only about 80% of what was added with manure (Figure 3A). Also, after 7, 28, and 56 days there was a strong correlation (r² _adj = 0.84–0.89***), but with somewhat flatter regression lines (Figure 3A). The later measurements indicated a net immobilisation of around 25% of the added ammonium N until day 56 in the treatments with the highest amounts of ammonium added, while for the manures with initial low contents of ammonium N added there was a slight net mineralisation of up to 4% of total N added. There was not any significant correlation (r² _adj = 0.18–0.30) between the C/N-ratio of the manure and net mineral N release to soil from manure addition, although there was a tendency (p = 0.06) for exponential decay (Figure 3B). However, none of the manures with C/N-ratios above 11 resulted in mineral N contents above 15% mineral N of added total N at any time throughout the measuring period.

The nitrogen concentration in drainage water was higher during certain periods in treatments B and I, which are the treatments where manure A (with a low C/N-ratio) is applied in early autumn on loamy sand and silty clay respectively (Figure 4). In treatment B (on sandy soil), the nitrogen concentration was significantly higher compared to other treatments on that soil in sampling dates from 19 January 2015 until 31 March 2015. This means a time lag of 3.5 months between surface application in beginning of October until significant effect appeared in leachate. In treatment I (on silty clay) the nitrogen concentration was significantly higher than in the treatment without manure on that soil, and occasionally also higher than in the treatment with later manure application, during the period 13 November 2014 until 7 January 2017, which is a time lag of only 1.5 months between surface application and effect on leachate.

During the first year of measurement (1 October 2013–30 September 2014), the nitrogen concentration was unusually high in all treatments, likely due to mineralisation of dead plant material during the period after drilling up the soil monoliths. Therefore, the focus is hereafter mainly given to the two following years (1 October 2014–30 September 2016), as they could be considered to better mirror a field situation. Due to the high nitrogen concentrations in drainage water during the first year of measurements, leaching during this year was also high and ranged from 58 to 119 kg N ha^–1. The highest leaching that was measured within each soil type was in the treatments with early autumn application of manure A, but there were no significant differences within the soil types. For the two following years (Figure 5), early autumn application of manure A was significantly higher in both years on the silty clay and in the year 2014/2015 on the loamy sand. The evaluation was on average for both years 10 kg N per ha on loamy sand (p = 0.07) and 15 kg ha^–1 on silty clay (p = 0.02) compared to application in November (Figure 5).

The nitrogen leaching during the first year after manure application, as calculated according to the NLeCCS system (Table 6), was higher the earlier the manure application was carried out before sowing. Leaching was higher after manure application in the autumn and when the ammonium content of the manure was higher and the C/N-ratio lower, but any differences depending on these manure characteristics were rather small (Table 6). Differences in leaching between soil types were high with small differences concerning the effect on leaching between earliest and latest application date on the clay soil compared to on the sandy loam (Table 6).

The lysimeter study showed a similar trend of leaching from the silty clay as from the loamy sand (Figure 5). Likewise, the mineral N content in the soil profiles that were taken in late winter or early spring in the field experiments (Figure 7) reveal that nitrogen from manure applied in early autumn has moved down to the subsoil on both soil types in 2014. However, in the lysimeters, the leaching effects tended to be higher on the silty clay, whereas in the field experiments the increased mineral N content in subsoil was half as much on the silty clay as on the loamy sand. The higher transport of mineral N in the lysimeter could be due to edge effects in the lysimeter. As the soil dries, large cracks are formed in this type of soil, both in the field and in the lysimeters. However, cracking along the edges in the lysimeter may have increased this effect, which is also indicated by the quite short time lag of only 1.5 months between manure application and effect on leachate on this soil compared to 3.5 months on the lighter soil. The lighter soil was lacking macropores, and therefor had a slower response on drainage from heavy rainfall than the structured silty clay soil. Another observed difference is that on the loamy sand, there was no difference in subsoil mineral N between the two different application times in autumn, whereas on the silty clay subsoil it was only elevated after late autumn application, similar to the lysimeter experiments. The somewhat lower effect on the clay soil shown in the field experiments exhibits a closer similarity with previous observations at Lanna (Aronsson and Stenberg, 2010) and from fine textured soils in general (Hina, 2024). Since low leaching from clay soils are, at times, explained by assumed higher denitrification losses (Aronsson and Stenberg, 2010) which occur on water saturated soil (Bouwman, 1996), one could speculate that the denitrification losses may have been smaller in the lysimeters. Indeed, because they often appeared to be dry, they may never have become as saturated with water, which can be the case under field conditions when ground water level occasionally rises above the drainage depth, since that should not be possible in the freely drained lysimeters. However, Wallman and Delin (2022) found from measurements of drainage water from lysimeters in the same facility and within the same soil from the same field, that leaching levels were similar to those measured from a tile drained field facility in that very same field where the lysimeters were collected. Hence, the large effects on leaching that also occurred on the silty clay cannot be ignored.

The modelled higher leaching with earlier manure application (Table 6) was in accordance with the results with Manure A in the lysimeter study. However, as opposed to the lysimeter study where the effects of Manure B on leaching were insignificant, the effect of manure application according to the model was very similar regardless of the ammonium content or the C/N-ratio. Negligible leaching after application of manure with a high C/N-ratio, as found in the lysimeter study, is supported by previous literature (Shepherd and Newell-Price, 2016; Pedersen B. N. et al., 2021). This implies that the effect of the C/N-ratio is underestimated by the model. Another disparity between the modelled and experimental results were the differences between the soil types. According to the model, leaching effects on the clay soil were rather small compared to lighter soil which is in line with other publications (Hina, 2024), as discussed above. Considering both the literature (Hina, 2024) and the results from the field experiments (Figure 7) the modelled effects of soil type may be more representative than what was observed in the lysimeter study.

The leaching results after application of the low C/N-ratio manure were in line with those found by Thomsen (2005), who also concluded that manure applied in early autumn increased leaching losses compared to later applications that were carried out in winter or spring. However, Thomsen (2005) and Hansen et al (2004) also found that early application improved crop N utilization, which was not applicable to the results in our field experiments, in which the yield was unaffected on the loamy sand and affected in the opposite direction on the clay soil. Reduced crop yield after late compared to early autumn ploughing was, on the other hand, in accordance with results from Stenberg et al. (1999), who suggested this to be an effect of increased growth of weeds in the late ploughed treatment. That there is a temperature dependence of ammonia emissions from manure is known from previous research and the increase from less than 5% to 20%–40% with higher emissions at fine textured soils is in line with what Pedersen J. et al. (2021) presented for animal slurries with high DM content.

The results indicate that the risk for leaching after autumn application differ depending on the composition of the manure. Therefore, restrictions may only be relevant for manure with C/N-ratios below 14 or based on the amount of ammonium nitrogen content. Applying the manure late rather than early was unbeneficial for yield as opposed for leaching and current restrictions that only allow application before a certain date does not serve any purpose when it comes to the limitation of leaching. If farmers find it better to apply the manure later, it could also benefit from lower leaching.