Slurry Acidiﬁcation as a Solution to Minimize Ammonia Emissions from the Combined Application of Animal Manure and Synthetic Fertilizer in No-Tillage

: The combined application of manure/slurry and synthetic fertilizer (SF) might be a solution to decrease transport and application costs involving those by-products as well as enable access to them in regions where availability is low. Moreover, their joint application can potentially reduce environmental pollution, enlarge the manure beneﬁts to more areas, and enhance the SF efﬁciency. However, such a strategy might result in increased ammonia emissions when applied to crop residues. Two experiments were implemented to assess ammonia emissions from stubble-covered soil fertilized with manure amended with SF. In Experiment 1 (E1), urea (U) and calcium ammonium nitrate (AN) were applied combined with dairy manure (MAN). In Experiment 2 (E2), urea was combined with acidiﬁed pig slurry (APS) and applied just after sowing (T0) or eight days later (T8). The combinations U + MAN and AN + MAN increased the ammonia emissions, while APS decreased the emissions from U, in APS + U combination, by more than 75%. Therefore, manure combined with SF applied on stubble-covered soil should not be recommended. T8 reduced ammonia emissions from U. APS enhanced the efﬁciency of U, being then an interesting strategy to mitigate ammonia emissions when applied on stubble-covered soil, as in no-tillage.


Introduction
Most of the nitrogen (N) used in crop nutrition comes from mineral/synthetic fertilizers, of which urea is the major source [1][2][3]. On the other side, the use of manure as fertilizer contributes to the increase in soil organic matter and adds beneficial microbes, besides also delivering N and other elements to plants [4]. Nevertheless, the transport costs within and between farmlands and greater application rates required in relation to synthetic fertilizers (SF) are some of the limitations related, particularly with slurry fertilization [5].
Slurry acidification before field application became popular in some regions due to its potential to decrease ammonia (NH 3 ) emissions, even after surface application without soil incorporation or injection [6,7]. Thus, slurry acidification may be an attractive option, especially in no-tillage. Moreover, it can be hypothesized that the acidified slurry, due to its ability to reduce ammonia volatilization, might enhance urea efficiency when mixed before application to the soil.
The NH 3 volatilized from livestock manure and SF is a significant loss of reactive N and also represents a threat to human health by reacting with acidic compounds in the atmosphere, resulting in fine particles (PM 2.5) emissions [8,9]. Moreover, the NH 3 -N drift may exceed the critical N load, negatively affecting ecosystems [9][10][11]. The nitrogen loss from fertilizer, by ammonia volatilization, may decrease the N fertilizer efficiency.
The manure was collected from the storage tank of a commercial dairy farm situated in the Setubal region, Portugal. First, 2 kg of dry clay loam soil (Vertic soil) with 50.5% sand, 20.9% silt, and 28.6% clay was used to fill each pot, and the soil surface was covered by wheat stubbles (300 g m −2 ). Some characteristics of the soil and manure can be found in Table 1. Table 1. Soil and dairy manure parameters (mean, n = 3).

Parameters Soil Dairy Manure
Total Nitrogen (g kg −1 ) 1.70 11.50 * NH 4 -Nitrogen (g kg −1 ) 0.01 3.86 * P (g kg −1 ) 0.21 ** 8.05 * K (g kg −1 ) 0. 40  The experiment was performed, with six treatments, in a completely randomized design that was replicated three times: Control/Unfertilized soil (N0); soil application of Dairy manure (MAN), Urea (U), Calcium ammonium nitrate (AN), Urea + Dairy manure (UMAN), and Calcium ammonium nitrate + Dairy manure (ANMAN). The manure, synthetic fertilizers, and their mixture were applied by hand on the stubbles at a rate of 0.5 g of total nitrogen (Total-N) per pot (0.25 g N/kg dry soil). For the manure/synthetic fertilizer combinations, the mixture was performed right before the application, and each component contributed to 50% of total-N. At first, soil humidity was set up at 60% of soil water-holding capacity by watering with deionized water before fertilizers application and on the fourth day. The urea (U) contained 46% of total-N, and the calcium ammonium nitrate (AN) contained 27% of total-nitrogen plus 4% of calcium oxide.

Experiment 2 (E2)
In the second experiment, pig slurry and sandy soil were used, in order to increase the potential for ammonia emissions. We hypothesized that ammonia emissions could be decreased by the slurry acidification, even if the slurry is mixed with urea, and by the postponed fertilization. Thereby, acidified pig slurry, urea, and the combination of both were evaluated concerning ammonia emissions after application on stubble-covered soil. The application was performed at two different times concerning the seeding.
A Haplic arenosol (texture characteristics: 92.2% sand, 4.5% silt, and 3.3% clay) was used to fill the pots (2 kg pot −1 ). The soil surface, in each pot, was covered by wheat stubbles (300 g m −2 ). The treatments, three times replicated, in a factorial design, consisted of the following: Control/Unfertilized soil (N0); Urea (U) applied at T0 or T8, Acidified pig slurry (APS) applied at T0 or T8, Acidified pig slurry + Urea (APSU) applied at T0 or T8, being T0: application at sowing, and T8: application 8 days after sowing. Oat seeds were sown in a nursery 8 days before the experiment started for the T8 treatment. On day 0, for the T0, 12 oat seeds were sown, in two rows per pot, and then, the soil was covered by stubbles. On the same day, for the T8, 10 newly emerged oat plants aged 8 days were transplanted in two rows per pot in the soil, and the stubbles were carefully placed close to the plants. After that, the fertilizers were applied in bands by hand along the sides of the rows over the stubbles in order to provide, in each pot, 0.5 g of total nitrogen (total-N). For the APSU, the mixture was executed right before the application, and each component contributed to 50% of the total-N. The pig slurry, from the storage tank of a regular commercial pig farm, was acidified with concentrated sulfuric acid, targeting a pH value of 5.5. Soil humidity was maintained at around 70% of soil water-holding capacity throughout the experimental period. The urea fertilizer used contained 46% of total nitrogen. The main soil and pig slurry characteristics are summarized in Table 2.

Analytical Analysis
Manure and stubble were dried at 105 • C, to constant weight, to determine the dry matter, and the organic matter (OM) content was obtained by loss on ignition after calcination at 550 • C (4 h). The pH was determined directly using a pH meter (Orion 3 star). Total N was obtained by Kjeldahl acid digestion, while the ammonium N concentration was determined directly, by distillation and titration [21]. Potassium (K) content was quantified after hydrochloric acid (HCl) treatment of the ash through graphite furnace atomic absorption spectrophotometry (Unicam M Series), and P was determined using the ammonium vanadomolybdate method [22] by molecular absorption spectrophotometry (Hitachi 2000).
For the determination of the soil texture, by sedimentation, the pipette method was used [23]. Organic carbon (OC) was obtained by catalytic oxidation of the sample at 1100 • C followed by CO 2 detection by NDIR in a Primacs TOC Analyzer (Skalar Analytical B.V., Breda, NL, USA), and the organic matter (OM) value was achieved assuming OM contains 58% OC [24]. The pH, in a soil:water (1:2.5 w/v) suspension, was verified using a pH meter after stirring. The Kjeldahl method was used to obtain total N [21]. The determination of mineral N content was performed by extraction of 6 g of soil in 30 mL of KCl solution (2 M) [25] followed by ammonium quantification through molecular absorption spectrophotometry in a SanPlus (Skalar Analytical B.V., Breda, NL, USA) segmented flow analyzer, respectively, by the Berthelot method and with the Griess-Ilosvay reagent [26]. K and P content were obtained according to the Egner-Riehm method [27].

Ammonia Measurements
The NH 3 emissions were gathered from each pot covered by a PVC chamber (0.035 m 2 area) after the application of the fertilizers, for 7 days in E1 and 8 days in E2, through a dynamic chamber system, as decribed by [11] (Figure 1). A continuous airflow rate of 3 L min −1 produced from a suction pump regulated by a needle valve was kept inside the chambers. Each chamber was linked to an acid trap containing 200 mL of orthophosphoric acid (0.05 M) to collect the ammonia emitted. For E1, the acid solution in each trap was substituted after 4, 8, and 12 h in the first 24 h, twice a day on days 2 and 3 and then every 24 h until day 7. For E2, the acid solution in each trap was substituted after 4, 8, and 12 h in the first 24 h, twice a day on day 2, and then every 24 h until the eighth day. The total ammonia N content in the solution of each acid trap, at the end of each sampling period, was analyzed by automated segmented-flow spectrophotometry [28]. NH 3 emission rates (E, mg N m −2 h −1 ) for each sampling period were calculated according to Equation (1).
where TAN = Total ammoniacal nitrogen concentration of the acid solution (mg L −1 ), V = Volume of acid solution (L), S = Soil surface area (m 2 ), and t = Duration of sampling period (h). Total ammonia emissions were also presented as the sum of the amount of NH 3 emitted in each time interval and as percentage of total nitrogen and percentage of NH 4 -N applied.
where TAN = Total ammoniacal nitrogen concentration of the acid solution (mg L −1 ), V = Volume of acid solution (L), S = Soil surface area (m 2 ), and t = Duration of sampling period (h). Total ammonia emissions were also presented as the sum of the amount of NH3 emitted in each time interval and as percentage of total nitrogen and percentage of NH4-N applied.

Statistical Analysis
The data collected were subjected to analysis of variance (one-way ANOVA in E1 and two-way ANOVA in E2). The least significant differences (LSD) test was used to compare means at p < 0.05 in E1. For the E2, the Tukey test was used to compare means, at p < 0.05, and log-transformations were performed to ensure the normality and homogeneity of the variances. The analyses were carried out using the software Statistix, version 9.

Experiment 1
The cumulative NH3 emissions expressed as mg NH3-N per pot, as percentage of Total-N and NH4-N applied, and also the differences between the observed and expected emissions, can be found in Table 3. The highest ammonia emissions (p < 0.05) were observed when manure was amended with both SF. Table 3. Cumulative NH3 emission (mg NH3-N pot −1 ) as a percentage of total-N and NH4-N applied (excluded control emissions). The expected NH3-N emissions considering the contribution of 50% from each one of the components of the mixture (U + MAN or AN + MAN) and the percentage of observed emission increase compared to expected. Values followed by different letters, in the same column, are significantly different based on the LSD test (mean, n = 3).

Statistical Analysis
The data collected were subjected to analysis of variance (one-way ANOVA in E1 and two-way ANOVA in E2). The least significant differences (LSD) test was used to compare means at p < 0.05 in E1. For the E2, the Tukey test was used to compare means, at p < 0.05, and log-transformations were performed to ensure the normality and homogeneity of the variances. The analyses were carried out using the software Statistix, version 9.

Experiment 1
The cumulative NH 3 emissions expressed as mg NH 3 -N per pot, as percentage of Total-N and NH 4 -N applied, and also the differences between the observed and expected emissions, can be found in Table 3. The highest ammonia emissions (p < 0.05) were observed when manure was amended with both SF. Table 3. Cumulative NH 3 emission (mg NH 3 -N pot −1 ) as a percentage of total-N and NH 4 -N applied (excluded control emissions). The expected NH 3 -N emissions considering the contribution of 50% from each one of the components of the mixture (U + MAN or AN + MAN) and the percentage of observed emission increase compared to expected. Values followed by different letters, in the same column, are significantly different based on the LSD test (mean, n = 3).

Treatments
Cumulative NH  MAN treatment lost 12.5% of total-N applied (36.5% of applied TAN) by ammonia volatilization, which was an amount significantly greater than those observed in SF (U and AN) but lower than N lost from UMAN (23.4%) and ANMAN (19.3%). Initially, the expected emissions from UMAN and ANMAN were the sum of the contribution of every single fertilizer (50% of NH 3 emission from MAN plus 50% of NH 3 emission from SF). However, the observed NH 3 -N emissions from UMAN and ANMAN were much higher than the expected emission. As a synergistic effect, UMAN emitted 165.8% more than the expected emissions from the relative contribution of U and MAN, and ANMAN also emitted 182.2% more than the relative contribution of AN and MAN.
The daily ammonia emission rates from all fertilizers peaked on the first day, which was followed by a reduction to negligible levels after the fourth day, except for U, which only reached the peak on the third day. Yet on the seventh day, U emitted more NH 3 , albeit in a small quantity, than the other fertilizers. The first and second highest peaks were reported in UMAN and ANMAN, respectively ( Figure 2). In Figure 3a,b, the dynamics of NH 3 -N daily emissions, expressed as the percentage of NH 3 -N daily emission rates in relation to the total emissions (100%), are presented. UMAN, ANMAN, and MAN had most of their emissions (≈80%), in the two first days. Diversely, the ammonia emissions from AN, U, and control were much less intense, namely U that only reached half of its total NH 3 -N emission on the fourth day.
MAN treatment lost 12.5% of total-N applied (36.5% of applied TAN) by ammonia volatilization, which was an amount significantly greater than those observed in SF (U and AN) but lower than N lost from UMAN (23.4%) and ANMAN (19.3%). Initially, the expected emissions from UMAN and ANMAN were the sum of the contribution of every single fertilizer (50% of NH3 emission from MAN plus 50% of NH3 emission from SF). However, the observed NH3-N emissions from UMAN and ANMAN were much higher than the expected emission. As a synergistic effect, UMAN emitted 165.8% more than the expected emissions from the relative contribution of U and MAN, and ANMAN also emitted 182.2% more than the relative contribution of AN and MAN.
The daily ammonia emission rates from all fertilizers peaked on the first day, which was followed by a reduction to negligible levels after the fourth day, except for U, which only reached the peak on the third day. Yet on the seventh day, U emitted more NH3, albeit in a small quantity, than the other fertilizers. The first and second highest peaks were reported in UMAN and ANMAN, respectively (Figure 2). In Figure 3a,b, the dynamics of NH3-N daily emissions, expressed as the percentage of NH3-N daily emission rates in relation to the total emissions (100%), are presented. UMAN, ANMAN, and MAN had most of their emissions (≈80%), in the two first days. Diversely, the ammonia emissions from AN, U, and control were much less intense, namely U that only reached half of its total NH3-N emission on the fourth day.

Experiment 2
The analysis of variance demonstrated a significant effect from the fertilizer type, application time and interaction between fertilizer type and application time. The cumulative NH3 emissions expressed as mg NH3-N per pot, as percentage of Total-N and NH4-

Experiment 2
The analysis of variance demonstrated a significant effect from the fertilizer type, application time and interaction between fertilizer type and application time. The cumulative NH 3 emissions expressed as mg NH 3 -N per pot, as percentage of Total-N and NH 4 -N applied, and also the differences between the observed and expected emissions from APSU at T0 and T8 as well as the two-way ANOVA results are displayed in Table 4. Table 4. Cumulative NH 3 emissions (mg NH 3 -N pot −1 ) as a percentage of total-N and NH 4 -N applied (excluding control emission), two-way ANOVA, the expected NH 3 -N emissions considering the contribution of 50% from each one of the components of the mixture (APS + U at T0 and T8), and the percentage of emission reduction observed compared to expected. Values followed by different letters, in the same column, are significantly different based on the Tukey test (mean, n = 3).

Treatments
Cumulative NH  The highest cumulative ammonia emissions were reported in urea treatment (p < 0.001) with values four to 10 times higher than in APSU. The time of application influenced significantly ammonia emissions from U. The urea application at T8 reduced the NH 3 -N emissions and consequently the nitrogen losses by half relative to T0. While the mixture of non-acidified manure and SF led to an increase in ammonia emissions (Experiment 1, Table 3), the combined application of U and APS led to a decrease in NH 3 emissions compared to urea treatment, regardless of the application time (Table 4, Figure 4). The observed emission from APSU was much lower than the expected emission (sum of 50% of APS emission and 50% of the U emission), 79.8% and 52.5%, respectively at T0 and T8. APS, as expected, led to the lowest ammonia emissions from fertilized soil, resulting in negligible nitrogen losses (≈0.2%) of the total-nitrogen applied), independently of the application time (Table 4, Figure 4).
The daily ammonia emissions rates from UT0 and UT8 peaked 4 days after application, keeping the trend observed in the previous experiment. APSU T0 and APSU T8 emissions reached their plateau from day 2 to day 4. APS at both application times kept its daily emission rates at very low levels, close to N0 (Figure 4).
Despite APSU emitting significantly less ammonia than U at both application times, the release of NH 3 from APSU was more intense in the first 4 days followed by a decrease in emissions rate, while urea released more than 80% of the total NH 3 after day 3 (Figure 5a,b). On the fourth day, despite marking the beginning of the decline in emission intensity, APSU T0 and APSU T8 emitted at least 25% more NH 3 -N than any other treatment. The dynamics of APS emissions (T0 and T8) behaved similarly to those of N0. sions compared to urea treatment, regardless of the application time (Table 4, Figure 4). The observed emission from APSU was much lower than the expected emission (sum of 50% of APS emission and 50% of the U emission), 79.8% and 52.5%, respectively at T0 and T8. APS, as expected, led to the lowest ammonia emissions from fertilized soil, resulting in negligible nitrogen losses (≈0.2%) of the total-nitrogen applied), independently of the application time (Table 4, Figure 4). The daily ammonia emissions rates from UT0 and UT8 peaked 4 days after application, keeping the trend observed in the previous experiment. APSU T0 and APSU T8 emissions reached their plateau from day 2 to day 4. APS at both application times kept its daily emission rates at very low levels, close to N0 (Figure 4).
Despite APSU emitting significantly less ammonia than U at both application times, the release of NH3 from APSU was more intense in the first 4 days followed by a decrease in emissions rate, while urea released more than 80% of the total NH3 after day 3 ( Figure  5a,b). On the fourth day, despite marking the beginning of the decline in emission intensity, APSU T0 and APSU T8 emitted at least 25% more NH3-N than any other treatment. The dynamics of APS emissions (T0 and T8) behaved similarly to those of N0.

Discussion
Notably, the combined application of MAN and U or MAN and AN led to a greater amount of NH3-N emitted than the sum of emissions from each of the components separately (Table 3). UMAN emitted 4.47 and 1.87 times more NH3-N than U and MAN, respectively. ANMAN emitted 16.4 and 1.54 times more than AN and MAN, respectively.
Ammonia emissions from manure, because of its alkaline pH and TAN content, normally exceed those from synthetic fertilizers [29], although ammonia emissions from manure fertilization, according to [18], are not as well comprehended as the emissions from the slurry. Therefore, the low cumulative NH3-N emission from MAN and low percentage

Discussion
Notably, the combined application of MAN and U or MAN and AN led to a greater amount of NH 3 -N emitted than the sum of emissions from each of the components separately (Table 3). UMAN emitted 4.47 and 1.87 times more NH 3 -N than U and MAN, respectively. ANMAN emitted 16.4 and 1.54 times more than AN and MAN, respectively.
Ammonia emissions from manure, because of its alkaline pH and TAN content, normally exceed those from synthetic fertilizers [29], although ammonia emissions from manure fertilization, according to [18], are not as well comprehended as the emissions from the slurry. Therefore, the low cumulative NH 3 -N emission from MAN and low percentage of total-N lost as ammonia might be justified by its low NH 4 -N content and the propensity to form surface crust because of its low water content [10,30].
The highest amount of ammonia emitted from manure amended with SF is possibly justified primarily by the manure pH, which favored the dominance of NH 3 over NH 4 as described by [8,[31][32][33]. In addition, as presented in Table 3, the observed NH 3 -N emissions from UMAN and ANMAN were much greater than expected, respectively 165.8% and 182.2% higher. Additionally, we can hypothesize that the highest and more intense emissions from UMAN (Figures 2 and 3a,b) are likely determined primarily by the contact between U and urease enzyme present in manure, in a medium wetter than the soil, favoring the conversion from U to NH 3 and subsequent high-intensity emission stimulated by the alkaline medium. The manure pH appears to be the trigger to the increase in ANMAN's ammonia emissions as well. Thereby, when mixed, the manure potentiated the emissions from U and AN.
The observed low NH 3 volatilization from calcium ammonium nitrate, which contains 50% of the N in nitric form, is in accordance with data from [11,34]. The N loss from urea, in E1, was below expectations, even being in the range of values reported by [34,35]. In addition, an initial lag phase was detected in the NH 3 -N daily emission rates from U in both experiments (Figures 2 and 4), influencing its dynamics (Figures 3b and 5a), which was mostly due to the time needed to convert urea into ammonia through the urease enzyme [18,35,36].
As discussed above, the high manure pH is likely the main cause for the surprising increase in the ammonia emissions from the applied mixture of raw dairy manure and mineral/synthetic fertilizers, especially the urea, on the soil covered with crop residues. Nevertheless, this assumption was evaluated in a subsequent experiment (E2) wherein urea was added to an acidified slurry (pH 5.5) and applied on stubble. In this case, the NH 3 emissions from urea in a low pH medium (acidified slurry) were assessed in more challenging conditions, which should potentially lead to higher NH 3 emissions, namely sandy soil (low cation exchange capacity) and pig slurry (low dry matter and high N-NH 4 content). Moreover, oat plants were included in this new experiment to allow assessing the influence of the application time on the amount of ammonia emitted, hoping to broaden the range of understanding over the ammonia emissions from organic-mineral fertilizer applied to stubble-covered soils.
The urea, in E2, was the fertilizer that emitted the greatest amount of ammonia (p < 0.001), which was significantly more than APSU, APS, and N0 ( Table 4). The soil surface application of urea, without soil incorporation, commonly represents a substantial amount of nitrogen lost as ammonia emission, especially when the soil is covered by crop residues [8,19,37]. Although there was an obvious reduction of emissions from the fourth day (Figure 5a), it is noticeable that the eight-day trial was not enough to assess the total amount of NH 3 -N emitted from urea applied on stubble-covered soil; therefore, the total cumulative ammonia emissions from U should be greater than the values reported here (Table 4, Figure 4). This point does not limit the value of our results or conclusions, but on the contrary, it reinforces the ability of APS to reduce ammonia emissions as well as its capability to enhance urea efficiency.
A meaningful decrease in the cumulative NH 3 emissions was observed when urea was mixed with the acidified slurry. The acidification of the pig slurry provided a low pH medium that acted on the balance of the TAN, favoring the predominance of NH 4 + over NH 3 and limiting ammonia emissions, even when mixed with urea. Consequently, the acidified pig slurry amendment reduced the susceptibility of urea to nitrogen losses by ammonia volatilization. Despite sulfuric acid already being used at a farm scale, promising additives should emerge from several studies [38,39], helping to make slurry acidification more viable by decreasing the risk associated with the handling of hazardous products as well as the cost involved.
The ammonia emissions observed in E2 were affected by the application time (T0, T8), fertilizer type (U, APS, APSU), and by the interaction of these two factors (Table 4).
Regarding the application time, the maximum amount of emitted ammonia (p < 0.05) occurred when U was applied right after the sowing (T0), which was probably because in T8, the plants were already able to uptake nutrients from the soil, namely nitrogen [29]. Therefore, some portion of ammonia might have been taken from the soil before it had the chance to be volatilized. This might be an indication that the application of the urea and ammoniacal sources of nitrogen (organic and synthetic) after plants' emergence can reduce ammonia emission, having the potential to increase the nitrogen use efficiency. Synchronizing N fertilization with plant nutrient demands may improve nitrogen availability for crops while decreasing the amount of N lost through gas emissions and leaching [13,20,29]. In addition, the post-seeding fertilizer application, besides delivering nutrients to the plants in a more opportune time to their needs, can also alleviate pressure on the farm's schedule such as the distribution of tasks, especially on the use of machinery [13]. Notwithstanding that the ammonia volatilized from fertilizers applied to soil can be absorbed in small quantities by plant leaves [40], it is unlikely to have occurred due to the unrepresentative leaf area of the oat plants in the first 8-16 days after sowing. The application time did not significantly affect the ammonia emissions from APSU and APS, which was likely because the lower levels of ammonia emissions from those treatments made it difficult to spot this trend. Despite the plant analysis being outside of the scope of this experiment, it is important to report the absence of any apparent damage in the oat plants caused by the band application of acidified pig slurry, urea, or the joining of these two fertilizers after plant emergence. On a field scale, the application of post-emergence slurry is usually carried out by side dressing or injection, avoiding the slurry spreading, which can damage the leaves [13].
The NH 3 -N emissions from APSU at both application times were much more intense than the emissions from the other treatments (Figure 5a,b), mainly until the 4th day, which was possibly because of the action of the urease, present in the pig slurry, on the urea. The urease enzyme, present in post-harvest residues, as well as in the soil microorganisms and animal fecal matter, enhance the urea hydrolysis to carbonic acid and ammonia [8]. However, from the fourth day, the emission intensity was reduced possibly because of the low pH of the acidified pig slurry, which influenced the TAN balance, favoring ammonium production over ammonia.
APS decreased nitrogen loss from urea by reducing NH 3 emissions. Therefore, the joint application of urea and acidified pig slurry on stubble-covered soil enhanced urea efficiency.

Conclusions
Making slurry application viable in no-tillage/conservation agriculture systems is a decisive challenge. This study helps to better comprehend the behavior of the nitrogen from manure and manure amended with SF as well as the consequences of the application on stubble-covered soil of a mixture of urea and acidified slurry applied regarding ammonia emissions. In addition, it presents some solutions that can encourage the sustainable use of manure in stubble-covered surfaces, as in no-tillage. The combined application of MAN and U or AN on wheat stubble stimulated ammonia emissions regarding the isolated application of MAN or SF. Thus, the mixture of MAN and U or AN for application on stubble-covered soil should be avoided.
The results evidenced the feasibility of the joint application of urea and acidified pig slurry regarding ammonia emission. The acidified pig slurry was effective in reducing NH 3 -N emissions when applied both alone or combined with urea, and it enhanced efficiency for the urea, reducing significantly the cumulative ammonia emission from that SF applied on crop residues. Moreover, the costs that involve slurry transport and application might drop by the combined application of slurry and mineral/synthetic fertilizer, since lower amounts of enriched slurry would be applied. In addition, the farmer can trim costs by reducing the need for commercial fertilizers and using slurry produced nearby as well.
The ammonia emissions from urea applied eight days after oat sowing were lower than urea fertilization on the sowing day. Thus, the efficiency of fertilizers prone to higher ammonia emissions, such as urea, can be improved by the post-emergence application of plants. Nevertheless, studies involving NH 3 emission from U applied on soil covered by stubble should last more than eight days.
Acidified pig slurry enhanced the efficiency of urea when in a combined application, being then an interesting strategy to decrease costs, promote the use of slurry as fertilizer, and mitigate ammonia emissions. Further studies over the joint application of acidified slurry and urea, that provide information on crop yield and greenhouse gas emissions, are required to reinforce this strategy as a solution for farm-scale adoption as well as contribute to new regulations that support slurry application in no-tillage and other conservation agriculture models.