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Published online 4 January 2008
Published in J Environ Qual 37:280-286 (2008)
DOI: 10.2134/jeq2007.0317
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

The Fate of Sulfate in Acidified Pig Slurry during Storage and Following Application to Cropped Soil

Jørgen Eriksen*, Peter Sørensen and Lars Elsgaard

Dep. of Agroecology and Environment, Faculty of Agricultural Sciences, Univ. of Aarhus, PO Box 50, 8830 Tjele, Denmark

* Corresponding author (Jorgen.Eriksen{at}agrsci.dk).

Received for publication June 18, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
Acidification of slurry with sulfuric acid is a recent agricultural practice that may serve a double purpose: reducing ammonia emission and ensuring crop sulfur sufficiency. We investigated S transformations in untreated and acidified pig slurry stored for up to 11 mo at 2, 10, or 20°C. Furthermore, the fertilizer efficiency of sulfuric acid in acidified slurry was investigated in a pot experiment with spring barley. The sulfate content from acidification with sulfuric acid was relatively stable and even after 11 mo of storage the majority was in the plant-available sulfate form. Microbial sulfate reduction during storage of acidified pig slurry was limited, presumably due to initial pH effects and a limitation in the availability of easily degradable organic matter. Sulfide accumulation was observed during storage but the sulfide levels in acidified slurry did not exceed those of the untreated slurry for several months after addition. The S fertilizer value of the acidified slurry was considerable as a result of the stable sulfate pool during storage. The high content of inorganic S in the acidified slurry may potentially lead to development of odorous volatile sulfur-containing compounds and investigations are needed into the relationship between odor development and the C and S composition of the slurry.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results
 Discussion
 Conclusions
 REFERENCES
 
ACIDIFICATION is a well-known technique to reduce ammonia volatilization from animal slurry (Sommer and Hutchings, 2001). Recently, a new technique for acidification has been developed and introduced in Denmark. The slurry is acidified in a tank by controlled application of sulfuric acid to give a final pH of around 5.5. The acidified slurry is then pumped back to the livestock buildings, resulting in a reduction of pH of the new excreta shortly after excretion. Ammonia emission from buildings has been reduced by 70% with this technique (Kai et al., 2007), and the subsequent utilization of nitrogen in the field has also been demonstrated to be higher (Sørensen, unpublished data, 2007). Slurry acidification to pH 5.5 requires approx. 5 kg sulfuric acid t–1 of slurry, equivalent to 1.6 kg SO4–S t–1 of slurry. Depending on manure S content, animal manured crops often need additional mineral sulfur fertilizer. Using acidified slurry may ensure sulfur sufficiency if the sulfur is plant-available at the time of application. Therefore, acidification with sulfuric acid could serve a double purpose: reducing ammonia emission and ensuring crop S sufficiency.

The S content and composition of manure from both monogastrics and ruminants may be extremely variable, depending on the S content of the feed. The composition of S in feces and urine is influenced by differences in the amount and form of dietary S. Bird and Hume (1971) found that supplementing a basal sheep diet with S mostly affected S in urine and inorganic sulfate excretion was dramatically increased. Although there have been relatively few investigations with monogastrics such as pigs, it is recognized that diet is important for the S content of manures. Focus has been mainly on the reduction of odorous S-containing compounds, and it has been shown that if pigs are fed essential nutrients based on their genetic potential and stage of growth, nutrient excretion is minimal and potential for creating odor-producing compounds is reduced (Sutton et al., 1999; Whitney et al., 1999).

After excretion, feces and urine are subject to anaerobic microbial conversion into microbial biomass and soluble and gaseous compounds. Animal manure may release volatile S compounds such as carbon disulfide (CS2), carbonyl sulfide (COS), dimethyl sulfide (CH3SCH3), dimethyl disulfide (CH3SSCH3), and methyl mercaptan (CH3SH), as well as hydrogen sulfide (H2S) (Banwart and Bremner, 1975). Production of gaseous S compounds by anaerobic bacteria involves sulfate reduction and metabolism of S-containing amino acids (Mackie et al., 1998).

The speciation of S in slurry and the proportion that is plant available may vary depending on both source and storage. As a consequence, different slurries may be expected to have different levels of plant-available S. Unfortunately, there is little information on the influence of feeding and slurry storage on plant availability of manure S, which makes it difficult to generalize results. Lloyd (1994) found S in cattle slurry to be 55% as effective as S in gypsum when applied to grass for silage. This is much higher than the effectiveness of 5% found by Eriksen et al. (1995b) when applying slurry to spring oilseed rape in a pot experiment. The differences in effectiveness between studies may be explained by differences in feeding and storage. Under Danish conditions the S content of feed is mostly balanced with animal requirements and the storage time is usually many months. This combination may minimize the content of inorganic, plant-available S. The findings of Eriksen et al. (1995b) are supported by data of Pedersen et al. (1998). In eight Danish field trials they found a response to a mineral fertilizer application of 40 kg S/ha to winter oilseed rape despite the application of organic manure.

The literature on utilization of manure S is limited and there is a need for more information about loss during storage and plant utilization in the field. Such investigations will be of economic benefit for the farmer, who can minimize fertilizer use without jeopardizing crop productivity, but may also lead to wider beneficial effects such as reduced emissions of odorous S-containing compounds. In this paper we investigated S transformations during storage of untreated and acidified slurry and plant availability of S following application. The specific objective was to evaluate if the additional S in slurry, after acidification with sulfuric acid for the purpose of reducing ammonia emissions, is still plant available after storage of the slurry.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 Conclusions
 REFERENCES
 
The turnover of S was investigated in untreated and acidified pig slurry stored for up to 11 mo at 2, 10, and 20°C. The fertilizer efficiency of sulfuric acid in acidified slurry was investigated in pot experiments with spring barley.

Sulfur Transformations during Storage
Two batches of fresh pig slurry (2 x 120 L) each had 9 L of slurry added from a slurry store at a farm which used continuous acidification. One batch was acidified with concentrated sulfuric acid through an inlet valve at the bottom of the reactor. The sulfuric acid (corresponding to 5.2 kg H2SO4 or 1.7 kg S t–1) reduced slurry pH to 5.5. The fresh and acidified slurries were each distributed in 8-L aliquots into 14 polyethylene containers of 11.5 L capacity. The polyethylene containers were immediately closed with a lid. For both the fresh and acidified slurry four containers were stored at each temperature of 2, 10, and 20°C. Two containers of each slurry type were frozen.

At Day 0, 4, 8, 12, 19, 29, 39, 51, 65, 78, 92, 164, 225, and 318, subsamples of, in total, 100 mL slurry were collected from each of the containers following gentle stirring and analyzed for pH, dissolved and total sulfide content, sulfate, and dry matter. The initial headspace of the containers was approx. 3.5 L, and no precautions were made to avoid entrance of air during each sampling. However, collected subsamples were handled immediately to minimize subsequent oxidation effects. Total S content was determined only at Days 0 and 225.

Determination of Slurry S Plant Availability
For the pot experiment, sandy loam Foulum soil (7.7% clay, 1.6% C, and 210 mg S kg–1) collected from the top 0 to 15 cm was sieved (<2 cm) to remove stones and plant residues (for full soil description see Eriksen et al., 1995a). Forty-seven rectangular polyethylene pots (surface: 0.1 m2, volume: 31 L) were each filled with 29 kg moist soil. A PVC grid covered by a glass fiber mat was placed at the bottom to ensure free drainage through a 12-mm hole at the bottom of the pot. The pots were placed outdoors under a transparent roof and leached with 20 L of demineralized water per pot over a period of 2 wk to remove soil-extractable sulfate. In mid April 0.4 kg slurry from each of the four replicated containers stored at different temperatures with and without acidification and four replicates of the frozen "fresh" slurries with and without acidification was applied to the pots. The amounts of S applied with the slurry are indicated in Table 1 . Eight kg of moist soil that had also been leached was placed on top of the 29 kg already in the pots. Thus, the slurry layer was positioned about 7 cm below the soil surface. The total content of dry soil in each pot was 33.4 kg. Reference pots without slurry application had five levels of mineral S fertilizer added in three replicates (0, 0.1, 0.2, 0.3, and 0.5 g S pot–1) using a solution of calcium sulfate. Pots were placed in a randomized block design.


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  		Table 1. S contents of fresh slurries or slurries stored at 2, 10, or 20°C for 78 d before soil incorporation. Values in each row within each treatment followed by the same letter are not significantly different (P < 0.05).

 
In mid April, 28 seeds of spring barley (Hordeum vulgare L.) were sown in each pot. Demineralized water was added every other day to water-holding capacity and excess water allowed to drain off to individual reservoirs under the pots. The drainage was recycled into the pots at next water application. We added 2 g N as Ca(NO3)2, 0.5 g P and 1.5 g K as K2HPO4 and KCl, 0.25 g Mg as MgCl2, 25 mg Mn as MnCl2, 5 mg Cu as Cu(NO3)2, and 1 mg Mo as Na2MoO4 to each pot.

The barley was harvested at maturity in mid August and grain and straw were separated. Dry matter content was determined after drying at 80°C to constant weight.

Chemical Analyses
For analysis of dissolved sulfide, a centrifuge tube was filled with slurry, closed with a diffusion-tight lid, and centrifuged for 10 min. at 5000 g and 4°C. From the supernatant 10 mL was quickly transferred to 50 mL of 20% zinc acetate and stored cold until further analysis of precipitated ZnS. For total sulfide and sulfate analysis 10 g of slurry was quickly transferred to 50 mL of 20% zinc acetate and stored cold until further analysis. For analysis of total S and dry matter content 40 mL slurry was freeze dried.

Dissolved and Total Sulfide Analysis
Five g of the zinc acetate-preserved slurry sample (with and without centrifugation before ZnAc addition) was transferred to a 300 mL diffusion flask. A tube with 15 mL oxygen-free 3% alkaline zinc acetate solution was placed in the diffusion flask before sealing with a rubber stopper. The flask was evacuated and filled with N2 three times to obtain anoxic conditions. Using a syringe 15 mL oxygen-free concentrated hydrochloric acid was added and the flask was left overnight. The sulfide trap was removed and concentrations were determined using the methylene blue method (Cline, 1969).

Sulfate Analysis
Thirty mL of the zinc acetate-preserved sample was centrifuged at 12,000 rpm for 2 h and activated charcoal was added to the supernatant to remove organic matter. The sulfate content was determined turbidimetrically after acidification with hydrochloric acid (Hoque et al., 1987).

Total Sulfur
Total S in freeze-dried slurry or oven-dried plant material was determined by turbidimetry after wet-ashing with magnesium nitrate and perchloric acid (Nes, 1979).

Statistical Analyses
Analysis of variance was performed using the GLM procedure in SAS (SAS Institute, 1989). Least significant differences were used to compare means, when differences were found to be significant at the P = 0.05 level.


   Results
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
Discussion
 Conclusions
 REFERENCES
 
Transformations during Storage
Addition of sulfuric acid initially decreased pH to 5.5 but over time it increased to above 7 (Fig. 1 ), probably as a consequence of microbial activity. Similarly, an increase in pH occurred in the untreated slurry, so a significant difference between the two treatments was maintained throughout the experimental period. At 20°C, pH decreased at the last two samplings in both treatments, which may indicate decreasing microbial activity. This also coincided with a decrease in sulfide concentrations (see below).


Figure 1
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  		Fig. 1. Dry matter content, pH and content of sulfate, dissolved and total sulphide in untreated and acidified pig slurry stored at 2, 10, and 20°C. Error bars: SE (n = 4).

 
Acidification significantly increased the content of sulfate, corresponding approximately to the amount added (Fig. 1). Sulfate was very stable and the amount added could be quantitatively recovered from the sulfate pool throughout the entire 11 mo storage period. At Day 0, sulfate constituted 50% of slurry total S content in untreated and 94% in acidified slurry (results not shown). Sulfide accumulated in the slurry, but the level of sulfide in acidified slurry did not exceed that of the untreated slurry (Fig. 1). The effect of temperature on sulfide production was very clear. This started immediately at 20°C, but took more than 92 d at 10°C, and at least 225 d at 2°C. Precipitated sulfide (difference between dissolved and total sulfide) was on average 17 mg S kg–1 and did not vary significantly with treatment or storage temperature. However, during the period with sulfide oxidation that occurred at 20°C (see below) precipitated sulfide was also oxidized. As a consequence of a relatively constant amount of precipitated sulfide, fluctuation in total sulfide was caused by fluctuations in dissolved sulfide.

The dry matter content of acidified slurry was significantly higher (mean 1.9%) than the untreated slurry (mean 1.53%). Dry matter content was also affected by temperature and toward the end of the storage period it averaged 1.70, 1.46, and 1.33% at 2, 10, and 20°C, respectively.

In untreated slurry stored at 20°C (Fig. 2 ), a picture of a very dynamic system appears with considerable transformation in both sulfate and sulfide pools. During the first 3 wk sulfate was reduced to sulfide. From Days 29 to 78, both sulfate and sulfide contents increased, indicating concurrent mineralization of organic S and sulfate reduction. Finally, during the latter part of the storage period sulfide was almost fully transformed back to sulfate, suggesting less reductive conditions in the slurry.


Figure 2
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  		Fig. 2. Content of sulfate and total sulphide in untreated pig slurry stored at 20°C. Error bars: SE (n = 4).

 
Total S content at Day 225 was not significantly different from that at the start of the experiment and there was no significant effect of storage temperature (Fig. 3 ). As expected, the total S content in acidified slurry was significantly higher (P < 0.001) than that of untreated slurry with averages of 2.43 g kg–1 (SD = 0.26) and 0.40 g kg–1 (SD = 0.13), respectively.


Figure 3
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  		Fig. 3. Content of total S at Day 0 and 225 in untreated and acidified pig slurry stored at 2, 10, and 20°C. Error bars: SE (n = 12 for Day 0 and n = 4 for Day 225).

 
Plant Availability of Sulfuric Acid Sulfur in Acidified Slurry
Stored or fresh (frozen and thawed) slurries were incorporated into soil in the pot experiment on Day 86 (mid April). Composition of the slurries is shown in Table 1. Significantly more sulfide was present in untreated than in acidified slurry (P < 0.001). In both untreated and acidified slurry, higher temperature resulted in increased concentrations of sulfide when compared to the fresh slurry. In untreated slurry, this was at the expense of sulfate concentrations. However, the sum of sulfide and sulfate were about the same giving similar addition of total inorganic S per pot. This was also the case for the acidified slurry, except that the level of sulfate was 1.87 g S kg–1 higher than in untreated slurry, approximately corresponding to the 1.7 mg added in sulfuric acid.

Yield only responded to S application when the addition of mineral S was increased from 0 to 100 mg S pot–1, showing that other factors were yield-limiting above this level (Fig. 4 ). Yield levels in the pots corresponded to optimal growth in previous pot trials with spring barley (Eriksen et al., 2004). There was a clear response to S application in S uptake; this was almost entirely in the straw fraction, due to luxury S assimilation. The level of S uptake in pots with untreated slurry corresponded on average to a mineral fertilizer S application of approximately 120 mg pot–1, much in agreement with the amount of inorganic slurry S applied (Table 1). It must be kept in mind that the untreated slurry was also inoculated with 7.5% acidified pig slurry, which compared to ‘raw’ slurry raised the sulfate content by approximately 200 mg kg–1. There was a significant effect of slurry treatment on both total yield and total S uptake (P < 0.001), but it was much more pronounced for S uptake. Clearly, S application in acidified slurry was larger than the crop was able to fully respond to.


Figure 4
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  		Fig. 4. Yield and S-uptake in spring barley in pot experiment with application of inorganic S fertilizer and untreated and acidified slurry either fresh or stored at 2, 10, and 20°C. Error bars: SE (n = 3 for inorganic S, n = 4 for slurry).

 

   Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
Conclusions
 REFERENCES
 
Sulfate Reduction
In terms of overall mass balance, sulfate from acidification with sulfuric acid was relatively stable. After 11 mo of storage, most S present was in the form of plant-available sulfate. Samples from two private farms which practiced slurry acidification confirmed these observations, as sulfate was not quantitatively reduced during storage (Sørensen, unpublished data, 2006). Sulfate-reducing bacteria can reduce sulfate to sulfide when provided with anoxic conditions and a suitable organic carbon source to act as an electron donor. Therefore, the excessive sulfate pool in acidified slurry could lead to a substantial generation of hydrogen sulfide. Organic electron donors for sulfate-reducing bacteria are low molecular weight compounds such as volatile and short-chain fatty acids and occasionally long-chain fatty acids and aromatic compounds (Hansen, 1992). However, complex substrates and polymers such as lignin, cellulose, protein, and lipids are not utilized (Zagury et al., 2006). Sulfate-reducing bacteria are terminal degraders that depend on low molecular weight substrates from fermentative bacteria degrading polymers and complex organic carbon compounds (Widdel, 1988; Neculita et al., 2007). In pig slurry most of the carbon is expected to be of a complex nature. Sørensen and Fernández (2003) found insoluble fiber and lignin to be major constituents of a wide range of feces from growing pigs. After storing different pig slurries (16 wk at 8°C and 4 wk at 15°C), these authors found that only 6 to 16% of total C was dissolved. Thus, as a substrate with high lignin and fiber content, pig slurry is a relatively poor substrate for sulfate reduction on a long-term basis (Coetser et al., 2006).

In untreated slurry, the initial sulfate concentration was approximately 300 mg S kg–1, equivalent to 9 mmol L–1 SO42–. At all incubation temperatures, a phase concurrent depletion of sulfate and accumulation of sulfide indicated dissimilatory sulfate reduction during incubation. At 20°C, this phase started immediately and resulted in a sulfate depletion of 266 mg S kg–1 during the first 29 d of incubation. The resulting rate of sulfate reduction was equivalent to 12 µmol L–1 SO42– h–1, which is comparable to rates observed in slurries of freshwater and marine sediments that are classical ecological niches of sulfate-reducing bacteria (Burdige, 1989; Elsgaard and Jørgensen, 1992). As a plausible estimate, this sulfate reduction rate could correspond to a cell number of 106 to 107 sulfate-reducing bacteria per mL, assuming a specific sulfate reduction rate (qSO42–) of 10–13 to 10–14 mol SO42– cell–1 d–1 as determined by Vester and Ingvorsen (1998) using a natural media most probable number method. At 20°C, subsequent to the phase of sulfate reduction, an increase in both sulfate and sulfide concentrations indicated concurrent reoxidation of reduced S compounds to sulfate together with liberation of hydrogen sulfide from organic sources of reduced S in the slurry.

The dynamics of concurrent sulfate reduction and sulfide production were not as distinct in the acidified slurry as in the untreated slurry. Generally, sulfide production was slower in acidified than untreated slurry or at least needed a longer incubation period to be activated. The acidification of slurry had an inhibitory effect on dissimilatory sulfate reduction, although an ample sulfate pool was provided. One possible reason for this observation could be the initial low pH induced when slurry is acidified. Most sulfate-reducing bacteria, except those from extreme environments, appear to be neutrophilic and inhibited by a pH below 6 or above 9 (Widdel, 1988). Likewise, cellulose decomposition has been found to be highly sensitive to pH values below 6 (Lynd et al., 2002). This may indirectly affect sulfate reduction due to reduced substrate availability. Similar observations on substrate limitation have been made in bioreactors for treatment of acid mine drainage, where sulfate reduction depends on products from plant polymer degradation (Logan et al., 2005).

Apart from substrate limitation, sulfide production in the tested slurries responded to incubation temperature. There were slower production rates and longer lag phases found before the onset of production at the lower incubation temperatures. However, the maximum levels of free and precipitated sulfides during the incubation period were similar in both untreated and acidified slurry.

The initial presence and persistence of free sulfide demonstrated an overall low redox potential in the slurries. It is generally believed that sulfate-reducing bacteria need an anoxic environment with a redox potential (Eh) lower than–100 mV, although sulfate reduction has been observed under positive Eh values (Neculita et al., 2007). This could be a result of Eh measurements not reflecting real values in pockets of organic matter where bacteria are present (Neculita et al., 2007) or that some sulfate-reducing bacteria can tolerate and even respire oxygen as suggested in a review by Baumgartner et al. (2006). In the tested slurries, mineralization of organic matter caused rapid oxygen consumption and low oxygen penetration into the slurry. However, when stored at 20°C for more than 92 d, sulfide was oxidized (Fig. 2), probably as a result of decreasing microbial activity and some access of oxygen to the slurries. In this experiment we allowed air to enter the headspace of the containers at each sampling, as some access to oxygen would also occur in real slurry storage. However, the effect of this on sulfate reduction is expected to be modest as long as a significant mineralization of organic matter is proceeding. Kjeldsen et al. (2004) found that sulfate reduction in activated sludge was unaffected by aeration for up to 9 h and quickly recovered even after 121 h of continuous aeration.

Fertilizer Value
The fertilizer value of sulfuric acid S in slurry was considerable, due to the result of the stability of sulfate during storage, and corresponded well with the inorganic S content at application. From an agronomic point of view, slurry acidification seems very advantageous, since it reduces ammonia emission and increases both the N and S fertilizer value of slurry. From an environmental point of view, the balance between S application with slurry and crop requirement must be considered. Application of 30 to 40 m3 acidified slurry ha–1 corresponds to 48 to 64 kg S ha–1, which in many cases exceeds crop requirements. Sulfur fertilizer recommendations vary with crop, soil type, and soil fertility status, but for cereals it is typically 15 to 25 kg ha–1 and for more S-demanding crops such as oilseed rape and grass the recommendation is typically 35 to 45 kg ha–1 (Eriksen, 2002). Retention of sulfate in soils depends on the nature of the colloidal system, pH, concentration of sulfate, and the concentration of other ions in the solution (Harward and Reisenauer, 1966). Curtin and Syers (1990) found that virtually all sulfate, even for a soil with a pH above 6, was in solution and even for soil with a marked capacity to retain sulfate, the strength of the retention seems weak as adsorbed sulfate can be removed by repeated extraction with water (Chao et al., 1962). As a consequence of the low adsorption capacities, many agricultural soils are prone to S leaching by percolating rainwater and excess sulfate is expected to leach. This is not expected to have any adverse natural environmental consequences as natural ecosystems are usually not sulfate-limited and the EEC guideline for sulfate in drinking water (250 ppm) is far from being exceeded under north-European climatic conditions. However, this limit will eventually be reached with long-term annual application of acidified slurry if application rates are not generally adjusted to crop requirements.

Potential for Odor Development
An important environmental aspect to consider when acidifying slurry with sulfuric acid is the potential for development of malodorous, volatile S-containing compounds. Several hundred malodorous compounds have been identified in pig manure. They can be divided into four groups: (i) volatile fatty acids, (ii) indoles and phenols, (iii) ammonia and volatile amines, and (iv) volatile S-containing compounds (Zhu, 2000). Of the compounds that have the lowest odor detection thresholds for humans, six contain S and the three with the lowest detection threshold all contain S (O'Neill and Phillips, 1992). Hydrogen sulfide and methanethiol are the sulfurous compounds most commonly causing offensive smells in pig manure (Spoelstra, 1980). There are correlations between hydrogen sulfide and odor concentration (Clanton and Schmidt, 2001; Hobbs et al., 2001; Sørensen, unpublished data, 2006), while others find no such correlations (Le et al., 2005). In this experiment the volatilization of H2S was not determined; it is probable that some H2S loss occurred due to sulfate reduction in the early phase of storage, as seen in Fig. 2 where sulfate and sulfide concentrations decrease during the same period (Day 19–29). It is also possible that H2S created by anaerobic decomposition of organic S compounds was emitted. Finally, the lower pH after acidification shifts the equilibrium between dissolved sulfide and hydrogen sulfide toward the latter (Stevens et al., 1993).

Apart from odor, hydrogen sulfide is increasingly being associated with health hazards. Tolerable concentrations in air have been derived based on respiratory effects for inhalation exposures (WHO, 2003). The focus of this study was the fate and especially plant availability of added sulfate, but in ongoing experiments more focus will be on emissions of H2S and other sulfurous compounds. It is particularly important to investigate under what circumstances sulfate reduction may sustain a continuing sulfide production in the acidified slurry, thus exploiting the coexistence of organic matter, anoxic conditions, and a high sulfate concentration.

Practical Implications
For practical reasons the present study was made with pig slurry that had accumulated in a pig-producing unit before sampling and acidification. When using the acidification technique in practice, pig excrement will be acidified shortly after excretion on reaching the slurry pit located below the floor. This means in practice, decomposition of fresh excrements in the accumulated slurry will be retarded due to both rapid acidification and the apparent inhibition of decomposition processes. However, we expect that this will have minimal influence on the turnover of S during subsequent slurry storage and effects similar to those reported here should be expected in a full-scale system.

The major benefits of slurry acidification are lower ammonia losses from buildings, during storage and after field application, and the increased S and N fertilizer value of the treated slurry. The savings in fertilizer alone may not make this technology profitable, but if a farm is located in an area with special restrictions on N emissions, slurry acidification may be a relatively cheap way of reducing these. The use of acidified slurry in the field does not make any extra requirements over and above normal farm practices in countries having legislative demands on the utilization of slurry which imply that all slurry should be applied to farm land in amounts corresponding to crop requirements.

Sulfide may also precipitate in the slurry with metals such as copper, zinc, manganese, and iron. The slurry in this study showed some capacity to precipitate sulfide and slurries from pig production systems that use copper as a dietary additive must be expected to have a considerable capacity to immobilize sulfide. This will lower the potential for H2S emission but is not expected to effect plant availability of S, as metal sulfides will be readily oxidized to sulfate when the slurry is applied to soil (Tributsch and Bennett, 1981; Howarth, 1984).

Farmers have been concerned about the effect of high sulfate concentrations on the life expectancy of concrete slurry channels and storage tanks. This problem is restricted to certain types of concrete and at least in the case of new constructions it should not present a problem, but needs to be considered when older facilities are in place.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results
 Discussion
 Conclusions
REFERENCES
 
The major benefits of slurry acidification are lower ammonia losses from buildings, during storage and after field application, and also the increased S and N fertilizer value of treated slurry. Sulfate reduction during storage of acidified pig slurry was limited presumably by pH effects and limitation in the content of easily degradable organic matter. However, the high content of inorganic S in the acidified slurry may potentially lead to development of odor from volatile S-containing compounds. We have to find out precisely under which conditions that happens in relation to both diet and storage.


   ACKNOWLEDGMENTS
 
We thank Karin Dyrberg for skilled technical assistance.


   NOTES
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