Agriculture, Ecosystems and Environment 131 (2009) 240–246

Effects of slurry acidification with sulphuric acid combined with aerationon the turnover and plant availability of nitrogen

Peter Sørensen *, Jørgen Eriksen

University of Aarhus, Faculty of Agricultural Sciences, Department of Agroecology and Environment, P.O. Box 50, 8830 Tjele, Denmark

A R T I C L E I N F O

Article history:

Received 10 July 2008

Received in revised form 29 January 2009

Accepted 29 January 2009

Available online 4 March 2009

Keywords:

Ammonia

Cattle manure

Fertilizer replacement value

Pig manure

Redox potential

Sulphur

VFA

A B S T R A C T

Ammonia (NH3) losses from animal houses, slurry storage and slurry application can be significantly

reduced by acidifying the slurry in livestock houses. The acidification may potentially influence organic

matter turnover during slurry storage and therefore also the following release of inorganic nitrogen (N)

after application to soil. We studied the effects of pig and cattle slurry acidification with sulphuric acid on

(1) N release in a loamy sand soil and (2) the N fertilizer replacement values after slurry application to

spring barley (Hordeum vulgare L.) by incorporation and to winter wheat (Triticum aestivum L.) by surface

application. Pig and cattle slurries were acidified to pH 5.5 before storage but pH increased to above 6 at

application in the field. The composition of slurry after storage indicated that the organic matter

turnover during storage is inhibited by acidification, probably due to the presence of acetate in

combination with low pH. The acidified slurry contained more butyric acid than the untreated slurry.

However, there was no clear effect of acidification on the subsequent mineral N release in soil. Effects of

aeration of the acidified slurry were also studied, but 4 days’ aeration had no detectable effects on slurry

composition and N availability. Slurry acidification had no detectable effect on the mineral N fertilizer

equivalence (MFE) when slurry was incorporated before sowing a barley crop, whereas after surface-

banding the MFE of cattle slurry N increased from 39 to 63% and of pig slurry N from 74 to 101% due to the

acidification.

After acidification, the MFE of slurry was similar in the two crops, indicating that NH3 volatilisation

from acidified cattle and pig slurry was low both after incorporation and after surface application.

� 2009 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Agriculture, Ecosystems and Environment

journa l homepage: www.e lsev ier .com/ locate /agee

1. Introduction

Ammonia (NH3) volatilisation from livestock manure is the mostimportant source of NH3 pollution in Denmark. Deposition of NH3 orparticulate NH4

+ to land or water causes eutrophication of naturalecosystems (Fangmeier et al., 1994), and airborne particulate NH4

+

may also be a health hazard (Erisman and Schaap, 2004).Volatilisation of NH3 can be significantly reduced by acidificationof the manure. A new technique for slurry acidification has beendeveloped and introduced in Denmark where the slurry is acidifiedin a process tank by controlled addition of sulphuric acid to a pH ofabout 5.5. The acidified slurry is then pumped back to the livestockbuildings resulting in a reduction of pH of the fresh excreta shortlyafter excretion. The process is repeated a few times per day to keepthe pH of the slurry in the building low. A 70% reduction of NH3

emission from pig production buildings has been documented withthis technique, and the utilization of nitrogen in the field may also beimproved (Kai et al., 2008).

* Corresponding author. Tel.: +45 89991748; fax: +45 89991719.

E-mail address: Peter.sorensen@agrsci.dk (P. Sørensen).

0167-8809/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.agee.2009.01.024

Previous studies have shown that NH3 volatilisation issignificantly reduced after addition of sulphuric acid or nitric acidto the slurry (Stevens et al., 1989, 1997). However, in these studiesthe acid was added just before application of slurry in the field.With the new acidification technique the slurry is stored for avariable time after acidification. The acidification may influencethe turnover of manure organic matter during storage and therebyalso the N availability.

After acidification it is possible to aerate the slurry withoutlosing NH3, and manure properties such as smell and N availabilitycould potentially be further improved by the aeration. Easilydecomposable compounds like volatile fatty acids (VFA) may bedecomposed by the aeration (Cooper and Cornforth, 1978; Zhangand Zhu, 2005). The VFA may cause N immobilisation in soil(Kirchmann and Lundvall, 1993) and thus reduce the first year Nfertilizer value, and some VFAs contribute significantly tomalodour (Zhu, 2000). A Danish company is marketing anacidification system where aeration is also included, but theeffects of the aeration have not been documented.

In Northern Europe much of the slurry is surface-applied towinter cereals in spring, as it is difficult to directly inject slurrywithout damaging the crop. Ammonia volatilisation can be

P. Sørensen, J. Eriksen / Agriculture, Ecosystems and Environment 131 (2009) 240–246 241

significant with surface application, and it can probably beconsiderably reduced with slurry acidification.

Animal-manured crops often need additional mineral sulphur(S) fertilizer due to a low content of plant-available S. The bufferingcapacity of slurry is variable and a variable amount of acid isneeded to reduce pH to a certain level, but acidification to pH 5.5usually requires approximately 5 kg sulphuric acid t�1 equivalentto 1.6 kg SO4-S t�1. There is a potential for sulphate reduction andloss as hydrogen sulphide (H2S) during storage, but this loss islimited after acidification (Eriksen et al., 2008) and most of theapplied S is still plant-available at the time of application. Theobjective of this study was to evaluate the effects of cattle and pigslurry acidification with and without slurry aeration on the Nturnover in soil and N utilization in spring barley and winter wheatcrops. In addition effects of slurry acidification and aeration on VFAconcentration in slurry was measured.

2. Materials and methods

2.1. Slurry sampling and treatment

Just before the start of the experiment samples of cattle and pigslurry were taken directly from the animal houses at two farmswith dairy cattle and fattening pigs. The chemical environment inacidified slurry is different from untreated slurry and therefore adifferent microbial population may develop in acidified slurry. Toensure that the experimental slurry contained a microbialpopulation adapted to the pH in acidified slurry, the non-acidifiedslurry was mixed with 10% acidified slurry from a farm whereacidification is used. This mixture was termed ‘untreated’. The pHin the mixture containing 10% acidified slurry was only 0.1 unitlower than in the non-acidified slurry due to buffering compoundsin the slurry.

A subsample of the slurry was kept as untreated, while theremaining slurry was acidified by slowly adding concentratedsulphuric acid during the mixing. Sulphuric acid was added until apH of about 5.5. Slurry foaming could not be totally avoided despitea slow application rate (foaming is not usually a problem on thefarms using this acidification technique). Two batches of both pigand cattle slurry were acidified in this way and 5.4 ml concentratedH2SO4 kg�1 and 2.85 ml H2SO4 kg�1 were added to, respectively,the pig and cattle slurry to reach a pH of 5.5. Each batch wasdivided into three 30 kg portions and added to cylindrical plasticcontainers with a total volume of 70 l. The containers were thencovered with a lid and stored at a temperature of 8 8C, which isabout the normal temperature in slurry storage tanks in spring inDenmark. The following three treatments were applied to theacidified slurry: (a) no further treatment, (b) aeration for 6 h with

Table 1Characteristics of experimental slurries at time of application and pH in slurry just aft

Treatment DM

(g kg�1)

VS

(g kg�1)

Total N

(g N kg�1)

NH4+–N

(g N kg�1)

Pig slurry

Untreated 23.8 15.2 4.37 3.66

Acid 34.4 23.2 4.43 3.64

Acid, 6 h aeration 35.9 24.5 4.44 3.57

Acid, 4 days aeration 35.5 24.2 4.43 3.64

Acid on farm 29.0 22.8 5.85 3.80

Cattle slurry

Untreated 47.0 39.3 2.65 1.50

Acid 50.3 41.3 2.58 1.52

Acid, 6 h aeration 51.5 41.3 2.58 1.52

Acid, 4 days aeration 50.8 40.0 2.60 1.51

Acid on farm 85.6 69.0 4.43 2.47

ND: not determined.a At field application.

atmospheric air (1.2 l s�1 m�3) and (c) aeration for 4 days withatmospheric air (1.2 l s�1 m�3). Each container had an inlet at thebottom centre to which an air pump could be connected for thecontrolled aeration of the slurry. The pig slurry aeration wasfinished 8 days after the acid application and the slurry was storedfor another 3 weeks. The cattle slurry aeration was finished 4 daysafter acid application and the slurry was stored for another 18 days.Due to experimental conditions the storage time was different forcattle and pig slurry.

The pig slurry used in the experiment came from a farm wherethe slurry is acidified in one half of the buildings and untreated inthe other half. The pigs were fed similarly in all sections, and theonly difference was the acidification. For reference, samples of theon-farm-acidified pig slurry were taken from the process tankwhere sulphuric acid is added while untreated pig slurry wassampled under the slatted floor in the building and used for theexperimental acidification. Similarly, reference samples weretaken of on-farm-acidified cattle slurry from the storage tank ata dairy farm with on-farm acidification while non-acidified cattleslurry for the experiment was sampled in the buildings of anotherdairy farm since acidified and non-acidified cattle slurry was notavailable from the same farm. The on-farm-acidified referenceslurries were stored under conditions similar to the experimentalslurries.

The Redox potential (Eh) of the experimental slurry wasmeasured before and after aeration by platinum and calomelelectrodes. The platinum electrodes (Royal Holloway EnterpriseLtd.) were fixed to the wall of each slurry container 10 cm abovethe base and 20 cm below the slurry surface throughout thestorage period. Redox potentials were measured with a portablestandard pH/mV-meter (PHM 80, Radiometer) and a saturatedcalomel reference electrode (Radiometer) that was placed in theslurry while measuring. Readings of Eh for the calomel referenceelectrode were corrected to the standard hydrogen electrode. Theaccuracy of the electrodes was tested using a hexacyanoferrate II/III, pH 7.0 Redox Buffer (BS870, Radiometer).

At the end of the storage period a subsample was taken fromeach container (two replicates of each treatment) for analysis oftotal N, ammonium N, dry matter (DM, 80 8C, 24 h), volatile solids(VS, loss of DM by ignition at 550 8C), total S, and content of short-chain VFA. The two replicates were combined to one sample to beused in the following incubation and field study.

2.2. Incubation study

The dynamics of inorganic N were measured after application ofthe eight differently treated slurries and two reference slurries(Table 1) to soil in an incubation experiment. Slurry samples

er acid application.

NH4–N/

total N

Total S

(g S kg�1)

N/S ratio pH after

acid application

pHa

0.84 0.68 6.43 7.22 7.45

0.83 3.46 1.28 5.47 6.17

0.81 3.44 1.29 5.47 6.19

0.82 3.33 1.33 5.47 6.24

0.65 4.44 1.32 5.48 6.25

0.56 0.49 5.41 ND 7.03

0.59 2.04 1.26 5.52 6.01

0.59 1.92 1.34 5.52 6.21

0.59 2.05 1.27 5.52 6.26

0.56 3.28 1.35 5.50 6.00

P. Sørensen, J. Eriksen / Agriculture, Ecosystems and Environment 131 (2009) 240–246242

equivalent to 12 mg total N were placed on 30 g soil (dry weightbasis) in 250 ml polyethylene containers and then covered withanother 30 g soil (200 mg N kg�1 soil) to simulate an incorporationof slurry in soil. The soil was a loamy sand (Typic Hapludult, mixed,mesic) with 9% clay (<2 mm), 12% silt (2–20 mm), 76% sand (20 mmto 2 mm), 0.165% N, 1.72% C and pH (H2O) was 6.8. The soil hadbeen taken from the top 0 to 20 cm of an arable field and sieved(5 mm) in moist form a few days before start of the experiment.After the manure application the water content was adjusted to55% of the water-holding-capacity where the conditions formicrobial activity are nearly optimal (Wang et al., 2003). Theincubation bottles were covered with Parafilm with holes foraeration and placed in a dark, temperature-controlled room at10 8C. The soil moisture content was kept nearly constant bycontrolling the weight of the bottles weekly and adding extrademineralised water if necessary. Similar soil samples withoutmanure addition were incubated under the same conditions. After2, 7, 14, 28 and 84 days three replicate samples were extractedwith 120 ml 2 M KCl for analysis of extractable mineral N. The netrelease of mineral N from slurry was calculated as the mineral N insoil fertilized with slurry minus the mineral N in unfertilized soil.

2.3. Field experiment

The utilization of N in the untreated and acidified cattle and pigslurries (Table 1) was compared in small field plots. The N fertilizervalue was measured in two situations: (1) after incorporationbefore sowing spring barley and (2) after surface-banding in awinter wheat crop in two fields with a similar soil type as used inthe incubation experiment.

Spring barley. In April 2005, 60 microplots confined in PVCcylinders (30 cm inner diameter, 0.0707 m2 area and 30 cm length)were established by pushing the cylinders to 25 cm soil depth in anarable field at Research Centre Foulum, Denmark (568290N, 98340E).

The slurry was applied by removing the upper 15 cm soil,placing the manure in a layer and then returning the soil to the plot(simulated ploughing). The slurry application rate was equivalentto about 120 kg ammonium N ha�1, which is the recommendedlevel for mineral N application to barley. The air temperature atslurry application was 12 8C. To establish a mineral fertilizerresponse curve, separate plots were supplied with 0, 50, 100, 125,and 150 kg N ha�1 as ammonium nitrate. There were fourreplicates of each treatment, and the experiment was designedin four randomized blocks with plots placed in four rows with 1 mbetween rows and 0.5 m between plots.

The plots and the surrounding soil were sown with springbarley (Hordeum vulgare L.). The soil surrounding the plots wasfertilized with 120 kg N ha�1, 17 kg P ha�1 and 57 kg K ha�1 in anNPK fertilizer.

Winter wheat. Similar cylinders as used in barley were installedin October 2004 in a field nearby already established with winterwheat (Triticum aestivum L.). The cylinders covered two plant rowssymmetrically with a distance of 12 cm between the rows. In lateMarch 2005 all plots received 45 kg N ha�1 in ammonium nitrate.The slurries (100 kg ammonium N ha�1) were applied to plots bysurface-banding on 3 May when the crop was 10–15 cm high. Atapplication the air temperature was 15–18 8C with sunshine. Amineral fertilizer response curve was prepared by includingtreatments with 0, 50, 100, 130 and 160 kg N ha�1 as ammoniumnitrate in separate plots. The soil surrounding the plots wasfertilized with 145 kg N ha�1, 21 kg P ha�1 and 69 kg K ha�1 in anNPK fertilizer.

For both crops essential plant nutrients except N were appliedto all plots at a level sufficient for optimal growth. During thegrowing season the field including the plots was treated withherbicides and fungicides according to normal practice in Den-

mark. The mature crops of barley and wheat were harvested 1 cmabove soil surface on 19 August and the harvested grain and strawwas analyzed for dry matter and total N content.

The experiments were carried out with four replicates of eachtreatment randomized in four blocks, giving a total of 120 plots forthe two crop experiments including mineral N plots.

2.4. Analytical methods

Total N in slurry samples was determined using a Kjeldahlmethod (Tecator Kjeltec Auto 1030, Tecator, Hoganas, Sweden).Inorganic N in soil and manure was extracted with 2 M KCl for 1 h(2.5:100 for manure; 1:2 for soil), followed by centrifugation andfiltration through glass filters. Ammonium and nitrite + nitrate N inextracts were measured by flow colorimetry (Autoanalyzer III,Bran+Luebbe GmbH, D-22803 Norderstedt, Germany). Total N inoven-dried plant material (80 8C) was determined by elementalanalysis.

Volatile fatty acids in slurry were analyzed as described by Jensenet al. (1995) with some modifications regarding the chromato-graphic equipment. A 10 g subsample was diluted 10-fold with a0.028 M Sodium hydroxide solution containing 11.1 mmol/l internalstandard (2-ethylbutyric acid) and homogenised for 2 min. Of thediluted sample 1 ml was extracted by adding 0.5 ml concentratedHCl and 2 ml diethyl ether. Quantification of VFA was performed on aHewlett Packard gas chromatograph (Model 6890) equipped with aflame ionisation detector and a 30 m ZB-5 column with an internaldiameter of 0.32 mm coated with 5% phenyl–95% dimethylpolysi-loxane with a film thickness of 0.25 mm and helium as carrier gas.Detector and injector temperatures were set to 250 8C. A samplevolume of 2 ml was injected with an autoinjector with a split ratio of20. The compounds were eluted with a temperature gradient of thefollowing shape: held at 70 8C for 3 min then increased to 110 8C(10 8C per min), further increased to 290 8C (20 8C per min) and heldfor 5 min.

Total S in freeze-dried slurry was determined by turbidimetryafter wet-ashing with magnesium nitrate and perchloric acid (Nes,1979).

The mineral fertilizer equivalence (MFE) of the slurries wascalculated by relating N uptake in grain on plots treated with slurryto the linear N uptake curve on plots receiving increasing amountsof mineral fertilizer N (Munoz et al., 2004), and the mineralfertilizer replacement value was expressed as a percentage of totalmanure N applied.

2.5. Statistical analysis

Analysis of experimental variance was carried out using the SASprocedure GLM (The SAS System for Windows version 8e) andLeast Significant Differences (LSD) were calculated if the maineffects were significant (P < 0.05). The on-farm-acidified slurrieswere included in the statistical analysis of the soil N turnover andthe field experiments.

3. Results and discussion

3.1. Effects of acidification and aeration on slurry composition

After the application of sulphuric acid, slurry pH was reduced toabout 5.5. However, pH increased again to about 6.0–6.2 after fewweeks’ storage and pH tended to increase slightly more when theslurry was also aerated (Table 1). Similarly, pH also increased afterstorage of the untreated slurry, probably due to decomposition oforganic compounds in the slurry (Sommer and Husted, 1995).Eriksen et al. (2008) found that the pH of acidified pig slurryincreased further over a longer storage period.

P. Sørensen, J. Eriksen / Agriculture, Ecosystems and Environment 131 (2009) 240–246 243

After storage the content of organic matter expressed as volatilesolids (VS) was lower in the untreated pig slurry than in theacidified slurry (Table 1). Thus more organic matter remained inthe acidified slurry, indicating that microbial decompositionduring storage was inhibited by the acidification. By contrast,there was no significant difference in VS between untreated andacidified cattle slurry, indicating no effect of the acidification ondecomposition of the cattle slurry. There was no detectable effectof the aeration of the acidified slurry on the VS content indicatingthat the aeration had no detectable effect on the decomposition oforganic matter in the cattle slurry. The dry matter content was alsolower in the untreated slurries, but this could be partly due to theextra dry matter added with the acid.

After slurry storage the acidified slurry had a significantly(*P < 0.05) higher content of butyric acid than the untreated slurry(Fig. 1). In the untreated pig slurry the concentration of butyric acidwas below the detection limit (0.6 mmol kg�1), while theconcentration in the acidified pig slurry was 1.5–1.9 mmol kg�1.In cattle slurry the butyric acid concentration was 0.9 mmol kg�1

in untreated and 1.8 mmol kg�1 in acidified slurry. The aerationhad no significant effect on the concentration of the VFAs (Fig. 1).Both cattle and pig slurry acidified on-farm had a higherconcentration of butyric acid than the experimental slurries, andthe cattle slurry acidified on-farm also had a higher content ofacetic acid. Butyric acid is malodorous and attention should begiven to this aspect, though we cannot generalize on effects ofacidification on odour emission based on this single investigation.

Fig. 1. Volatile fatty acids in untreated and acidified pig and cattle slurry at time of

field application. Bars indicate standard errors (n = 2).

The VFA concentrations were generally within the range observedin slurry samples by Sommer and Husted (1995).

The acidification and aeration had no significant influence onthe concentration of total N, ammonium N and total S in slurry(Table 1). Thus, there was no detectable influence of the treatmentson the turnover of N and losses of N and S. The on-farm-acidifiedpig slurry showed a relatively low content of ammonium relativeto total N, while the reduced ammonia volatilisation in the buildingdue to acidification should result in a higher proportion ofammonium in the slurry. However, the proportion of ammoniumwas not higher than normally found for untreated slurry. Thisindicates that microbial decomposition of organic N compoundswas inhibited by on-farm acidification where acidification wasmade shortly after the excretion from animals. The untreated pigslurry was sampled at the same farm in a building section withoutacidification, but for practical reasons it was not possible to obtainuntreated pig slurry identical to that acidified on the same farm.The untreated as well as the experimentally acidified pig slurrycontained a high proportion of ammonium N relative to total N.Because slurry was sampled directly in the building, it was difficultto effectively mix the slurry before sampling and this could explainthe high proportion of ammonium N in the untreated slurry.

The cattle slurry acidified on-farm had higher dry mattercontent and a correspondingly higher concentration of nutrientsthan the cattle slurry used for acid treatment, whereas theproportion of ammonium N relative to total N was similar in thecattle slurries (Table 1).

The N/S ratio was similar and about 1.3 in all the acidifiedslurries. This indicates that the amount of sulphuric acid neededfor the acidification to pH 5.5 is related to the role of N in the slurrybuffer system. Stevens et al. (1989) similarly found that theamount of acid needed for slurry acidification to pH 5.5–6 wasrelated to the ammonium content of slurry. The most importantbuffer compounds in slurry are NH4

+/NH3, CO2/HCO3�/CO3

2� andacetic acid/acetate (Sommer and Husted, 1995). Urea hydrolysis toammonium bicarbonate is likely to be the dominant carbonate-generating reaction in slurry, and acetic acid formation is likely tobe related to the protein decomposition in the manure. Thus all theimportant buffer compounds are related to slurry N content andcan explain the close relationship between acid application neededand slurry N content.

The aeration treatment applied to the acidified slurry tempora-rily increased the redox potential of the slurry. The redox potentialof the pig slurry increased from about�100 mV prior to aeration toabout 200 mV just after aeration periods of either 6 h or 4 days(Fig. 2). However, the redox potential decreased again indicatingcontinued oxygen consumption through microbial activity in theslurry. There were large differences in the decline of the redoxpotential after aeration between the two replicated batches(Fig. 2). The first measurement of redox potential was made justafter acid application. Here the potential was about 0 declining toabout�100 after 4 days. Similarly, after aeration of the cattle slurrythe redox potential increased from about �200 mV to about100 mV and then declined to the original level within a few days(data not shown). These measurements document that the appliedaeration effectively increased the redox potential at leasttemporarily.

The aeration of the acidified slurries had no influence on thecontent of volatile fatty acids and the content of organic N in slurry.In contrast, Cooper and Cornforth (1978) and Zhang and Zhu(2005) found significant decomposition of VFA by aeration of pigslurry at a temperature of 18–20 8C. The aeration was made at aslurry temperature of about 8 8C, which is a normal slurry storagetemperature in spring in Denmark, and the absence of an effect ofaeration on decomposition of VFA could be due to the lowtemperature. However, the most likely explanation for the lacking

Fig. 2. Redox potential in acidified pig slurry before and after aeration for 6 h or 4

days. Single results from two replicates are shown.

Fig. 3. Net release of inorganic N in soil during 12 weeks’ incubation (10 8C) after

application of differently treated pig and cattle slurries to a loamy sand soil.

Inorganic N in soil without slurry application was subtracted. Bars indicate standard

errors (n = 3).

P. Sørensen, J. Eriksen / Agriculture, Ecosystems and Environment 131 (2009) 240–246244

microbial activity in the acidified slurry is the combination of lowpH and a high concentration of VFA. The concentration ofprotonized acid increases when pH decreases (pKa is 4.76 foracetic acid), and acetic acid can cross biological membranes only inuncharged form. At high concentrations of uncharged short-chainacids the pH gradient of cell membranes is destroyed (Kell et al.,1981). As consequence of the acetic acid penetration, the reducedpH in cytoplasm inhibits cellular reactions and renders themembranes permeable to protons (Baronofsky et al., 1984). Thisinhibitory effect of acetate in combination with low pH onmicrobial growth is also well known from the conservation ofvegetables and silage. At pH 5.5, where 15.5% of the acetate ispresent as undissociated acid, Baronofsky et al. (1984) foundeffects of acetate on internal pH of nongrowing cells at acetateconcentrations below 10 mM. In the present study the acetateconcentration was about 80 mM in acidified pig slurry and 50 mMin the cattle slurry. Apparently these conditions inhibitedmicrobial decomposition, and no detectable effects of the aerationon decomposition of VFA and organic matter were observed.

In stored slurry there is a potential for gaseous loss of sulphateafter reduction to H2S (Eriksen et al., 2008). The amount of Sapplied with sulphuric acid was an estimated 3.24 g S kg�1 for thepig slurry and 1.7 g S kg�1 for the cattle slurry. The extra amount oftotal S in the acidified slurry indicated that 85–90% of the applied Sremained in the stored slurry (calculated from Table 1). Most of theS in acidified slurry was still present as sulphate after storage (datanot shown) as also observed by Eriksen et al. (2008). The aerationtreatment had no influence on the content of S in slurry afterstorage. The reference slurries acidified on-farm had a similar N/Sratio, indicating that losses of S were also limited here. The limitedturnover of S in the slurry could also be due to the microbialinhibition caused by acetate at low pH as described above.

3.2. N turnover in soil

The dynamics of soil mineral N were followed for 12 weeks afterslurry application. The soil incubation test was more sensitive todifferences in slurry composition than the field experiments, but itshowed no particular effects of the slurry treatments. Theacidification of pig slurry with or without aeration had noinfluence on the dynamics of inorganic N in soil after applicationof the slurry (Fig. 3). Slurry acidification had also no effect on thenitrification rate in soil (data not shown). A slight immobilisationof N was observed within the first 14 days after applicationfollowed by a period with net N mineralisation. Thus, at the end ofincubation no net mineralisation of N had occurred. The pig slurry

acidified on-farm had a lower proportion of total N in ammoniumform, and 2–4 weeks after application of this slurry a slightly morepositive net N mineralisation occurred. Apparently more miner-alisable organic N was present in the on-farm-acidified slurry,which is in accordance with the inhibitory effect of acidification onmicrobial decomposition, as discussed above. After 12 weeks thenet release of mineral N was still lowest (*P < 0.05) from the on-farm-acidified pig slurry.

All the differently treated cattle slurries had nearly the sameproportion of ammonium N to total N at application. The cattleslurry acidified on-farm resulted in a larger net N immobilisationwithin the first week in soil followed by a higher net Nmineralisation in the following weeks. This could be attributedto a higher content of decomposable organic matter in this slurrycausing a more intense N mineralisation–immobilisation turnoverin soil.

The untreated cattle slurry showed a slightly different patternand the final soil mineral N content was significantly lower(*P < 0.05) in this treatment. The lower mineral N content could bedue to increased N immobilisation or to increased gaseous N losses.We cannot exclude that some ammonia volatilisation occurredafter application of the untreated cattle slurry, although similarexperiments with 15N-labelled ammonium in slurry have indi-cated insignificant gaseous N losses (unpublished results).

3.3. Fertilizer efficiency of slurry N

The mineral N fertilizer replacement value of slurries wasestimated both in spring barley and winter wheat based on

P. Sørensen, J. Eriksen / Agriculture, Ecosystems and Environment 131 (2009) 240–246 245

measured N uptake in grain. A linear response between mineral Napplication and total N uptake in grain was observed. The MFE ofuntreated pig slurry applied by surface-banding to winter wheatwas equivalent to 74% of total N while the MFE of the acidified pigslurry was 101–103% (Fig. 4). The MFE of cattle slurry was 39% forthe untreated and 63–66% for the acidified slurry applied to winterwheat. Thus acidification resulted in an MFE that was about 25%point higher for the surface-banded slurry. A similar increase ofMFE after surface application of acidified pig slurry to wheat wasfound by Kai et al. (2008).

In barley the MFE was 93% for the untreated pig slurry and about100% for the acidified pig slurry (90% for on-farm acidified), and theMFE of cattle slurry was 59% for untreated and 61–68% for theacidified cattle slurries (Fig. 4). After the application of slurry to abarley crop by incorporation there were only non-significant,positive effects of acidification on N utilization in the field, indicatingthat ammonia volatilisation was low with all slurry treatments.

The net release of mineral N after 12 weeks’ soil incubation wasequivalent to the initial ammonium N content in the slurry,indicating no net N mineralisation of manure N (Fig. 3). By contrast,the MFE of slurry incorporated in barley was higher than theammonium content, especially in the pig slurries (Fig. 4). Such adifference between net release by incubation and MFE has alsobeen observed in previous studies (Sørensen and Fernandez, 2003).We have no definite explanation for this. Since the MFE of pigslurry total N was close to 100%, priming effects of slurry on soil Cand N turnover could be involved, as a total mineralisation of theentire organic manure N applied would be unlikely. Apparently

Fig. 4. Mineral fertilizer equivalence (MFE) of differently treated pig and cattle

slurries applied in spring by simulated injection to barley or surface-banding to

winter wheat. Bars indicate standard errors (n = 4).

these effects only occurred in the field experiment and mainly afterapplication of pig slurry.

3.4. Practical implications

With on-farm acidification the excrements are acidified shortlyafter excretion. For practical reasons the untreated experimentalslurry used in this study was sampled in the building shortly beforeapplication of the acid treatment, and here some of the slurry wouldhave been under microbial decomposition for more than a weekbefore the acid treatment. Also some ammonia volatilisation wouldhave occurred before the acid application. That means that theexperimental conditions were slightly different from the conditionson farms practising on-farm acidification. However, the slurriesacidified experimentally behaved almost similarly to the slurriesacidified on-farm, and the difference can be considered to be of littleimportance.

After on-farm acidification the acidified slurry contains more N.Kai et al. (2008) calculated that ammonia emission is reduced from15% to 4.8% of total N from buildings and from 9% to 1% fromstorage by acidification of pig slurry to pH 5.5 under Danishconditions. That means that the acidified pig slurry contains 26%more total N than untreated slurry after storage. This calculation isfor uncovered storage. Under conditions with a covered storage itcan be calculated that acidified pig slurry contains about 15% moreN than untreated slurry based on Kai et al. (2008). In the presentstudy a lid covered the slurry during storage and ammonia losseswere expected to be low from both acidified and untreated slurry.

There is concern that the high content of inorganic S in acidifiedslurry may potentially lead to development of malodour fromvolatile sulphur-containing compounds (Eriksen et al., 2008). Ahigh content of organic acids, especially butyric acid, as observed inacidified slurry also contributes to malodour. This subject needsfurther investigation.

The costs of using the acidification technique have beencalculated by Kai et al. (2008) to be 60 Euro per livestock unit.There has been concern that the high sulphate concentration couldcause destruction of concrete in slurry channels and storage tanks.This problem is restricted to certain types of concrete, but for newconcrete types with a high content of fly ash it should not be aproblem.

4. Conclusion

The study indicated that acidification to pH 5.5 inhibited theturnover of organic matter in slurry during storage, probably dueto the presence of acetate and other organic acids in a protonizedform. After storage the acidified slurry contained significantlymore butyric acid than the untreated slurry. For surface-appliedslurry a higher crop N utilization can be attained after slurryacidification. When slurry was directly incorporated in soil therewas no detectable effect of acidification on the relative N fertilizervalue. However, the total fertilizer value is higher when taking intoaccount the higher N content of the acidified slurry due to thereduced ammonia emission from buildings and storage. Insignif-icant effects of aeration of the acidified slurry on slurrycomposition and N turnover were detected when aeration wascarried out at a slurry temperature of 8 8C. Further studies wouldbe useful to see if aeration of acidified slurry at highertemperatures would have beneficial effects on slurry composition.

Acknowledgements

The study was supported by funding from the Danish Ministryof Food, Agriculture and Fisheries. We thank the staff of ‘The SoilOrganic Matter Group’ for skilled technical assistance and the staff

P. Sørensen, J. Eriksen / Agriculture, Ecosystems and Environment 131 (2009) 240–246246

at Infarm A/S (formerly ‘Staring Miljø’) for helping with slurrysampling.

References

Baronofsky, J.J., Schreurs, W.J.A., Kashket, E.R., 1984. Uncoupling by acetic acidlimits growth of and acetogenesis by Clostridium thermoaceticum. Appl.Environ. Microbiol. 48, 1134–1139.

Cooper, P., Cornforth, I.S., 1978. Volatile fatty acids in stored animal slurry. J. Sci.Food Agric. 29, 19–27.

Eriksen, J., Sørensen, P., Elsgaard, L., 2008. The fate of sulphate in acidified pig slurryduring storage and following application to cropped soil. J. Environ. Qual. 37,280–286.

Erisman, J.W., Schaap, M., 2004. The need for ammonia abatement with respect tosecondary PM reductions in Europe. Environ. Pollut. 129, 159–163.

Fangmeier, A., Hadwiger-Fangmeier, A., Van der Eerden, L., Jager, H.J., 1994. Effectsof atmospheric ammonia on vegetation: a review. Environ. Pollut. 86, 43–82.

Jensen, M.T., Cox, R.P., Jensen, B.B., 1995. Microbial production of skatole in the hindgut of pigs given different diets and its relation to skatole deposition in backfat.Anim. Sci. 61, 293–304.

Kai, P., Pedersen, P., Jensen, J.E., Hansen, M.N., Sommer, S.G., 2008. A whole-farmassessment of the efficacy of slurry acidification in reducing ammonia emis-sions. Eur. J. Agron. 28, 148–154.

Kell, D.B., Peck, M.W., Rodger, J., Morris, J.G., 1981. On the permeability to weakacids and bases of the cytoplasmic membrane of Clostridium pasteurianum.Biochem. Biophys. Res. Commun. 99, 81–88.

Kirchmann, H., Lundvall, A., 1993. Relationship between N immobilization andvolatile fatty acids in soil after application of pig and cattle slurry. Biol. Fertil.Soils 15, 161–164.

Munoz, G.R., Kelling, K.A., Powell, J.M., Speth, P.E., 2004. Comparison of estimates offirst-year dairy manure nitrogen availability or recovery using nitrogen-15 andother techniques. J. Environ. Qual. 33, 719–727.

Nes, P., 1979. Routine measurement of total sulphur in biological material. N. Z. J.Sci. 22, 269–272.

Sommer, S.G., Husted, S., 1995. The chemical buffer system in raw and digestedanimal slurry. J. Agric. Sci., Camb. 124, 45–53.

Stevens, R.J., Laughlin, R.J., Frost, J.P., 1989. Effect of acidification with sulphuric acidon the volatilization of ammonia from cow and pig slurries. J. Agric. Sci., Camb.113, 389–395.

Stevens, R.J., Laughlin, R.J., O’Bric, C.J., Carton, O.T., Lenehan, J.J., 1997. The efficiencyof the nitrogen in cattle slurry acidified with nitric acid for grass production. J.Agric. Sci., Camb. 129, 335–342.

Sørensen, P., Fernandez, J.A., 2003. Dietary effects on the composition of pig slurryand on the plant utilization of pig slurry nitrogen. J. Agric. Sci., Camb. 140, 343–355.

Wang, W.J., Smith, C.J., Chen, D., 2003. Towards a standardised procedure fordetermining the potentially mineralisable nitrogen of soil. Biol. Fertil. Soils37, 362–374.

Zhang, Z., Zhu, J., 2005. Effectiveness of short-term aeration in treating swinefinishing manure to reduce odour generation potential. Agric. Ecosyst. Environ.105, 115–125.

Zhu, J., 2000. A review of microbiology in swine manure odor control. Agric. Ecosyst.Environ. 78, 93–106.